Tectonophysics 570–571 (2012) 42–56

Contents lists available at SciVerse ScienceDirect

Tectonophysics

j ourna l homepage: www.e lsev ie r .com/ locate / tecto

Review Article

Earthquake nests as natural laboratories for the study of intermediate-depthearthquake mechanics

Germán A. Prieto a,⁎, Gregory C. Beroza b, Sarah A. Barrett b, Gabriel A. López c, Manuel Florez a

a Departamento de Física, Universidad de los Andes, Colombiab Department of Geophysics, Stanford University, Palo Alto, CA, USAc Department of Imaging Science and Technology, TU Delft, The Netherlands

⁎ Corresponding author.E-mail address: gprieto@uniandes.edu.co (G.A. Priet

0040-1951/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tecto.2012.07.019

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 January 2012Received in revised form 9 July 2012Accepted 21 July 2012Available online 1 August 2012

Keywords:Intermediate-depth earthquakesEarthquake nestsEarthquake clusteringRupture mechanismEarthquake locationStress drops

The physical mechanism of intermediate-depth earthquakes is still under debate. In contrast to conditions in thecrust and shallow lithosphere, at temperatures and pressures corresponding to depths >50 km, rocks ought toyield by creep or flow rather than brittle failure. Some physical process has to enable brittle or brittle-like failurefor intermediate-depth earthquakes. The two leading candidates for that are dehydration embrittlement and ther-mal shear runaway. Given their great depth, intermediate-depth earthquake processes can't be observed direct-ly. Insteadwemust rely on a combination of seismology and the study of laboratory analogs to understand them.Earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity. In thispaper we focus on three intermediate-depth earthquake nests — Vrancea, Hindu Kush and Bucaramanga, andwhat they reveal about the mechanics of intermediate-depth earthquakes. We review published studies oftectonic setting, focal mechanisms, precise earthquake locations and earthquake source physics at theselocations, with an emphasis on the Bucaramanga nest. All three nests are associated with subducting litho-sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity.In contrast, the Bucaramanga nest has a larger b-value, larger variability of focal mechanisms and showsno evidence of aftershock sequences unlike the other two. We also report for the first time finding a signifi-cant number of repeating earthquakes, some with reverse polarity.Given the nature and characteristics of earthquake nests, they can be thought as natural laboratories. Futureseismological studies of intermediate-depth earthquakes in nests will likely enlighten our understanding oftheir physical mechanisms.

© 2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432. Proposed mechanisms for intermediate depth earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.1. Dehydration embrittlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.2. Thermal shear runaway instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3. Earthquake nests in the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444. Tectonic setting of earthquake nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.1. Vrancea nest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.2. Hindu Kush nest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.3. Bucaramanga nest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5. Location of earthquakes within the Bucaramanga nest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.1. Repeating earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6. Temporal behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506.1. Aftershock sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

7. Earthquake source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507.1. Size distribution (b-values) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507.2. Focal mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517.3. Stress drops and source physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

o).

rights reserved.

43G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1. Introduction

Just what constitutes an intermediate-depth earthquake? Early defi-nitions seem to come from the expectation that improved understandingof earth structurewould provide clear definitions for distinct populations.For example,Wadati (1929) defines shallow earthquakes as b60 km andintermediate earthquakes as 100–200 km. While Gutenberg and Richter(1949), use 70–300 km in most (but not all) of their publications.Presently, without fully understanding the mechanism by which inter-mediate and deep earthquakes occur, it is difficult to define a depthcut-off. Another definitionmight use the distribution of global seismicity,defining the start of intermediate-depth seismicity at the onset of theapproximately exponential decay in seismic frequency, extending thisrange to ~300 km. We choose a definition based on the maximumdepth of inter-plate subduction zone earthquakes of ~50 km (Bileket al., 2004). This is also the depth at which well-developed aftershocksequences become less frequent (Frohlich, 2006).

Earthquakes deeper than 50 km represent approximately 25% ofglobal seismicity (Frohlich, 2006). Intermediate-depth earthquakes,the focus of this contribution, are those earthquakes with depthsranging from about 50 to 300 km (Frohlich, 2006; Houston, 2007).They occur exclusively at convergent plate boundaries withinsubducting lithosphere, although in some cases the subduction thatgave rise to the lithosphere at depth may no longer be active at thesurface (Chung and Kanamori, 1976).

Intermediate-depth earthquakes occur at temperatures and pres-sures above the point where ordinary fractures ought to occur, anddespite their abundance, the physical mechanism behind them remainsuncertain (Frohlich, 1989; Green and Houston, 1995; Houston, 2007).Perhaps the leading hypothesis is dehydration embrittlement, inwhich hydrated minerals release fluids at particular pressures andtemperatures allowing brittle failure to occur (Frohlich, 1989; Greenand Houston, 1995; Hacker et al., 2003; Kirby et al., 1996a; Wiens,2001). Another plausible mechanism is known as thermal shear run-away instability (Ogawa, 1987; Hobbs and Ord, 1988; Frohlich, 2006;Houston, 2007; Keleman and Hirth, 2007; John et al., 2009), whichwould occur through a positive, rapid feedback between shear strainlocalization and thermal heating.

A particular kind of intermediate-depth earthquake concentrationor clustering known as an earthquake nest is characterized with highactivity rate that is isolated from nearby activity. An earthquake nestis different from an aftershock sequence or an earthquake swarmbecause the activity rate at nests is both high and persistent overtime.

The three most famous and well-studied earthquake nests arethe Vrancea, Romania, Hindu-Kush, Afghanistan; and Bucaramanga,Colombia nests. In this review, we discuss all three of these nestsbut devote most of our attention to the Bucaramanga nest, wherewe have started a thorough analysis of regional seismic data fromthe Colombian seismic network. Although there may be other nests(Pulpan and Frohlich, 1985; Sacks et al., 1967; Zarifi and Havskov,2003) we will concentrate on these three in this paper.

Earthquakenests provide perhaps the best setting for understandingthe physical mechanism responsible for intermediate-depth earth-quakes. They represent high temporal and spatial concentrations ofearthquakes that have been recorded at teleseismic and in some cases,regional and local distances. The magnitudes range from very small

(Mb2.0) to large (M>7.0), and they occur within a limited volume,so that attenuation and other path corrections will have similar effectson their waveforms. Complex tectonic and geodynamic models havebeen proposed to explain the high earthquake concentration at eachearthquake nest, but less has been done on the physical mechanismthat enables brittle-like failure.

Intermediate-depth earthquake nests also pose a significant seismichazard. Significant earthquakes with ~1000 fatalities have occurred inRomania (Mw 7.5, 4 March, 1977; Frohlich, 2006), and Hindu Kush(Mw 6.9, 20 August, 1988; Frohlich, 2006) and significant damage inBucaramanga and nearby cities (M 6.3, 27 July, 1967; Ramirez, 2004).

We do not attempt to cover the entire literature on earthquakenests, but rather we wish to highlight some key aspects and seismolog-ical observations of earthquake nest seismicity and how these may beuseful for constraining the physical mechanism of intermediate-depthearthquakes. The reader may find useful reviews on deep earthquakes(Frohlich, 1989; Green and Houston, 1995; Green and Marone, 2002;Houston, 2007; Kirby et al., 1996a), on earthquake nests (Zarifi andHavskov, 2003), and of course an entire book dedicated to deep earth-quakes (Frohlich, 2006).

This paper is organized as follows: First we describe candidate physi-cal mechanisms that may explain intermediate-depth earthquake occur-rence, thenwe describe earthquake nests from a general perspective anddiscuss the tectonic setting of the Vrancea, Hindu Kush and Bucaramanganests. Next we discuss in more detail precise earthquake locations andrepeating events, aftershock productivity, Gutenberg–Richter statistics,focal mechanisms, and source parameters with a special attention torecent results from the Bucaramanga nest and how it compares to theother nests. Finally, we discuss how all of these seismological observablesmay contribute to the main goal of better defining the mechanism ofintermediate-depth earthquakes and how this may point towards futureresearch on earthquake nests.

2. Proposed mechanisms for intermediate depth earthquakes

In the following we outline the two primary andmost widely accept-ed mechanisms proposed for the generation of intermediate-depthearthquakes. A third mechanism known as metastable phase transition(Frohlich, 2006; Green and Houston, 1995; Kirby et al., 1996a) but itis unlikely to be relevant for intermediate-depths (50–300 km) and wewill not discuss it further.

2.1. Dehydration embrittlement

In this mechanism, hydrated minerals in the subducting slabundergo phase changes to anhydrous forms, releasing fluids in theprocess that counteract the high normal stresses expected at depth.The reduced effective normal stress that results is hypothesized toenable brittle failure (Frohlich, 1989; Green and Houston, 1995; Hackeret al., 2003; Kirby et al., 1996a,b). Rayleigh and Paterson (1965)suggested that partial dehydration of serpentinite caused the embrittle-ment and weakening of the samples in experiments performed at700 °C and 0.5 GPa. Multiple experiments at different pressures andtemperatures confirm that dehydration embrittlement can operategiven the availability of fluids (Chollet et al., 2009; Jung et al., 2004;Meade and Jeanloz, 1991).

44 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

The transformation of basaltic oceanic crust to eclogite provides alikely source of fluids (Kirby et al., 1996b). The distribution of earth-quakes in Japan supports the dehydration embrittlement hypothesis,in that it is consistentwith the predicted depth of dehydration reactions(Peacock and Wang, 1999). Probably the most compelling argument infavor of dehydration embrittlement comes from thermo-petrologicalmodels that are able to explain double seismic zones in a wide varietyof slab conditions (Hacker et al., 2003). Seno and Yamanaka (1996)and Jiao et al. (2000) found that intermediate-depth earthquake faultorientations could be explained by re-activation of faults that wereformed, and hydrated, in the outer-rise. Kiser et al. (2011) imagedlarge M>7.0 intermediate-depth earthquakes using backprojectionand found that many of these large events are comprised of largesub-events that are separated in depth along sub-horizontal faults.They suggested a combination of dehydration that preferentiallyweakens pre-existing sub-horizontal faults in the subducting crustand dynamic triggering to explain their observations.

2.2. Thermal shear runaway instability

The shear runaway hypothesis holds that a localized shear or plasticinstability forms and leads to a runaway process due to increasedweak-ening with increasing temperature (Griggs and Handin, 1960; Ogawa,1987; Hobbs and Ord, 1988; Frohlich, 2006; Houston, 2007; Kelemanand Hirth, 2007; John et al., 2009). In this model, localized shear pro-duces a positive feedback between temperature dependent rheologyand shear deformation that generates viscous heating (Houston,2007) and leads to the exponential acceleration and extreme localiza-tion of shear strain.

Geological observations support this possibility through brittleand viscous deformation and the formation of shear zones andpseudotachylite (e.g., Andersen et al., 2008; John and Schenk, 2006;John et al., 2009). Numerical experiments also provide support forthis mechanism (Kelemen and Hirth, 2007; John et al., 2009). Ahybrid model invokes a twist in which local perturbations to materialproperties, perhaps due to the presence of a dehydrating phase, initi-ate a process that leads to a thermally induced shear runaway (Johnet al., 2009).

