ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2020

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1918

Source analysis of multipletearthquakes (two case studies inIran)

SAMAR AMINI

ISSN 1651-6214ISBN 978-91-513-0909-5urn:nbn:se:uu:diva-407247

Dissertation presented at Uppsala University to be publicly examined in Hambergsallen,Geocentrum, Villavägen 16, Uppsala, Friday, 12 June 2020 at 10:00 for the degree of Doctorof Philosophy. The examination will be conducted in English. Faculty examiner: ProfessorKuvvet Atakan (University of Bergen, Norway).

AbstractAmini, S. 2020. Source analysis of multiplet earthquakes (two case studies in Iran). DigitalComprehensive Summaries of Uppsala Dissertations from the Faculty of Science andTechnology 1918. 45 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0909-5.

Multiplet earthquakes are large earthquakes of similar magnitude which occur close in timein the same limited geographical area. They are not common but they considerably increasethe potential hazard in the area in which they occur. This thesis studies source properties andtriggering mechanisms of two sets of multiplet events in Iran, which both occurred in unexpectedareas, but close to some major active fault systems. The first multiplet is an earthquake doublet(Mw 6.5 and Mw 6.4) which occurred in northwestern Iran and caused more than 300 fatalitiesand significant injuries. In paper I, a teleseismic body-waveform inversion was used to deducethe slip distribution pattern on the fault plane of the first mainshock. The estimated slip patternwas utilized to calculate the Coulomb stress changes on the second fault plane and on thefollowing aftershocks. Based on this analysis, the ambiguity between the primary and auxiliaryfault plane of the second mainshock could be resolved. The second set of events is a triplet (Mw6.1 - 6.0) that occurred in eastern Iran, close to the Kerman province. In paper II, the rupturepropagation patterns of the three mainshocks were analyzed using Empirical Green’s Function(EGF) deconvolution. Two different approaches were used: One, the analysis of the azimuthalvariation of the apparent rupture duration based on the width of the observed relative source timefunctions, and the second, the analysis of along-strike rupture directivity by assessing azimuthalvariations of the relative amplitude spectra. The second approach was also used to investigatethe rupture directivity of the largest aftershocks in the sequence (Mw 5 - 5.5). A clear tendencyfor rupture propagation towards the northwest was observed for the sequence, which suggeststhat the regional stress field has a central role in controlling the rupture propagation direction. Inpaper III, the slip distribution patterns of the triplet earthquakes were analyzed using teleseismicbody-waveform inversion, and the similarities and differences in the rupture processes of thethree mainshocks were investigated. Using the Coulomb stress analyses, the stress interactionsbetween the mainshocks were examined, leading to identification of the primary and auxiliaryplanes. Finally, we suggest that the hazard estimates in complex continental regions such as Iranneed to consider the probability of multiplets, which might allow a reduction of the seismic riskassociated to the occurrence of further large earthquakes subsequent to a devastating earthquake.

Keywords: Multiplet earthquakes, slip inversion, Coulomb stress, rupture directivity

Samar Amini, Department of Earth Sciences, Geophysics, Villav. 16, Uppsala University,SE-75236 Uppsala, Sweden.

© Samar Amini 2020

ISSN 1651-6214ISBN 978-91-513-0909-5urn:nbn:se:uu:diva-407247 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-407247)

Dedicated to those,who made this happen

SupervisorRoland RobertsProffessor at Department of Earth Sciences, GeophysicsUppsala University, Uppsala, Sweden

Assistant SupervisorBjörn LundAssociate Professor at Department of Earth Sciences, GeophysicsUppsala University, Uppsala, Sweden

Assistant SupervisorHossein ShomaliResearcher at Department of Earth Sciences, GeophysicsUppsala University, Uppsala, Sweden

OpponentKuvvet AtakanProffessor at Department of Earth SciencesUniversity of Bergen, Bergen, Norway

List of papers

This thesis is based on the following papers, which are referred to in the textby their Roman numerals.

I Amini, S., Roberts, R., Raeesi, M., Shomali, Z.H., Lund, B., Zarifi, Z.(2018) Fault slip and identification of the second fault plane in theVarzeghan earthquake doublet. Journal of Seismology, 22, 815-831

II Amini, S., Roberts, R., Lund, B.(2020) Directivity analysis of the 2017December Kerman earthquakes in Eastern Iran. Journal of Seismology,DOI: 10.1007/s10950-020-09913-8

III Amini, S., Raeesi, M., Roberts, R. (2020) Fault slip and ruptureproperties of the December 2017 Hojedk triplet in Eastern Iran.submitted to Geophysical Journal International

Reprints were made with permission from the publishers.

An additional journal article, published during my Ph.D. studies, that is notincluded in the thesis is:

Raeesi, M., Zarifi, Z., Nilfouroushan, F., Amini, S., Tiampo, K. (2017)Quantitative Analysis of Seismicity in Iran. Pure Appl. Geophys. 174, 793-833

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 Study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.1 Seismotectonic setting of northwestern Iran and the doublet on

August 11, 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Seismotectonic setting of eastern Iran and the triplet on

December 2017 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.1 Waveform Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.2 Coulomb stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.3 Directivity analysis and Empirical Green’s Function . . . . . . . . . . . . . . . . . . 20

4 Summary of papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1 Paper I: Fault slip and identification of the second fault plane in

the Varzeghan earthquake doublet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2 Paper II: Directivity analyses of the 2017 December Kermanearthquakes in Eastern Iran . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.3 Paper III: Investigations of rupture properties of the December2017 Hojedk triplet in Eastern Iran . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.3.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6 Sammanfattning på svenska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Abbreviations

3D Three-dimensionalEGF Empirical Green’s FunctionE-W East-Westg-CMT Global Centroid Moment TensorHz Hertzkm kilometerkm/s kilometer per secondMl local MagnitudeMw moment MagnitudeNE NortheastNW NorthwestN-S North-Souths secondSE SoutheastSW SouthwestIRIS Incorporated Research Institutions for SeismologyIRSC Iranian Seismological CenterRSTF Relative-source-time-functionSTF Source-time-function

1. Introduction

Throughout human history some areas have been repeatedly devastated byearthquakes, the origins of which remained a mystery. As civilization andhuman constructions developed, understanding and mitigating the effects ofthese destructive events became ever more important. A major step towardsthe development of our understanding of earthquakes occurred in 1668 whenHooke introduced the theory of elasticity, explaining the deformation of solidobjects due to external forces. Hooke’s law was extensively used to explainvarious aspects of the mechanical behavior of material and physical phenom-ena including earthquakes. Two and a half centuries later, the faulting the-ory of earthquakes was presented by Reid (1910) whose analysis was basedon investigations of the San Andreas fault and the 1906 San Francisco earth-quakes. Reid’s concept, known as the elastic rebound theory, explains earth-quakes as the sudden release of strain energy which has been accumulatedslowly on two sides of a fault plane over an extended period of time. Thetheory was a major conceptual development in understanding the mechanismsof earthquakes. Further important developments towards understanding thelarge scale geological processes causing earthquakes came in the mid-1960’safter the installation of the World Wide Standardized Seismograph Network(WWSSN). The WWSSN allowed monitoring of earthquake activity all overthe Earth with instruments that had similar, and well-known, characteristics.It was observed that most earthquakes occurred in long, narrow zones, lateridentified as boundaries between the tectonic plates. It is now understood thatthe surface of the Earth consists of a number of semi-rigid plates, which arein constant slow motion relative to each other, the motions being ultimatelydriven by the export of heat from the Earth’s hot interior. The idea that earth-quakes mainly reflect the relative motions of the tectonic plates was soon ex-tensively accepted.

