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Journal of Geodynamics 52 (2011) 34–44

Contents lists available at ScienceDirect

Journal of Geodynamics

journa l homepage: ht tp : / /www.e lsev ier .com/ locate / jog

ectonic movements monitored in the Bohemian Massif

lahoslav Kost’áka,∗, Jan Mrlinab, Josef Stemberka, Bohumil Chánb

Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holesovickách 41, 182 09 Prague 8, Czech RepublicInstitute of Geophysics, Academy of Sciences of the Czech Republic, Bocní II/1401, 141 31 Prague 4, Czech Republic

r t i c l e i n f o

rticle history:eceived 29 March 2010eceived in revised form5 November 2010ccepted 15 November 2010vailable online 9 December 2010

eywords:ressure pulse

a b s t r a c t

This paper provides evidence for recent geodynamic activity within the Sudeten and Krusné Hory Mts.Fault Zones of the Bohemian Massif, Central Europe. Data were recorded using crack gauges and tilt-meters located on specific geological structures within caves and galleries. These data are supported byrangefinder, seismic, and groundwater observations. It is shown that a significant pressure phenomenon,here termed a pressure pulse, occurred during 2003. The pressure pulse initiated a series of tectonicdeformations. In the Krusné Hory Mts., the pulse was preceded by chaotic tilt movements followed bysignificant tilt reorientation. Several stages of the deformation process were determined, analysed, anddescribed. These stages represent stability, relaxation, compression, compaction, and later relaxation.

ectonic displacementarthquake micro swarmrack gaugeiltmeterroundwater monitoringohemian Massif

The pressure pulse itself was associated with the compressional stage. Moderate, but regionally signif-icant, earthquakes occurred during the later stages of the deformation process. This precludes the ideathat they might be responsible for the initiating the recognised movements. At the same time, an unusualsequence of earthquake micro-swarms occurred in West Bohemia. These events should all be seen as theresult of tectonic deformation initiated by the pressure pulse. Supplementary data indicate an affinitybetween the deformation process and large global disturbance within the Earth’s crust (Stemberk et al.,2010).

. Introduction

The Bohemian Massif is a large stable body of crystalline rockssociated with only limited seismic activity located in Centralurope. The massif is, therefore, highly suitable for research intohe microdeformation of tectonic structures. Data interpretation isreatly simplified as a result of the low levels of seismic noise andhis, in turn, allows recent aseismic movements to be constrained

ore readily. An international monitoring program (Cello andost’ák, 2003) stimulated greater research in the Bohemian Massif.ata obtained from the dense monitoring network has revealedsignificant increase in tectonic activity during the last decade

Stemberk and Kost’ák, 2007; Stemberk et al., 2008a, 2008b). Anal-ses have shown that this activity started with a pressure pulse in003. This was followed by specific movements or unusual localarthquakes. It has been shown that this activity could also be

ecognised on some of the major continental tectonic structuresf Europe such as those in the Rhinegraben in Germany, the Cen-ral Apennines in Italy, and the Gulf of Corinth in Greece (Stemberkt al., 2010).

∗ Corresponding author. Tel.: +420 266009369; fax: +420 284 680 105.E-mail addresses: kostak@irsm.cas.cz (B. Kost’ák), jan@ig.cas.cz (J. Mrlina),

temberk@irsm.cas.cz (J. Stemberk), bch@ig.cas.cz (B. Chán).

264-3707/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.jog.2010.11.007

© 2010 Elsevier Ltd. All rights reserved.

Much of the documented research focused on the SudetenMarginal Zone that separates the Bohemian Massif in the southwestfrom Lower Silesia in the northeast. These results were obtainedwith the use of specially designed monitoring instruments (Kost’ák,2006). However, it has not hitherto been known whether the tracesof such tectonic effects are recorded by other instrumentationlocated in the region. Fortunately, thorough geotechnical investi-gations have recently been undertaken as a result of extensive openpit coal mining in the basin that lies at the forefront of the KrusnéHory Mts. The fault slopes of the massif have been studied to pre-vent possible instability. This fault zone borders the northwesternBohemian Massif towards Saxonia. The two fault directions (Fig. 1)represent the principal structural orientations most active in theBohemian Massif during the Quaternary.

Over the past 30 years, a variety of instruments have beeninstalled there to monitor a range of rock movements. Investigatedphenomena include creep, superficial slope movements, and desta-bilisation within the massif. An extensive discussion of the variousmethods and results was published by Rybár et al. (1990) and sum-marised by Rybár and Kost’ák (1998). Indication of slope instability

in the mountains, as reflected in various geophysical and geodeticobservations, was presented by Mrlina et al. (1997). The relation-ship between tilts and micro tremors associated with mining wasstudied by Skalsky and Tobyás (1996). The analyses of differenttypes of data make it possible to differentiate between mining

B. Kost’ák et al. / Journal of Geodynamics 52 (2011) 34–44 35

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ig. 1. Location of observation sites in the northern part of the Bohemian Massif, Ceell; 5 – The main West Bohemia earthquake focal zone, Novy Kostel. KHFZ – Krus

nd natural effects (Kost’ák, 1998, 2000). For example, significantovement anomalies have been reported from precise geophys-

cal tiltmeters (Kost’ák et al., 2006). A particularly notable tiltnomaly developed in 2002. This anomaly was shown to be causedy the extreme precipitation recorded in August 2002, which ledo unprecedented floods across Bohemia (Chán et al., 2003; Chán,005).

The present paper analyses a range of data (tiltmeter,angefinder, hydrological, seismic) that have the potential to recordatural tectonic effects for the period relevant to the pressure pulseescribed in Stemberk et al. (2010). It was considered to be par-icularly advantageous to exploit data derived from a geophysicaliltmeter station located in a gallery deep within the crystallineocks that form the mountains. These data were then comparedo the results derived from the specially designed monitoringnstruments used in the study of Stemberk et al. (2010). The tec-onic pressure pulse and subsequent geodynamic processes wereemonstrated by a range of instrumental methods. It is hoped thathese results will stimulate research into aseismic fault tectonic

ovements and demonstrate suitable methods for their detection.