Seismological evidence for a possible thermal shear runaway asso-ciated with partial melting was presented by Kanamori et al. (1998).Estimates of temperature rise for the 1994 M8.2 deep Bolivia earth-quake suggest that melting would occur if fault zone thickness isless than 0.3 m. Based on estimates of radiation efficiency (see alsoVenkataraman and Kanamori, 2004) the authors conclude that a largeamount of the available energy was dissipated, perhaps by melting. Aspointed out by Andersen et al. (2008) the geological evidence ofpseudotachylite formation even for small faults at intermediate-depthssuggests that very high stress drops (200–500 MPa) are observed and asignificant amount of energy is converted to heat during faulting.

3. Earthquake nests in the Earth

An earthquake nest is a volume of intense seismic activity that isisolated from nearby activity (Richter, 1958). Nests are distinct fromaftershock sequences because they have a persistent high activityrate while an aftershock sequence has high activity rate after themain-shock but that dies down over a period of time (Pavlis andDas, 2000). Nests are also distinct from earthquake swarms, becauseeven though earthquake swarms are concentrated areas of intenseseismicity, they are localized in time. Earthquake nests, on the otherhand, persist for (at least) decades.

The definition of what represents an earthquake nest is somewhatarbitrary. There are many regions where clustering of seismic activityis observed, many times associated with subducting slabs, that may becategorized as earthquake nests. Zarifi and Havskov (2003) suggest

that a seismic nest is defined by stationary seismicity within a volumethat is substantially more active than its surroundings.

There are strong similarities and strong differences between theseismic nests discussed in this contribution so that a single set ofcriteria to define seismic nests is not available. In fact, other seismicnests have been suggested in the past (see for example an attemptto evaluate some of them in Zarifi and Havskov, 2003) but very littlework has been published on these clusters.

So, in conclusion even though we will use the term earthquakenest throughout the paper, this term represents a particular kind ofintermediate-depth clustering of seismicity that has been observedfor a long time. Other possible nests may be found or confirmed asglobal and regional networks improve. Also, the use of the termnest here does not preclude other clustering as being relevant anduseful for understanding the mechanism of intermediate-depthearthquakes.

It was probably Santo (1969a, 1969b) who first used the termseismic nest to refer to the intermediate depth earthquake cluster inBucaramanga (Schneider et al., 1987). The term nest was used todistinguish it from earthquake swarms or aftershock sequences. Atthat time, it was evident that at least for the Bucaramanga nest, theseismicity was isolated in space, surrounded by areas of much lowerseismic activity rates (Frohlich, 1989).

Several other intermediate-depth clusters have been called earth-quake nests and have been compared to the Bucaramanga nest. Thetwo most well-known of these earthquake nests are the HinduKush, Afghanistan (Chatelain et al., 1980) and the Vrancea, Romania(Oncescu, 1984; Oncescu and Trifu, 1987). Although other earth-quake nests have been suggested in the literature (Socampa Nest inPeru, Sacks et al., 1967; the Iliamma Cluster beneath Cook Inlet, Alaska,Pulpan and Frohlich, 1985; Fiji, Argentina-Chile border region andEcuador, Zarifi and Havskov, 2003), the Vrancea, Hindu Kush, andBucaramanga nests will be used throughout this paper for discussionbecause a significant literature by multiple authors exists for each. Inthis paper much attention is centered in the Bucaramanga nest due tolocal seismic data provided by the Colombian Seismic network (RSNC).

Fig. 1 shows the location of the three prominent seismic nests andlists some of themost significant features for comparison. The numbersshown about their seismic activity correspond to locations from theISC catalog (International Seismological Centre, 2001) and additionalreferences (see figure caption).

The Vrancea seismic nest is located around 45.7°N and 26.5°E,with an areal extend of around 20×50 km and depths between 70and 180 km (Sperner et al., 2001). Significant earthquakes have beenreported (e.g., Böse et al., 2009; Oncescu et al., 1999; Oth et al., 2008,2009) with magnitudes of Mw 7.4 (1940), Mw 7.7 (1977), and Mw7.1(1986), in some cases with more than 1000 fatalities (Oncescu et al.,1999). It is probably one of the best-studied cases due to its proximityto a large urban area.

The Hindu Kush nest is broadly associated with the collision of theIndian and Eurasian plates and shows an S-shaped form (Pegler andDas, 1998). It is the most active of the three nests discussed here.Santo (1969b) first suggested that the nest had dimensions of about30 km at a depth of around 215 km and multiple researchers haveproposed widely different tectonic interpretations (see Section 4.2).This nest is also capable of significant earthquakes M>7.0, with atleast 15 earthquakes in the last century, some of them with over1000 fatalities (Frohlich, 2006).

The Bucaramanga nest is a unique concentration of seismicactivity with a depth concentration around 160 km below the Earth'ssurface and with approximately one M4.7 earthquake per month,near the city of Bucaramanga, Colombia (Trygvasson and Lawson,1970; Pennington et al., 1979; Schneider et al., 1987, and manyothers). Compared with other concentrations of intermediate-depthearthquakes, the Bucaramanga nest has a higher activity rate, a smallersource volume, and a clearer isolation from nearby activity (Schneider

Fig. 1. Location map of the Vrancea, Hindu Kush and Bucaramanga nests. Insets list relevant features for each based on the ISC catalog from 2000 to 2010. Extension and depth rangeof each nest are taken from Sperner et al., 2001 (Vrancea), Nowroozi, 1971 (Hindu Kush) and Schneider et al., 1987 (Bucaramanga). Number of earthquakes with M>4 is listedbased on ISC data from 2000 to 2010. Histogram shows seismicity depth concentration for each nest.

45G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

et al., 1987). Zarifi and Havskov (2003) showed that the Bucaramanganest has at least five times more events per unit volume compared toother nests. It is believed (Ramirez, 2004) that the Bucaramanganest has experienced at least a M6.3 earthquake according to the ISC,although it was Mb 6.0 according to Cortes and Angelier (2005), inthe last century, leading to significant damage in Bogota and nearbycities.

As suggested in Fig. 1, the Bucaramanga nest represents thehighest concentration of intermediate-depth seismicity in the world.Even though the Hindu Kush area presents a larger number of events,the Bucaramanga nest has the largest number of earthquakes per unitvolume of the three as can be seen from the histogram inset. As wewill show later, using local earthquake data, that the concentrationand isolation from nearby activity becomes even more apparent.

4. Tectonic setting of earthquake nests

The tectonic setting of each of the nest is complex and in all casesthere is debate on important aspects of the tectonic setting, specificallywhether subduction or delamination is involved, or if plate collisionat depth is responsible for the high activity rates. Below we brieflydescribe the general tectonic setting and discuss some of the proposedmodels for each of the three regions. This is not intended to be compre-hensive and the reader is encouraged to consult the literature in eachcase for further information.

4.1. Vrancea nest

The Vrancea region is located in the SE-Carpathian mountainsystem and is part of the greater Alpine fold-and-thrust system(Koulakov et al., 2010). The tectonic history of the Carpathians isassociated with the retreating subduction of the Tethys Ocean towardsthe SW–W (Csontos, 1995; Sperner et al., 2002; Stampfli and Borel,2002). Subduction ceased in the northern part about 12–14 Ma agowith the arrival of the buoyant continental East European lithosphereinto the subduction zone and later continued towards the SE (Jiricek,1979). The progression of subduction from NW to SE is corroboratedby foredeep depocenter ages (Meulenkamp et al., 1997) and systematicdecrease in age of volcanic activity (Linzer, 1996; Pècskay et al., 1995;Szakacs and Seghedi, 1995). Plate convergence in the northern andeastern Carpathians seems to be currently inactive, while in the SECarpathians, where the Vrancea nest is located, active processes arestill taking place. The Vrancea region thus marks the youngest part ofthe subduction/collision along the Carpathians.

Fig. 2 shows the location of seismicity in the Vrancea region basedon the ISC catalog (2000–2010). The map view shows the highlyconcentrated seismicity of the Vrancea nest over an area approxi-mately 30×70 km. The NW-SE cross-section shows almost vertical,finger-shaped intermediate-depth seismicity that is spatially separatedfrom the shallow activity.

Sperner et al. (2001) proposed one of the most well-accepted inter-pretations of the tectonics in the SE Carpathian region (e.g., Martin et al.,2006; Wenzel et al., 2002). Based on seismic tomography, earthquakelocations, and stress patterns Sperner et al. suggested that there is aclear separation between the shallow crust and the subducted slab,either by slab break-off or delamination. This decoupling is completein the northern and eastern Carpathians where subduction ceasedwhile in the Vrancea region (Fig. 2) it is in the process of detachmentand some coupling is still present. Fig. 3 illustrates the slab break-offmodel of Sperner et al. (2001) where slab segments are detached inthe northern Carpathians while in the southern part, slab segmentsmay still be mechanically coupled to the shallower lithosphere. Thismodel explains the absence of intermediate-depth seismicity in thenorthern and eastern Carpathians, due to diminished slab-pull oncedetachment was completed.

Previous tomographic studies (Fan et al., 1998; Fuchs et al., 1979;Koch, 1985; Lorenz et al., 1997; Oncescu, 1982, 1984; Popa et al., 2001;Wortel and Spakman, 1992) had been performed based on regionaland local seismicity. More recent tomographic results (Koulakov et al.,2010;Martin et al., 2006) based on teleseismic and local data respectivelyhave confirmed the presence of a nearly vertical high-velocity regionextending from about 60 to more than 200 km depth (Martin et al.,2006 suggest that the high-velocity region extends to 350 km depth).

The geodynamic model of Martin et al. (2006) based onteleseismic tomography suggests a high-velocity volume that theyinterpret as a subducted, but not fully detached, slab. The slab hoststhe Vrancea nest where it is coupled with the continental block andis aseismic where it is not.

Koulakov et al. (2010) do not interpret the presence of thishigh-velocity body as a slab break-off, but rather as delamination ofcontinental material. Their interpretation is as follows: Due to conti-nental collision, lithospheric thickening leads to higher P-T conditionsand the formation of eclogite from the mafic crust and upper mantle.Once a critical mass of eclogite is concentrated, the denser layerbegins to drop and may produce the high stress conditions that driveseismic deformation in the Vrancea nest. Koulakov et al. (2010) suggestthat slab break-off may not be able to explain the presence of seismicityin the central part (highest velocity anomaly) of the interpreted slab.

Fig. 2. Vrancea area seismicity based on the 2000–2010 ISC catalog. Left panel shows the regional location of the seismicity (color coded by depth) and the areal extent of theVrancea nest. Right panel shows a cross section between A-A′ showing the vertical extend of the Vrancea nest and its apparent separation from shallow seismicity. Only welllocated events, based on low rms arrival time residuals and number of reporting stations are shown.

46 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

4.2. Hindu Kush nest

The Hindu Kush and Pamir areas are located in the westernsyntaxis of the India–Eurasia collision zone. The tectonic history(Windley, 1988) is mainly associated with the closure of the TethysOcean, the northward accretion of multiple plates and the final colli-sion with the Indian plate along the Indus suture zone (Dewey andBird, 1970; Gansser, 1966, 1977). After collision, significant crustalshortening has taken place as the Indian plate indents into Eurasia(DeMets et al., 1990; Pegler and Das, 1998).

