Crustal deformations in the epicentral area of the West Bohemia 2008 earthquake swarm in central Europe

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jb009053, 2012 Crustal deformations in the epicentral area of the West Bohemia 2008 earthquake swarm in central Europe Vladimír Schenk, 1,2 Zdeňka Schenková, 2 Zuzana Jechumtálová, 3 and Richard Pichl 4 Received 23 November 2011; revised 31 May 2012; accepted 2 June 2012; published 24 July [1] In West Bohemia, central Europe, during October 2008 an earthquake swarm of 25,000 shocks with a maximum event of M L occurred at depths of 7 11 km. In 2007, annual GPS campaigns were launched. During the co-seismic phase, displacements of a few centimeters were detected at GPS sites. Maximum displacement was revealed at the KOPA site, which subsided by 167 mm. The epicentral area is covered by eluvium of 4 10 m thick, and is located in undulating pastures and well-forested valleys where visible surface soil effects could not be observed. To test possible fault manifestations, rough geomorphologic, geoelectric, and geochemical surveys were performed. GPS and seismic data, with geologic materials, were used to build a forward model for surface displacements, crustal deformations, and shear and normal stress fields. The fields enabled us to better determine crustal deformations and stresses that appeared within the seismic cycle, during the pre-, co-, and post-seismic phases. During the co-seismic phase, modeled fault motions along N-S faults located within the epicentral zone reached mm/day. Possible structural block rotations were comprised of these motions. A dominant role for stress accumulation, release, and relaxation was assigned to the Mariánské Lázně fault zone and the Nový Kostel zone. Strain loads slowly, and when local PT conditions with an action of deep magmatic fluids reach instability, the strain is released and stress balancing occurs. The process leads to the reversible motions known for silent earthquakes. A forward crustal deformation model for West Bohemia is also presented within. Citation: Schenk, V., Z. Schenková, Z. Jechumtálová, and R. Pichl (2012), Crustal deformations in the epicentral area of the West Bohemia 2008 earthquake swarm in central Europe, J. Geophys. Res., 117,, doi: /2011jb Introduction [2] The western portion of the Bohemian Massif is characterized by the repeated occurrence of earthquake swarms. The first written reference to swarms dates back to the 12th century [Kárník et al., 1957]. The impetus for systematic monitoring of earthquake activity within the area was a high energy swarm having a shock of M L 4.6. Since 1994, when seismicity in the region began to be monitored by the local seismic network WEBNET [Horálek et al., 2000], a number of swarms have been detected. However, as determined by the network, which is able to record events of M L 0.5 or less, the majority of these swarms have consisted of micro- or weak earthquakes. Only 1 Geo-Service Praha, Prague, Czech Republic. 2 Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, Prague, Czech Republic. 3 Department of Seismology, Institute of Geophysics, Academy of Sciences of the Czech Republic, Prague, Czech Republic. 4 Air Navigation Services of the Czech Republic Department of Safety, Jeneč, Czech Republic. Corresponding author: V. Schenk, Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41, CZ Prague 8, Czech Republic. (schenk@irsm.cas.cz) Published in 2012 by the American Geophysical Union. a few of the swarm shocks have macroseismic effects. Thus, the origin of the swarms can be explained by deep magmatic activity, which has been documented in this region as spanning the late Tertiary to the Middle Pleistocene [Wagner et al., 2002], as a result of the common occurrence of hot springs and CO 2 emanations (Figures 1c and 1d). [3] The October 2008 earthquake swarm was registered by the WebNET network and had an energy similar to the swarm. During the 2008 swarm, approximately 25,000 shocks were recorded, with the strongest events reaching a magnitude of M L [Fischer et al., 2010]. GPS campaign measurements began in the West Bohemia geodynamic network in August GPS campaigns during permitted the detection of crustal deformations in the area before, during, and after the 2008 swarm, and were used to determine their relationship to swarm sequences. 2. The Geological Structure of the Area [4] The study area is located between the Krušné hory (Erzgebirge) and Smrčiny (Fichtelgebirge) blocks, two major structural units of the Bohemian Massif, where an intersection of the NW-SE and NE-SW tectonic systems is found. At the intersection, the Cheb (Eger) Basin originated during the Oligocene and from that time forward volcanic activity intermittently persisted until the Middle Pleistocene 1of19

2 Figure 1. The location of the Nový Kostel seismogenic zone in West Bohemia, central Europe. The black rectangle represents the study area. White squares represent the positions of the GPS sites. Lines represent the main tectonic zones. Dots represent earthquake epicenters. (a) Central Europe; (b) the Bohemian Massif and the main fault zones; (c) the West Bohemia region, with earthquake epicenters of ; MLz represents the Mariánské Lázně fault zone; Khz represents the Krušné hory fault zone; and (d) is the Nový Kostel seismogenic zone within the earthquake epicenters of 14 October [Kopecký, 1978; Wagner et al., 2002]. Volcanic activity in the area has been accompanied by earthquake occurrences that have lasted until the present-day. [5] A recent seismoactive zone, the NNW-SSE Nový Kostel zone, is located in the crystalline units of the Krušné hory Mts, near the eastern border of the Cheb Basin, that coincides with the NW-SE striking Mariánské Lázně fault zone [Malkovský, 1976] ( mapy/mapy-online/mapserver) (Figures 1 and 2). Detailed studies of earthquake focal mechanisms, as carried out for the 1997 swarm [Vavryčuk, 2002], have revealed the following two distinct types of faulting in this zone: 1) tensile faulting linked mainly to NE-SW fractures (N39E), and 2) shear faulting linked to NW-SE fractures (N53W). The directions coincide with two dominant fault systems within the region, the NE-SW striking Krušné hory zone (Khz) and the NW-SE striking Mariánské Lázně zone (MLz) (Figure 1d). The shape of the Horka valley reservoir (Figures 1d and 2) follows these two systems. 3. GPS Data [6] In order to measure motions between structural blocks in the West Bohemia area, three GPS sites within the geodynamic network (Figures 1c and 1d) - Kopanina (KOPA), Nový Kostel (NOKO), and Černá near Luby (CERN) - were determined in and close to the Nový Kostel zone (Figures 1d and 2). Two additional sites - Lazy near Lázně Kynžvart (LAZY) and Svatý Kříž near Cheb (SAKR) - were chosen some 15 km to the SE and SW, respectively, in the crystalline units of the Slavkovský Les Mts and the Cheb-Dyleň complex (Figure 1c). The sites consisted of rectangular concrete 2of19