. Observation techniques

The data investigated in this work are primarily based on twopecial monitoring techniques. The first is dilatometric and is per-ormed using optical–mechanical crack gauges, known as TM71s.he second is inclinometric and is performed using geophysical tilt-eters. In addition, some more general techniques have also been

onsidered. These comprise rangefinder measurements, ground-ater level observations, and seismic data.

.1. Crack gauge TM71

The optical–mechanical crack gauge, known as a TM71, is ahree dimensional indicator of the relative movements betweenwo blocks separated by a discontinuity. The principles underlyinghe function of the instrument were described by Kost’ák (1991).t is a mechanical instrument that does not include any electricalarts. Therefore, it is able to record data over protracted periodsven in harsh outdoor conditions. This is important as the instru-ent is permanently connected via a special bridge fixed into the

ock on either side of the discontinuity under consideration. Conse-uently, data obtained over periods of several decades are readilyomparable. The principles underlying the instrument are based onechanical interferometry, i.e. registered moiré patterns observed

etween special optical grids. Information about the movement is

urope. 1 – crack gauge; 2 – tiltmeter; 3 – rangefinder; 4 – groundwater monitoringry Fault Zone, SFZ – Sudeten Fault Zone.

represented by the moiré patterns in two planes and this defines thethree-dimensional vector of displacement plus angular deviations.

The main application of the instrument in geotechnical researchwas suggested and discussed by Kost’ák (1993, 2002, 2006). Thelong-term application of this instrument has been studied in detailas a result of a protracted observation period in Poland (Kontnyet al., 2005). It was demonstrated that seasonal and climatic vari-ations can be detected and separated from the results. Therefore,long-term relative displacements can be studied successfully ontectonic structures. The resolution of the installed device variesfrom 0.05 to 0.0125 mm depending on the particular grid gauge.Excessive displacements are read with decreasing accuracy in allthree spatial co-ordinates. The angular deviation between twoblocks separated by a discontinuity is indicated with a resolu-tion of 3.2 × 10−4 rad ≈ 0.02 grad. Under field conditions, where theinstrument is subject to natural perturbations, data are found to beaccurate to better than 0.03 mm and rates of 0.1 mm per year can beverified. The instrument is produced by GESTRACZ, Czech Republic.

It is preferred that the investigations are undertaken under-ground in caves and galleries. The ability to register lateral andshear displacement components is a result of the three dimen-sional operation of the instrument. This represents the fundamentalmethodical advantage of the TM71.

2.2. Tiltmeter ASNS

The tiltmeter observatory is located near Jezerí. The observa-tory is set in a gallery driven into the fault slope of the KrusnéHory Mts. It is equipped with Automatic Seismo-Tiltmeter Stations,knows as ASNS (Inst. of Physics of the Earth, Russia). The ASNSis a modernised version of the well-known Ostrovskij Pendulum(Ostrovskij, 1961), in that the pendulum photoelectric displace-ment converter has been replaced by a capacitive analogue–digitalconverter. The feedback circuit was added to decrease the magni-tude of nonlinearity in the pendulum system. The control system ofthe pendulum position was added to extend the effective dynamicrange of measurements up to ±20′′. An open instrument is shownin Fig. 2.

The technical parameters of the ASNS are as follows:

(a) The dynamic range of the tilt record is ±10−5 rad without

pendulum position current compensation and 10−4 rad withpendulum position current compensation (compensation isachieved by shifting the instrument sensor measuring positionusing electromagnetic pulses. A set of such shifts can increasethe dynamic range by up to 10 times). The frequency range

36 B. Kost’ák et al. / Journal of Geod

Fig. 2. The ASNS tiltmeter. A robust measuring system contains a horizontal pen-d(di

(

(

wT(o(psa

TCKe

ulum in the black box at the base of the instrument. The small light-grey cylinderon the right-bottom side of the uncovered instrument) holds the capacitive trans-ucer of tilts as well as magnetic coils for calibration and compensation. The total

nstrument height is about 40 cm.

(with AFR deviations ±3 dB) is 0–5 mHz whilst the transducercoefficient from input tilt to output voltage is ±106 V/rad.

b) The maximum output voltage is ±10 V with 10 k� loading.(c) The pendulum free period is 5 s ± 10%.d) ASNS tilt channel sensitivity has been determined in the factory

using a reference tilt platform. Periodical verification of thesestability parameters is provided with a pulse calibrator thatproduces double-sided current pulses (each of 120 s duration)twice a day and is synchronised with hourly time marks.

Two additional accelerometer channels were added to recordithin the frequency range of long period seismic oscillations.

hese channels were designed as second-order band-pass filters0.2–5 mHz) followed by amplifiers but these have not been used in

ur study. Two temperature channels with sensors in each tiltmetersensitivity 5 V/◦C) enable the identification of temperature relatederturbations. Air-pressure is also recorded so that its impact on tiltignals can be accurately assessed. Due to the pervasive difficultiesssociated with the mathematical correlation of air-pressure and

able 1omparison of the tectonic deformation process in the Krusné Hory Mts. Fault Zone withrusné Hory Mts. Fault Zone are based on tiltmeter monitoring at Jezerí-1 (see Fig. 12). Ret al., 2010).

Deformation process

Krusné Hory Mts. Fault Zone Tiltmeter results Significant eve

Period Trend Deformationcharacteristic

November 01–December 01 W CreepJanuary 02–July 02 W → E → W: S-like

loopStability

August 02–February 03 SW RelaxationRains – August

March 03–May 03 W → S → NE → SE RelaxationJune 03–November 03 NW Compression

Groundwater rDecember 03–January 04 W → E loop CompactionFebruary 04–June 04 SW → SSW CompactionJuly 04–December 04 W → WSW Compaction →

stability� Zakopane (SE�Macquarie (N� Sumatra (Ind

January 05–November 05 Chaotic → SW →S → SE → WSW → NW

Relaxation � Islamabad (Kn.M. (NE Bohem

December 05–February 07 Chaotic → SW →W → N

Relaxation �Vrbové (W Sloore mines (S Po

March 07–December 07 NW Compaction

ynamics 52 (2011) 34–44

tilts, we simply qualitatively estimate its impact on the derived tiltdata. Over the long-term, these impacts upon general tilt trend areinsignificant.

