Geophysical Field Pattern in The West Bohemian Geodynamic Active ...

GEOPHYSICAL FIELD PATTERN IN THE WEST BOHEMIAN GEODYNAMIC ACTIVE AREA JAN SVANCARA Institute of Physics of the Earth, Masaryk University, Brno, Czech Republic1

IVAN GNOJEK2 FRANTISEK HUBATKA AND KAREL DEDACEK

Geofyzika a.s., Brno Czech Republic3

S u m m a r y : Regional geophysical data from detailed gravity survey, airborne magnetometry and gamma-ray spectrometry were analysed in order to determine the subsurface extent of contrasting geological bodies and to highlight subtle anomalies which can be related to the occurrence of earthquake swarms. Potential field data were compiled into contour and colourshaded relief maps suitable for detecting structural tectonic elements. A shaded relief map of the horizontal gradient of gravity was used to detect considerable structural and tectonic features. The results of airborne gamma-ray spectrometry, showing the regional total gamma-ray activity, abundance of uranium, thorium and potassium, were included in this study. Only the two most instructive maps - the total gamma-ray activity and the abundance of potassium are shown. The main line of epicentres Novy Kostel - Pocdtky coincides well with the N-S configuration of abundances of these natural radioactive elements. The epicentres of micro-earthquakes detected by the local seismological network KRASLICE for the 1991 to 1998 period were plotted in the geophysical maps. The hypocentres of earthquakes in the main epicentral zone at Novy Kostel were projected onto the crustal density model based on the interpretation of seismic reflection profile 9HR and gravity data. The average distance between the Novy Kostel epicentral zone and the seismic profile was 4-5 km. Based on the interpretation of gravity data the hypocentres of the main epicentral zone seem to be associated with the western margin of the Eibenstock - Nejdek (Karlovy Vary) Pluton and, beside that, they follow the depth level where the allochthonnous part of the Saxothuringian Zone is thrust over the " European parautochton". A drawing of the geodynamic model of the area is also shown.

Key w o r d s : Earthquake swarm, Bohemian Massif, Saxothuringian Zone, Eibenstock - Nejdek (Karlovy Vary) Pluton, Cheb Basin, gravity, airborne magnetometry, airborne gamma-ray spectrometry, seismic profile 9HR, density model, stress field, geomorphology

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Address: Tvrdeho 12, 602 00 Brno, Czech Republic ([email protected]) Address: M.Hubnerove 42, 621 00 Brno Address: Jecna 29a, 621 00 Brno, Czech Republic