3 Table 2. The Campaign Dates, the Monitored Periods, and the Sites Where GPS Signals Were Recorded Campaign Date GPS DOY/DOW Sites Aug 30 31, , 243/1438 CERN, KOPA, LAZY, NOKO, SAKR Aug 28 29, , 242/1492(3) CERN, KOPA, LAZY, NOKO, SAKR Nov 1 2, , 307/1503 CERN, KOPA, NOKO May 13 14, , 134/1531 KOPA, NOKO Aug 29 30, , 242/1546(7) CERN, KOPA, LAZY, NOKO, SAKR Figure 2. A geologic map of the eastern portion of the Cheb Basin and the adjacent Krušné hory crystalline ( The dashed line represents faults; boxes represent GPS campaign sites; and the blue ellipse represents the Nový Kostel zone. blocks, with bases of 40 cm by 40 cm and heights of 60 to 120 cm anchored into bedrock. A steel plate with a screw thread for mounting a GPS antenna was cemented into the top of each concrete block [Schenková et al., 2007]. Table 1 contains the geographic coordinates of the sites, as well as information on the types of GPS receivers and antennas used. [7] At all five of the sites within the WEST BOHEMIA network, three annual , 2008, and GPS campaign measurements were performed, and two additional campaigns were carried out at selected sites during , following the 2008 earthquake swarm (Table 2). Each Table 1. The Site Coordinates and GPS Equipment Site Latitude (deg) Longitude (deg) Height (m) Antenna GPS Receiver CERN Geodetic Ashtech Z-MAX KOPA Marine III Ashtech Z-12 Surveyor LAZY Marine III Ashtech Z-12 NOKO Geodetic Ashtech Z-12 SAKR Marine III Geodetic Ashtech Z-12 Surveyor campaign lasted 48 h. To eliminate problems of phase center effects, the same GPS antennas were always installed at the same site. [8] To obtain site movement velocities, the data from these five network sites, three EPN permanent stations (BORI, PENC, and ZIMM), and the surrounding GNSS permanent stations (KYNS, LUBY, MARJ, POUS) of the GEONAS network [Schenk et al., 2010a] were obtained, and the solution for each GPS observation day was computed using Bernese GPS v.5 software [Hugentobler et al., 2005]. The following complementary data were applied during processing: (a) the precise satellite orbits, the satellite clock data, and the Earth s rotation parameters obtained from the Centre for Orbit Determination in Europe (CODE), Bern; (b) the antenna phase center characteristics obtained from the Geosciences Research Division of the U.S. National Geodetic Survey; (c) the stochastic model of ionosphere GLOBAL (CODgpswd.ION) used for estimating the ionospheric correction; (d) the linear combinations of L3 observations (ionosphere free); (e) the Quasi Ionosphere Free Strategy for ambiguity resolution; and (f) the DRY NIELL atmospheric model for estimating the tropospheric correction. Geocentric and geographic coordinates were computed in the ITRF2005 reference frame by keeping identical baselines. Daily solutions were determined using the ADDNEQ2 program. [9] To determine site position changes within the West Bohemia area that originated as a result of the dynamic processes of the 2008 swarm, Eurasian plate positions in the ITRF2005 reference frame were subtracted from the Bernese calculated site positions (Tables 3 and 4). The time series of site position differences (Figure 3) indicated that the horizontal and vertical movements were initiated by the earthquake swarm. [10] Schenk et al. [2010b] analyzed the uncertainties of the north (N), east (E), and vertical (Up) components, as determined by the Bernese GPS software for one-day GPS campaign observations, and found that for any site the standard deviations of the positions could be ten times higher than the errors evaluated by the Bernese GPS software. Therefore, by multiplying the sn, se, and sup errors (Table 3) by ten, we believe that realistic accuracy for the position estimates were obtained. In the views of this analysis, the positions changed toward centimeters and detects in the horizontal and vertical directions could be considered as reliable. The 167 mm change in the vertical position at the KOPA site during the 2008 swarm, most likely reflects reverse fault deformations induced by deep magmatic processes and the related ascent of fluids into the upper crust. 3of19