One observation site comprises two ASNS tiltmeters installedon two granite plates (1.2 m × 0.6 m × 0.1 m). These are oriented NSand EW, determined using geodetic instrumentation to within 0.2deg. The instruments are protected with a foam plastic case in caseof convection air noise and temperature impacts. In addition, a pro-tective hood of polyethylene film was stretched across the woodenbars. Such simple precautions reduce noise by an order of one mag-nitude. The data acquisition system (10 min sampling rate) containsa personal computer and a 16-channel 16-bit analogy–digital con-verter (ADC).

2.3. Groundwater measuring devices and methods

Groundwater monitoring is performed in a series of wells withthe hydro-sensor DIVER DI240. This sensor has a range of 5 m anda resolution of 1 mm. The barometric impact is corrected usingBaroDIVER with a range of 240 hPa and an accuracy of 0.1%. Theinstrument is produced by Van Essen Instruments, Holland.

Initially, Well JZ41 near Jezerí was tested to select the most suit-able equipment. These results showed that relatively high waterlevel fluctuations passed beyond the acceptable range of the moresophisticated instrumentation. Therefore, we decided to proceedusing a manual water level electric sensor dropped into the well.Monitoring took place at an interval of once every two weeks. Thewell is located at the toe of the slope of Jezerí. It has a depth of123 m and reaches the underlying beds of the coal basin.

3. Pressure pulse observations

The pressure pulse can be studied on the presented displace-ment graphs. These data are recorded from two distinct fault zoneslocated in the northern sector of the Bohemian Massif; the Krusné

Hory Mts. Fault Zone and the Sudeten Fault Zone. The location ofeach instrument is shown in Fig. 1. All of the sites demonstratedthe effects of the pressure pulse. A summary of the timing of theobserved effects and phases is presented in Table 1. This table alsoallows a comparison of the results derived from the Sudeten and

the Sudeten Fault Zone. Also shown are significant seismic events. Results from thesults from the Sudeten Fault Zone are based on crack gauge monitoring (Stemberk

nts Sudeten Fault ZoneTM71 resultsStage

Not analysed(1) January 02–September 02 Stability

02(2) October 02–June 03 Relaxation

ise – October 03(3) October 03–December 03 Pressurepulse

(4) January 04–July 04 CompactionPoland) 30.11.04; M = 4.8

ew Zealand) 23.12.2004; M = 8.1on.) 26.12.2004 M = 9.0 and M = 7.5

(5) September 04–October 04 Downthrust(6) November 04–December 04Compaction

ashmir) 8.10.05; M = 7.5 �Hronovia) 25.10. 2005; M = 4.8

(5)–(6) January 05 → August05 → Downthrust and compaction cycles

vakia) 13.03.2006; M = 3.6 �Lubinland) 14.03.2006; M = 3.7

(7) May 06 → Relaxation

(8) Not analysed

B. Kost’ák et al. / Journal of Geodynamics 52 (2011) 34–44 37

Fig. 3. Long-term crack gauge displacement graphs displaying the pressure pulse. Sites a, b, and c are located underground whereas Site d is located at the surface. The recordingresolution is ≤0.013 mm. (a) The total length of the displacement vector u registered in Na Spicáku Cave, NW Moravian (Sudeten Fault Zone). The inverse peak (i.e. the increasein displacement vector length) registered from July 2003 to December 2003 was followed by a period of increased geodynamic activity that persisted until May 2007. (b) Thetotal length of the displacement vector u registered in Na Pomezí Cave, NW Moravia (Sudeten Fault Zone. The inverse peak registered from October 2003 to December 2003 wasfollowed by a period of increased geodynamic activity that persisted until March 2007. Specific pressure conditions in the Sudeten Fault Zone are responsible for the decreasei displaZ perios at Mt.e e toe

Km

3

ai

n vector length observed here (Stepancíková et al., 2008). (c) The total length of theone). The peaks registered from May 2003 to September 2003 were followed by ahear displacement z registered in a fossil slope fissure composed of ortho-gneissnd of 2003 and the beginning of 2004 represents a short period of uplifts within th

rusné Hory Mts. The data obtained from the tiltmeter requiresore detailed study and will be analysed separately in Section 4.

.1. Crack gauge measurements

A series of crack gauges located in caves and galleries recordedtectonic pressure pulse across the Bohemian Massif. This was

ndicated by sharp peaks in the long-term displacement data that

cement vector u registered in the gallery of Janowice Stare, W Silesia (Sudeten Faultd of increased geodynamic activity that persisted until June 2006. (d) The verticalJezerka, NW Bohemia (Krusné Hory Fault Zone). The peak registered between theslope block, i.e. the movement contradicts usual rock slope movements (sliding).

appeared almost simultaneously during the second half of 2003.This signal was recorded for several months. Fig. 3 shows some rep-resentative graphs of the displacement vector length, u. The pressure

pulse initiated several years of increased tectonic deformation. Thisis described as a phase of increased tectonic dynamics.

The first three graphs were obtained from underground mea-surements within the Sudeten Fault Zone. At Na Spicáku Cave(Fig. 3a), the pressure pulse produced an overthrust to the north-

38 B. Kost’ák et al. / Journal of Geodynamics 52 (2011) 34–44

Fprm

eaptmtdpcF

Jitr

lgea

Ktfmrsao2d

srdtpsFwo

a

Fig. 5. Displacements registered in the wall of Jezerí Gallery. The instrument is

of 388 m; and Point K3 at a distance of 380 m. The results for theperiod between 1996 and 2008 are given in Fig. 7. These indicatethat the maximum drop between the measured distances occurredbetween November 2002 and November 2003, i.e. during the period

ig. 4. A fissure monitored on the slope of Jezerka near Jezerí, Krusné Hory Mts. Theressure pulse produced a movement in which the lower block (right) moved upelative to the upper block (left), i.e. it was uplifted. The observed vertical shear zovements on the investigated slope fissure are displayed in Fig. 3d.

ast. At Na Pomezí Cave (Fig. 3b), the pressure pulse producedn overthrust to the northwest. At Janowice Gallery (Fig. 3c), theressure pulse produced downthrust to the north whilst horizon-al dextral (right-lateral) fault movements prevailed. Whilst the

ovements were clearly locally complex, all can be explained byhe impact of a bulk pressure force. The detailed analyses of theseata indicate an increased dynamic phase as a result of the pressureulse (Stemberk et al., 2010). This increased dynamic phase is asso-iated with several specific stages of collision within the Sudetenault Zone.