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J. Svancara et al. 1. INTRODUCTION The analysis of regional geophysical data is very important for the geological interpretation of frequent earthquake swarms in the western part of the Bohemian Massif. Most hypocentres of this seismoactive area are in the depth interval of 5 to 18 km but the micro-earthquakes of the main zone at Novy Kostel originate in the depth interval of 5 to 9 km. Because of the absence of deep boreholes in the area, the hypocentres can be attributed to a specific seismoactive geological structure using geophysical data only. Regional geophysical data play an important role in the study of gas flux distribution in mineral springs and of the tectonic structure as well (Weinlich et al.]998). Understanding the gravity field is of principal importance for interpreting the deep geological structure. The Bouguer anomaly map, compiled for a reduction density of 2670 kg/m3 has enabled the location of density-contrasting geological bodies, or even an estimation of the depth range of granite bodies. The shaded relief of the total horizontal gradient of gravity has provided information on fault trends and other tectonic lines which could not be identified in usual contour maps. The results of airborne magnetometry completely differ from those of gravimetry. This difference is attributed to the fact that the granite plutons, which are dominant in the upper part of the Earth's crust in the study area, are not magnetic. On the other hand, magnetic anomalies can characterize the composition of deep-seated rocks and direct the attention to various volcanic bodies within the Earth crust. The phenomenon of volcanism is considered by several authors to be a common denominator of the European earthquake swarm occurrences (Spicdk et al. 1999). The very shallow depth of penetration is typical for the airborne gamma-ray spectrometric survey. This method measuring potassium, equivalent uranium and equivalent thorium contents in the near-surface rocks is able to supply valuable petrological and geochemical information especially in the areas built by magmatic rocks. Moreover, the distribution pattern of the natural radioactive elements can provide an indirect indication of some tectonic features. Deep reflection seismic profile 9HR and seismological records were further geophysical data sources utilized in this study. A crustal-scale 2.5 D balanced density model (Svancara and Chlupdfovd, 1997) has been computed along seismic profile 9HR. Since the northwestern part of the 9HR profile runs only 4 - 5 km from the main Novy Kostel epicentral zone, the authors tried to project the hypocentres of earthquakes onto the density cross-section. All the hypocentres from the records of the KRASLICE network in the period 1991 - 1998 (Skdcelovd et al.,1998) were used for these analyses. These hypocentres have a high degree of coherence because they were recorded within a fixed network geometry. The algorithm for locating events and the 3D block velocity model remained unchanged during the whole 1991 - 1998 period of recording. The maps shown rely on the geophysical data acquired, processed and stored by Geofyzika Brno prevailingly in projects for the Czech Geological Survey, Prague. 2. GEOLOGY Almost the whole area under study belongs to the Saxothuringian Zone (Fig. 1). Only the SE corner of the geological scheme presented in this figure pertains to the Marianske Lazne" high-grade ophiolitic complex, which separates the Saxothuringian Zone from the Tepla - Barrandian Zone in the SE. The geological structure of this area has recently been explained in the monograph edited by Vrdna, Stedrd (1997). The segment of the Saxothuringian Zone representing the geodynamic active area is predominantly built by the Krusne hory - SmrCiny Crystalline Unit, Horni Slavkov Crystalline Unit and the Cheb - Dylen Crystalline Unit, which are understood to be complexes consolidated both during the Cadomian and the Variscan

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orogeny. The Krusne hory - SmrCiny Crystalline Unit is covered (in the NW) by the Lower Paleozoic of Vogtland and Saxony consisting of weakly metamorphosed sedimentary rocks with a low proportion of metavolcanites. Three granite plutons intruded into the area during the Variscan orogeny. The largest is the Eibenstock - Nejdek (Karlovy Vary) Pluton (NE), rather smaller is the SmrCiny (Fichtelgebirge) Pluton (W) and the smallest body to the SE is the Zandov Pluton. The Kladska Unit cropping out NW of the Marianskd LaznS Complex is a denudation remnant of metasediments with concordant bodies of mostly basic metavolcanites tectonically overlying the Horni Slavkov and the Cheb - Dylefi Crystalline Units. (See Fig. 1) Along the young OhFe rift zone, two basins, filled by Tertiary and Quarternary sediments, are developed in the area under interest. In the SW part of the area there is the Cheb Basin, in the E there is the rather narrower Sokolov Basin. Not only the sediments of the basins, but also the granite plutons and complexes of metamorphites are locally penetrated by Cainozoic volcanites.

3. GEOMORPHOLOGY

The area under study belongs to four mountain systems among which two young basins are developed. In the NW, there are the SmrCiny (Fichtelgebirge) Mts., the NE part pertains to the large Krusne hory (Erzgebirge) Mts. System. The SE corner of the area is occupied by the Slavkovsky les Mts. and the small SW part belongs to the Cesky les Mts. The Cheb Basin (W) and the Sokolov Basin (E) are situated between the pairs of the mentioned mountain systems in the North and those in the South. The Smrdiny Mts. are separated from the Krusne hory Mts. by a flat depression spread in the surroundings of the small town of Luby. The KruSne hory Mts. System begins (at its SW margin) with the Krajkova Highland continuing to the NE with the Nejdek Highlands which gradually rise to the highest partial area - the Klfnovec Mts.. Also the second largest mountain part of the area studied, i.e. the Slavkovsky les Mts., is composed of the Kynlvart Highland (in its SW part) and of the Hornf Slavkov Highlands (in the NE). The Cheb Basin, as the broader, is sharply tectonically limited in the E and NE (towards the Krusne hory Mts. System) by a fault slope with the relative offset reaching 150 m. The remaining N, W and S margins of the Basin pass almost gradually to the surrounding uplands. The S part of the Cheb Basin is separated from the Tachov Depression, developed further to the S, by a ridge cropping out between the villages of Dolnf 2andov and LaznS Kynzvart. The Sokolov Basin, the narrower, is almost systematically limited by a set of faults from both the NW (the Krusne hory Mts.) and the SE (the Slavkovsky les) sides. These two basins are separated from one another by a small ridge of crystalline rocks linking up the KruSne hory Mts. and the Slavkovsky les Mts. Systems. The area of interest is drained by the Ohfe river and its tributaries. The Ohre river flows from the flat western part of the Cheb Basin (elevation about 460 m) to the Sokolov Basin with a considerably more variable relief through a narrow ridge near the village of Chlum nad Ohfl