About 90% of the total seismic energy around the world is released by earth-quakes that occur at plate boundaries, known as interplate earthquakes. How-ever, some earthquakes occur within the interior of tectonic plates, away fromthe plate boundaries. These are known as intraplate regions. Intraplate earth-quakes can be catastrophic since they are rare and may happen in unexpectedareas. They indicate that plate interiors are not fully rigid and include weak-ened zones where regional tectonic strain can be released. There are manypopulated areas with significant risk of large intraplate earthquakes, and it

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is usually challenging to assess the seismic hazard in these areas, especiallywhen there is a lack of historical earthquake records.

Globally, only a small extent of the large earthquakes belong to multipletsets, but multiplets have been observed not infrequently on active tectonicboundaries, especially on subduction zones. Multiplets occur when adjacentplate segments or faults of comparable size break with a short time delay. Onemore specific definition of multiplets is that they are clusters of moderate tolarge earthquakes (M ≥ 6) with magnitude differences of no more than 0.2units (e.g. Astiz and Kanamori, 1984), temporal separation of a small frac-tion of the average recurrence time of the earthquake cycle (Nomanbhoy andRuff, 1996; Kagan and Jackson, 1999), and with spatial separation betweenthe centroids of the earthquakes less than the rupture size of the events (Kaganand Jackson, 1999). Aftershocks following large earthquakes are known to bedangerous, because even relatively small aftershocks may cause the collapseof structures weakened by the preceding large event. Multiplets, where thefollowing event or events are comparable in size to the first large event, areespecially dangerous. It follows that better understanding of multiplets hasthe potential both to help us to understand earthquake systems and to improvehazard estimates and thereby reduce risk, possibly very significantly.

The proximity in space and time of multiplet events makes it clear thatthe first event has a significant role in triggering the following events, but themechanisms of this triggering is not yet well-understood. However, statisticalevidence suggests that the same physical triggering mechanism is responsi-ble for the occurrence of multiplets as for aftershocks and foreshocks (Felzer,2004). A common theoretical framework for investigating the earthquake trig-gering process is to estimate stress interaction between the events. One of themost used methods is Coulomb stress transfer (Stein, 1999) which estimatesthe stress changes close to a fault caused by an earthquake’s fault displace-ment, and evaluates where these changes may enhance or suppress the likeli-hood of a subsequent event. The method has been successfully used to explainthe triggering mechanisms of several successive mainshocks (e.g. King et al.,1994) and multiplets (e.g. Lin et al., 2008).

In this thesis, two multiplet sequences are discussed from the point of viewof the slip distribution and rupture process. The events investigated occurredin Iran, which is an active continental deformation zone with significant levelsof seismic hazard and where numerous destructive earthquakes are known tohave occurred in both historical and recent times.

This dissertation consists of five chapters including this introduction as thefirst chapter. Chapter two describes the tectonic and seismicity characteristicsof the areas studied. A concise description of the methods used for the follow-

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ing investigations is presented in chapter three. Chapter four is a summary ofthe papers included in this thesis and in chapter five conclusions and outlookare discussed. Following this, the papers are included in the printed versionsand pre-prints.

In paper I, entitled ”Fault slip and identification of the second fault planein the Varzeghan earthquake doublet”, a teleseismic body-waveform inversionmethod was used to deduce the slip distribution for the first mainshock ofa doublet in 2012. The deduced slip pattern was then used to estimate theCoulomb stress changes on the two nodal planes of the second mainshock andthe largest aftershocks.

In paper II, ”Directivity analysis of the 2017 December Kerman earth-quakes in Eastern Iran”, the rupture propagation of the Kerman triplet wasinvestigated, utilizing Empirical Greens Function (EGF) deconvolution. Twodifferent approaches were applied separately to P and S phases to detect therupture propagation direction for the three mainshocks and six of the largestaftershocks.

In paper III, ”Investigations of rupture properties of the December 2017Hojedk triplet in Eastern Iran”, the fault plane parameters and the slip distri-bution pattern of the three mainshocks were investigated using a teleseismicbody-waveform inversion method. The estimated slip models were used to as-sess the Coulomb stress changes on the fault planes and to examine the stressinteraction between them.

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2. Study area

Iran is part of the enormous Alpine-Himalayan orogenic belt. Active conver-gence of the Arabian plate to the southeast and the Eurasian plate to the north-west of Iran cause intense seismicity, taking place on active thrust and strike-slip faults (e.g. Jackson, 1992) (Figure 2.1). The crustal deformation resultingfrom the continental collision has generated various tectonic and topographicfeatures in a relatively small area, dividing Iran into five major seismotectonicprovinces (Mirzaei et al., 1998).

Figure 2.1. Tectonic map of the Arabian-Eurasian collision. Focal mechanisms arefrom the global-CMT catalog for the period from 1976 to 2020. Major faults are in red.Arrows and the numbers beside are the GPS-derived plate velocities (mm/year) rela-tive to Eurasia according to Reilinger et al. (2006). Inset shows the Alpine-HimalayanOrogenic belt. Location of other figures is marked by green boxes.

Iran have experienced frequent destructive earthquakes back in historicaltimes and during recent decades which totally ruined several populated cities

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and caused large numbers of fatalities and injuries (e.g. the 1990 Mw 7.4Rudbar and 2003 Mw 6.6 Bam earthquakes). In the research described below,two seismic sequences in two different seismogenic zones are investigated; anearthquake doublet in northwestern Iran and a triplet in eastern Iran.

2.1 Seismotectonic setting of northwestern Iran and thedoublet on August 11, 2012

The most dominant tectonic element of northwestern Iran is the North Tabrizfault which is a large strike-slip fault with NW-SE strike. With an overalllength of more than 200 km along several right-stepping en-echelon segments,it accommodates about 7 mm/yr of right-lateral motion of the Arabia-Eurasiaconvergence (e.g. Masson et al., 2006). The North Tabriz fault has experi-enced a significant number of historical earthquakes leading to repeated de-struction of Tabriz city, which now has a population of over 2 million. How-ever, recent seismicity data from 1960 to 10 August 2012 from EHB Bulletin(http://www.isc.ac.uk) do not include any earthquake of larger than magnitude5 and the most recent major movement was a sequence in 1721-1786 (M 6.3to 7.3) (Figure 2.2). According to the slip accumulation rate and historicalseismicity data, the average recurrence time of a magnitude 7-7.4 earthquakeon the North Tabriz fault is 250-300 years which suggests a high risk of a sig-nificant earthquake in the coming decades. The Varzeghan doublet, occurredabout 50 km north of the North Tabriz fault in the area where there are noreports of previous seismicity. The sequence caused about 300 fatalities andmore than 3000 injuries.