Fig. 3d is recorded at the surface of a disturbed fault slope underezerka Hill (Fig. 4). This site is far from the Sudeten Fault Zone and isnstead located in the Krusné Hory Mts. Fault Zone. The graph showshe most typical slope fissure reaction, i.e. vertical movement zegistered during and after the indicated pressure pulse.

In Fig. 3d, the slope fissure movement z represents uplift of theower block. The displacement is opposite to that expected for aravitational slope movement. It is, therefore, a response to anxternal pressure. This indicates that uplift of the lower block islso associated with the documented pressure pulse.

Fig. 5 presents the results of crack gauging in the Jezerí Gallery,rusné Hory Mts. The gallery portal is situated about 650 m from

he site shown in Fig. 4. The monitoring point is located 389 mrom the portal, deep in the crystalline ortho-gneiss rocks of the

assif. In this example, some fractures show slow slipping as aesult of the pressure pulse (Fig. 5a: fracture dip-slip component,). Between 2003 and 2006, the displacement subsides slowly frombout 0.05 mm to a new level at about −0.21 mm. Two shorter peri-ds of acceleration are observed during summer 2003 and spring006. The period of slow movement coincides with the increasedynamic phase triggered by the pressure pulse.

The first acceleration developed along with the peak of the pres-ure pulse during September 2003. Fig. 5b shows displacement peaction across the inclined fracture. This represents compressionue to the pressure pulse. The fracture was dry and tightly closed inhe rock wall during the pressure pulse in 2003. Therefore, the peakin the graph represents an elastic rock deformation of negative

train rather than simply fracture narrowing. The site is shown in

ig. 6. It changed in character during 2009, as the fault zone becameet as a result of draining groundwater from the rock massif. This

ccurred due to stress relaxation.The geophysical tiltmeters are installed in the same gallery but

bout 16 m deeper (see Section 4).

located at a depth of 389 m. It is set across a major rock fracture in the crystallinerocks of Krusné Hory Mts. The investigated structure slopes 70◦ with an azimuth of270◦ . (a) Dip slip s along the structure and (b) pressure displacements p across thefracture. The recording resolution is ≤0.013 mm.

3.2. Supplementary observations

3.2.1. Rangefinder measurementsOn top of Jezerka Hill, rangefinder measurements are performed

regularly once a year (every November) with a Mekometer. The aimof these measurements is to check the distance between a refer-ence point at the top of the cliff that marks the crest of the slopeand several other points further to the northwest in the mountains.These are: Point K1, at a distance of 589 m; Point K2 at a distance

Fig. 6. The monitoring point in Jezerí Gallery. The photograph was taken in Novem-ber 2008, when the structure drained a significant amount of groundwater from themassif. This is interpreted as a sign of pressure release.

B. Kost’ák et al. / Journal of Geod

FKau

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3

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Foarr

ig. 7. Variations in the distance between points located around Jezerka Hill in therusné Hory Mts. The distances between the reference point and Points K1, K2,nd K3 are 589 m, 388 m, and 380 m, respectively. These measurements have beenndertaken using an electrooptical rangefinder (Mekometer).

f the pressure pulse. During this period, the distances had shortenedy about 3 mm. This observation confirms the compressional effectf the pressure pulse. The value can also be understood to expresshe horizontal strain produced by the pressure pulse.

.2.2. Groundwater level observationGroundwater observations taken from a borehole located at the

oe of Jezerí Hill have shown unusual characteristics (Chán, 2005).t depth, this borehole (JZ41) reaches the crystalline basement of

he massif. An almost constant groundwater level was recordedetween January 2002 and April 2003. The level increased signifi-antly by 11 m between May 2003 and September 2003. This wasollowed by an unprecedented increase of 33 m in October 2003Fig. 8a). The groundwater then remained at this elevated level fornother eight months. Thus, groundwater level increased by a totalf 44 m in 2003. Groundwater slowly returned to its earlier level byovember 2004. It remained at this level until monitoring ceased

n March 2005.It is important to consider the effect of precipitation on ground-

ater level, especially with regard to the previously mentionedxtreme events of 2002 that led to widespread flooding in Bohemia.his association was studied by Rybár and Novotny (2005). It was

oncluded that, for the climatic belt of Central Europe, increasedroundwater levels are a consequence of the effects of spring thaw-ng following late autumn and winter snow precipitation. Specificnalyses are presented for the critical period between 2002 and003. The derived precipitation totals are based on observations

ig. 8. (a) Groundwater level in borehole JZ41. The borehole is located at the slope toe of 2003. The rise can be correlated with anomalies registered at the tiltmeter station of Jeutumn when precipitation is regularly at its minimum and no coeval precipitation extreelative to the long-term monthly averages recorded at Teplice Station (after Rybár and Nains of August 2002; – groundwater level rise of 2003.

ynamics 52 (2011) 34–44 39

recorded at the nearby Teplice Station. Fig. 8b shows the deviationbetween monthly precipitation totals and the long-term monthlyaverage. This best represents the effect of rock water saturation. Theextreme summer precipitation of 2002 caused groundwater accu-mulation in August 2002 and then again in December 2002 andJanuary 2003. An increase in the precipitation balance stopped inJanuary 2003. From that point, a continual decrease in the precipi-tation balance was observed across western and northern Bohemia.