Since the 1970s, the tectonic arrangement that would explain thedistribution of earthquakes in the Pamir Hindu Kush has been underdebate. The main feature clearly observed since Nowroozi (1971)has been an “S-shaped” configuration of the intermediate-depthseismicity. Billington et al. (1977) used earthquake locations andfocal mechanisms to suggest that a preexisting oceanic lithospheresubducted due to converging Indian and Eurasian plates. They identi-fied a contorted (S-shaped) Benoiff zone with a north-dipping zone inthe Hindu Kush and a south-east dipping zone in the Pamir area. Theyinterpreted the contorted volume as a single slab but did not discountthe possibility of two opposing subduction zones.

In contrast, Chatelain et al. (1980) and Roecker et al. (1980) usingdata from microseismic studies concluded that two subducted slabs

Fig. 3. Model for slab break-off around the Carpathian arc. The subducted slab is alreadycoupled and hosts the Vrancea earthquake Nest (From Sperner et al., 2001 with permission

with opposing directions were involved in the region. Later studieshave reached similar conclusions (Burrman and Molnar, 1993; Fanet al., 1994; Hamburger et al., 1992), although it must be noted thatin some cases it has been the presence of seismic gaps that has leadto the idea of a two slab model. More recently, using teleseismictomography, seismicity and thermo-kinematic numerical modelingNegredo et al. (2007) also suggested a two-slab model. Lou et al.(2009) relocated seismicity using hypoDD (Waldhauser and Ellsworth,2000) and proposed a two-slab model, but with a collision of the slabsat about 130 km depth.

A thorough relocation of more than 6000 earthquakes in the areain combination with focal mechanisms led Pegler and Das (1998) andlater Pavlis and Das (2000) to support the initial model of Billingtonet al. (1977). Pegler and Das (1998) suggest that the seismicity andthe alignment of the T-axes along the Pamir area with the contortedsubduction can be explained with a single S-shaped seismic zone.Similar to what is observed in Vrancea, the lack of seismicity above70 km depth in the Pamir region suggests that the slab has becomedecoupled from the surface.

Fig. 4 illustrates the seismicity features in the Hindu Kush, usingwell-located earthquakes from the ISC catalog between 2000 and2010. Both overall seismicity and depth sections clearly show theS-shaped Benioff zone (or zones). As previously suggested in the

detached from the continental lithosphere while the SE segment is still mechanicallyby Wiley).

Fig. 4. Seismicity in the Hindu Kush Nest area. Middle panel shows overall seismicity color coded by depth. Red box shows area plotted in the right panel rotated 30° clearly showingthe S-shaped nature of subduction in the area as well as the different maximum depths of the Hindu Kush and Pamir areas. Red lines (A-A′ and B-B′) correspond to thecross-sections shown in the left panel. Note how in the western cross-section (Hindu Kush) subduction dips towards the north, while in the Pamir region (B-B′) subductiondips towards the S–SE.

47G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

literature, the cross sections across the Hindu Kush and Pamir regionsshow opposing dips. It is also evident that the maximum depth ofseismicity is different in the two regions.

A recent work by Lister et al. (2008) suggests that the Hindu Kushnest seismicity is associatedwith slab break-off, similar to some sugges-tions for the Vrancea nest. Using the global CMT solutions, the authorssuggest that the intermediate-depth seismicity in the area is associatedwith slab beak-off, ductile faults and shear zones, similar to what isobserved at a much smaller scale in gneisses. Their model is focused onthe Hindu Kush area and more specifically on the reduced and thinnedseismicity at around 150 km (see cross-section A-A′) and does notdiscuss in detail whether one or two slabs are present.

4.3. Bucaramanga nest

The tectonic setting in the region surrounding the Bucaramanga nestis complicated and there are a variety of competing models to explainthe active tectonics of the region (Christeson et al., 2008; Cortes andAngelier, 2005; Higgs, 2009; Pennington et al., 1979; Pindell andKennan, 2009; Suter et al., 2008; Taboada et al., 2000; van der Hilstand Mann, 1994; Zarifi et al., 2007). The models differ to such a degree

Fig. 5. Seismicity in northern South America based on the RSNC catalog color-coded by depand south of about 5°N, which some authors associate with a continental tear (C. Vargas, per(B) and the interpreted Caribbean subducting slab (A), where the Bucaramanga nest is loca

that it is difficult to say with certainty which slab, or slabs, theBucaramanga nest seismicity is associated with. The seismicity couldoccur within a subducted portion of the Caribbean plate, within theNazca plate, or perhaps at the interaction/collision of two plates(Zarifi et al., 2007).

Northern Colombia is located at the intersection of three plates:the Nazca to the west, the Caribbean to the north and the SouthAmerican. Several models (Cortes and Angelier, 2005; Pennington,1983; Taboada et al., 2000) suggest that a portion of the Caribbeanplate is subducting southeastward, and the Bucaramanga nest is locatedwithin it. In thesemodels, theNazca plate also subducts eastward but tothe south of the Bucaramanga nest. Anothermodel proposed by van derHilst andMann (1994) suggests that the Bucaramanga nest is located inthe Nazca plate in a segment they call the redefined Bucaramangaslab. A third model suggested recently by Zarifi et al. (2007) based onlocations and focal mechanisms of Bucaramanga nest earthquakes sug-gests that the collision between theNazca and Caribbeanplates at depthis responsible for the Bucaramanga nest seismicity.

Fig. 5 shows the general seismicity in northwestern South America.From seismicity located by the RSNC (Colombian seismic network)one can easily define two distinct regions, one on the south (south of

th. Map view shows major plate boundaries. Seismicity shows distinct behavior norths. comm., 2011) Right panels show two cross-sections along the Nazca subduction zoneted.

48 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

about 5°N) and one to the north. Both regions show a dipping Benioffzone, but it is not clear whether these two regions are connected(similar to the S-shaped zone in Hindu Kush) or if they represent twodifferent subducting plates.

It may seem surprising, but the Bucaramanga nest is not easilydiscerned from the seismicity shown in map view or in cross section(Fig. 5, cross section A). If the Bucaramanga nest is the highestconcentration of intermediate-depth seismicity, shouldn't it be obviousfrom the seismicity? Fig. 6 shows the location of the Benioff zone basedon the seismicity of Fig. 5. It also shows (bottom panel) a 3D isosurfacemap representing the number of earthquakes per unit volume (numberof earthquakes per 5×5×5 km volume). The Bucaramanga nest is nowclearly marked by the volume with a density of over 2000 earthquakes,while the density in other parts of the country is well below 20 earth-quakes. The Bucaramanga nest is interpreted inmany cases as occurringon the subducting Caribbean plate (Cortes and Angelier, 2005;Pennington et al., 1979; Schneider et al., 1987; Taboada et al., 2000),but there are few events above 70 km depth (see Fig. 5, cross-sectionsA) that may suggest that, similar to what is observed in Vrancea orHindu Kush (Lister et al., 2008), the slab is mechanically detached oris in the process of break-off.

In contrast to Zarifi et al. (2007) the earthquake locations andBenoiff zones (Figs. 5 and 6) do not support a collision between twosubducting plates. Instead it supports the model of Taboada et al.(2000) and Cortes and Angelier (2005) in that the nest is associatedwith a subducting Caribbean plate, although the data does not provideinformation on whether the plate is in the process of slab break-off.

5. Location of earthquakes within the Bucaramanga nest

Trygvasson and Lawson (1970) used seismicity located in the1960s and concluded that in principle the distribution of hypocenterswas co-located, and suggested a volume of about 10 km in extent.Similarly Santo (1969a, 1969b) were not able to distinguish the

Fig. 6. Benioff zone and earthquake density in northern South America. Top panel showsthe estimated top of the Benioff zone based on the seismicity in Fig. 5. Bottompanel showsthe Benioff zone (transparency of 40%) and isosurface volumes representing differentnumber of earthquakes (20, 200, 2000) on a small volume (5×5×5 km). A clear yellowiso-surface represents a region with over 2.000 earthquakes inside the small volumeand where the Bucaramanga nest is located, clearly isolated from any significant nearbyseismicity.

Bucaramanga nest from a point source. Later, portable seismic networkswere deployed in 1976 and 1979, which recorded 27 nest earthquakes(Penington, 1981; Pennington et al., 1979; Schneider et al., 1987). Using89 well-located events (Schneider et al., 1987) suggested that thevolume of the Bucaramanga nest is 8×4×4 km. Both Pennington etal. (1979) and Schneider et al. (1987) using these local deploymentswere able to show that the seismicity actually came from a volumeand not from a point source.

Frohlich et al. (1995) relocated Bucaramanga nest earthquakesusing teleseismically determined phases (including depth phases)and estimated a 13×18×12 km volume. Zarifi et al. (2007) usinglocal earthquake data suggest an elongated Bucaramanga nest withmajor axes of 24 km and 15 km width. But neither of the relocationstudies so far has found planar features in the Bucaramanga nest.Recent results of intermediate-depth earthquakes elsewhere suggestthat seismicity may align on sub-horizontal (or sub-vertical) planes(Kiser et al., 2011; Warren et al., 2007, 2008).

Using the data from the Colombian national seismic network(RSNC) we perform earthquake relocations with a double-differencealgorithm (Waldhauser and Ellsworth, 2000). We started from catalogphase picks from the national calatog and were reviewed manually.Waveform cross-correlations were also used to improve the selectedpicks. Most seismic stations have both P and S wave picks and in somecases additional arrival times were picked during processing. Fig. 7shows relocated Bucaramanga nest earthquakes M>3.5 using 15regional seismic stations from the RSNC. The error ellipses shown inFig. 7 are estimated using a bootstrap resampling technique (Prietoet al., 2007; Shearer, 1997; Waldhauser and Ellsworth, 2000).

The high-quality locations have average errors of about 2 kmhorizontally and 1 km vertically, as the long axis is generally on ahorizontal plane. Fig. 7 suggests that the Bucaramanga nest is not avolume of seismic activity, but that it shows planar features, somesub-horizontal and some sub-vertical. This observation, if corroboratedwith waveform cross-correlations and more dedicated deploymentscould prove fundamental for better understanding the physical mecha-nismand structural features associatedwith seismic nests. These resultsalso suggest that the volume of the nest is likely larger than what wasproposed by Schneider et al. (1987) but it has internal structures andplanar features that were not evident until now.

5.1. Repeating earthquakes

Earthquake nests have seismicity, which is highly localized inspace and has the tendency to have earthquakes that occur repeatedlyat the same or almost the same location. The question that arises iswhether a significant number of these earthquakes represent repeatedrupture of the same fault plane. This feature has certainly been observedat shallow depths (Marone et al., 1995; Poupinet et al., 1984; Schaff andBeroza, 2004; Schaff and Richards, 2004; Schaff et al., 1998; Uchida etal., 2007) and may be present for deep moonquakes as well (Frohlichand Nakamura, 2009).

Typically, repeating earthquakes can be identified because theyexhibit nearly identical seismograms at common stations, suggestingnearby locations, similar focal mechanisms and rupture of the same as-perity or patch (see for example Uchida et al., 2007). As far as we know,there have not been any systematic searches for repeating earthquakesin intermediate-depth earthquake nests or even for intermediate-depthearthquakes. An example of repeating deep earthquakes was presentedbyWiens and Snider (2001), where they found clusters of M4-5 earth-quakes with identical waveforms in the Tonga slab (560–600 kmdepth). Zhang et al. (2008) searched for doublet or multiplet clustersof similar waveforms at the Bucaramanga nest for studying the rotationof the inner core and found a significant number of similar waveformsthat they actually designated as a 9-event multiplet.