4 Table 3. N, E, and Up Site the Relative Positions of the Geographic Components Determined With Respect to Eurasian Plate Motion Using the ITRF2005 Reference Frame of Altamimi et al. [2002] a Site DOW DOY N (mm) s N (mm) E (mm) s E (mm) Up (mm) s Up (mm) CERN KOPA LAZY NOKO SAKR a Here s N, s E, and s Up are the standard deviations for N, E, and Up, respectively, determined from GPS data processing in the sense of the ITRF2005. There is no explanation for the N position difference of the KOPA site between DOY305 and DOY306 in DOW1503. [11] A time history of recorded earthquakes from August through December 2008 with a plot of cumulative seismic moment, and the periods of the two double-day GPS campaigns (DOW1492(3)) and DOW1503) are displayed in Figure 4. As shown, between the GPS campaigns nearly the entire amount of seismic swarm energy was released. Such a finding enabled us to assume that site position changes observed prior to the DOW1492(3) campaign could already be the result of pre-seismic energy loading. Therefore, the changes that appeared between both campaigns had to depend closely on seismic energy unloading during the co-seismic phase of the swarm; and, lastly, the surface changes detected after the DOW1503 campaign are related to the post-seismic processes of energy release balancing. 4. The 2008 Earthquake Swarm [12] To explain the surface deformation detected by the GPS campaigns, the focal depth distributions of swarm events were analyzed and the possible extension of the seismogenic zone to the surface was assessed. Seismic data, available from were used to draw focal depth maps for 250 m intervals between the depths of 6.5 to 11.5 km (Figure 5). The maps indicate that the swarm shock sequences commenced at depths of approximately 11 km and progressed upward in time to a depth of 6 km. A similar upward migration of hypocenters was also observed during the previous swarm of 2000 [Fischer and Horálek, 2005]. Such focal migration is compatible with the postulate that fluids released in the upper part of the lithosphere, ascending though the crust, participated in the shock origin. Maps of the 2008 swarm show that shocks in the Nový Kostel seismogenic zone were primarily generated along several parallel NNW-SSE striking faults. Deeper shocks occurred at the western boundary of the Nový Kostel seismogenic zone in the basement of the Cheb Basin, while shallower shocks were generated eastward in Krušné hory crystalline rocks. The depth distribution of swarm events demonstrates that the seismoactive zone does not consist of one tectonic fault zone, but rather, is built from sets of NNW-SSE parallel or subparallel faults (Figure 6a) that are intersected by crosscutting faults. [13] The volume and pressure of ascending fluids determine the rate of rock cracking and the extent of their upward progress through fracture zones. Since the October 2008 swarm events were the strongest of the past 20 years, the ascension of large volumes of fluids close to the surface cannot be excluded. Such a possibility is in keeping with observed mantle-derived CO 2 emanations [Heuer et al., 2006; Bräuer et al., 2008, 2009], which also frequently arise in places not characterized by the occurrence of seismic events. For the 2008 swarm, it was also observed that despite negligible precipitations the discharge of some springs located within the epicentral area increased throughout the culminations of earthquake energy release. Furthermore, a month before swarm initiation, continuous gravity monitoring was began in the epicentral zone and indicated an increase in gravity by more than mg that possibly reflected the intrusion of fluids (V. Schenk et al., manuscript in preparation, 2012). [14] The probable position of fluid emissions on the surface seems to be located 1 to 2 km ENE of the hypocenters observed at depths of approximately 7 km (Figure 6b). Therefore, the surface deformations detected between the KOPA and NOKO sites could involve a reactivation of local fracture systems. Although most foci of the swarm sequences were associated with NNW-SSE striking faults (Figure 5), at the time of low post-swarm activity shocks also originated along NE-SW striking fractures that crosscut the NW-SE and NNW-SSE striking faults (Figure 7). Presumably, these NE- SW striking fractures were reactivated by fluids rising upwards, allowing the possible development of flat to furrowed valleys or surface scarps. As known from seismically active areas, V-shaped valleys are often associated with tensional faults [Lienkaemper et al., 1987]. 5. Evidence of the Surface Deformation [15] Topographic maps and a digital elevation model with a horizontal resolution of 20 m indicated a number of terrain undulations between KOPA and NOKO that included 4of19

5 Table 4. The Site Latitude and Longitude Displacements With Standard Deviations Shown as the Difference Between Observed Site Movements and Movements Calculated From the Eurasian Plate Motions of the ITRF2005 Reference Frame a Site Latitude Differences (mm) Site Longitude Differences (mm) DOW / DOY CERN KOPA LAZY NOKO SAKR CERN KOPA LAZY NOKO SAKR 1438 / / / / / / / / / / a DOW1438/DOY242 are the initial site positions, for details see Table Figure 3. The time series of coordinate differences for West Bohemia network site positions. Points represent site positions [mm]; bars represent velocity estimate uncertainties equal to ten times the calculated s N, s E and s Up errors (Table 4); and the gray zone represents the 2008 earthquake swarm. 5of19