The course of the groundwater level recorded in borehole JZ41(Fig. 8a) is markedly different to the course of the precipitationbalance (Fig. 8b). The unprecedented groundwater increase during2003 evidently occurred during a deficit phase of the precipitationtotals (Fig. 8b). Therefore, there is no reason to connect the unusualgroundwater level observation with the effects of precipitation. Theonly explanation for the sudden rise is a shock in pore pressure.Following this, the rock structures through which water percolatedin and out of the borehole have been blocked at depth. This blockagelasted for a period of eight months. Slow pressure release began inthe second half of 2004. This observation shows that the pressurepulse of 2003 is also responsible for the rise in groundwater level. Itis coeval with peak compressive displacement indicated by fracturegauging in Jezerí Gallery (Fig. 5b).

3.3. Geodynamic phenomena in West Bohemia

West Bohemia is well known for its geodynamic activity. Forexample, the region has recently been analysed for seismicity (e.g.Fischer and Michálek, 2008) and for surface kinematics (Mrlina andSeidl, 2008). Here, we examine the development of seismicity andgroundwater level through time as these factors may relate to thegeodynamic pressure pulse under investigation.

3.3.1. West Bohemia seismicityEarthquake activity in the Novy Kostel Focal Zone had previously

been thought to be characterised by repeated weak earthquakeswarms with a local magnitude of up to M = 4.5 (1985/1986). Dur-

ing the 1990s, these swarms occurred every 3–4 years (1991, 1994,1997, and 2000). After the significant swarm of autumn 2000(M = 3.5) and the associated series of aftershocks in 2001, there wasa clear break in swarm activity that lasted until the end of 2003.However, a long lasting sequence of frequent micro swarms (2–3

f Jezerí Hill, and reaches crystalline rocks at depth. A clear rise occurred at the endzerí and in crack gauge monitoring at Jezerka. The effect was observed during themes were recorded at this time. (b) Deviations in the monthly precipitation totalsovotny, 2005). Data source: Czech Hydro-Meteo Service, Prague. – the torrential

40 B. Kost’ák et al. / Journal of Geodynamics 52 (2011) 34–44

Koste

por

(mmbcolmttt2iyiottsf

3

fleKiisct

a2yd

Flep2

Fig. 9. Temporal overview of seismic activity in the Novy

er year: M < 2) spanned the period from 2004 to 2007 (Fig. 9). Thisccurrence was unique in the long-term earthquake observationsecorded in the region.

Using GPS and precise levelling observations, Mrlina and Seidl2008) proved that minor horizontal and vertical surface move-

ents were found to be related to the swarms. Such surfaceovements were especially notable during the 1990s. However

etween 2001 and 2007, the only notable event was a locally signifi-ant forward/reverse movement in the Novy Kostel Focal Zone. Thisccurred between October 2003 and March 2004. This period fol-ows the peak of the pressure pulse and includes the first significant

icro swarm in February 2004. Taken as a whole, the specific pat-erns of seismicity do not show any remarkable characteristics inhe period between 2004 and 2007 (i.e. that can be associated withhe pressure pulse of 2003). In fact, the opposite can be observed;003 is characterised by low or even minimum seismicity. However,

ncreased micro seismicity developed gradually in the proceedingears. This coincides with the increased dynamic phase recognisedn the Sudeten Fault Zone (Stemberk et al., 2010). During this periodf increased dynamic activity, earthquakes only appeared towardshe end of the phase. Therefore, observations from the Novy Kos-el Focal Zone support the view that the pressure pulse itself waspecifically aseismic but that it also created the conditions requiredor the increased seismicity activity that appeared later.

.3.2. West Bohemia groundwaterIn the seismoactive region of West Bohemia, groundwater level

uctuations have been monitored in two wells since 1994 (Mrlinat al., 2003). Well P1A is located close to the focal centre in Novyostel and exhibits a strong tidal influence. In contrast, Well H3

s located about 4 km to the west and does not exhibit any tidalnfluence. Both wells show a notable correlation with earthquakewarms. Fig. 10 shows clearly that most of the earthquake swarmsoincide, or are close to, the groundwater minima. This suggestshat pore space is opened considerably during the swarms.

Following a comparatively deep groundwater minimum associ-ted with the earthquake swarm of autumn 2000 (Mrlina et al.,003), groundwater levels continued to rise for more than twoears to reach a positive peak in spring 2003. The groundwater thenropped. It continued to oscillate around this generally lower level

ig. 10. Variations of groundwater level at Well H3 in West Bohemia. The well isocated near the main earthquake focal zone. The peak recorded in mid 2003 clearlyxceeds the usual water level. Relative minima correspond with earthquake swarmeriods. Striking noise was recorded during the Sumatra earthquake in December004.

l Zone, West Bohemia (after Fischer and Michálek, 2008).

until 2007. It is considered that these phenomena provide furtherevidence for the pressure pulse (here, locally, compression) andsubsequent period of relaxation, as recognised in the earthquakemicro swarms described above.

An exact hydrological model that describes the relationshipbetween groundwater level and earthquakes has not been devel-oped yet. It is only possible to say that the data do not provide aclear correlation between precipitation and groundwater level (seealso Section 3.2.2 and Fig. 8b).

The conclusions derived from Section 3 suggest that evidencefor the pressure pulse is recorded across a comparatively wide areaby different monitoring methods. The monitoring sites displayed inFig. 3a and c indicate extension. These two gauges are clearly set onstructures that are orientated incongruently to the local pressure.However, in general, the observation that the displacement vectoru has either increased or decreased in length does not indicate thecharacter of the pulse, i.e. whether it was compressive or exten-sive at that particular locality. However, observations recorded byfour different methods (slope fissure monitoring (Figs. 3d and 4),fracture monitoring at depth in the rock massif (Fig. 5), rangefinderobservations (Fig. 7), and borehole groundwater level monitoring(Fig. 8a)) reveal that the pulse is compressive. This corresponds togroundwater observations in West Bohemia (Fig. 10). Therefore,the geodynamic pulse of 2003 is shown to be a tectonic pres-sure pulse that may have also affected the character of earthquakeswarms in West Bohemia. Earthquakes only appeared later duringthe increased dynamic phase (Fig. 9). The delayed appearance ofearthquakes was also reported by Stemberk et al. (2010).