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The elevation map (Fig. 2) was compiled on the basis of a digital elevation model. The map is supplemented by a schematic network of hydrography (without contouring water reservoirs) compiled and provided to the authors by Man (1997).

4. GRAVITY FIELD

The map of Bouguer anomalies computed for the reduction density of 2670 kg/m3 was used to interpret the geological structure of the area of interest. This map is based on a detailed gravity survey with 4-6 gravity points per km2. Topographic corrections were calculated for Hayford zones A-C>2 , i.e. up to a distance of 166.7 km. The gravity map shown in Fig. 3 was computed in the Potsdam gravity system using the Helmert formula for normal gravity. The Bouguer anomalies computed in this way are approximately 17 mGal = 170 um/s2 higher than the anomalies calculated in the IGSN71 gravity system and using International gravity formula 1967. The extensive gravity minimum caused the by the light granites of the Eibenstock Nejdek (Karlovy Vary) Pluton is a dominant feature of the gravity field (Fig. 3). This Pluton is the largest granite body in the whole Saxothuringian Zone (Conrad, 1994; Svancara, Chlupacova 1997). The gravity effect of the light sediments of the Sokolov Basin is superimposed on this regional negative gravity anomaly. The light granites of the Zandov intrusion of the Eibenstock - Nejdek (Karlovy Vary) Pluton account for the negative anomaly at the southern margin of the area under study. The gravity minimum bounded by the FrantiSkovy Lazn6 - Kynsperk - Novy Kostel Plesna - Hazlov polygon is attributed to the light granites of the SmrCiny (Fichtelgebirge) Pluton (Hecht et al. 1997; Srdmek, Mrlina 1997) as well as to the sediments of the Cheb Basin. The sources of the positive anomaly in the area of Luby - Novy Kostel - Krajkova Stribrna include dense phyllites of the Vogtland - Saxony Paleozoic and mica schist of the Krusne hory Mts. crystalline complex. The positive anomaly to the west of Cheb is caused by various kinds of phyllites and mica schist of the Cheb - Dyleft Crystalline Unit. The horizontal gravity gradient is presented in the form of a gray-shaded relief map (Fig. 4). This derived gravity field shows the high frequency component of the gravity field that may not be visible in the Bouguer anomaly map. Fig. 4 shows all the pronounced density contacts, especially those of the upper part of the Earth's crust. Tectonic lines are depicted in a similar way because they are usually accompanied by zones of reduced density. Some indications from Fig. 4 have a known geological explanation, but many of them provide new information and enable the correlation of spatially distant phenomena. A pronounced indication of the density contact is associated with the eastern marginal fault of the Cheb Basin in the Luby - Kynsperk nad OhFi interval. Many E-W trends are pronounced in Fig. 4 (N of Nejdek, SE of Kostelni BFi'za and between Soos and Liba, etc.). Indications of density contacts between Kostelnf Bfiza and Lazy correlate with earthquake occurrences near Lazy. The line connecting the main epicentral zone of Novy Kostel with the epicentres N of Franti§kovy Lazn6 within the Cheb Basin is also shown in the horizontal gravity gradient map.