As pointed out by Houston (2007), the detection of similar wave-forms does not directly mean that the events are rupturing the same

Fig. 7. Relocated Bucaramanga Nest earthquakes. E-W and N-S cross sections of relocated seismicity and error ellipses using hypoDD (Waldhauser and Ellsworth, 2000).Sub-horizontal features of concentrated activity are evident from the relocations and within errors at least three distinct regions (160, 151 and 148 depths) can be identified.

49G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

fault patch. Evenusing precise locations, uncertainties are at least 1–2 km(see Fig. 7) meaning earthquakes may be occurring on sub-parallel faultplanes. Nevertheless, the observation of possible repeating eventsand their precise location is key for constraining the physical mechanisminvolved in intermediate-depth and deep earthquakes.Wiens and Snider(2001) interpret the repeating earthquakes they found as rupturingthe same fault patch, and suggest that these earthquakes are due to

Fig. 8. Similar and reverse polarity waveform records for repeating Bucaramanga nest earthleast 5 stations) with respect to a “master” event (ID 63214) are shown. For each panel greenegative CC), while red waveforms have equal polarity. Bottom signals (thick waveforms) rehas been flipped for comparison purposes). Extremely precise locations are required to detpolarity events occur on faults sub-parallel to the normal polarity earthquakes.

a thermal shear runaway process. Mechanisms involving dehydrationembrittlement or phase transformations might not be expected to fosterrepeating earthquakes.

We have recently started a systematic search for repeating earth-quakes in the Bucaramanga nest using the local seismic network(RSNC). We have found a significant number of very similar waveforms(correlation coefficient CC>0.9) at multiple stations. Waveform

quakes (yellow star). Records at multiple stations of similar waveforms (CC>0.9 in atn waveforms represent waveforms with reverse polarity (w.r.t. the master event, largepresent the stack of the reverse and normal polarity waveforms (note that green stackermine whether the events are occurring on the same faults, or whether the reversed

Fig. 9. Aftershock decay properties for the Hindu Kush, Vrancea and Bucaramanganests. Data for Hindu Kush (top panel) taken from Pavlis and Hamburger (1991),each color symbol represents one of the three earthquakes that showed clearaftershock-like behavior. Vrancea data (middle panel) from Enescu et al. (2008). TheRSNC catalog is used for the Bucaramanga nest data (bottom panel). Each panelshows the minimum magnitude of events based on estimated catalog completeness.The Bucaramanga nest shows no clear Omori-like behavior, while the other twonests show aftershock sequences. Note the large value for background seismicity forthe Bucaramanga nest.

50 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

similarity holds for at least 15 s after P and S wave arrivals, suggestingthat these earthquakes occur at very short separation distances fromeach other and that focal mechanism and source complexity are similar.

Perhaps more interesting, however, we found also reversed polaritywaveforms, that show the same waveform complexity, but with thepolarity of ground motion reversed (Fig. 8). The presence of reversedpolarity earthquakes has been previously observed in volcanic regionsas well as in deep moonquakes (Frohlich and Nakamura, 2009). Thepresence of reversed polarity earthquakes is also in agreement withthe observed variation of focal mechanisms in the Bucaramanga nest(Cortes and Angelier, 2005; Frohlich and Nakamura, 2009; Frohlichet al., 1995). We have made a careful quality control and in Fig. 8 pres-ent those events where abs(CC)>0.9 is observed in at least 5 stations(negative CC for reverse polarity events). The DD relocations of therepeating events show that reversed polarity earthquakes are locatedon sub-parallel faults with respect to normal polarity ones, and areseparated by 1–2 km. Unfortunately this distance is similar to theuncertainties in relocating these events, so it is not possible to concludewhether these are repeating and “anti-repeating” earthquakes, but thisis clearly a topic for future research.

6. Temporal behavior

6.1. Aftershock sequences

Trygvasson and Lawson (1970) analyzed seismicity in theBucaramanga nest between 1962 and 1968 and noted that there didn'tseem to be any evidence of swarm, aftershock or foreshock sequences.The behavior of aftershock sequences for intermediate-depth anddeep earthquakes is still under debate, mainly because the number ofaftershocks is small at these depths, and in just a few cases have 20 ormore aftershocks been reported (Frohlich, 2006). There are someexamples of very large deep earthquakes with no aftershocks at all(Frohlich, 2006). There are other examples with well-developed after-shock sequences (Wiens et al., 1997) such as the Bolivian 1994 M8.3earthquake where a significant number of aftershocks were detected(Myers et al., 1995).

In the three earthquake nests variable aftershock productivity isobserved. Pavlis and Hamburger (1991) presented strong evidencefor aftershock sequences in the Hindu Kush area. They studied 40earthquakes (M>5.5) and found that only 3 of these earthquakespresented clear aftershock productivity. The three events that showedaftershock sequences had very similar locations, focal mechanismsand magnitudes and seem to follow an Omori law with p ~1.0.

In the Vrancea nest, very clear aftershock sequences have beendetected after the major M ~7.0 earthquakes in 1977, 1986 and 1990(Enescu et al., 2008; Fuchs et al., 1979). Even though the number ofaftershocks is far less than for shallow sequences, Enescu et al. wereable to detect more than 300 aftershocks M>2.8 for all three examplesand showed that at least for the 1990 sequence, aftershock productivityfollowed an Omori law with p=0.83.

In the Bucaramanga nest aftershock productivity has not beenreliably detected. Trygvasson and Lawson (1970) found that theseismicity in the Bucaramanga nest followed a Poisson distributionin time. Frohlich et al. (1995) used the ratio test (e.g., Van der Elstand Brodsky, 2010), where one takes the ratio of the time of a preced-ing event and the time of the event following the mainshock, andfound that the ratios obtained were undistinguishable from a Poissondistribution. No evidence of aftershock sequences was observed.

Fig. 9 shows aftershock productivity for the three earthquakenests, as discussed above, taken from Pavlis and Hamburger (1991)and Enescu et al. (2008). For the Bucaramanga nest we take the cata-log from the RSNC and use the method proposed by Davis andFrohlich (1991) by aligning all earthquakes with ML>4.5 at timezero and counting the number of events after the mainshock time.As can be observed and compared to the results in Hindu Kush and

Vrancea, the Bucaramanga nest has little evidence of aftershockproductivity. It is possible that this productivity is masked by thelarge background seismicity and the fact that there haven't beenany large (M>6.5) earthquakes in the Bucaramanga nest.

7. Earthquake source

7.1. Size distribution (b-values)

Both shallow and deep earthquakes follow Gutenberg–Richterstatistics, i.e., that over a given area or volume, the number of smallearthquakes compared to the large ones fits a power low distributionof the form:

logN ¼ a−bM;

51G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

where N is the number of earthquakes of magnitudeM, a is a constantwhich describes the productivity of earthquakes, and b (sometimesknown in the literature as b-value) gives the fall-off rate in thenumber of large earthquakes compared to the smaller ones. Thefrequency–magnitude relation was originally defined in this manner,but it is commonly applied with M representing the cumulativenumber of earthquakes of that magnitude or greater.

It is clear that a values for the Hindu Kush nest are higher thanthose in the Vrancea and Bucaramanga nests (Fig. 1); there is a largertotal number of earthquakes (more productivity) in Hindu Kush. Theb-value is a different story, and based on global catalogs of both deepand shallow earthquakes the b-value is close to 1.0 (Frohlich andDavis, 1993; Kagan, 1999). A b-value of 1.0 indicates that there are10 times more earthquakes for every unit magnitude decrease. If weassume b=1 in the Hindu Kush nest and we have 549 M>4 earth-quakes in the ISC catalog, we would expect about 5500 M>3 andonly 55 M>5 earthquakes (see Fig. 10 below).

Spatial and temporal variations of b-values for shallow and deepearthquakes have been reported (Frohlich and Davis, 1993; Trifuand Radulian, 1991), but it is more evident for spatial variation withdeep earthquakes (e.g., Frohlich, 1989; Frohlich and Davis, 1993;Giardini, 1988). A very significant and robust difference is observedbetween the slopes of deep earthquakes at Tonga-Karmadec andSouth America (e.g., Frohlich, 2006; Houston, 2007). Some authorshave interpreted the spatial variations of b-values as a consequenceof the thermal parameters of slabs (Wiens, 2001; Wiens and Gilbert,1996) or shear stresses near the source region (Amitrano, 2003;Scholz, 1968; Schorlemmer et al., 2005; Wiemer and Wyss, 2002;Wyss, 1973) although there doesn't seem to be any such relationshipwith stress drops of small earthquakes in southern California (Sheareret al., 2006).

Fig. 10 shows the frequency–magnitude relationship for the threeearthquake nests. For the Vrancea nest the b-values is 1.15, slightlylarger than previous estimates between 0.5 and 1.0 (Mantysniemiet al., 2003; Mârza et al., 1991; Trifu and Radulian, 1991; Zarifi andHavskov, 2003) and for the Hindu Kush nest it is 0.95. Drakopoulosand Srivastava (1972) and Zarifi and Havskov (2003) report valuesof 1.4, while Gutenberg and Richter (1954) and Chouiian andSrivastava (1970) report 0.6 for the b-values for this nest.

What is more striking in Fig. 10 is the distinct behavior of thefrequency-magnitude statistics for the Bucaramanga nest. Using ISCdata the b-value of 1.35 is larger compared to the other two nestsand is slightly smaller than previous studies. Frohlich et al. (1995)reported a value of 2.0, while more recently Frohlich and Nakamura(2009) obtain 1.6.

Fig. 10. Frequency–magnitude relation for the Hindu Kush, Vrancea and Bucaramanga nestsalog, inset shows the b-value fit for each region. The gray symbols are not used in fitting the(Red triangles) and the local RSNC catalog. Note that the ISC uses Mb while the RSNC uses ML

catalogs. The blue symbols show corrected ML for the RSNC data with a resulting change in

The ISC data for the Bucaramanga nest (Fig. 10, right panel) showsa dip in the number of earthquakes of large magnitude (data does notfollow a straight slope), but the local RSNC data show a consistentestimated b-value of 1.05. It is evident that there is a discrepancybetween local magnitude and ISC magnitudes, and this is evident asthe largest ISC earthquake in the period of interest (2000–2010) is amagnitude 5.2, while for the local RSNC data it is a M6.0. We correctlocal magnitudes using earthquake magnitudes of the same eventfrom both catalogs and finding a linear relationship between bothmagnitude estimates (e.g., Scordilis, 2006). The corrected ML shows amagnitude frequency relationship with a constant slope and a b-valueof 1.6, similar to recent results presented by Frohlich and Nakamura(2009) for the Bucaramanga nest. It is important to notice that themuch higher b-value of the Bucaramanga nest is both observed usingthe ISC and the corrected local magnitudes.

7.2. Focal mechanisms

Focalmechanisms in the Vrancea nest have been studied extensively(Bala et al., 2003; Constantinescu and Enescu, 1964; Oncescu andBonjer,1997; Oncescu and Trifu, 1987; Radulian et al., 2000; Telesca et al., 2011;Trifu, 1991). Most studies agree that the shallow crustal seismicityshows a wide variety of plane orientations, but when only theintermediate-depth earthquakes are compared, a clear pattern isobserved, indicating a reverse-faulting environment with verticalextension and horizontal compression. For the larger earthquakes(M>6.0) the prevalent fault strike is NE-SW with a few cases ofNW-SE striking fault planes (e.g., Radulian et al., 2000). Recent resultsby Bala et al. (2003) and Telesca et al. (2011) agree in finding mostlyvertical T axes, and horizontal P axes. For the more than 700 combinedearthquakes studied in these two papers, there is a slight clustering ofNW-SE P axes orientation.