4. Geophysical tiltmeter observation

Jezerí-1 tiltmeter station is set deep in the crystalline of theKrusné Hory Mts. It is located in Jezerí Gallery, 410 m from theportal. Two tiltmeter stations (Jezerí-1 at depth, Jezerí-2 in thesubsurface zone), orientated N–S and E–W, provide the course ofthe vector movement. A hodograph represents the development ofinclination in Jezerí Block, an outstanding marginal massive rockblock of gneiss within the fault zone. The medieval Jezerí Castle islocated on the slope of this block. Monitoring its stability is one ofthe aims of the tilt observations in addition to assessing the safety ofmining operations (Mrlina et al., 2006). The resulting tilts at Jezerí-1are presented, but with the influence of tidal effects removed.

4.1. Pressure pulse period of 2003

Fig. 11 provides a detailed record of tilting during the criticalperiod of the pressure pulse. A loop develops from April 2003 thatends in June 2003. The loop represents a transitional period dur-ing which a significant change in vector orientation developed,from SW to NW. The direction of tilting to the northwest is unique

within the whole investigated period from 2001 to 2007. This coin-cides with the tectonic pulse period. In addition, the two peaks ofanomalous tilt velocity recognised in March 2003 and October 2003represent the maxima of the whole investigated time span (see alsoFig. 13). The characteristic tilt periods are as follows:

B. Kost’ák et al. / Journal of Geod

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ig. 11. Vector development of tilts recorded at Jezerí-1 between 2003 and 2004.he circled numbers are summarised in Table 1. [ ] ≈ arcsec; – the torrentialains of 2002; – groundwater level rise of 2003.

1 February–1 March 2003 → SW, slow1 March–1 April 2003 → W, acceleration

1 April–1 June 2003 → loop back to E, chaotic1 June–1 November 2003 → NW, month to month acceleration

In February 2004, a significant change returned the tilt vectorrientation from NW back to SW (Fig. 12). A very low tilt velocity

ig. 12. The long-term development of the tilt vector registered at Jezerí-1 between 7 Sable 1. [ ] ≈ arcsec; – the torrential rains of 2002; – groundwater level rise of 200

Fig. 13. The long-term tilt velocity (′′) in Jezerí Gallery. Monthly average values with

ynamics 52 (2011) 34–44 41

was observed at the time of this change (Fig. 13). The southwest-ern trend remains constant until the end of 2004, although in somecycles it is deflected (e.g. SW until July, then WSW (Fig. 11)) with tiltvelocity acceleration until November 2004 (Fig. 13). This complexdevelopment is characteristic for post-pulse geodynamics. Detailedanalysis of this phase has been undertaken by Stemberk et al.(2010).

The analysis revealed several stages of the deformation pro-cess and specified its characteristics (Table 1). We introduced thesecharacteristics into the tiltmeter graph with respect to the timingof the particular phase in order to incorporate results from theSudeten and Krusné Hory Mts. Fault Zones. In Fig. 11, it can beseen that the loops or inflection points only appear around phases(2)–(5). This indicates rock massif stress changes, i.e. the onset ofnew deformation phases (cf. Table 1 and Fig. 11).

Another correlation can be found with regard to the rise ingroundwater level (Figs. 8 and 11). The first rise of groundwaterlevel appeared in April 2003. At the same time, the first tilt loopstarted following the total maximum of tilt velocity in March 2003.The unprecedented rise occurred in October 2003. This was thenlater than phase (3), at the time when the effects of the increased

pressure was in operation and tilt velocity culminated for the sec-ond time (Fig. 13). This high groundwater level started to drop withthe compaction phase (4), and again during cycles (5) and (6). In theperiod from April 2003 to June 2003, the H3 groundwater level alsoreached its total maximum in West Bohemia (Fig. 10).

eptember 2001 and 1 January 2008. Supplementary remarks and symbols refer to3.

polynomial trend (6◦). Supplementary remarks and symbols refer to Table 1.

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.2. Long-term tilts

Fig. 12 shows the association between tilting and the long-erm deformation process. Following the pressure pulse, a longereriod of geodynamic activity developed. This is referred to as the

ncreased Dynamic Phase in accord with investigations in the Sude-en Fault Zone (Table 1). The phases of mechanical tilt developmentnd tilt velocities are based on monthly average values. These showignificant reversals in the long-term tilt vector as well as periodsf more or less constant trends. As a whole, the tilt vector trendevelops towards the west.

Clearly this orientation does not reflect the steepest slope of theezerí Block, which is inclined to the southeast into the sedimentaryasin. Therefore, the overall westerly trend may instead reflect localorphology, i.e. natural creep of the massif towards the adjacent

alley beneath the Jezerí Block. This process certainly seems moreffective than the consequences of mining.

It is also pertinent to note the results of tilt measurementsecorded during the great flood of August 2002. Jezerí-2 tiltmetertation, located in the sliding slope zone, showed a strong impact ofhis flooding on the observed tilts (Chán, 2005; Kost’ák et al., 2006).ezerí-1 tiltmeter station, located deep in the massif, did not showstrong impact of this flooding on the observed tilts (see Fig. 12:– torrential rains). Thus, deviations from the westward trend

an be considered as evidence of ongoing tectonic interference inhe creep (Fig. 12). Observations concerning deviations from theong-term tilt trend are listed in Table 1.

The westerly trend is typically observed when the phase ofncreased geodynamic activity commences (Fig. 12: March 2003).he westerly trend indicates a moment of disorientation. Onlyhen the pulse commenced, did reorientation begin and this finally

tabilised tilting towards the northwest. By the end of 2003, theressure culminated with a final regression to the southwest whenhe massif redistributed its collision points and adapted to a newituation within the compaction process.