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5. MAGNETOMETRY The airborne magnetic survey was performed using a G-801/3B proton magnetometer (accuracy 1 nT). The distance between the flight profiles was 250 m with tie lines every 2500m. The prevailing ground clearance was 80- 100 m, the sampling rate was 1 s (approx. 35 m along the flight profiles). The finally processed data - AT anomalies - are stored in a 125-m square grid. Fig. 5 shows the magnetic map generated from this grid. The magnetic field of the Bohemian Massif is characterized by the zonal character of major long-wavelength anomalies (Pokorny, Benes 1997). Two large magnetic elevations can be distinguished within the predominantly positive field of the area under study. The first situated in the southern half of the Cheb Basin is attributed to the magnetized rocks which build the basement of the basin. Local linear anomalies trending WSW-ENE are interpreted as a system of tufaceous horizons consisting of magnetite-bearing phyllites (Mazdc, Pokorny 1962) which may appertain to the Ordovician Phycode complex. Local magnetic anomalies are often caused by strongly magnetized young volcanics of Miocene to Pleistocene age; e.g. basalts near the village of Stary Hrozftatov (S of Cheb) and/or nephelinites of the Komornf Hurka Hill. The second magnetic elevation Sokolov - Olovf is assumed to be caused by a buried basic body of unknown type (Pokorny 1969). The ground survey performed by Pokorny et al. (1966) disclosed that many local anomalies of the Vogtland Saxony Paleozoic were caused by phyllites with magnetite and pyrrhotite. The most intensive anomaly of the area is produced by a strongly magnetized serpentinite body near the village of Prameny pertaining to the Marianske LaznS Complex of mostly basic rocks. The pronounced anomaly north of the village of Lazy is generated by metasediments and metebasites of the recently defined tectonic unit of Kladska (Kachlik 1994). The granites of the Eibenstock - Nejdek (Karlovy Vary) Pluton are non-magnetic and also the Smrc'iny (Fichtelgebirge) Pluton has a low magnetization. The fault system following the boundary between the phyllite and mica-schists complexes (SW) and Eibenstock - Nejdek Pluton (NE) along the Kraslice - Rotava - Jindfichovice - Chodov line can be recognized in the magnetic field pattern. On the contrary, the structure of the Cheb Basin is not reflected in the magnetic field. No evident correspondence of the main Novy Kostel - PoCatky epicentral zone and the magnetic field pattern has hitherto been found. A detail study of some small-sized magnetic anomalies, which might be connected with buried volcanic bodies, could bring new interpretation results in future. 6. GAMMA-RAY SPECTROMETRY Airborne gamma-ray spectrometry was applied simultaneously with airborne magnetometry. The 256-channel airborne gamma-ray spectrometer GR-256 with a volume of 33 litres of Nal(Tl) crystal detectors was used. The resolution was 1 ppm of uranium, 1.5 ppm of thorium, 0.25% of potassium and 2 ppm Uequiv. in the total count channel. The processed gamma-ray spectrometric data - abundance of U, Th, K and total gammaray activity of the Earth's surface - are also stored in 125-m square grids. Fig. 6 displays Studia geoph. et geod. 44 (2000)