6 Figure 4. A time history of seismicity from August through December 2008 from GPS campaigns performed before and after the 2008 swarm; dots show events of M L > 0.5; stepped line is the cumulative seismic moment calculated for all events; and arrows indicate the GPS campaign periods given in DOW. flat and shallow valleys (Figure 8a), and slope steps and scarps (Figure 8b) that were locally developed into V-shaped valleys (Figure 8c), mostly overgrown by forests. Figure 8d provides an outcrop of gneiss with a fault plane (strike N10 E / dip 80 W) on which slicken sides (dip 12 N) could be observed (Figure 8d1). The sides presumably developed in response to recent fault movements. Within the epicentral zone of the 2008 swarm, the orientation of valleys and scarps correlated well with NE-SW and NNW-SSE seismoactive faults. The presented rough structural and morphotectonic survey was guided in order to identify the dominance of some valleys in manifesting swarm motions associated with fluid upwelling, and indicates that the reactivation of faults could reflect the existence of upper crustal discontinuities along which fluids rose to the surface. 6. The Geoelectrical and Geochemical Detection of Faults [16] A geoelectrical resistivity survey supported by geochemical soil analyses was carried out on the meadows of a flat valley (Figure 8, S place) that were located close to a ridge that separates the Cheb Basin and the Horka water reservoir. The valley was selected as a result of the following: (a) its NNE SSW elongation roughly in the direction of surface motions determined by GPS measurements at KOPA and NOKO; (b) local residents reporting that in the spring snowmelts faster in this valley than in surrounding areas; and (c) its location to the area where fluids can percolate to the surface (Figure 6). Additionally, two local farmers reported that during the 2008 earthquake swarm the topography of their pastures was slightly reshaped, but that they were unable to specify these alterations. They also reported that the yield of several wells decreased following the swarm for the first time in many years. [17] Three, 300 m profiles, N78 W perpendicular to the valley axis, with points spaced at 10 m were surveyed. The Schlumberger resistivity method (configuration A30M10N30B) was used to assess whether fault indications existed in the valley. Profiling was supplemented with the pole-dipole resistivity method of symmetric dipole configurations (C 1 30P 1 10P 2 and P 1 10P 2 30C 1 ) with a remote C 2 current electrode. The forward dipole was oriented along the profile WNW and in the reverse dipole ESE. The depth range of both resistivity methods was approximately 20 m. A conductive zone detected in all three profiles was interpreted as highly associated with a fault or a fault zone that dipped 75 to the west (Figure 9). [18] Methods to undoubtedly prove the presence of fluids at this fault could not be applied due to their complexity and expense. Therefore, four samples of eluvial soil covering the underlying muscovite mica schist were collected approximately 1 m below the surface in order to detect possible chemical differences in elements that could indicate the presence of an active fault within the valley. Two samples were taken within the valley axis and two from its flanks. The samples were analyzed for trace elements (Cd, Cr, Cu, Pb, Li, Mn, Ni, Rb, Sr, Zn), water, and the alkali (K, Na) content of clay minerals since anomalous concentrations could be attributed to the upwelling of fluids along the fault. The results summarized in Table 5 indicate that samples collected around the postulated fault clearly have an increased content of K, Pb, Cr, Mn, water, and illite, indicating that mineralized fluids had indeed percolated up to the surface along the fault. [19] The results of the geoelectrical and geochemical survey were compatible with the concept that within the epicentral area the development of many smaller or larger valleys is controlled by a crustal deformation associated with crypto-magmatic activity, and that the ascent of fluids, at times, evokes a stress-field instability released by the triggering of earthquake swarms [Schenk and Schenková, 2011]. 7. Crustal Deformations [20] Prior to interpreting site displacement changes, it is essential to realize the distinctive nature of the West Bohemian area. The geological environment consists of crystalline rigid rocks that are strongly tectonically crushed due to frequent earthquake occurrences. If these structures are dynamically impacted they will internally manifest themselves as slightly ductile units and not only as rigid ones: crushed zones not only do fault shifts exist but that block rotations can also occur. [21] In the area, rock behavior and the character of faulting are closely connected to magmatic and seismic activities. Two-decades of seismic activity monitoring by the network WEBNET [Horálek et al., 2000] has recorded more than 16 thousand events of M L > 0.5; with 95% of the events located between 6 and 11.5 km in depth [Fischer et al., 2010]. In the case of earthquake swarms, 98 99% of the 6of19

7 Figure 5. The depth distributions of the 2008 swarm foci at depth intervals of 250 m between depths of 6.5 to 11.5 km. 7 of 19

8 Figure 6. The Nový Kostel seismogenic zone. (a) A 3D view of the 2008 swarm events; (b) the vertical WSW-ENE cross-section - black points represent the 2008 swarm foci, and the gray area represents a probable path of fluid welling up to the Earth s surface. events have been concentrated within the referenced depth. Therefore, it is obvious that the earthquake depth distribution is closely related to local magmatic processes that preserve the base of the brittle crust from 11 to 11.5 km in depth, and that all faults below that level in the ductile crust are already under a steady creep. If any tectonic processes impact them, their frictional resistance is minimal and fault creep does not allow them to cumulate strain, leading to earthquakes. In the course of the pre-seismic phase, faults placed above this depth are locked. When the co-seismic phase begins, magmatic fluids rise to the upper brittle crust, and the PT conditions and the frictional resistance of crushed rigid rocks changes. During this time interval, faults switch their lock states to creep-slip states and the tectonic strain that cumulated on the faults during the interseismic period begins to be released by earthquakes. Following the dominant release process, in which post-seismic phase stress-strain balance seeks equilibrium among geological structures, the creep-slip state of the faults slowly comes back to the lock state. [22] In West Bohemia, as has been observed [Fischer et al., 2010], seismic events have not occurred above 6 km in depth (Figures 5 and 6), and shallow crustal faults up to 6 km in depth have remained almost in a lock state. However, no one can ignore that during the co-seismic phase of intensive swarms some faults can be unlocked and that moderate displacements impact the surface. During the 2008 swarm such an outcome likely occurred, and due to a thick eluvial grass-covered soil layer within the epicentral area, the displacements were not visible and only detectable by GPS. Below, the presented forward model includes all of the structural geological aspects and indicates that the seismic energy accumulation and the release processes are reversible in West Bohemia, which is known for silent earthquakes. Regional long-term, right-lateral slip motions with shortterm, left-lateral swarm strike-slips accumulate and release stresses and strains on faults that can be accompanied with structural block rotations. The finding clarifies the fact that not only could immediate displacements not be visible on the surface, but that long-term structure shifts can also not be found. [23] From the view of interpretations of GPS displacements, we had to take into account the fact that GPS sites lie at fixed places on the surface of structural crustal blocks. Position changes only represent the displacements of these places and not the displacements of the entire structure as one compact body. In contrast, the fault motions obtained from focal solutions determine strike- and/or dip-slip movements that originate along faults during earthquake ruptures. The differences between these types of motions demonstrate a simple tectonic model of two block structures (Figure 10) with clockwise rotations. Due to these rotations, the blocks are separated by faults with left-lateral strikes. The blue arrows indicate the GPS site displacements of places on the surface of the blocks. The black arrows denote the fault motions Surface Deformations Observed by GPS [24] In the course of the seismic cycle, stress accumulates and is then released by earthquakes, and finally seeks balance in structures. The positions of the five GPS sites (Figure 3) permitted determination of the relative changes of the locations between two successive GPS campaigns (Figure 11). Since the campaigns were carried out during periods closely matching the pre-, co-, and post-seismic phases of the 2008 swarm (Section 3; Figure 4), we could assume that the observed position changes reflected inter-seismic motions 8of19