2.2 Seismotectonic setting of eastern Iran and the tripleton December 2017

Eastern Iran is an almost flat, rigid block, which is surrounded by large strike-slip faults to the east, west and north. The triplet occurred on the western mar-gin of this rigid block and is surrounded by several major right-lateral strikeslip faults and some smaller reverse faults (Figure 2.3). The recent seismic ac-tivity is mostly concentrated on the Gowk fault to the south, hosting destruc-tive earthquakes in 1981(Mw 7.1 Sirch, Mw 6.6 Golbaf) and 1998 (Mw 6.6Fandoqa) (Berberian et al., 2001). The Gowk fault with a length of ∼ 150 kmand a strike of ∼ 155◦, breaks into the Nayband, Lakar-Kuh, and Kuh-Banan

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Figure 2.2. Seismicity of northwestern Iran for M ≥ 3 events based on the ISC catalogfor the period 1960 to 10 August 2012. Stars and beach balls (from the global-CMTcatalog) mark the Ahar-Varzeghan doublet. The North Tabriz Fault is plotted in red.Green ellipses are the last four historical events based on Berberian and Yeats (1999).Gray boxes are main cities.

faults to the north. The first two faults striking N-S with right-lateral shearmotion have experienced relatively small seismicity, but the third fault has along record of earthquakes back in 1875 (e.g. Ambraseys, N.N. and Melville,C.P., 1982). The Kuh-banan fault extending for ∼ 200 km along NW-SE waslately ruptured in 1977 (Mw 5.8 Gisk) and 2005 (Mw 6.4 Zarand). Whilethe first event indicated an almost pure strike slip motion (Berberian et al.,1979), the latter one had a pure reverse mechanism on an E-W trending, northdipping splay fault which was assumed to be the southern termination of theKuh-banan fault. The 2017 triplet occurred 40 km east of the 2005 event withall mainshocks presenting oblique reverse movement. Further investigationsof faulting in the area (Walker et al., 2010), have detected a series of shortstrike-slip and reverse faults along the margins and even within the mountain-ous regions which suggests evolution of a ∼ 40 km wide left step restrainingbend between the Gowk and Kuh-Banan faults.

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1977

2005

1998

1981

1989

Figure 2.3. Seismicity map of the triplet area. All focal mechanisms are from theglobal-CMT catalog. Blue circles are events with M ≥ 5 between 1964 to Nov 2017,from the ISC catalog. Stars and colored focal mechanisms are the 2017 triplet events.Faults are in red. White boxes are main cities.

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3. Methodology

Earthquakes radiate mechanical (seismic) waves which propagate from theearthquake in all directions through the Earth’s interior and are recorded bysensitive instruments placed on or close to the surface. The record of themotion, called a seismogram, includes information about both the source andthe medium it passed through. In a general form the observed seismogram canbe written as:

u(t) = s(t)∗g(t)∗ i(t) (3.1)

where u(t) is the seismogram, * is the convolution operator, s(t) is the seis-mic source signals, g(t) is the propagation effect, and i(t) is the instrument re-sponse of the seismograph. It is possible to study a specific component of theabove equation if this effect can be isolated from the effects of the other com-ponents. Approaches to extracting information on the seismic source (s(t)) byuse of waveform inversion and Empirical Green’s function methods are goingto be discussed in this thesis.

3.1 Waveform Modeling

Waveform modeling is an iterative process of comparing synthetic and ob-served seismograms to minimize the difference between them by adopting thepropagation structure or the seismic source process. The propagation effect(g(t) in Eq. 3.1), also known as the Earth transfer function or the Green’sfunction, is the most complex parameter since it needs to account for elasticphenomena, such as reflections, multiplications, diffractions and phase con-versions, as well as attenuation effect, including scattering and geometricalspreading. However, at teleseismic epicentral distances (30◦− 90◦), we canreasonably avoid the crustal multiplications and core-mantle boundary diffrac-tions. Then, considering the early P-wave arrivals, g(t) can be regarded asbasically consisting of three pulses; P, pP and sP, i.e. the P wave which hastraveled directly from source to receiver, and the signals which have first prop-agated to the surface as P or S waves before propagating onwards as P. Re-flected and phase-converted pulses will arrive at the receiver after the direct

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(primary) waves. Arrival time differences mainly depend on source depth andEarth structure and the relative amplitudes depend primarily on radiation pat-tern and epicentral distance (Lay and Wallace, 1995, chapter 10).

Seismic source parameters generally include fault orientation, epicenter,depth, seismic moment, slip direction and magnitude, and source time function(STF). As mentioned above, the Green’s functions produce synthetic wave-forms that traveled along the path from the source to the seismometer. Hence,the seismic moment and source time function can be estimated by compari-son of the observed and synthetic waveform amplitudes. Once the source timefunction is determined, the other source parameters may be estimated undersome assumptions, including an assumed rupture velocity. Unfortunately, suchanalyses are often not so straightforward because there is a trade-off betweenthe assessed parameters. The strongest trade-off is mostly found between thesource depth and duration of the STF (Lay and Wallace, 1995, chapter 10),where the latter is proportional to the rupture velocity. In the other words,a deeper source with higher rupture velocity would be similar to a shallowersource and lower rupture velocity. However, use of broadband data and mul-tiple stations for modeling is helpful to reduce the trade-offs (Christensen andRuff, 1985).

In the following, the source parameters and slip distribution are estimatedby inversion of body waves at teleseismic distances using the computer codeof (Kikuchi et al., 1993). In this method the fault plane is discretized into anumber of elements or sub-faults with equal dimensions, covering the fault,which has predefined strike and dip angles. The kinematic parameters, slipand source time function, are retrieved for each sub-fault by adjusting the nu-merical model of these to obtain a good fit between the observed seismogramsand the model response. The far-field displacement u j (t) at station j due toshear dislocation on a fault surface S can be represented as (Olson and Apsel,1982; Hartzell and Heaton, 1983):

u j (t) =2

∑q=1

∫Gq j (t,ξ )∗ Dq (t,ξ )dξ (3.2)

where * is the convolution in the time domain, Gq j are the synthetic dis-placement waveforms (Green’s functions) for a source at ξ and receiver atstation j computed for two orthogonal unit steps q. Dq is the q− th compo-nent of spatio-temporal slip rate which can be written as a linear combinationof space and time basis functions:

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Dq (t,ξ )'K

∑k=1

L

∑l=1

aqklXK(ξ )Tl(t− tK) (3.3)

with k and l as space and time steps, aqkl is an array of coefficients thatneeds to be determined, XK(ξ ) is the space basis function at grid node k, Tl(t−tK) is the time basis function where tk is the time that rupture starts at grid nodek. By substituting Eq. 3.3 in Eq. 3.2:

u j (t) =2

∑q=1

K

∑k=1

L

∑l=1

aqklTl(t− tK)∗∫

Xk (ξ )Gq, j (t,ξ )dξ (3.4)

We can write the time and space functions as a single parameter Hqkl j andsubstitute in Eq. 3.4 to:

u j (t) =2

∑q=1

K

∑k=1

L

∑l=1

Hqkl jaqkl (3.5)

In vector form, Eq. 3.5 can be written as u = Ha. Solving for a using anon-negative least squares inversion method retrieves and estimates slip oneach fault element, which is zero or positive. The least squares misfit definedas ∑(xobs− xcal)

2/∑(xobs)2 is minimized during the inversion. xobs is the ob-

served data and xcal is the model data.

To select a model among a group of possible models the Akaike BayesianInformation Criterion (ABIC; Sclove, 1987) was used here. The Bayesian pro-cess chooses two weights for the orthogonal Green’s functions and one for theSTF duration, when evaluating the goodness of fit between the observed andmodeled data. The two weights that scale the orthogonal Green’s functionsdefine the slip amplitude and rake angle, where the rake angle is calculatedat ±45◦ from the predefined average rake. The STF is described by over-lapping triangles in which the amplitudes of the triangles are parameters inthe inversion. A smoothness constraint is applied as prior information and aconstant rupture velocity is assumed during the inversion. Theoretically, bysolving an inverse problem, the kinematic parameters can be retrieved at eachpoint of the fault. One problem with this type of analysis is the high degreeof non-uniqueness in the solution (Das and Kostrov, 1994), in the sense thatdifferent models may fit the data almost equally well. Therefore, other typesof information such as the existence of a surface rupture, aftershock distribu-

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tions, geological information, or information from other types of geophysicalanalysis, may be very helpful in stabilizing the results.