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the pattern of the total gamma-ray activity and Fig. 7 shows the abundance of potassium. (The reader is asked to take into account the fact that most of the extensive local minima - with the exception of the SE corner - are water reservoirs and peateries). The western part of the Bohemian Massif is proved to be a region with high uranium potential. A remarkable abundance of the radioactive elements was found in the plutonic rocks. The Eibenstock - Nejdek (Karlovy Vary) Pluton is relatively rich in uranium on the Czech side. The content of this element commonly varies between 4 to 10 ppm U with many local anomalies reaching tens ppm U. Similar results were presented in the study by Forster et al.1999 from the German part of the pluton. As a thorium is concerned, the autometamorphosed types of granites have a reduced content of thorium (6- l O p p m T h ) while the other-mostly biotite - granites contain 14-30 ppm Th. The SmrCiny (Fichtelgebirge) Pluton also pertains to notable granite bodies from the radiometric point of view. Especially the uranium contents were found to be rather high - mostly from 5 tolO, but locally up to 25 ppm U. Chlupacova et al. 1998 ascertain that the regional distribution of uranium is distinctly independent of granite petrology. The content of thorium, generally varying at medium values, rapidly decreases to 4-2 ppm Th in the easternmost outcropping part of the pluton near the Cheb Basin. Detailed information about the geochemistry of the German side of this pluton was published by Hecht et al. 1997. Remarkable contents of radioactive elements were also observed in the Zandov granite body to the SSE of the Cheb Basin. The most striking values relate to the content of uranium (10 - 20 ppm U); thorium as well as potassium contents (15-20 ppm Th and 3.5 - 4% K) are less pronounced. A notable abundance of uranium was even found in several localities within the Cheb and Sokolov Basins where ceramic raw materials and brown coal are exploited in open pits. Radioactive anomalies, found in these excavated parts of the basins, provided the proof that the supply of uranium to the basins was not negligible. The N-S trending minima in the map of the total gamma-ray activity and the maps of distribution of natural radioactive elements (K, U and Th), which correlate with epicentres near Novy Kostel, may indicate - through the presence of local streams - faults of lower order with which the location of epicentres could be linked. 7. INTERPRETATION OF SEISMIC, SEISMOLOGICAL, AND GRAVITY DATA The completion of the 9HR deep seismic profile initiated a new period in the interpretation of geophysical data in western Bohemia. The 9HR profile provided information about the acoustic impedance changes for the whole Earth's crust and even several upper mantle reflections. Fig. 8 shows the northwestern part of the crustal density model of the geological structure along the straightened 9HR profile (Svancara, Chlupacova 1997). The initial geometry of the density model was derived from the line-drawing of the essential reflections along the profile (Tomek et al., 1997) and also from the correlation of areas with equal echogenity. Particular attention was paid to the natural rock densities used for

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the gravity modelling. The resultant density model of the Earth's crust is in full agreement with the measured gravity data. The gravity low of the light granites of the Eibenstock - Nejdek (Karlovy Vary) Pluton (density 2600 kg/m3) is a dominant feature of the gravity profile. The thickness of the Eibenstock - Nejdek (Karlovy Vary) Pluton is 11-12 km. (In a similar study performed by Conrad (1994) along the MVE 90 profile, the thickness of the Eibenstock granite was estimated to be 10 km.) The increase in gravity values towards the northwest is attributed to a dense Ordovician phyllites (density 2800 kg/m3) and mica schists (density 2706 kg/m3). The abrupt increase in gravity along the south-eastern margin of the Eibenstock - Nejdek (Karlovy Vary) Pluton coincides with the contact of this granite massif with the dense rocks of the Marianske Laznfi Complex. As an attempt to display possible correlation between the density model of the geological structure and the located hypocentres we prepared montage shown in Fig. 8. The hypocentres of earthquakes located by the Kraslice network in the period 1991 - 1998 closer than 10 km to the 9HR profile were projected on the density model. The hypocentres of the main epicentral zone of Novy Kostel (4 - 5 km of the seismic profile) occur along the western margin of the granites of the Eibenstock - Nejdek Pluton. Here the authors assume a body with a density of 2720 kg/m3 that forms the bedrock of the Krusne hory Mts. crystalline unit. The montage shown in Fig. 8 is twofold ambiguous. The first is the principal ambiguity of the inverse gravity problem, and the other is the unproven assumption of the two-dimensionality of the geological structure between the 9HR profile and the hypocentres. Under similar assumptions we compiled Fig. 9, which displays envelopes of hypocentres in the NW part of the migrated time section of seismic profile 9HR. Here the authors observed groups of pronounced dipping reflections that can be regarded as manifestations of tectonic displacements. The high intensity of these reflections that disturbs the relatively isotropic reflection image may indicate recurrent tectonic displacement. The crossing of these reflections at 4 km and the two-way time of 2500 ms may suggest the presence of two tectonic systems. The majority of hypocentres registered in the broader vicinity of Novy Kostel is situated in the depth interval from 5 to 9 km. It is the very place of the geological section, in which the allochthonous complex of the Saxothuringian Zone is thrust over the 'European parautochthon' as depicted in the ray tracing section of the 9HR seismic line and subsequently interpreted by Tomek et al. 1997 The analysis of the structural and tectonic features recorded in geophysical fields and the study of regional stress fields published by Skdcelovd et al. (1998) and Havif (2000) enabled a simplified dynamic scheme to be designed and a possible movement mechanism which could explain the occurrence of earthquake events to be outlined (Fig. 10). The authors attribute great importance to the existence of two subparallel NW-SE trending fault systems, i.e. the Marianske LaznS regional fault and the Kraslice - Rotava - Chodov fault system (near the SW margin of the Eibenstock - Nejdek Pluton) both well reflected in the geophysical data. Between these two faults exposed to the NW-SE (NNW-SSE) regional stress field, an 'en-echelon' fault structure was created. The nearly N-S oriented line of epicentres Novy Kostel - PoCatky could be a manifestation of the young 'enechelon' fault (Nehybka, Skdcelovd, 1995) which seems to be a hosting structure of microearthquake events. The continuation of this N-S line to the south is clearly seen in all the Studia geoph. et geod. 44 (2000)