9 Figure 7. Earthquake epicenters in the Nový Kostel area displayed for two-month periods during Dashed lines indicate the directions of the NW-SE Mariánské Lázně (MLz) and the NE-SW Krušné hory (Khz) tectonic systems. 9of19

10 Figure 8. Geomorphological features between the KOPA and NOKO sites, as follows: (a) flat and shallow valleys; (b) slope steps; (c) sharply shaped V-valleys; (d) an outcrop with a fault plane on which (d1) slickensides are presented; and (e) an aerial map with the locations of the presented geomorphological features and the outcrop; S indicates the location of the geoelectric profile (photos by T. Marek). 10 of 19

11 Figure 9. The apparent resistivity curves along the geoelectric profile crossing the valley axis (location see Figure 8) indicating the presence of a steeply dipping fault at point 145; the black line indicates the Schlumberger resistivity method; and red and blue lines indicate the pole-dipole resistivity method. among the phases. To explain and generalize the GPS site displacements detected during the entire seismic cycle of the 2008 swarm, forward modeling, based on observed regional faulting and complex tectonic knowledge of West Bohemia, was applied. [25] The earthquakes that occurred during displayed a conspicuous distribution in the area (Figure 11), as follows: for the pre- and post-seismic phases earthquakes were spread over the area, while in the co-seismic phase they were concentrated only in the Nový Kostel zone. For example, the relative displacements of the KOPA and NOKA sites, as monitored during the GPS campaigns, showed that the epicentral zone of the 2008 earthquake swarm caused deformations of the order of centimeters (Figure 3), especially in the vertical direction. Eluvial soils 4 10 m thick within the epicentral area are grass-covered. Therefore, the deformations detected by GPS measurements could hardly be determined at the surface. [26] The pre-seismic phase: For the period prior to the swarm (DOW1438 DOW1492(3)) the displacements monitored at the GPS network sites (except the LAZY site) exhibited regional extensions arising from the Nový Kostel zone which indicated an opposite, inward displacement. Thus, the CERN, NOKO, and KOPA sites located within and near the epicentral zone likely reflect the beginning of seismogenic processes of the swarm; while the LAZY and SAKR sites revealed a final energy accumulation along the MLz due to emerging left-lateral regional motions. The extensions apparently reflect the possible ignitions of magmatic fluids upwelling from the lower to the upper crust. [27] The co-seismic phase: The accumulated energy was released by seismic events in distinctly concentrated small parts within the Nový Kostel zone (Figures 4 and 11). The NOKO and KOPA site displacements observed during the DOW1492(3) DOW1503 period did not follow the leftlateral strike-slips of the swarm events [Fischer et al., 2010], but reflect the clockwise block rotations assumed by Schenk et al. [2009a]. Block rotations are opposite the fault slips [Freund, 1970a, 1970b; Garfunkel, 1974]. As mentioned above, the Nový Kostel zone is built by sets of parallel and shear faults (Figures 5 and 6) whose strike-slips predetermine block rotations. The focal solutions indicated not only leftlateral, but also right-lateral strike-slips along these faults during the 2008 swarm [Vavryčuk, 2011]. [28] The post-seismic phase: The decelerating slip phase of the 2008 swarm, lasting several months, began to spread the earthquakes over the area, and reflects the recovery of the stress field back to its pre-seismic state. The displacements of the CERN, NOKO, and KOPA sites, detected during the (DOW1503 DOW1546(7)) period, still led to the existence of clockwise block rotations (Figure 11c). Both long-term and short-term motions indicate that West Bohemia swarms have not only unloaded the accumulated stress energy, but have also accomplished a balance between ongoing dynamic processes, leading to the greater steady state of crustal blocks. Until now, as a result of deep magmatic processes, no isostatic adjustments have been identified within these blocks. Table 5. The Geochemical Analyses of Eluvial Soil Samples a Abundance in Sample Element the Earth Crust [Taylor, 1964] K 20.9% Pb 12.5 ppm Cr 100 ppm Mn 950 ppm Content of water (%) Content of illite (%) a The pickup place for samples 1 and 2 is in the valley axes tectonic zone ; for samples 3 and 4 is out of the valley axes non-tectonic zone. 11 of 19