3.2 Coulomb stress

A sudden release of the accumulated shear stress is the cause of most natu-ral earthquakes. This stress release causes changes of the shear and normalstresses in the volume surrounding the source. Such a stress change can sig-nificantly affect the seismicity rate, provoke earthquake sequences, clusteringand aftershocks (e.g. Harris, 1998; Stein, 1999). One of the most known meth-ods for studying earthquake interactions is the Coulomb stress change:

∆σ f = ∆σs + µ∆σn (3.6)

The Coulomb stress change (∆σ f ), also sometimes known as the Coulombfailure stress (CFS), considers the changes in shear stress (∆σs) and normalstress (∆σn) on a chosen or predefined ”receiver” fault with an effective fric-tion coefficient (µ). Shear stress is defined as positive in the direction of faultslip and the normal stress as positive for opening (unclamping). Theoretically,failure is encouraged where the Coulomb stress change is positive and is dis-couraged for negative values.

To evaluate the static stress changes caused by a significant earthquake, weuse the Coulomb3 software (Toda et al., 2011). The slip distribution patternsobtained by waveform inversion were used as input for the calculation. Thecode uses the slip values to calculate the 3D strain field, multiplies this byelastic stiffness, and produces the stress changes due to the source fault plane(the fault that has ruptured). The stress changes are then used to estimatethe shear and normal stress changes on specified receiver fault planes. Eachreceiver fault plane is defined by its position, strike, dip and rake values. Theshear stress changes are sensitive to the position of the receiver fault relativeto the source fault and the relative geometries (rakes and strikes) of the faults.The normal stress changes are affected by the geometry and position, but notby the rake of the receiver fault.

An earthquake can thus enhance or suppress the likelihood of subsequentevents, depending on their location and orientation. There are some implicitassumptions about the pre-existing stress situation before the main event, butnumerous studies have proved this type of modeling to be a useful analysis

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tool (e.g. King et al., 1994; Stein, 1999). The seismic hazard in an area may besignificantly affected by stress transfer from earlier earthquakes. However, theCoulomb stress criterion only indicates if the receiver fault has been broughtcloser to failure or not, and nothing about how close it is. Evaluation of thelatter demands accurate assessment of the pre-existing stress fields.

3.3 Directivity analysis and Empirical Green’s Function

A common representation of the seismic source signal is the source time func-tion, which is the time evolution of the seismic moment. The STF consists ofthe slip history (TD) and rupture time (TR). Slip during an earthquake is oftenassumed to increase gradually on the fault plane and may be modeled as aramp function where the derivative is a box car with its length equal to the slipduration (e.g. Lay and Wallace, 1995, chapter 9). The rupture time describesthe rupture duration along the finite fault. Considering that the waves radiatedfrom the initial part of the rupture would propagate first and then the pointsfurther along the fault, the rupture-function also may be assumed to be welldescribed by a box car. The convolution of these two box cars gives a trape-zoid with its area equal to the seismic moment (Figure 3.1a). If we assume afinite fault of length L, and a simple pulse type rupture model (e.g. Haskell,1964) propagating from one end through the other end of the fault with rupturevelocity vr, and a receiver recording the rupture at distance r0 and azimuth θ

(Figure 3.1.b), then the variation of the rupture duration with azimuth (Tθ ) canbe written as:

Tθ =Lvr

(1− vr

ccosθ

)(3.7)

According to Eq. 3.7, unilateral rupture, would manifest itself as an appar-ently shorter duration STF when observed at stations in the direction of rupturepropagation, and an apparently longer duration when observed from the otherside. Since the area of the source time function is constant (because M0 isconstant) shorter duration is accompanied by higher amplitude observed STFsignals, and vice versa (Figure 3.1.c) (after Stein and Wysession, 2003).

As mentioned above, to extract the source properties from the seismogramswe need to filter out the wave propagation effects as well as site and instru-mental effects. A practical method to suppress these effects is the EmpiricalGreen’s Function (EGF) method (Hartzell, 1978; Mueller, 1985). The methodinvolves deconvolution of the target event with smaller events which are suffi-

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Figure 3.1. (a) Convolution of tow box-car functions representing the slip history(TD) and the rupture time (TR) results in a trapezoid source time function. (b) A finitefault of length L and a recording station at distance r0 and azimuth θ from the ruptureinitiation point. Rupture propagates with rupture velocity vr from one end of the faultto the other.(c) Variation of the source time function at different azimuths as an effectof rupture directivity (after Stein and Wysession, 2003)

ciently close to the target event such that the propagation paths can be assumedto be essentially similar. The smaller event should also have a similar fault-ing mechanism and be at least one order of magnitude smaller than the targetevent, so that it can be regarded as a point source. The deconvolution providesa relative source time function (RSTF), which contains information on sourceproperties such as rupture extent and propagation direction.

In paper II, we applied the EGF method to capture the variation of ruptureduration with respect to azimuth and so to detect potential rupture directiv-ity. Two different approaches were used in order to validate the results. Oneapproach was to retrieve the RSTF and assess the variations of its width atdifferent stations. The other approach was the comparison of the displace-ment amplitude spectrum observed at the stations along and opposite to thefault strike. The latter approach stems from the fact that the corner frequency,defining a change of gradient in the observed source spectrum, is inverselyproportional to the source duration ( fc = 1/TR +2/TD). Hence, the amplitudespectrum will exhibit an azimuthally dependent corner frequency (Figure 3.2)(Calderoni et al., 2013, 2015). By averaging the spectra at stations alignedalong fault strike and comparing with the average of stations in the oppositedirection, one can detect the propagation direction of the rupture.

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Figure 3.2. Illustration of the along strike rupture directivity based on the spectralvariation at stations at opposite azimuths (Calderoni et al., 2015),

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4. Summary of papers

4.1 Paper I: Fault slip and identification of the secondfault plane in the Varzeghan earthquake doublet

4.1.1 Motivation

On August 11, 2012, northwestern Iran experienced two destructive earth-quakes with Mw 6.5 (at 12:23 GMT) and Mw 6.4 (at 12:34 GMT) followedby numerous aftershocks with magnitudes up to Mw 5.6. According to theglobal-CMT(g-CMT) solution, the first mainshock ruptured with almost purestrike-slip motion, and the second main shock with an oblique thrust mech-anism. Due to the short time delay between the mainshocks, and the smallepicentral separation between them, ∼ 6 km (Ghods et al., 2015), we considerthem as doublet.

A surface rupture trace of about 12 km length with an E-W trend has beentracked in the field. The trace was associated to the first mainshock, since itwas favorably oriented along one of its nodal planes (Donner et al., 2015). Thesecond mainshock has no associated surface rupture, nor a clear aftershocksignature to identify the fault plane. There is also no recognized active faulttrace in the area where the doublet occurred and an absence of significantseismic activity for at least the past 200 years. However, the doublet locatesabout 50 km northeast of the North Tabriz Fault, a large active strike-slip fault(Figure 2.2) with high risk of a significant earthquake in the coming decades.Therefore seismic investigations of the doublet have important implicationsfor hazard assessment in the area.

4.1.2 Results

A teleseismic body waveform inversion method (described in section 3.1) wasused to estimate the slip distribution pattern associated with the first main-shock. The data for the second mainshock was highly contaminated by thesurface waves and coda from the first event, thus preventing reliable results

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from body wave inversion with this data for the event. P and SH waveformsfrom 93 stations at teleseismic distances, obtained from the IRIS data center(http://www.iris.edu/), were used for the inversion. A regional velocity modeland the CRUST2 model (Bassin et al., 2000) were chosen to describe the nearsource and near receiver structures, respectively. A zero-phase band-pass fil-ter of 0.015 to 0.5 Hz was applied to the data to enhance the body waves.The fault plane was discretized with 4 km node spacing, a total of 32 kmlength along strike, and 20 km width along dip direction. The initial strike anddip values were taken from the g-CMT solution and then altered gradually toobtain a minimum misfit between the observed seismograms and the modelresponse. The hypocenter depth and the rupture velocity was controlled man-ually to provide a good match with the observed surface rupture as well as agood fit between the observed and synthetic data.