Chatelain et al. (1980) studied fault plane solutions in the HinduKush area, and although significant variations were observed, thesolutions showed predominantly reverse faulting with nearly verticalT axes (e.g., Billington, et al., 1977). Pavlis and Das (2000) agree thatin the Hindu Kush region vertical T axes suggest that focal mechanismswere aligned with a dipping slab, but found that in the Pamir area, theT axes were horizontal and followed the trend of the contortedS-shaped Benioff zone. This was interpreted as not being a reflectionof subduction but rather of bending of a single subducting plate. Khan(2003) has a different interpretation of focal mechanisms, and findmostly vertical T axes, but with a significant scatter of compressionalstresses. Lister et al. (2008) found no obvious preferred orientationof Hindu Kush earthquake focal mechanisms at shallower depths

. Left panel shows the relationship for the three nests based on the 2000–2010 ISC cat-b-values. Right panel shows results for the Bucaramanga nest based on the ISC catalog(green triangles) with a significant discrepancy between magnitudes for events in boththe b-value.

52 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

(b180 km) but found that between 180 and 280 km reverse-faultinggeometry dominatedwith sub-parallel and conjugate vertically dippingslip directions.

The Bucaramanga nest shows highly variable focal mechanisms(Frohlich andNakamura, 2009; Schneider et al., 1987) and in a significantnumber of focal mechanisms, a large percentage of CLVD (compensatedlinear vector dipole) components is observed (Frohlich, 2006; Zarifi etal., 2007). Schneider et al. (1987) based on fault plane solutions of 59micro-earthquakes found no discernable trend and suggested that thismay be due to ascent of fluids. In contrast, using teleseismically deter-mined mechanisms, Frohlich et al. (1995) found some variation, but aclear tendency for P axes dipping towards the W and T axes dipping to-wards the east. Several of the largest earthquakes in the Bucaramanganest have substantial CLVD components (Frohlich, 2006; see Fig. 11). Itis possible that these may be composite ruptures of multiple sub-eventsas observed in the Hindu Kush nest by Kiser et al. (2011). Additionally,previous studies observe shorter durations for large intermediate-depthearthquakes than expected using a relationship for shallow events(τ∝Mo

0.33). The simultaneous (or partially simultaneous) rupture ofsubevents might explain this discrepancy.

This last conclusion (Frohlich et al., 1995) is in agreement with re-cent results by Cortes and Angelier (2005) who found a similar trendand interpret this as due to a sinking slab that is being torn or is break-ing off (similar to what is observed in Vrancea). Zarifi et al. (2007)performed stress inversion of Bucaramanga nest earthquakes andagree with these results in that there may be dominant down-dip ten-sion, but suggest that the observed variability of micro-earthquakefocal mechanisms can be explained by an interaction between twocolliding slabs.

Careful review of the estimated focal mechanisms by Cortes andAngelier (2005) still find a significant scatter with strike–slip, normaland reversemechanisms. From the global CMT catalogwefind19 earth-quakes with focal mechanism solutions, as shown in Fig. 11. Even forthe largermagnitude earthquakes, focalmechanisms in the Bucaraman-ga nest are highly variable. A number of focal mechanisms show strike–slip mechanisms and some have large CLVD components.We plot the Pand T axes of each of the CMT solutions and find that on average focalmechanisms show a SE dipping T axis and a W dipping P axis, in agree-ment with previous work, but significant scatter is evident.

7.3. Stress drops and source physics

Understanding the nature and the characteristics of earthquakerupture based on seismological observables (seismic waveforms) is

Fig. 11. Focal mechanisms of earthquakes in the CMT catalog for the Bucaramanga nest. Leftestimated principal directions of stress as estimated from Cortes and Angelier (2005)with dowshown in the map, with reverse, normal and strike–slip faulting on a very tight volume.

fundamental to understanding the physical mechanism responsiblefor intermediate-depth earthquakes. Earthquake nests are particularlyuseful for this purpose, because they provide a unique view of earth-quake rupture at a wide range of magnitudes, but over a small sourceregion such that source effects can be more effectively separated frompropagation effects than would otherwise be possible.

There are many important earthquake source parameters that canprovide insight into the static and dynamic character of earthquakerupture, including: stress drop, rupture size, rupture velocity, radiatedseismic energy and seismic efficiency. Based on some of these sourceparameters, Kanamori et al. (1998) argued that frictional meltingcould be invoked during the rupture of the 1994 Bolivian deep focusearthquake, whichwould lead to very low estimates of seismic efficien-cy and correspondingly high stress drops. The high stress drops and lowseismic efficiencies suggest some energy dissipation during seismicrupture, and points toward a thermal shear runaway process (Johnet al., 2009). Does this behavior stand for different earthquake sizes, ordo small earthquakes at depth behave differently than the larger ones?Do all intermediate-depth earthquakes have particularly high stressdrops and low seismic efficiencies? How do these earthquakes compareto their shallow counterparts? Earthquake nests may be the key foranswering these questions.

Few studies have investigated the scaling characteristics ofintermediate-depth earthquakes (Radulian and Popa, 1996; Gusev etal., 2002; Oth et al., 2007, 2009, mainly for the Vrancea nest). Exceptfor the larger earthquakes, stress drops, radiated seismic energy oreven seismic efficiency is not routinely estimated. Gusev et al. (2002)found that 16 intermediate to large magnitude, intermediate-depthVrancea earthquakes followed a constant stress-drop model, exceptfor the larger ones, which had relatively high-stress drops (>10 MPa),similar to previous results in the region (Radulian and Popa, 1996).For the larger Vrancea nest earthquakes: the 1977 M7.5 earthquakestress drops of 4.4 MPa and 90 MPa, and for the 1986 M7.1 values of5 MPa and 30–85 MPa were obtained by Radulian and Popa (1996)and Oth et al. (2007, 2009) respectively. It is important to note thatstress-drop estimates can be subject to large uncertainties, and signifi-cant differences between estimates can be obtained for the same dataunder different measurement techniques or model assumptions (seefor example uncertainty estimates of source parameters in Prietoet al., 2007; Kane et al., 2011).

Other than in the Vrancea nest, we are unaware of any systematicstudy of earthquake stress drops in earthquake nests. Fig. 12 showsstress drops of intermediate-depth and deep earthquakes as reportedby Frohlich (2006) compared to estimated stress drops in the Vrancea

panel shows the P and T axes of the focal mechanisms shown in the map as well as then dip extension. Note the widely variable focal mechanisms present in the 19 earthquakes

53G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

nest and our recent work on the Bucaramanga nest based on RSNCseismic data (Lopez and Prieto, 2010; Prieto et al., 2011). The resultsare similar to other earthquakes deeper than 50 km, in that the nestearthquakes in Vrancea and Bucaramanga have higher stress drops thantheir shallow counterparts, with an average stress drop Δσ>10 MPa.Further work is needed to estimate radiation efficiency, radiated seismicenergies and other dynamic source parameters that may constrain themechanism of these nest earthquakes.

8. Discussion

Dehydration embrittlement provides a good explanation of the loca-tion of intermediate-depth earthquakes, especially as it is able to predictthe presence of double-seismic zones (Hacker et al., 2003; Yamasaki andSeno, 2003). It is also a good candidate for earthquake nests because acommon constituent of subducting lithosphere, antigorite serpentinite,is expected to dehydrate up to 250 km depth. Nevertheless, dehydrationembrittlement may not explain the presence of repeating ruptures alongthe same fault planes as suggested here (Figs. 8 and 9).

Once a section of the subducted slab has been dehydrated, onewould not expect it to trigger a repeating earthquake. It is possiblethat locations of these repeating events, or similar locations alongsub-horizontal faults as suggested in Fig. 7 are due to location accura-cy and that in reality earthquakes occur on close but yet different faultplanes. Pre-existing fault planes along bend-related faults at the outerrise (Ranero et al., 2003, 2005) may provide the explanation forboth sub-horizontal and repeating earthquakes. Sub-horizontal faultplanes have been observed in various locations (Warren et al., 2007,2008) and recently have been imaged by Kiser et al. (2011) in theHindu Kush area for example. These sub-parallel fault planes mayconcentrate volatiles from dehydration reactions elsewhere andallow for repeating rupture along the same faults.

Thermal shear instability is another viable mechanism. This mech-anism does allow for repeating earthquakes (see for example Wiens

Fig. 12. Estimated static stress drop for intermediate-depth and deep earthquakes.Vrancea (green) and Bucaramanga nest earthquake (red) stress drop estimates haveaverage between 40–70 and 20–40 MPa respectively. Compilation of results forintermediate-depth and deep earthquakes by Frohlich (2006) is shown for comparisonpurposes (gray bars). Vrancea results (M3.5–7.1) are taken from Oth et al. (2009).Magnitude range for Bucaramanga nest earthquakes is M4.0–5.7.

and Snider, 2001) and is also consistent with high stress drops asshown in Fig. 12 for the Bucaramanga nest. High stress drops areexpected based on geologic observations of shear zones andpseudotachalyte formation (Andersen et al., 2008; John et al., 2009).Partial melting and high stress drops have been inferred in particularcases (Kanamori et al., 1998) pointing towards a thermal instabilitymechanism. The high stress drops shown in Fig. 12 agree with the ob-servations above, but are insufficient to confirm whether thermalshear instability is responsible for those earthquakes. Estimates ofseismic efficiency (not shown) for the data shown in Fig. 12 are sim-ilar to those of the deep Bolivia earthquake (Kanamori, et al., 1998)and agree with expected large energy dissipation during rupture forsmall and large intermediate-depth earthquakes. It may be difficultto estimate other source parameters like rupture velocity in the Buca-ramanga nest because the largest events are around M5.0, but this ispossible in the Hindu Kush area with much higher productivity oflarge M>6.5 earthquakes.

Other possible seismic observables that have not been document-ed carefully include high-resolution tomographic Vp/Vs in the sourcearea. We expect to observe a Vp/Vs signature if dehydration embrit-tlement is responsible, although care must be taken since antigoritemay have strong anisotropy and high Vp/Vs (Reynard et al., 2010).Some results of anisotropy in earthquake nests can be found in the lit-erature but have not been discussed here (e.g., Shih et al., 1991a,1991b). Earthquake nests are ideal candidates for high-resolution to-mography at the source region due to their compact nature and con-centration of large number of events.

9. Conclusions

Some of the key observations discussed above for earthquakenests are consistent with one or both of the proposed mechanismsfound in the literature. It is this kind of seismological observationthat may help resolve this issue in the future. Table 1 lists some ofthe relevant observations in the literature and as presented in thispaper.

In particular, for the Bucaramanga nest we show for the first timethat precise locations suggest that the nest has linear structures, per-haps sub-horizontal planes of seismicity. By searching the recordedseismograms we have found a significant number of repeating earth-quakes and more interestingly reversed polarity waveforms. Re-versed polarity may suggest repeating rupture on the same orsub-parallel faults but with reversed slip directions. A simple modelthat may explain these observations is an “extruding block model”,where a block moves faster with respect to the surrounding material,leading to sub-parallel faults with contrary slip directions.