12 Figure 10. The tectonic model of two clockwise rotated crustal block document relationships between focal mechanism displacements (black arrows) and GPS site movements (blue arrows) Inputs for Deformation Modeling [29] To qualitatively evaluate regional displacements during the 2008 swarm, the observed site displacements were numerically modeled. Forward mechanical dislocation Coulomb software [Toda et al., 2010] was applied in order to simulate the size and orientation of surface displacements, the stress fields in the crust, and, in this manner, the dynamic fault parameters. The crustal deformations and the shear and normal stress fields were modeled with respect to the stress field acting during the course of the 2008 swarm. [30] The recent stress field in the West Bohemia area has been investigated over the last two decades. The main stress field determined from the measurements of breakouts, tensile fractures, etc., carried out within the German Deep Drilling Program (KTB) [Brudy et al., 1997] indicates that the greatest horizontal stress, azimuths of N E, was remarkably uniform over a depth interval from 3.2 to 8.6 km. Bohnhoff et al. [2004] investigated 125 fault plane solutions in the KTB borehole for microearthquakes induced during a fluid injection experiment, and for the P axes found dominant azimuths of N E. [31] The stress field of the West Bohemia area coincided with N E[Havíř, 2000]. Ibs-von Seht et al. [2006] delimited the local stress field to the N E azimuth sector and indicated a clockwise rotation around 30 from N to S. The focal mechanism solutions for the 1997 and 2008 swarms [Vavryčuk, 2002, 2011] defined the stress tensor (azimuth/plunge) as follows: for 1997 s 1 = 156 /33, s 2 = 20 /48, s 3 = 262 /23, and the shape ratio R = [2002]; and similarly for 2008 s 1 = 146 /48, s 2 = 327 /42, s 3 = 237 /1, and R = 0.55 [2011]. All of the stress data presented above showed a good mutual coincidence. For our model, Vavryčuk [2011] data with a friction coefficient of m = was applied. [32] The regional distribution of tectonic ruptures and faults within in the area were also investigated. An explicit structural geological study of the area with regard to tectonics had not been investigated. Therefore, for the quantitative forward modeling we elaborated on a scheme of fault sets (Figure 12) that comprised available tectonic [Misař, 1983; Špičáková et al., 2000; Bankwitz et al., 2003] ( geology.cz/extranet/mapy/mapy-online/mapserver) and seismological [Havíř, 2000; Fischer and Horálek, 2005; Vavryčuk, 2002, 2011] data. To examine its ability to model the observed site displacements, the compiled faults were tested by means of the Coulomb software [Toda et al., 2010]. The faults for which dynamic effects allowed a successful fit to surface displacements were selected. In the scheme (Figure 12), these faults are drawn by heavy lines. Every such fault should be understood as a fault set with a larger or smaller number of faults due to highly crushed rocks. [33] In this section we provide the GPS positions of the regional geodynamic WEST BOHEMIA network sites. In West Bohemia, we had to expect position changes evoked by faults that related both to short-term earthquake strike- Figure 11. The horizontal displacements of the five GPS sites (blue arrows) for the pre-, co-, and post-seismic phases. Displacements correspond to the differences between observed site movements and Eurasian plate ITRF2005 reference frame motions; red circles indicate earthquake epicenters. 12 of 19

13 Figure 12. The scheme of fault sets applied in the modeling (numbered heavy lines); continuous and dashed lines indicate other faults in the area. and dip-slip motions, as well as to long-term, inter-seismic creeps. Therefore, the quantitative modeling presented for surface deformations was accomplished for the following three periods: (1) Aug (DOW1438) Aug (DOW1492(3)) the period during which dynamic processes were beginning to relate to the pre-seismic phase of the swarm; (2) Aug (DOW1492(3)) Nov (DOW1503) the period in which the co-seismic phase of the swarm was detected, after a quiescent period in September 2008 a rather intensive seismically active period occurred during October 2008; and (3) Nov (DOW1503) Aug (DOW1546(7)) - the period that included the post-seismic phase of the swarm Fault Motions and the Surface Displacement Field [34] Site displacements disclosed by modeling the pre-seismic phase confirmed the extension trends within the epicentral zone of the 2008 swarm (Figure 13a). The modeled fault motions (Table 6) specified that during this phase energy accumulation was the result of right-lateral motions along N-S faults in the epicentral zone and left-lateral motions along the NW-SE faults of the MLz tectonic system, in close vicinity (Figure 13b). [35] Numerical modeling of fault movements in the co-seismic phase exhibited not only opposite motions (Figures 13b and 13e), but also multiple increases in size (Table 6) as a result of rapid energy release. In the epicentral zone, N-S faults followed left-lateral movements and most of the NW-SE faults turned back to their long-term, right-lateral trends. The presence of these two movement trends also confirmed the focal plane solutions made for the 2008 swarm [Vavryčuk, 2011]. Opposite actions in the rock environment compensated instabilities, and subsiding trends accompanied by horizontal WSW motions were observed (Figure 13e) in the central part of the Cheb Basin. In the southern part rightlateral motions originated along marginal MLz faults. Likewise, as in the pre-seismic phase, in the southern and eastern sections of the area, these motions created conditions for anticlockwise block rotations, while structures in the northern part of the Basin displayed clockwise trends. [36] The large change of 167 mm in the vertical position at the KOPA site can explain the motions modeled for fault sets 2, 4, and 7 (Table 6 and Figure 3). Modeling was undertaken for 65-day period, right-lateral strike-slips of 63.9 mm along fault set 7 (=65 days * mm/day); and left-lateral motions of 4.8 and 32 mm, respectively, along fault sets 3 and 4, with set 3 taken as a reverse fault with a dip angle of 85. The orientations and the dynamic interactions of these three fault sets apparently led to right-lateral motions along fault set 7, and created extension conditions for fault sets 3 and 4 accompanied by reverse fault motions on the eastern side of the Cheb basin. Considering that the total horizontal extensions could have reached 64 mm in this area, and with the reverse subsidence along fault planes with 13 of 19