Figure 4.1. The estimated slip distribution pattern for the first mainshock of the 2012Varzeghan doublet. Horizontal axis is distance along strike and vertical axis is dis-tance along dip of the fault planes. Yellow star is the hypocenter. Red dots are theaftershocks with magnitude larger than 3. The topography profile along the strike ofthe fault is plotted above the slip map, and the red triangles on it marks the two endsof the observed surface rupture.

We conducted separate and joint inversions of P and SH data and examinedvarious combinations of predefined parameters. The preferred slip pattern ob-tained for the first event, resolves surface slip vectors which matches reason-ably well with the observed surface rupture (Figure 4.1). The slip model witha hypocenter depth at 12km and rupture velocity of 2.8 km/s, yields a slip

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distribution pattern that extends more than 30 km along the strike and 20 kmalong the dip of the fault plane. The maximum slips occur around 4 km depthand to the west of the hypocenter. The rupture propagation directionality ismainly from the east to the west.

Figure 4.2. Coulomb stress changes caused by the first mainshock on the two nodalplanes of the second mainshock, (a) North-dipping nodal plane, (b) East-dipping nodalplane. (c), (d) The Coulomb stress changes on nodal planes of the largest aftershocks,due to the slip on the first event combined with the north-dipping and east-dippingnodal planes of the second event, respectively. Star is the second mainshock, circlesare the ten largest aftershocks. F1 and F2 are the surface traces of the fault planesassociated to the first and second mainshocks.

The slip model obtained for the first event was used as the source fault tocalculate the Coulomb stress changes on the two possible fault planes of thesecond mainshock as the receivers (section 3.2). We compared the deducedCoulomb stress changes on both these nodal planes (Figure 4.2) to investigatewhich was the more probable fault plane. The fault plane solutions (strike, dip,rake) and the hypocenter depth for the second event were defined accordingto the g-CMT solution and the relocations of Ghods et al. (2015), respectively.The sections of the Coulomb stress along these two planes indicate that theCoulomb stress changes are almost neutral around the hypocenter for the E-Wnodal plane (Figure 4.2a), while they are strongly positive on the N-S nodalplane (Figure 4.2b).

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Furthermore, we investigated Coulomb stress changes caused by both main-shocks on the nodal planes of the ten largest aftershocks. For this analysis wedefined a simple slip model for the second event based on its focal mechanismsolution and scalar moment (from g-CMT). Nine out of ten of the aftershocksexhibited positive Coulomb stress changes when the N-S nodal plane of thesecond mainshock was combined with the first fault plane, but only half werepositive when the E-W plane was used instead (Figure 4.2 d, c).

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4.2 Paper II: Directivity analyses of the 2017 DecemberKerman earthquakes in Eastern Iran

4.2.1 Motivation

Three moderately large events with Mw 6.1-6.0 occurred in eastern Iran be-tween December 1st and 12th 2017. The first two events are estimated to have3 km epicentral separation (based on the IRSC catalog), 11 days time separa-tion, and very similar focal mechanism solutions (based on the g-CMT). Thethird event is located about 10 km NW of the first two, it occurred 13 hoursafter the second one, and had a slightly different mechanism. The sequencewas followed by seven moderate aftershocks with Ml 5.0-5.2 and a significantnumber of smaller events. We studied the rupture propagation direction of themainshocks and six of the largest aftershocks in the sequence, and comparedthe results with the regional tectonic stress state.

4.2.2 Results

In order to detect the rupture directivity of the events in the triplet, we appliedthe EGF approach (section 3.3) using P and S wave data separately. ThreeEGF events were selected for each mainshock analysis (EGF1 to EGF9 re-spectively), to reduce the risk of radiation pattern mismatch between the targetand EGF event. The P-waves were used to retrieve the RSTF and compare itswidth at different stations. For this approach, first a 10 s window bracketingthe maximum P-wave amplitude was selected on vertical seismograms of boththe mainshock and EGF waveforms, which were all band-pass filtered 0.5 to1.2 Hz before the selection. Then, a 3 s window of the EGF waveforms waschosen at each station in such a way as to include the most similar part ofthe P-waveforms. Next, the target and the EGF event were deconvolved witha 1% water-level regularization. The resulting RSTF waveforms have beenplotted according to the stations’ azimuth to assess the relative duration of theobserved pulses.

The S-wave data was used in a different approach, to reach an additionalsemi-independent analysis allowing assessment of the consistency of the re-sults. 30 s data segments including a significant part of the S-wave energy,were chosen on both radial and transverse S-waveforms of the mainshock andEGF events. Amplitude spectra of the waveforms were calculated and spectraldivision of the mainshocks and EGF’s was performed. Then stations aligned

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within ±45◦ with fault strike were chosen and used to calculate the averagespectra along fault strike.

Figure 4.3. (a) Retrieved width of the RSTF for the first mainshock. The waveformsare normalized (dotted plots). Solid plots are the absolute values of the waveforms andgray curve draws an envelope over it. Dash-dotted vertical line marks the beginningof the RSTF and triangles mark the ends, both determined based on 35% drop fromthe peak value of the envelope. (b), (c) Results of the S-wave spectral division for thefirst mainshock using two different EGF events, EGF1 and EGF3, respectively. Redis the average spectra of the stations around the NW end of the fault plane, blue is theaverage around the SE end, and black is the average for the rest of the stations. Thebars in the insets show the mean amount of the amplitude spectra in the frequencyrange of 0.5 to 1.2 Hz, using the same color scheme as the spectra curves.

Our directivity analyses for the three mainshocks are summarized in Figures4.3, 4.4, and 4.5. The left columns of each figure displays the RSTF calcula-tions where a shorter width at stations oriented toward one direction wouldsuggest a rupture directivity in this direction. The column to the right showsthe S-wave analysis, where average spectra over stations in the NW and SEdirections of the fault strike are plotted in red and blue, respectively, and av-erage spectra of the stations in the directions perpendicular to the fault strikeare plotted in gray. The largest average spectra should define the prevailingrupture propagation direction, but if the gray spectra has greater amplitude,the test is considered as unreliable. For a more convenient comparison, the av-erage spectral values are summed in a band width of 0.5 to 1.2 Hz, and plotted

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as bar graphs shown as the inset in the spectral diagrams. The choice of fre-quency band was constrained to be adequately high relative to the frequencycontent of the mainshocks, and sufficiently low for the EGF events. The latteris such that the EGFs can effectively be considered as point sources, and theformer is to capture the finite fault properties of the mainshocks.

Figure 4.4. (a) Retrieved width of the RSTF for the second mainshock. (b), (c), (d)Results of the S-wave spectral division for the second mainshock using three differentEGF events, EGF5, EGF6 and EGF7, respectively. Details as in Figure 4.3.