The phase following January 2004 does not represent the end ofectonic activity, but rather an adjustment phase which did not endefore 2007. The period between 2005 and 2006 was characterisedy peculiar movements in which there were chaotic low-velocityilts to the south. It was at this time that significant earthquakesccurred. The period between 2005 and 2006 exhibits a low tilt ratef less than 0.05′′/month whilst during the period of the pressureulse the tilt rate was greater than 0.10′′/month, i.e. about threeimes greater (see trend line in Fig. 13). In 2007, tilting became

ore regular and the rate of tilting became comparable with itsarlier velocity. However the long-term trend has changed to NNW,imilar to that observed during the pressure pulse of 2003. Thisay indicate the possible occurrence of another pressure effect. Of

ourse, to demonstrate that, further studies with sufficiently longbservations will be necessary.

.3. Irregular tilts in 2005–2006

Fig. 14 shows details of the complex period between 2005 and006 (for other important data, see also Fig. 13 and Table 1). Thereak in the long-term trend is coeval with the great Sumatra Earth-uake that occurred on 26 December 2004. It was well documentedy the instruments in the gallery as well as at other locationsPetrovsky and Pegrimek, 2005; Bousková et al., 2007). The break isharacterised by chaotic movements during which extremely lowate tilts were orientated to the south. These movements lasted

or three months and reflect the slowest rates recorded for theecorded period (2001–2007). Further chaotic movement loopsppeared between October 2005 and May 2006. At this time, strongarthquakes occurred in Asia as well as locally significant eventsn Central Europe. The first tilting stage, in October 2005, is asso-

Fig. 14. A detailed record of tilt development between 1 November 2004 and 1 April2007 at Jezerí-1. [ ] ≈ arcsec.

ciated with the Islamabad and Hronov Earthquakes. The second,most chaotic, tilting stage, in March 2006, is associated with theVrbové Earthquake. During the five month period between Novem-ber 2005 and March 2006, the total tilt to the SSW slowed to onlyabout 0.05′′/month. Later, the movement consolidated to the south-east. This is interpreted as a sign of gradual relaxation within themassif. From July 2006 to October 2006, tilts developed regularlyto the SSW. These turned again, in November 2006 and Decem-ber 2006, to westerly and northerly for the last chaotic loop duringJanuary 2007 and February 2007. This period is coeval with lastand strongest earthquake micro swarm recorded during the studyperiod in West Bohemia (Fig. 9: February 2007). Then, eventually,the trend returned to a new regular NWW direction and acceleratedto about 0.10′′/month (Fig. 12).

It was only in February 2007 that the investigated period ofincreased geodynamic activity ended. Since March 2007, a newdevelopment has taken place. Tilts have accelerated and stabilised.This suggests a process in which the natural creep in rocks inter-feres with some compaction in the massif (see Figs. 12–14).

5. Pressure pulse analysis

5.1. The pulse

The displacement variations measured perpendicular to theinvestigated fracture plane in Jezerí Gallery (Fig. 5b: value p ofcompressive displacement) represent, in general terms, rock strainvariations. The maximum increase of p was registered from May toAugust 2003. To calculate the strain level from the p co-ordinate, weconsidered the geometric setting of the gauge and its orientationwith regard to the fracture plane.

During the relatively short period of the two critical months

of the pressure pulse we can assume conditions such as that ofhard rock and of an elastic body. In this period, gauge observedcompression reads �p = −0.133 mm. Strain can be calculated fromthe compression �p observed on the distance l measured between

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he end points of the gauge, perpendicular to the fault planel = 620 mm); ε = �p/l = −0.133 mm/620 mm = −2.145 × 10−4.

The calculated two month strain rate during critical periodf the pressure pulse reads �ε = −2.145 × 10−4/2 months. Theegative sign shows compressive strain rate. Considering theechanical properties of the Jezerí rock, taken from Mejzlík

nd Mencl (1981) in their finite-element model that investi-ated stability conditions of the massif under the Jezerí Castle,nd thus applying Young’s modulus of sound ortho-gneiss= 27 300 MPa, we calculated stress � induced by the pressureulse in question and two months in force. We arrived to thealue of � = ε · E = −2.145 × 10−4 × 27 300 MPa = 5.8 MPa. The stressurcharge reads �� = 5.8 MPa. This stress surcharge caused the dis-lacement observed in the crystalline of the Jezerí Block within theritical two month period of the pressure pulse.

The application of Young’s modulus in relation to sound ortho-neiss may be questioned by some investigators as measurementsere made within a fracture zone. However, this value can be

ubstantiated as the finite-element model application was basedpon a massif that includes fractures. All evidence shows that theoids of the fracture were tightly closed during this period. Inhe gallery, at a depth of about 120 m under the surface, the rock

assif was under considerably high pressure due to the overbur-en. No water percolation through the fracture could be observedt this time, contrary to later periods. This indicates that duringhe pressure pulse the rock was deformed under elastic pressureonditions.

The rangefinder measurements (Section 3.2.1) provide supple-entary data for rock strain determination. The strain due to the

ressure pulse in the superficial zone behind the crest of the hillas evaluated using the same formula. Observed compression on

he line lK1 reads �lK1 = −2.6 mm during 2003. This was observedn line K1 with a length of 589 m, which cuts a saddle behindhe marginal cliffs. Clearly, the marginal slope section cannot beounted in the deformation. The morphology of the saddle sug-ests a relatively narrow fault zone, about 15 m wide, which ishought to be responsible for the deformation. The zonal strain forK1S = 15 m reads ε = �lK1/lK1S = −2.6 mm/15 m = −1.73 × 10−4. Thisalue was calculated from displacement observations recordednnually. Assuming that the �lK1 = −2.6 mm displacement origi-ated mainly during the two months critical for the pressure pulse,e find a good agreement with the result from the gallery. The

alue of the two month strain rate is only 20% lower.