14 Figure 13. The site displacement fields of the 2008 swarm. Blue arrows indicate GPS site displacements; red arrows indicate modeled site displacements. a dip angle of 85, a vertical subsidence of 167 mm could have occurred in the KOPA site area located at a close distance east of fault set 3. [37] GPS site movements observed during the post-seismic phase modeled the displacement field, reflecting a gradual stress balance to the equilibrium state. In the central portion of the Cheb Basin fault motions did not change their trends, ran to the WSW, and, in a similar manner as in the co-seismic phase, may be accompanied by anticlockwise motions along marginal faults of the southern portion of the Basin (Figures 13c and 13f). In the northern portion of the Basin the conditions for the clockwise rotation of structures exists. Forward modeling for observed GPS motions proved the dynamic pattern for West Bohemia in which stress balancing changes, using antithetic and synthetic stresses, are expected [Schenk et al., 2009a, 2009b] since they can cause clockwise Table 6. The Modeled Fault Motion Rates Along the Given Fault Sets (Figure 13) a Pre-seismic Co-seismic Post-seismic Tectonic Fault Set Tectonic System a s d s d s d 1 N-S N-S N-S N-S NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NNW-SSE NNW-SSE NW-SE a Dip angle of the fault a [ ]: a =0 horizontal, a =90 vertical; strike along the fault plane s [mm/day]: s >0 right-lateral, s <0 left-lateral; dip on the fault plane d [mm/day]: d >0 reverse, d <0 normal. 14 of 19

15 Figure 14. The computed shear and normal stress fields for the pre-, co-, and post-seismic phases of the 2008 swarm; stress values are shown in arbitrary [bar] units. and anticlockwise rotations of structural blocks. The change also illustrates motions modeled along the N-S fault sets 1 to 4 (Table 6). Initially, the right-lateral strike-slips occurred along inner sets 2 and 3, whereas the left-lateral strike-slips continued along outer sets 1 and 4. The strike-slips modeled for fault set 1 were one order of magnitude larger than those of the other N-S fault sets. 8. The Upper Crust Stress Fields [38] The shear and normal stress fields for the pre-, co-, and post-seismic phases of the 2008 swarm were simultaneously computed together with the modeling of the displacement fields (Figure 14). The shear and normal stress fields determined for the pre-seismic phase (Figures 14a and 14d) considerably decreased during the co-seismic phase (Figures 14b and 14e). To explain these changes one procedure should be remembered, a hydraulic fracturing technology applied in oil extraction that is based on pumping a fluid down into the rocks. When the pore fluid pressure significantly increases, the normal stress field in the rocks is reduced and the rocks break up. If we analogically transfer the procedure to the process of lithospheric magmatic fluids penetrating into the crustal rock environment, the obtained changes of the normal stresses between the pre- and co-seismic phases can be explained. [39] The stress map of Europe [Müller et al., 1992] (www. world-stress-map.org) displays an impressive N-S stress action in the Alpine portion of southern Germany that naturally compresses the structures of the western portion of the Bohemian Massif. Compressions clutch the faults in structures along which fluids ascended. Restricting the ascent of the fluids led to their accumulation in the lower crust, and to the possible uplift of reverse faulting in upper crustal structures. The region became unstable, and when the fluid pressure reached instability, fluids penetrated along the faults upward and initiated earthquakes. The deepest hypocenters of the 2008 swarm earthquakes were initiated at a depth of approximately 11 km and occurred along NW-SE and/or NNW/SSE striking faults (Figure 5), indicating that at this depth fluid pressure built up a critical value for fluid injection into the upper crust. Pore pressure in the rocks had to increase enough to open up faults and fractures, and then the fluids rose upward. Simultaneously, the normal stresses decreased. [40] To support the above mentioned explanation, the degassing activity of a few mineral sources and mofettes between Kopanina and Nový Kostel is provided. To evaluate the 3 He/ 4 He ratio, during the period a few degassing sites were sampled monthly in the area [Bräuer et al., 2009] to determine the amount of deep mantle fluids penetrating through the crust to the surface. We determined that before two 2007 microswarms, the 3 He/ 4 He ratios likely decreased as a result of the stress increase in the area surrounding faults and rock fissures, so the rocks became less permeable. The gradual increase of fluid pressure in the lower crust led to an increase in stress. When the pressure reached a critical value the fluid components began to penetrate upward, initiated earthquakes, and led to a stress decrease in the upper complexes. The ratio variations observed at one of the degassing locality, the Bublák mofette, correlated well with the strain variations measured in the direction along the MLz [Schenk and Schenková, 2011]. The findings agree well with the shear and normal stress 15 of 19