For the first mainshock, the retrieved width of the RSTF shows a relativelyshorter duration for stations oriented towards the NW (Figure 4.3a). With thespectral analyses, the test was assessed to be invalid using EGF1, due to thehigher gray curve (Figure 4.3b). The S-wave data was corrupt at some sta-tions for EGF2 and analysis with this event was not feasible, but with EGF3the results suggest directivity towards the NW (Figure 4.3c). For the secondmainshock, not only the RSTF width comparison suggests rupture directiv-ity toward NW (Figure 4.4a), but also the spectral division analysis all showa clear rupture propagation direction to the NW direction (Figure 4.4b,c,d).For the third mainshock, the waveform deconvolution results do not exhibit aclear rupture propagation direction (Figure 4.5a). Besides, the spectral analy-ses display an almost uniform rupture propagation using EGF7 (Figure 4.5b),

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and apparent rupture directivity toward SE and NW using EGF8 and EGF9,respectively (Figure 4.5c,d). This discrepancy of the obtained results couldsuggest the lack of significant along strike directivity, or a bilateral rupturepropagation.

Figure 4.5. (a) Retrieved width of the RSTF for the third mainshock. (b), (c), (d)Results of the S-wave spectral division for the third mainshock using three differentEGF events, EGF5, EGF6 and EGF7, respectively. Details as in Figure 4.3.

Using S-wave spectral division analysis, six of the largest aftershocks ofthe sequence with Ml 5.0 to 5.2, have been analyzed to detect any preferredrupture propagation direction which four of them showed rupture propaga-tion towards the northwest. Our directivity analysis suggests a predominantrupture propagation direction towards the NW for the seismic sequence. Thedirectionality seems to be driven by the regional tectonic stress field due to thenorthward Arabian-Eurasian convergence.

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4.3 Paper III: Investigations of rupture properties of theDecember 2017 Hojedk triplet in Eastern Iran

4.3.1 Motivation

The 2017 seismic sequence in eastern Iran consisted of three mainshocks withMw 6.1-6.0 and numerous aftershocks with Mw up to 5.2. The sequence oc-curred in a sparsely-populated area and caused minor damage, but it was just60 km north of Kerman city, with a population of almost one million and sev-eral earlier destructive earthquake incidents. The triplet area is surroundedby four major right lateral strike slip faults: one to the southeast, two to thenorth, and one to the northwest (Figure 2.3). The triplet is located in a gapbetween these faults and rupturing was predominantly reverse faulting, incon-sistent with the mechanisms observed for the surrounding faults. The faultingpatterns and field observations in the area suggest a wide restraining bendconnecting these strike-slip faults together by creating a series of short reversefaults (Walker et al., 2010). The 2017 seismic sequence took place within therestraining bend and at the eastern edge of it. Therefore, investigating sourceproperties of the three mainshocks is helpful to better understand the faultingmechanism and stress interactions in a restraining bend.

4.3.2 Results

To estimate the slip distribution pattern on the fault plane of the three main-shocks, we applied the waveform inversion method by Kikuchi et al. (1993),described in section 3.1. The waveform data was from 40 stations at tele-seismic distances, obtained from the IRIS data center. After removal of theinstrument responses and bandpass filtering 0.01 – 1 Hz, a window of 25 slength starting 3 s before the first P and S arrivals was used for the inver-sion. The near-source and receiver structures were defined according to theCrust2 (Bassin et al., 2000) and Iaspei91 (Kennett and Engdahl, 1991) veloc-ity models, respectively. The initial strike and dip angles of the faults weredefined based on the g-CMT solution and the hypocenter depths and locationsare from the IRSC catalog. These predefined values are then adjusted man-ually and iteratively to fulfill four criteria including reduction of the misfitvalue, consistency with the observed surface rupture, compatibility of the es-timated moment magnitude with the g-CMT moment, and smoothness of theslip model. The first two mainshocks did not have any geological indicationsor field observation of rupture that could be applied to deduce which of thetwo nodal planes was the fault plane. Therefore, we estimated the slip distri-

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Table 4.1. Fault plane parameters for the slip models presented in Figure 4.6.

Model strike,dip,rake depth(km),vr(km/s)

M0(e16),misfit

strike,dip,rake depth(km),vr(km/s)

M0(e16),misfit

ruptureduration

a, b 305◦,52◦,103◦ 8, 2.0 165, 0.26 105◦, 40◦, 70◦ 8, 1.9 171, 0.26 6 sc, d 310◦, 50◦, 99◦ 9, 1.8 123, 0.39 115◦, 40◦, 81◦ 8, 1.6 134, 0.38 12 se 110◦, 40◦, 85◦ 6, 2.2 143, 0.40 8 s

bution on both nodal planes of the first two mainshocks. The third mainshockhowever, was observed to have surface rupture and a SW-dipping nodal planewas suggested as the fault plane (Savidge et al., 2019).

The estimated slip distribution patterns and the fault parameters are dis-played in Figure 4.6 and Table 4.1. Comparing the results, we observe that thefault strike, dip, rake, hypocenter depth and rupture velocity are quite similarfor the first two mainshocks (Table 4.1), but the rupture propagation pattern isdifferent. For the first mainshock, the rupture propagation is almost bilateralwith only slight further extension to the northwest (Figure 4.6a, b), but forthe second mainshock, slip vectors show a predominant rupture propagationdirection towards the northwest (Figure 4.6c, d). The third mainshock appearsdifferent from the first two in several ways; shallower hypocenter, 6 km com-pared to 8 - 9 km, faster rupture velocity, 2.2 km/s compared to 1.8 - 2.0 km/s,a clear bilateral rupture propagation, and the surface rupture which was absentin the first two events.

The estimated slip models have been used to assess the Coulomb stresschanges on the faults in two steps. First, the slip models for the first main-shock were defined as the source and the Coulomb stress changes calculatedon the possible fault planes of the second mainshock. For an easier visualcomparison, the contour lines of the slip pattern for the receiver fault is over-lain on the calculated Coulomb stress changes and the results presented for thefour possible combinations of the nodal planes (Figure 4.7a, b, c, d). Compar-ing the correlation between the areas with increased Coulomb stress changes(red cells) and the overlain slip distribution pattern, the better correlation isobserved for the source fault as the SW-dipping nodal plane of the first main-shock and the receiver fault as the NE-dipping plane of the second mainshock(Figure 4.7c).

In the next step, Coulomb stress was calculated on the fault plane of thethird mainshock, due to the rupture of the first two mainshocks, consideringthe four combination of the nodal planes (Figure 4.8a, b, c, d). As above, theincreased Coulomb stress changes are most compatible with the slip distribu-tion pattern when the SW-dipping plane of the first mainshock and the NE-

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Figure 4.6. Slip distribution pattern estimated for (a), (b) The first mainshock of theKerman triplet, (c), (d) the second mainshock, and (d) the third mainshock. In the rightcolumn are the solution for SW-dipping nodal planes and to the left are the NE-dippingones. Hypocenter is at node (0,0) and is marked with a star. Numerical parameters ofthe models are listed in Table 4.1.

dipping plane of the second mainshock are combined together (Figure 4.8c).Therefore, we suggest these two planes as the actual fault planes that haveruptured during the mainshocks.

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Figure 4.7. Coulomb stress changes due to the first mainshock on the fault plane ofthe second mainshock. Four possible combinations of the nodal planes are presented.The second fault plane (receiver fault) is outlined by the green frame and the contourlines of the slip pattern obtained by the waveform inversion is overlain on it. Graystar marks the hypocenter. Fault labels are ”F1” as the first mainshock and ”F2” asthe second mainshock. ”-NE” means the NE-dipping nodal plane and ”-SW” meansSW-dipping. Northwest and southeast directions along the receiver fault strike areindicated by the NW and SE labels.

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Figure 4.8. Coulomb stress changes due to the first and second mainshocks on thefault plane of the third mainshock. Four possible combination of the source faults arepresented. The third fault plane (receiver fault) is outlined by the magenta frame andthe contour lines of slip pattern obtained by the waveform inversion is overlain. Graystar marks the hypocenter. Fault label ”F3” stands for the third mainshock. Otherlabels as in Figure 4.7.