Previous studies and our own results show high stress drops in theVrancea and Bucaramanga nests. This may be due to small ruptureareas and/or slow rupture velocities compared to shallow earth-quakes. To evaluate shear instability as a mechanism, a complete en-ergy budget is needed and estimates of radiated seismic energy andseismic efficiency are required. We are working on a more completeanalysis of Bucaramanga nest earthquakes to address this issue.

As discussed in this review, seismological characterization of earth-quake nests may provide a unique tool for constraining the physics ofrupturemechanism.More precise earthquake locations and focalmech-anisms will be valuable, but additionally high-resolution source-regiontomography and earthquake physics studies are all needed for improv-ing our understanding of this behavior. The results presented herecannot conclusively favor a particular mechanism but are in the rightdirection to help in deciphering this key seismological question.

Acknowledgments

We would like to thank two anonymous reviewers for thoughtfulcomments on an earlier version of this paper. We thank Adrien Oth

Table 1Comparison of some observed features of the three nests studied in this study. Depthranges and number of earthquakes are estimated from the ISC locations. b-valuesfrom the ISC catalog and local catalog for the Bucaramanga case. Focal mechanism asreported in the references, Global CMT for the Bucaramanga nest. Average stressdrops from Oth et al. (2007, 2009) for the Vrancea nest and from local data for theBucaramanga nest.

Nest Hindu Kush Vrancea Bucaramanga

Location (lat, lon) 36.5, 71.0 45.7, 26.5 6.8, 73.1Depth range (km) 175–250 70–180 145–165# M>4 (2000–2010) 549 50 151b-value 0.95 1.15 1.35 (Local) 1.60Focal mechanism Vertical T-axes

Variable P-axesVertical T-axesHoriz. P-axes

Highly variableEast Dipping T-axesSome cases CLVD

Avg. stress drop N.A. 45 MPa 30 MPa

54 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

for sharing his stress drop estimates from Vrancea. We also wouldlike to thank the Editor for inviting us to contribute this article. Thiswork was partially supported by NSF Grant EAR-1045684.

References

Amitrano, D., 2003. Brittle-ductile transition and associated seismicity: experimentaland numerical studies and relationship with the b value. Journal of GeophysicalResearch 108 (B1), 2044. http://dx.doi.org/10.1029/2001JB000680.

Andersen, T.B., Mair, K., Austrheim, H., Podladchikov, Y.Y., Vrijmoed, J.C., 2008. Stress-release in exhumed intermediate-deep earthquakes determined from ultramaficpseudotachylyte. Geology 36, 995–998.

Bala, A., Radulian, M., Popescu, E., 2003. Earthquakes distribution and their focal mecha-nism in correlationwith the active tectonic zones of Romania. Journal of Geodynamics36, 129–145.

Bilek, S.L., Lay, T., Ruff, L.J., 2004. Radiated seismic energy and earthquake source dura-tion variations from teleseismic source time functions for shallow subduction zoneearthquakes. Journal of Geophysical Research 109. http://dx.doi.org/10.1029/2004JB003039, B00308.

Billington, S., Isacksa, B., Barazangi, M., 1977. Spatial distribution and focal mechanismsof mantle earthquakes in the Hindu Kush-Pamir region: a contorted Benioff zone.Geology 5, 699–704.

Böse, M., Sokolov, V., Wenzel, F., 2009. Shake mapmethodology for intermediate-depthVrancea (Romania) earthquakes. Earthquake Spectra 25, 497–514. http://dx.doi.org/10.1193/1.3148882.

Burrman, V.S., Molnar, P., 1993. Geological and geophysical evidence for deep subductionof continental crust beneath the Pamir, Spec. Pap. Geological Society of America 281,1–75.

Chatelain, J.L., Roecker, S.W., Hatzfeld, D., Molnar, P.m, 1980. Microearthquake seismicityand fault plane solutions inHindu Kush region and their tectonic implications. Journalof Geophysical Research 85, 1365–1387.

Chollet, M., Daniel, I., Koga, K.T., Petitgirard, S., Morard, G., 2009. Dehydration kineticsof talc and 10 Å phase: consequences for subduction zone seismicity. Earth andPlanetary Science Letters 284, 57–64.

Chouiian, R., Srivastava, V., 1970. Global variation of b in the Gutenberg Richter'srelation log N=a−bM with depth. Pure and Applied Geophysics 124–132.

Christeson, G.L., Mann, P., Escalona, A., Aitken, T.J., 2008. Crustal structure of theCaribbean–northeastern South America arc-continent collision zone. Journal ofGeophysical Research 113, B08104. http://dx.doi.org/10.1029/2007JB005373.

Chung, W.-Y., Kanamori, H., 1976. Source process and tectonic implications of theSpanish deep focus earthquake of March 29, 1954. Physics of the Earth and PlanetaryInteriors 13, 85–96.

Constantinescu, L., Enescu, D., 1964. Fault-plane solutions for some Romanian earth-quakes and their seismotectonic implication. Journal of Geophysical Research 69,667–674.

Cortes, M., Angelier, J., 2005. Current states of stress in the northern Andes as indicatedby focal mechanisms of earthquakes. Tectonophysics 403, 29–58.

Csontos, L., 1995. Tertiary tectonic evolution of the Intra-Carpathian area: a review.Acta Vulcanologica 7 (2), 1–13.

Davis, S.D., Frohlich, C., 1991. Single-link cluster analysis of earthquake aftershocks: decaylaws and regional variations. Journal of Geophysical Research 96 (B4), 6335–6350.

DeMets, C., Gordan, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. GeophysicalJournal International 101, 425–478.

Dewey, J.F., Bird, J.M., 1970. Mountain belts and new global tectonics. Journal ofGeophysical Research 75 (14), 2625–2647.

Drakopoulos, J.C., Srivastava, H.N., 1972. The dependence of earthquake frequencymagnitude relationship and strain energy release upon the focal depth in HinduKush region. Annali di Geofisica 25 (1972), 593–606.

Enescu, B., Struzik, Z., Kiyoto, K., 2008. On the recurrence time of earthquakes: insightfrom Vrancea (Romania) intermediate-depth events. Geophysical Journal International172, 395–404.

Fan, G., Ni, J.F., Wallace, T.C., 1994. Active tectonics of the Pamirs and Karakoram.Journal of Geophysical Research 99, 7131–7160.

Fan, G., Wallace, T.C., Zhao, D., 1998. Tomographic imaging of deep velocity structurebeneath the eastern and southern Carpathians, Romania: implications for continentalcollision. Journal of Geophysical Research 103, 2705–2724. http://dx.doi.org/10.1029/97JB01511.

Frohlich, C., 1989. The nature of deep-focus earthquakes. Annual Review of Earth andPlanetary Sciences 17, 227–254.

Frohlich, C., 2006. Deep Earthquakes. Cambridge University Press, Cambridge.Frohlich, C., Davis, S.D., 1993. Teleseismic b-values — or, much ado about 1.0. Journal of

Geophysical Research 98, 631–644.Frohlich, C., Nakamura, Y., 2009. The physical mechanisms of deep moonquakes and

intermediate-depth earthquakes: how similar and how different? Physics of theEarth and Planetary Interiors 173, 365–374.

Frohlich, C., Kadinsky-Cade, K., Davis, S.D., 1995. A reexamination of the BucaramangaNest. Bulletin of the Seismological Society of America 85 (6), 1622–1634.

Fuchs, K., Bonjer, K.-P., Bock, G., Cornea, I., Radu, C., Enescu, D., Jianu, D., Nourescu, A.,Merkler, G., Moldoveanu, T., Tudorache, G., 1979. The Romanian earthquake ofMarch 4, 1977: II. Aftershocks and migration of seismic activity. Tectonophysics 53,225–247.

Gansser, A., 1966. The Indian Ocean and the Himalayas a geological interpretation.Eclogae Geologicae Helvetiae 59 (831–848), 1966.

Gansser, A., 1977. The great suture zone between Himalaya and Tibet — a preliminaryaccount. Himalaya: Sciences de la Terre. Centre National de la Recherche Scientifique,Paris, pp. 209–212.

Giardini, D., 1988. Frequency distribution and quantification of deep earthquakes.Journal of Geophysical Research 93, 2095–2105.

Green, H.W., Houston, H., 1995. The mechanics of deep earthquakes. Annual Review ofEarth and Planetary Sciences 23, 169–213.

Green, H.W., Marone, C., 2002. Instability of deformation. In: Korato, S., Wenk, H. (Eds.),Reviews in Mineralogy and Geochemistry: Plasticity of minerals and Rocks, 51.Blackwell, Oxford, pp. 181–199.

Griggs, D.T., Handin, J.H., 1960. Observations on fracture and a hypothesis of earth-quakes. In: Griggs, D.T., Handin, J.H. (Eds.), Rock Deformation: Mem. Geol. Soc.Am., 79, pp. 347–373.

Gusev, A., Radulian, M., Rizescu, M., Panza, G.F., 2002. Source scaling of intermediate-depth Vrancea earthquakes. Geophysical Journal International 151, 879–889.

Gutenberg, B., Richter, C.F., 1949. Seismicity of the Earth and Associated Phenomena,2nd edn. Princeton University Press, Princeton, NJ. 310 pp.

Gutenberg, B., Richter, C., 1954. Seismicity of the Earth and Associated Phenomena.Princeton University Press, Princeton.

Hacker, B.R., Peacock, S.M., Abers, G.A., Holloway, S.D., 2003. Subduction factory 2. Areintermediate-depth earthquakes in subducting slabs linked to metamorphicdehydration reactions? Journal of Geophysical Research 108, 24627–24637.http://dx.doi.org/10.1029/ 2001JB001129.

Hamburger, M.W., Sarewitz, D.R., Pavlis, T.L., Popandopulo, G.A., 1992. Structural andseismic evidence for intracontinental subduction in the Peter the First range,central Asia. Geological Society of America Bulletin 104 (397–408), 1992.

Higgs, R., 2009. Caribbean-South America oblique collision model revised. GeologicalSociety of London Special Publication 328, 613–657.

Hobbs, B.E., Ord, A., 1988. Plastic instabilities: implications for the origin of intermediateand deep focus earthquakes. Journal of Geophysical Research 93, 10,521–10,540.

Houston, H., 2007. Deep earthquakes. In: Schubert, G. (Ed.), Treatise on Geophysics.Elsevier, Amsterdam, pp. 321–350. http://dx.doi.org/10.1016/B978-044452748-6.00071-7.

International Seismological Centre, 2001. On-line Bulletin. Internatl. Seis. Cent.Thatcham, United Kingdom. http://www.isc.ac.uk.

Jiao, W., Silver, P.G., Fei, Y., Prewitt, C.T., 2000. Do intermediate- and deep-focus earth-quakes occur on preexisting weak zones? An examination of the Tonga subductionzone. Journal of Geophysical Research 105, 28,125–28,138.

Jiricek, R., 1979. Tectogenetic development of the Carpathian arc in the Oligocene andNeogene. In: Mahel, M. (Ed.), Tectonic Profiles Through the West Carpathians.Geol. Inst. D. Stur, Bratislava, pp. 2003–2214.