16 field changes gained from GPS displacement modeling (Figures 14a 14d). [41] In the epicentral zone of the 2008 swarm, the positive normal stress was modeled for the pre-seismic phase (Figure 14d) and its marked change to negative stress for the co-seismic phase (Figure 14e). With high probability, this change could be interpreted as an improvement in rock conditions for easier fluid penetration into the upper crust. Analogous with hydraulic fluid injections, the change provided us with evidence of an enormous crushing of structural blocks in this area. Then, in the post-seismic phase, in order to separate themselves into partial manifestations of individual fault systems, the stress fields were launched in the narrowly limited northeastern section (Figure 14f). Simultaneously, the shear stress field (Figure 14c) began to turn to its previous pre-seismic pattern. Briefly, the forward stress field models of the post-seismic phase represent a gradual balancing to reach the original, relatively balanced regional state. 9. Discussion [42] Due to deep magmatic activity in the western portion of the Bohemian Massif, accompanied by phenomena such as earthquakes, natural springs, mofettes, weak fumaroles, etc., intensive investigations commenced approximately two decades ago. When a relatively strong earthquake swarm occurred, in October 2008, studies were begun to better understand the nature and interrelationships among these phenomena. One year before the onset of the swarm, when annual GPS campaigns on the local geodynamic WEST BOHEMIA network were launched, comprehensive evaluations of displacements and stress fields over the course of the 2008 swarm were performed. [43] A compilation of the observed data and modeled fields allowed us to innovate and to amend recent views of local seismotectonic processes. The first concepts were formed on the base of limited 1985 swarm data [Antonini, 1988; Grünthal et al., 1990]. Previous authors had assumed that the MLz fault system was the fundamental structural system responsible for swarm origins, and that the energy loading and unloading process was under way in a mutually reversible shifting zone along internal quazi-parallel faults. [44] Site displacements determined from the GPS data and additional numerical models of the displacement and stress fields agreed with previous concepts and extended them for complementary findings. The observations displayed good similarity to swarm occurrences and to the silent earthquake concept, in which after long-term energy loading a reversible short energy unloading arises due to earthquakes. Since no pronounced unidirectional displacement shifts of structures within the Cheb Basin have been discovered by geological and geophysical surveys (Figure 2), it is evident that reversible motions must exist between structures. The high probability of the existence of this process in the Nový Kostel zone is supported by specific PT conditions within this region. [45] Even if there was no evidence of high regional shear surface deformations, the observations indicated that they are a part of measured GPS rates and that block-bounding faults slip at the surface (i.e., are not locked). During interseismic periods, the most active faults are often locked at the surface and slip at depth until they rupture in an earthquake. As faults become unclamped and as regional crustal pressure increases, fluids move inside and change the pore pressure, while faults open and tensile fashions emerge. At this moment faults become ready for the slip and dip motions that are associated with earthquake origins. [46] We assume that anomalous PT conditions form a local hot spot due to deep magmatic processes in the region in which the rock ductility level is positioned around km in depth. Above this level, where earthquake activity originates, the rock environment is said to be densely crushed and disrupted by interconnected faults. According to this view, although the rocks should be in nearly a brittle state in the depth interval from 11 to 7 km, they can, nevertheless, dynamically react under quasi-ductile conditions. Therefore, when structural blocks composed of such rocks are impacted by variable stresses they will conform to these conditions, and an optimal stress balance for different depths using horizontal and vertical motions including rotations is inevitable. The limited extent of a hot spot area in West Bohemia can be assessed from the maximum focal depths of seismic events occurring in its broad vicinity. In Germany, km northwestward of the Nový Kostel zone, hypocenters already reach a depth of 20 km, and in a distance of km the focal depths can attain km. The existence of such a hot area is compatible with the thermal destabilization of the West Bohemia lithosphere, as determined by magmatic activity as recent as the mid-pleistocene [Wagner et al., 2002]; and the thermally stabilized lithosphere of North Germany [Ziegler and Dèzes, 2005, 2007]. [47] An example of mutual movements of two structural blocks in the epicentral area of the 2008 swarm is shown in Figure 15. Site displacements between the NOKO and KOPA blocks caused a slow lift and the right-lateral movements of the KOPA block during the pre-seismic phase. At any moment (the circumstances and the mechanisms, not yet defined) the KOPA block rose relatively to the NOKO block and stress accumulation was ongoing along the faults between the blocks. When accumulated energy and acting fluids reached a critical PT state, the process of seismic energy release commenced with earthquake events. In the co-seismic phase, the KOPA and NOKO blocks began to move reversibly and the KOPA block subsided. GPS data indicated that the distance between the KOPA and NOKO sites was reduced by 7 to 8 mm and that KOPA subsidence reached 167 mm (Figure 3). [48] In addition to the presented seismotectonic model, the existence of which is closely connected to the NW-SE (MLz) tectonic system [Schenk et al., 2009a], there is a concept of preferring dynamic activities within the N-S Počátky-Plesná zone (PPZ) (Figure 16). The zone is a part of the Regensburg- Leipzig zone [Bankwitz et al., 2003]. The concept assumes that in the Cheb Basin area the PPZ is under long-term leftlateral creep movements [e.g., Peterek et al., 2011] that initiate energy accumulations on MLz fault segments located inside the PPZ and that act as sinistral Riedel shears. In addition, the validity of the PPZ concept can be questioned due to (a) geological and/or geophysical evidence of longterm, left-lateral unidirectional movements along the PPZ in West Bohemia, (b) no acceptable focal plane solutions for the 1997 swarm (DC and non-dc components) and a number of the right-lateral plane solutions for the 2008 swarm 16 of 19

17 Figure 15. The deformation model for the pre-, co-, and post-seismic phases of earthquake swarms in West Bohemia. The relationship of synthetic and antithetic shear stress to energy accumulation and release processes, and to rock relaxing recovery. White arrows represent block motions; the circle with points inside represents forward motion; and the circle with the cross represents backward motion. Figure 16. The two seismotectonic concepts of the Nový Kostel zone; full lines represent the MLz concept; black arrows represent the long-term movement trend along the MLz zone; dashed lines represent the PPZ concept; white arrows represent the long-term movement trend along the PPZ zone; thin dotted lines represent other faults within the MLz system; blue circles represent the epicenters of the 2008 swarm; and the attached figure indicates two types of fault plane solutions for the events of the 2008 swarm [Vavryčuk, 2011]. 17 of 19