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5. Concluding remarks

In this research we studied source properties and triggering mechanisms oftwo multiplet sequences in Iran, which both happened in unexpected areas butclose to major active fault systems. The first event was the Ahar earthquakedoublet in 2012 (Mw 6.5 and Mw 6.4) that occurred in northwestern Iran.Before the doublet, the knowledge of active faulting in the region was verylimited and it was thought that the North Tabriz Fault was accommodating allthe shortening in northwestern Iran (e.g. Masson et al., 2006), with the areanorth of it being an effectively rigid block (e.g. Djamour et al., 2011). Thiswas consistent with the seismic records until 2012, but, then the Ahar doubletoccurred 50 km north of the North Tabriz Fault, with a different strike andsense of motion. After the doublet, studies suggested that the shear strain inthe area is not fully compensated by the North Tabriz Fault and the remainingshear strain is transferred further north and accommodated during the 2012earthquake sequence (e.g. Donner et al., 2015). The occurrence of the 2012doublet provided an opportunity to improve our knowledge of active faults inthe region. It also provided more general insights into the seismic hazard ofareas adjacent to continental active faults, which are usually assumed to bealmost rigid blocks with relatively little internal deformation.

The first mainshock in the 2012 doublet sequence ruptured with a strikeslip mechanism on an E-W trending fault plane. The second mainshock wascharacterized by oblique thrust motion, but the most important issue to be ad-dressed for this event was to identify the fault plane to determine if it wasstriking E-W, similar to the first event, or if it was perpendicular to that. Us-ing Coulomb stress analysis we suggested that the fault plane of the secondmainshock was the NNE-SSW striking plane, which intersects with the firstfault plane at depth. Fig 5.1a displays 3D illustration of the fault planes wherethe intersection is visible. Moreover, our results for the slip model of thefirst mainshock resolved two distinct slip patches which have different pre-dominant slip directions. Since the intersecting fault planes and different slipdirections occur in a relatively small area and in a very short time span, wesuggested a complex and segmented fault system for the area, which mightimply either a complex stress situation prior to the events or significant lateralvariations in shear strength along the fault area.

The second event studied in this thesis was the Kerman triplet of 2017 (Mw6.1 - 6.0) in eastern Iran. The triplet occurred at the eastern border of a re-

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Figure 5.1. 3D slip map of the fault planes for (a) the 2012 Varzeghan doublet, (b) the2017 Kerman triplet.

straining bend, where the western border had been identified as a reverse faultin 2005. Using the Haskell line source directivity definition and a simple de-convolution method, we could detect the rupture propagation direction of thelarge earthquakes (M ≥ 5) in the sequence. Based on our directivity analy-sis, a predominant rupture propagation direction from southeast to northwestis attributed to the sequence, and we consider that as a result of the regionalstress field controlling rupture directions in this area. Focal mechanism solu-tions show predominant reverse faulting mechanisms for all three main events,but several differences have been detected in the rupture processes of the threemainshocks (such as hypocenter depth, slip duration, slip distribution pattern)during our slip inversion analysis. Moreover, using Coulomb failure stress, weassessed the stress interaction between the mainshocks and suggested a modelfor the faulting configuration (Fig 5.1b) and hence the mechanical evolutionof the area. We suggest that the third mainshock formed a barrier which pro-hibited further rupture propagation during the first two mainshocks, but whichthen ruptured after a short delay because of the stress redistribution. As wesee, the 2017 triplet yielded a chance to assess faulting mechanisms and rup-turing processes within a restraining bend, which showed activation of faultsin the bend area in order to link the surrounding fault systems together.

The 3D representation of the fault planes for the doublet and triplet main-shocks (Fig 5.1a , b) have common features, notably intersection of the faultplanes at depth, where there were small to zero slip during the previous event.In the other words, though intersecting, slip was not repeated on the same faultsegment during the successive earthquakes.

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OutlookMultiplets are commonly known from subduction zones, often with largermagnitudes (M≥ 7), distances between their centroids which may exceed tensof kilometers, and time delays between events as large as a few years. Thetwo multiplet sequences studied here are intracontinental multiplets which arevery close in epicentral distance, 3 to 10 km, and occurred in a very short timespan, 11 minutes to 11 days. Both the sequences showed a complex interactionbetween strike-slip and reverse faults which characterizes the active tectoniczones in convergent plate-tectonic settings (Berberian and Yeats, 1999). Dueto these complexities the seismic hazard may be difficult to quantify in theseareas. Meanwhile, not only classical methods used to estimate earthquake haz-ard (such as concept of a seismic cycle), but also modern earthquake ruptureforecast models (such as the UCERF3 model for California, Field et al., 2014),do not include the potential of multiple fault rupture in their analysis, possiblyleading to misleading hazard assessment.

We suggest that future studies to update the hazard maps and stress regimesin such complex continental regions need to consider the probability of mul-tiplets and the possibility of intersecting fault planes. They also need to becautious in assuming rigid crustal blocks, and take into account the transfer ofshear strain from the main fault to reactivation of hidden faults, many kilome-ters away. It is also important to note that two successive events, even whenvery close in time and space, can exhibit noticeably different rupture behav-ior. A simple and practical way suggested here is the Coulomb stress analysis,which in spite of its assumptions about the pre-existing stress state provides auseful tool to describe the stress interaction between the faults.

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6. Sammanfattning på svenska

”Skalvmultiplette” är sekvenser av större (magnitud M ≥ 6) jordbävningarsom inträffar nära varandra i tid, rum och magnitud (t.ex. Astiz Kanamori,1984). Skalvmultipletter är inte vanliga men de ökar signifikant den seismiskarisken i de områden där de inträffar. Ett viktigt steg i bedömningen av den seis-miska risk som skalvmultipletter utgör är att undersöka deras källegenskaperoch de mekanismer som ligger till grund för att de sker. I denna studie under-sökte vi källegenskaper och källmekanismer för två skalvmultipletter i Iran,vilka båda inträffade i oväntade områden men i närheten av större förkast-ningszoner. Den första var en dublett som skedde i nordvästra Iran den 11augusti 2012 (Mw 6.5 och Mw 6.4) och den andra en triplett i östra Iran den1:a och 12:e december 2017 (Mw 6.0 - 6.1).

I artikel I studerade vi skalvdubletten i nordvästra Iran 2012 som orsakademer än 300 dödsfall och en mängd skadade. Det första huvudskalvet, medmagnitud Mw 6.5, var av horisontalförskjutande typ och associerades medett 12 km långt öst-västligt markgenomslag. Det andra huvudskalvet, Mw 6.4,karakteriserades av sned glidning på en reversförkastning men utan markgenom-slag eller efterskalv som definierade förkastningsplanet. Teleseismiskt datafrån mer än 100 P- och S-vågor användes för att invertera för förskjutnings-fördelningen på det första huvudskalvets förkastningsplan. Både P- och S-vågor användes för att stabilisera inversionen och markgenomslaget gav yt-terligare bivillkor till förskjutningsmodellen. Förskjutningsmodellen gav tvåväldefinierade områden med rörelse, ett centralt område och ett sidoområde.De största förskjutningarna skedde i det centrala området från nära markytanner till 10 km djup. I sidoområdet, på 12 – 18 km djup i förkastningens västradel, var förskjutningen mindre och med en annan rörelseriktning än i den cen-trala delen. Möjligen representerar sidoområdet rörelser på sekundärförkast-ningar som skedde i samband med skalvet. Med hjälp av förskjutningsfördel-ningen beräknades Coulombspänningsförändringen för det andra huvudskal-vets nodalplan och för de största efterskalven. Med antagandet att den statiskaspänningsförändringen från det första skalvet utlöste det andra visar resultatenav Coulombberäkningarna att det nord-sydliga nodalplanet bör ha varit merinstabilt, och därmed sannolikt det korrekta skalvplanet.