John, T., Schenk, V., 2006. Interrelations between intermediate-depth earthquakes andfluid flow within subducting oceanic plates: constraints from eclogite-faciespseudotachylytes. Geology 34, 557–560.

John, T., Medvedev, S., Rupke, L.H., Andersen, T.B., Podladchikov, Y.Y., Austrheim, H.,2009. Generation of intermediate-depth earthquakes by self localizing thermalrunaway. Nature Geoscience 2, 137–140. http://dx.doi.org/10.1038/NGEO419.

Jung, H., Green, H.W., Dobrzhinetskaya, L.F., 2004. Intermediate-depth earthquakefaulting by dehydration embrittlement with negative volume change. Nature 428,545–549.

Kagan, Y.Y., 1999. Universality of the seismic moment–frequency relation. Pure andApplied Geophysics 155, 537–573.

Kanamori, H., Anderson, D.L., Heaton, T.H., 1998. Frictional melting during the ruptureof the 1994 Bolivian earthquake. Science 279, 839–842.

Kane, D.L., Prieto, G.A., Vernon, F.L., Shearer, P.M., 2011. Quantifying seismic sourceparameter uncertainties. Bulletin of the Seismological Society of America 101 (2),535–543. http://dx.doi.org/10.1785/ 0120100166.

Keleman, P.B., Hirth, G., 2007. A periodic shear-heating mechanism for intermediate-depth earthquakes in the mantle. Nature 446, 787–790.

Khan, P.K., 2003. Stress state, seismicity and subduction geometries of the descendinglithosphere below the Hindukush and Pamir. Gondwana Research 6, 867–877.

Kirby, S., Stein, S., Okal, E.A., Rubie, D.C., 1996a. Metastable mantle phase transformationsand deep earthquakes in subducting oceanic lithosphere. Reviews of Geophysics 34(2), 261–306.

Kirby, S.H., Engdahl, E.R., Denlinger, R., 1996b. Intraslab earthquakes and arc volcanism:dual physical expressions of crustal and uppermost mantle metamorphism in

55G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

subducting slabs, in Bebout, G.E., et al., eds., Subduction: top to bottom: AmericanGeophysical Union. Geophysical Monograph 96, 195–214.

Kiser, E., Ishii, M., Langmuir, C.H., Shearer, P.M., Hirose, H., 2011. Insights into themechanism of intermediate-depth earthquakes from source properties as imagedby back-projection of multiple seismic phases. Journal of Geophysical Research.http://dx.doi.org/10.1029/2010JB007831.

Koch, M., 1985. Nonlinear inversion of local seismic travel times for the simultaneousdetermination of the 3D-velocity structure and hypocenters—application to theseismic zone Vrancea. Journal of Geophysics 56, 160–173.

Koulakov, I., Zaharia, B., Enescu, B., Radulian, M., Popa, M., Parolai, S., Zschau, J., 2010.Delamination or slab detachment beneath Vrancea? New arguments from localearthquake tomography. Geochemistry Geophysics Geosystems 10, Q03002.http://dx.doi.org/10.1029/2009GC002811.

Linzer, H.G., 1996. Kinematics of retreating subduction along the Carpathian arc,Romania. Geology 24, 167–170.

Lister, G., Kennett, B., Richards, S., Forster, M., 2008. Boudinage of a stretching slabletimplicated in earthquakes beneath the Hindu Kush. Nature Geoscience 1, 196–201.http://dx.doi.org/10.1038/ngeo132.

Lopez, G., Prieto, G.A., 2010. Earthquake scaling of intermediate-depth earthquakes inthe Bucaramanga Nest. Abstract S21C-2065 presented at 2010 Fall Meeting. AGU,San Francisco, Calif. 13-17 Dec.

Lorenz, F.P., Martin, M., Sperner, B., Wenzel, F., Popa, M., 1997. Teleseismic travel-time tomography of the compressional-wave velocity structure in the Vranceazone, Romania. EOS Transactions American Geophysical Union 78 (46) (FallMeet. Suppl., F497).

Lou, X.T., Cai, C., Yu, C.Q., Ning, J.Y., 2009. Intermediate-depth earthquakes beneath thePamir-Hindu Kush Region: evidence for collision between two opposite subductionzones. Earthquake Science 22, 659–665.

Mantysniemi, P., Marza, V.I., Kijko, A., Retief, P., 2003. A new probabilistic seismichazard analysis for the Vrancea (Romania) seismogenic zone. Natural Hazards29, 371–385.

Marone, C., Vidale, J.E., Ellsworth, W.L., 1995. Fault healing inferred from time depen-dent variations in source properties of repeating earthquakes. Geophysical Re-search Letters 22, 3095–3098.

Martin, M., Wenzel, F., the CALIXTO Working Group, 2006. High-resolution teleseismicbody wave tomography beneath SE-Romania—II. Imaging of a slab detachmentscenario. Geophysical Journal International 164, 579–595. http://dx.doi.org/10.1111/j.1365- 246X.2006.02884.x.

Mârza, V.I., Kijko, A., Mäntyniemi, P., 1991. Estimate of earthquake hazard in theVrancea (Romania) region. Pure and Applied Geophysics 136, 143–154.

Meade, C., Jeanloz, R., 1991. Deep-focus earthquakes and recycling of water into theEarth's mantle. Science 252, 68–72.

Meulenkamp, J.E., Kova c, M., Cicha, I., 1997. On Late Oligocene to Pliocene depocentremigrations and the evolution of the Carpathian–Pannonian system. Tectonophysics266 (1±4), 301–318.

Myers, S.C., Wallace, T.C., Beck, S.L., Silver, P.G., Zandt, G., Vandecar, J., Minaya, E., 1995.Implications of spatial and temporal development of the aftershock sequence forthe Mw 8.3 June 9, 1994 deep Bolivian earthquake. Geophysical Research Letters22, 2269–2272.

Negredo, A.M., Replumaz, A., Villase˜nor, A., Guillot, S., 2007. Modeling the evolutionof continental subduction processes in the Pamir–Hindu Kush region. Earth andPlanetary Science Letters 259, 212–225.

Nowroozi, A.A., 1971. Seismo-tectonics of the Persian Plateau, Eastern Turkey, Caucasus,andHindu Kush regions. Bulletin of the Seismological Society of America 61, 317–341.

Ogawa, M., 1987. Shear instability in a viscoelastic material as the cause of deep focusearthquakes. Journal of Geophysical Research 92, 13801–13810.

Oncescu, M.C., 1982. Velocity structure of the Vrancea region, Romania. Tectonophysics90, 117–122. http://dx.doi.org/10.1016/0040-1951(82)90256-6.

Oncescu, M.C., 1984. Deep structure of the Vrancea region, Roumania, inferred fromsimultaneous inversion for hypocenters and 3-D velocity structure. Annals ofGeophysics 2 (1), 23–28.

Oncescu, M.C., Bonjer, K.-P., 1997. A note on the depth recurrence and strain release oflarge Vrancea earthquakes. Tectonophysics 272, 291–302.

Oncescu, M.C., Trifu, C.I., 1987. Depth variation of moment tensor principal axes inVrancea (Romania) seismic region. Annals of Geophysics 5B, 149–154.

Oncescu, M.C., Bonjer, K.-P., Rizescu, M., 1999. Weak and strong ground motion ofintermediate-depth earthquakes from the Vrancea region. In: Wenzel, F., Lungu, D.,Novak, O. (Eds.), Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation. Springer,New York, pp. 27–42.

Oth, A., Wenzel, F., Radulian, M., 2007. Source parameters of intermediate-depthVrancea (Romania) earthquakes from empirical Green's functions modeling.Tectonophysics 438, 33–56.

Oth, A., Bindi, D., Parolai, S., Wenzel, F., 2008. S-wave attenuation characteristics beneaththe Vrancea region in Romania: new insights from the inversion of ground motionspectra. Bulletin of the Seismological Society of America 98 (5), 2482–2497.

Oth, A., Parolai, S., Bindi, D., Wenzel, F., 2009. Source spectra and site response fromS waves of intermediate-depth Vrancea, Romania, earthquakes. Bulletin of theSeismological Society of America 99 (1), 235–254.

Pavlis, G.L., Das, S., 2000. The Pamir‐Hindu Kush seismic zone as a strain marker forflow in the upper mantle. Tectonics 19, 103–115.

Pavlis, G.L., Hamburger, M.W., 1991. Aftershock sequences of intermediate-depthearthquakes in the Pamir-Hindu Kush seismic zone. Journal of Geophysical Research96 (18107–18117), 1991.

Peacock, S.M., Wang, K., 1999. Seismic consequences of warm versus cool subductionmetamorphism: examples from southwest and northeast Japan. Science 286,937–939.

Pècskay, Z., Lexa, J., Szaka cs, A., et al., 1995. Space and time distribution of Neogene-Quaternary volcanism in the Carpatho-Pannonian region. Acta Vulcanologica 7 (2),15–28.

Pegler, G., Das, S., 1998. An enhanced image of the Pamir-Hindu Kush seismic zonefrom relocated earthquake hypocenters. Geophysical Journal International 134,573–595.

Penington, W.D., 1981. Subduction of the eastern Panama basin and seismotectonics ofnorthwestern South America. Journal of Geophysical Research 86, 10,579–10,597(1981).

Pennington, W.D., 1983. The role of shallow phase changes in the subduction of oceaniccrust. Science 220, 1045–1047.

Pennington, W.D., Mooney, W.D., van Hissenhoven, R., Meyer, H., Ramirez, J.E., Meyer,R.P., 1979. Results of a reconnaissance microearthquake survey of Bucaramanga,Colombia. Geophysical Research Letters 6, 65–68.

Pindell, J.L., Kennan, L., 2009. South America in the mantle reference frame: an updateTectonic evolution of the Gulf of Mexico, Caribbean and northern South Americain the mantle reference frame: an update. Geological Society of London SpecialPublication 328, 1–35.

Popa, M., Kissling, E., Radulian, M., Bonjer, K.P., Enescu, D., Dragan, S., the CALIXTOWorking Group, 2001. Local source tomography using body waves to deduce aminimum 1D velocity model for the Vrancea (Romania) zone. Romanian Reportsin Physics 53, 519–536.

Poupinet, G., Ellsworth, W.L., Fréchet, J., 1984. Monitoring velocity variations in thecrust using earthquake doublets: an application to the Calaveras Fault, California.Journal of Geophysical Research 89, 5719–5731.

Prieto, G.A., Thomson, D.J., Vernon, F.L., Shearer, P.M., Parker, R.L., 2007. Confidenceintervals of earthquake source parameters. Geophysical Journal International168, 1227–1234. http://dx.doi.org/10.1111/j.1365- 246X.2006.03257.x.

Prieto, G.A., Lopez, G., Barrett, S., Beroza, G., 2011. Earthquake source scaling, stressdrops and radiated seismic energies of intermediate depth earthquakes. AbstractT22A-06 Presented at 2011 Fall Meeting. AGU, San Francisco, Calif. 5- 9 Dec.

Pulpan, H., Frohlich, C., 1985. Geometry of the subducted plate near Kodiak Island andLower Cook Inlet, Alaska, determined from relocated earthquake hypocenters.Bulletin of the Seismological Society of America 75, 791–810.

Radulian, M., Popa, M., 1996. Scaling of the source parameters for the Vrancea interme-diate depth earthquakes. Tectonophysics 261, 67–81.