.2. The pulse and the natural state

The assumptions of our calculations are substantiated by earlierock creep studies at this site (Kost’ák, 1998, 2000). The studiesn the crystalline rock indicated a natural process of local long-erm creep with a strain rate of −1.7 × 10−5 per year, i. e. whenompared with the two month strain rate of the pressure pulse, itould get natural creep compressive displacements 60 times lower

han those observed during 2003.It is possible to compare the above mentioned pressure pulse

arameters with the stress and strength data pertinent to higherepths, even for different rock types reported in the literature. Therincipal determination of vertical and horizontal stress, as a func-ion of depth, was reported by Herget (1988). These conclusionselate mainly to the Canadian Shield, which is lithologically com-arable to the Bohemian Massif. It is important to point out that,ccording to Herget (1988), the horizontal stress component does

ot generally show linear increase with depth as the increase isonsiderably reduced below 800 m. According to Herget (1988),orizontal stress in the massif reaches approximately �H = 47 MPat a depth of 1 km and 58 MPa at a depth of 2 km. Therefore, the sur-harge caused by the pressure pulse of August 2003 is about 12% or

ynamics 52 (2011) 34–44 43

10% of the value indicated as natural horizontal pressure at depthsof 1 km and 2 km, respectively.

Peska (1992) reported borehole tests that examined stress andstrength in sandstones and siltstones at various locations in theBohemian Massif. It was concluded that consistent breakout initia-tions due to horizontal mean stress appear at depths of 900–1000 munder condition of the mean rock strength of 45–107 MPa. Closerinspections of the results show that the conclusion indicates abreakout stress level comparable with that given by Herget (1988).The principal stress orientation observed by Peska was NE–SW.

There are other indications that the registered pressure pulsewith its level of surcharge could significantly affect stress concen-trations on low stability fractures. This is valid for faults producingearthquakes, as well as for unstable slopes. The risk analysis of theslope stability of the Jezerí Castle massif, undertaken by Kost’ák(2005), showed that the stress increase due to the recorded pres-sure pulse would surpass the critical stress level. This wouldthereby significantly increase the risk of slope failure. However,the duration of the pulse was relatively short and this also has tobe considered in the assessment of actual risk.

6. Conclusions

In the Bohemian Massif, monitoring of tectonic movements hasprimarily focused on the Krusné Hory Mts. and Sudeten Fault Zones.Here, a pressure pulse was registered during the second half of2003. The pulse was followed by a phase of increased geodynamicactivity that ended in early 2007. The pulse reoriented secular tiltsof the Jezerí Block for a period of five months between June 2003and November 2003. During this period, tilts were reorientatedfrom SWW to NW whilst tilt velocity significantly increased. Atthe same time, mountainous blocks behind the fault crest sufferedcompression. Rock strain recorded deep in the Jezerí Gallery inthe Krusné Hory Mts. during the peak of the pulse between June2003 and August 2003 provided data to characterise the pulse withsurcharge nominal pressure level by �� = −5.8 MPa.

The period of increased geodynamic activity induced by thepulse developed in individual stages of specific character. Char-acteristic stages were noted for chaotic tilting, sharp changes oftilt orientation and velocities, and anomalies in fracture microdis-placements. The three main stages of this phase were recognisedfrom the tilts:

1. Initiation of the process by the pressure pulse between June 2003and November 2003.

2. Compaction period when increased local deformations alongfracture contacts took place between February 2004 and Decem-ber 2004.

3. Regression when movements slowed down progressivelybetween January 2005 and February 2007. The process slowlydied out during a relaxation phase.

A number of moderate, yet regionally major, earthquakesoccurred during the period of increased geodynamic activity inCentral Europe. These events occurred only during relaxation, inthe later stage of the deformation process. It is clear that theearthquakes can be understood as phenomena resulting from thedeformation process when it is subject to relaxation. The earth-quakes should not be seen as independent, self-originating, anddominant in initiating fault movements. The same argument is valid

for the earthquake micro swarms in West Bohemia that started inFebruary 2004.

The increased geodynamics of tectonic displacements was ofcontinental extent as accelerated movements could also be recog-nised on some of the major continental tectonic structures of

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urope such as the Rhinegraben in Germany, the Central Apenninesn Italy, and the Gulf of Corinth in Greece (Stemberk et al., 2010).he continental extent of tectonic events that involve strong earth-uakes was recently indicated by Kost’ák et al. (2007). There are

ndications that the period of increased geodynamic activity wasssociated in some way with coeval earthquake activity in Asia. Theatastrophic Sumatra Earthquake was evidenced in fracture gaug-ng and tilt measurements. It marked sudden tilt reorientation inezerí Block at the end of December 2004. This coincidence evokeshe idea that far reaching global tectonic disturbance may havenfluenced the pressure pulse and increased geodynamic activitybserved in Central Europe between 2003 and 2006.

There are some open questions. The origin of the pressure pulses still unknown and this remains a crucial question that needs toe addressed. However, it is clear that the pressure pulse and sub-equent period of increased geodynamic activity extended overlarge territory covering different regions. It cannot simply beregional phenomenon. Other questions relate to the impact of

he pressure pulse in the Earth’s crust. It is not known whethert is exceptional or periodical. Nor is it known if there are corre-ations between the pressure pulse and global stress evolution ortrong earthquakes. These questions remain a challenge for futureesearch.

cknowledgements

Research was undertaken between 2000 and 2006 withinhe framework of the international European Science Foundationction COST 625 “3D monitoring of active tectonic structures”, sup-orted by the Czech Ministry of Education (Project OC 625.10).he next period of research proceeded within the Czech Rep. grantroject GACR 205/06/1828 “3D monitoring of micromovements inhe collision zone between African and Euro-Asian tectonic plates”etween 2006 and 2008. The interpretation was done in the frame-ork of the grant project GACR 205/09/2024 “Time development

nalysis of micro-displacement monitored in the collision zoneetween African and Euro-Asian plates”. Tiltmeter monitoring androundwater evaluation was also supported by the Czech Academyf Sciences (Projects S3012353 and IAA300120905, respectively).e would like to thank the mining company Mostecká Uhelná Ltd.

or allowing us to access data from Jezerí-1. We are grateful to V.olák and P. Skalsky at the Institute of Geophysics in Prague forechnical support during tilt observations and data processing. Were also indebted to Prof. Cacon at the Wroclaw University of Envi-onmental and Life Sciences for supplying data recorded north ofhe Sudeten Fault. Dr. Matt Rowberry provided a critical revision ofhe English.

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