I artikel II studerade vi i vilken riktning brottet utbredde sig på förkast-ningarna i skalvtripletten 2017 i östra Iran. Vågformsdata från regionala avstånd

39

(90 - 400 km) och avfaltningsmetoden med Empirical Green’s Functions (EGF)användes för att undersöka brottriktningsrörelsen på de tre huvudskalven (ML6.0 - 6.1) och de sex största efterskalven. Två olika metoder användes förrespektive P- och S-vågsdata. P-vågsdata användes till att beräkna relativakälltidsfunktioner och med dessa undersöka riktningsberoende variationer ibrottets utsträckning i tid. Samma fenomen undersöktes med S-vågsdata genomatt studera hur amplitudspektra varierade med riktningen från förkastningen.Resultaten visar att brottet i de två första skalven startade i sydost och utbreddesig åt nordväst. I det tredje skalvet startade brottet centralt på förkastningenoch utbredde sig både åt nordväst och sydost. Riktningsanalysen av efterskal-ven visade att fyra av sex efterskalv också hade brottutbredning från sydostmot nordväst. Jordskalvssekvensen som helhet dominerades alltså av brottut-bredning från sydost till nordväst, vilket tyder på att det regionala spännings-fältet kontrollerar brottutbredningen.

I artikel III undersöktes förkastningsparametrar och förskjutningsfördel-ningen på de tre huvudskalven i tripletten 2017 med hjälp av teleseismiskavolymvågor. Fler än 50 vågformer från P- och SH-vågor för varje skalvanvändes i inversionerna och resultaten anpassades till de skalära seismiskamomenten från gcmt-lösningarna samt till observationer (eller bristen på de-samma) av markgenomslag. Fokalmekanismerna visar att reversförkastnings-rörelse dominerar hos alla tre skalven men endast det sista producerade ettmarkgenomslag. Skillnader i brottprocessen detekterades hos de tre skalven,bland annat hade det andra skalvet en signifikant längre brottutbredningstidän de övriga (12 s istället för 6 - 8 s), det tredje skalvet hade ett grundarehypocentrum (5 - 6 km jämfört med 8 - 9 km) och spänningsfallet var ocksåhögre i det tredje skalvet. Förskjutningsfördelningarna i skalven användesför att beräkna Coulombspänningsförändringar på förkastningarna. Dennaspänningsväxelverkan mellan huvudskalven tyder på att det sydvästlutandenodalplanet i det första skalvet, och det nordostlutande nodalplanet i det andraskalvet, är de korrekta skalvplanen.

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7. Acknowledgements

The first time I visited the Uppsala University was in Google Maps! It capturedme. I started picturing myself walking in the streets, riding a bicycle andfeeling the breeze on my face. In a year, it became a reality and I cannotexplain how cheerful I was. Now here I am, a different me, who thrived onendless struggles and kept hope after all failures for the past nine years. I hadawesome people beside me during this wonderful journey and I want to takethe chance to thank them.

First, I would like to express my sincere gratitude to my supervisor, Roland,whose wisdom, expertise, generosity, patience, and attitude is something I ad-mire. Whenever I went to him with hopeless results, he brought out somethingvaluable from it and helped me see things in a different light. Special grati-tude goes to Hossein, as I luckily had the chance to be his student back in mymaster studies, and also benefited from his endless support and advice duringmy PhD. Hossein, it was actually your outstanding knowledge in seismologyand your professional behavior which motivated me to continue my studiesto a PhD. You became a true friend to our family and the one who was al-ways there to help us. I am also thankful to Björn, for his guidance, criticalcomments, subtle points, and his openness even at busy times.

I would also like to express my deep gratitude to Mohammad and I believethat this success would not have been possible without his generous assis-tance, technical support and practical contribution. And Zoya, thank you somuch for granting me the opportunity to be a PhD student at Uppsala Uni-versity, and for a year of intimate supervision, gentle guidance and share ofexperiences. I lost you as my supervisor, but I gained a valuable friendship. Iam particularly grateful to Hemin, for his priceless comments and ideas whichhave remarkably promoted my research work.

I also appreciate my friends and colleagues at Geocentrum. Special thanksto Rebekka and Ashkan for reviewing this thesis and providing me worthycomments and feedback. To Aggela, with whom we started as office matesand then became friends, thanks for inspiring me when I was frustrated, and foryour patience and thoughtfulness. I would also like to thank Claudia, Zeinab,Frederic and Silvia, Karin, Giulia, Sissa, Hamzeh and Ka Lok for the mem-orable moments we had during the meetings, fikas and conferences. Thanks

41

for Ari for the fun fikas we had during the first year, and thanks to Oli andChristoph for the courses we had together. Many thanks to the geophysicsdepartment for making me feel welcomed, including (in no particular order)Azita and Alireza, Sahar and Reza, Saeed and Farzaneh, Faramarz, Maria andMichael, ChunLing, Darina, Daniel, Magnus, Monika, Bojan, Remi, Tegan,Georgiana. Thanks to Peter, Emil and Arnaud who kindly helped me withtechnical problems; to my friends in Hydrology: Farzad, Saba and Faranak; toTaher and Anita for their kind hospitality. Also thanks to the new generation:Mohsen for your humour which was always refreshing me, Alex and Joshifor welcoming me to your office, Mahshad for your candid friendship, and toAyse, Alba, Christian, Ruth, Paula, Laura, Sebastian, Ruixue, Michael, Mag-dalena, George, Jan, Tatiana, and those I might miss to mention, for providinga friendly environment.

I am grateful to my friends in Sweden, Marie and Omid, Behnoush andAshkan, Mahshid and Behzad, Mahsa and Hossein, who offered me theirpriceless friendship and support and with whom I have shared laughter, frus-tration and companionship.

I also want to thank some people from my undergraduate studies. Dr.Sharghi and Dr. Shadmanaman who introduced me to geophysics in a veryattractive way and encouraged me to pursue my studies in seismology. Alsospecial thanks to Najmeh, my friend from the master program with whom weshared a lot of scientific and non-scientific discussions and she provided mewith all kinds of data I needed during my PhD studies.

There are no words to express my gratitude to my family, to my Mom andDad, and my lovely brother, Sahand, for their unconditional love, support, en-couragement and prayers they have sent my way. To Mohsen’s family andespecially his Mom, for all her help with the kids when we needed it the most.I deeply appreciate the warm hospitality of my uncle Hossein and Mitra, es-pecially when I arrived in Sweden for the first time and of course our visits allafter. My deepest gratitude goes to my life time friend and love Mohsen, whostood by me throughout this long journey, patiently listened to me, supportedme, encouraged me and was always there for me at the end of the day. Tomy lovely angels, Ariana and Romina, you cannot imagine how grateful I amfor having you in my life. Your shiny eyes, warm hugs and innocent kisseswipe out all my tiredness and are the most powerful treatment for my hopelessmoments. Thank you for making me stronger and better and for inspiring meto thrive.

Samar Amini, Uppsala, April 2020

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Acta Universitatis UpsaliensisDigital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1918

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A doctoral dissertation from the Faculty of Science andTechnology, Uppsala University, is usually a summary of anumber of papers. A few copies of the complete dissertationare kept at major Swedish research libraries, while thesummary alone is distributed internationally throughthe series Digital Comprehensive Summaries of UppsalaDissertations from the Faculty of Science and Technology.(Prior to January, 2005, the series was published under thetitle “Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology”.)

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