Radulian, M., Mandrescu, M.N., Panza, G.F., Popescu, E., Utale, A., 2000. Characterizationof seismogenic zones of Romania. Pure and Applied Geophysics 157 (2000), 57–77.

Ramirez, J.E., Instituto Geofísico Universidad Javeriana, 2004. Actualización de lahistoria de los terremotos en Colombia. Pontificia Universidad Javeriana, Bogotá,D. C.

Ranero, C.R., Phipps-Morgan, J., McIntosh, K., Reichert, C., 2003. Bending-relatedfaulting and mantle serpentinization at the Middle America trench. Nature 425,367–373.

Ranero, C.R., Villasenor, A., Morgan, J.P., Weinrebe, W., 2005. Relationship betweenbend-faulting at trenches and intermediate-depth seismicity. Geochemistry GeophysicsGeosystems 6, Q12002.

Rayleigh, C.B., Paterson, M.S., 1965. Experimental deformation of serpentinite and itstectonic implications. Journal of Geophysical Research 70, 3965–3985.

Reynard, B., Nakajima, J., Kawakatsu, H., 2010. Earthquakes and plastic deformation ofanhydrous slab mantle in double Wadati‐Benioff zones. Geophysical ResearchLetters 37, L24309. http://dx.doi.org/10.1029/2010GL045494.

Richter, C.F., 1958. Elementary Seismology. W.H. Freeman, San Francisco, Calif, p. 342.Roecker, S.W., Soboleva, V., Nersesov, I.L., Lukk, A.A., Hatzfeld, D., Chatelain, J.L., Molnar,

P., 1980. Seismicity and fault plane solutions of intermediate depth earthquakes inthe Pamir-Hindu Kush region. Journal of Geophysical Research 85, 1358–1364.

Sacks, I.S., Suyehiro, S., Kamitsuki, A., Tuve, M.A., Otsuka, M., et al., 1967. A tentativevalue of Poisson's coefficient from the seismic “nest of Socampa”. Annual Reportof the Director. Carnegie Inst. Dep. Terr. Magn, pp. 43–45. 1965-1966.

Santo, T., 1969a. Characteristics of seismicity in South America. Bulletin of the Earth-quake Research Institute University of Tokyo 47, 635–672.

Santo, T., 1969b. Regional study on the characteristic seismicity of the world, I, HinduKush region. Bulletin of the Earthquake Research Institute University of Tokyo47, 1035–1049.

Schaff, D.P., Beroza, G.C., 2004. Coseismic and postseismic velocity changes measuredby repeating earthquakes. Journal of Geophysical Research 109, B10302. http://dx.doi.org/10.1029/2004JB003011.

Schaff, D.P., Richards, P.G., 2004. Repeating seismic events in China. Science 303, 1176–1178.Schaff, D.P., Beroza, G.C., Shaw, B.E., 1998. Postseismic response of repeating after-

shocks. Geophysical Research Letters 25, 4549–4552.Schneider, J.F., Pennington,W.D.,Meyer, R.P., 1987.Microseismicity and focalmechanisms

of the intermediate-depth Bucaramanga Nest, Colombia. Journal of GeophysicalResearch 92, 13913–13926.

Scholz, C.H., 1968. The frequency–magnitude relation of microfracturing in rock andits relation to earthquakes. Bulletin of the Seismological Society of America 58,399–415.

Scordilis, E.M., 2006. Empirical global relations converting M S and m b to momentmagnitude. Journal of Seismology 10, 225–236.

Seno, T., Yamanaka, Y., 1996. Double seismic zones, compressional deep trench-outerrise events, and superplumes. In: Bebout, G.E., et al. (Ed.), Subduction: Top toBottom: Geophys. Monogr. Ser., 96. AGU, Washington, D. C., pp. 347–355.

Schorlemmer, D., Wiemer, S., Wyss, M., 2005. Variations in earthquake-size distributionacross different stress regimes. Nature 437, 539–542.

Shearer, P., 1997. Improving local earthquake locations using the L1 norm and wave-form cross correlation: application to the Whittier Narrows, California, aftershocksequence. Journal of Geophysical Research 102, 8269–8283.

56 G.A. Prieto et al. / Tectonophysics 570–571 (2012) 42–56

Shearer, P.M., Prieto, G.A., Hauksson, E., 2006. Comprehensive analysis of earthquakesource spectra in southern California. Journal of Geophysical Research 111, B06303.http://dx.doi.org/10.1029/2005JB003979.

Shih, X.R., Meyer, R.P., Schneider, J.F., 1991a. Seismic anisotropy above a subductingplate. Geology 19, 807–810.

Shih, X.R., Schneider, J.F., Meyer, R.P., 1991b. Polarities of P and S waves, and shearwave splitting observed from the Bucaramanga Nest, Colombia. Journal of GeophysicalResearch 96, 12069–12082.

Sperner, B., Lorenz, F., Bonjer, K., Hettel, S., Muller, B., Wenzel, F., 2001. Slab breakoff—abrupt cut or gradual detachment? New insights from Vrancea Region (SE Carpathians,Romania). Terra Nova 13, 172–179.

Sperner, B., Ratschbacher, L., Nemcok, M., 2002. Interplay between subduction retreatand lateral extrusion: tectonics of the Western Carpathians. Tectonics 21 (6),1051. http://dx.doi.org/10.1029/2001TC901028.

Stampfli, G.M., Borel, G.D., 2002. A plate tectonic model for the Paleozoic and Mesozoicconstrained by dynamic plate boundaries and restored synthetic oceanic isochrons.Earth and Planetary Science Letters 196 (1–2), 17–33.

Suter, F., Sartori, M., Neuwerth, R., Gorin, G., 2008. Structural imprints at the front ofthe Choco-Panama indenter. Tectonophysics 460, 134–157.

Szakacs, A., Seghedi, I., 1995. Time-space evolution of Neogene-Quarternary volcanismin the Calimani-Gurghiu-Harghita volcanic chain. Romanian Journal of Strategy 76(4), 24.

Taboada, A., Rivera, L.A., Fuenzalida, A., Cisternas, A., Philip, H., Bijwaard, H., Olaya, J.,Rivera, C., 2000. Geodynamics of the northern Andes: subductions and intracontinentaldeformation (Colombia). Tectonics 19 (5), 787–813.

Telesca, L., Alcaz, V., Sandu, I., 2011. The stress field of Vrancea region from fault planesolution (FPS). Natural Hazards and Earth System Sciences 11, 2817–2820.

Trifu, C.-I., 1991. The study of Vrancea earthquake mechanisms by a compositetechnique. Revue Roumaine de Physique 36, 457–469.

Trifu, C.-I., Radulian, M., 1991. Frequency–magnitude distribution of earthquakes in Vracea:Relevance for a discrete model. Journal of Geophysical Research 96 (B3), 4301–4311.

Trygvasson, E., Lawson, J.E., 1970. The intermediate earthquake source near Bucaramanga,Colombia. Bulletin of the Seismological Society of America 60, 269–276.

Uchida, N., Matsuzawa, T., Ellsworth, W.L., Kazutoshi, I., Okada, T., Hasegawa, A., 2007.Source parameters of aM4.8 and its accompanying repeating earthquakes offKamaishi, NE Japan: implications for the hierarchical structure of asperities and earth-quake cycle. Geophysical Research Letters 34, L20313 (doi:1029/2007GL031263).

Van Der Elst, N., Brodsky, E., 2010. Connecting near-field and far-field earthquaketriggering to dynamic strain. Journal of Geophysical Research 115, B07311.

van der Hilst, R., Mann, P., 1994. Tectonic implications of tomographic images ofsubducted lithosphere beneath northwestern South America. Geology 22, 451–454.

Venkataraman, A., Kanamori, H., 2004. Observational constraints on the fracture ener-gy of subduction zone earthquakes. Journal of Geophysical Research 109, B05302.http://dx.doi.org/10.1029/2003JB002549.

Wadati, K., 1929. Shallow and deep earthquakes (2nd paper). Geophysical Magazine 2,1–36.

Waldhauser, F., Ellsworth, W.L., 2000. A double-difference earthquake locationalgorithm: method and application to the Northern Hayward Fault, California.Bulletin of the Seismological Society of America 90, 1353–1368. http://dx.doi.org/10.1785/0120000006.

Warren, L.M., Hughes, A.N., Silver, P.G., 2007. Earthquake mechanics and deforma-tion in the Tonga-Kermadec subduction zones from fault-plane orientations ofintermediate- and deep-focus earthquakes. Journal of Geophysical Research 112(B05314). http://dx.doi.org/10.1029/2006JB004677.

Warren, L.M., Langstaff, M.A., Silver, P.G., 2008. Fault-plane orientations of intermediate-depth earthquakes in the middle America trench. Journal of Geophysical Research113 (B01304). http://dx.doi.org/10.1029/2007JB005028.

Wenzel, F., Sperner, B., Lorenz, F., Mocanu, V., 2002. Geodynamics, tomographic imagesand seismicity of the Vrancea region (SE-Carpathians, Romania): Stephan MuellerSpec. Publ. Ser., 3, pp. 95–104.

Wiemer, S., Wyss, M., 2002. Mapping spatial variability of the frequency–magnitudedistribution of earthquakes. Advances in Geophysics 45, 259–302.

Wiens, D.A., 2001. Seismological constraints on the mechanisms of deep earthquakes:temperature dependence of deep earthquake source properties. Physics of theEarth and Planetary Interiors 127, 145–163.

Wiens, D.A., Gilbert, H.J., 1996. Effect of slab temperature on deep-earthquakesaftershock productivity and magnitude–frequency relations. Nature 384, 153–156.

Wiens, D.A., Snider, N.O., 2001. Repeating deep earthquakes: evidence for faultreactivation at great depth. Science 293, 1463–1466.

Wiens, D.A., Gilbert, H.J., Hicks, B., Wysession, M.E., Shore, P.J., 1997. Aftershocksequences of moderate-sized intermediate and deep earthquakes in the Tongasubduction zone. Geophysical Research Letters 24, 2059–2062.

Windley, B.F., 1988. Tectonic framework of the Himalaya, Karakoram and Tibet, and theproblem of their evolution. Philosophical Transactions of the Royal Society of LondonA326, 3–16.

Wortel, M.J.R., Spakman, W., 1992. Structure and dynamics of subducted lithosphere inthe Mediterranean region. Proceedings of the Koninklijke Nederlandse Akademievan Wetenschappen 95, 325–347.

Wyss, M., 1973. Towards a physical understanding of the earthquake frequency distri-bution. Geophysical Journal of the Royal Astronomical Society 31, 341–359.

Yamasaki, T., Seno, T., 2003. Double seismic zone and dehydration embrittlement of thesubducting slab. Journal of Geophysical Research 108. http://dx.doi.org/10.1029/2002JB001918.

Zarifi, Z., Havskov, J., 2003. Characteristics of dense nests of deep and intermediate-depth seismicity. Advances in Geophysics 46, 237–278.

Zarifi, Z., Havskov, J., Hanyga, A., 2007. An insight into the Bucaramanga nest.Tectonophysics 443, 93–105.

Zhang, J., Richards, P.G., Schaff, D.P., 2008. Wide scale detection of earthquake wave-form doublets and further evidence for inner core super-rotation. GeophysicalJournal International 174, 993–1006.