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airborne gamma-ray spectrometric maps as N-S trending minima in the gamma-ray response. (The minima in these maps are caused by water-saturated alluvial deposits, which fill the valleys of local streams. The N-S orientation of the streams may correspond with the mentioned 'en-echelon' faults.) From the geomechanical point of view, the sinistral displacement along the above-mentioned young fault following the line of epicentres Novy Kostel - PoCatky should be accompanied by a dextral displacement along the Marianske' LaznS fault coinciding there with the eastern marginal fault of the Cheb Basin. The Novy Kostel - Pocatky line and the Marianskd LaznS fault in the vicinity of Novy Kostel form a conjugate system. The partial blocks delimited by the abovementioned faults and controlled by the regional stress field are compressed and pushed against one an other. The regional extension in the NE-SW to ENE-WSW direction enables the release of accumulated elastic-strain energy. 8. CONCLUSIONS Based on the gravity data interpretation we conclude that the hypocentres of the Novy Kostel epicentral zone are associated with the western margin of the Eibenstock - Nejdek (Karlovy Vary) Pluton. The line of epicentres Vackovec - Novy Kostel - PoCatky correlates with the N - S trending abundances (both minima and maxima) of the all natural radioactive elements detected by airborne gamma-ray spectrometry. This trend correlates slightly with geomorphology but does not correlate with the potential field data - Bouguer anomaly map and derived gravity maps and aeromagnetic total intensity map. The Novy Kostel - PoCatky line and the Marianske LaznS fault create a conjugate system. The sinistral shear sense along the Novy Kostel - PoCatky line should be accompanied by dextral displacement along the Marianske Lazn6 fault. The partial blocks delimited by the above-mentioned regional faults are controlled by a regional NW-SE (NNW-SSE) stress field and compressed and consequently pushed against one an other. The regional extension in the NE-SW to ENE-WSW direction enables the accumulated elastic-strain energy to be released.

Acknowledgement: Authors would like to thank Dr. Vladimir Nehybka for providing the database of hypocentres of microearthquakes recorded by the Kraslice network in the period 1991 - 1998, and Dr. Josef Havif for valuable consultations on the regional stress field. We would also like to thank Prof. Tomek, Dr. Jahr, and Dr. Kampf for the careful review of the article that greatly helped to eliminate inaccuracies in the text and provided many stimulating suggestions for extending this paper. The review by Prof. Tomek led to a major revision of the manuscript which was very valuable especially because of his different opinion in some interpretation questions.

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Fig. 1. Simplified geological map of the geodynamic active area. This map is a part of the digital geological map published by the Czech Geological Survey in 1998 as a digital atlas of the Czech Republic - GEOCR 500. Explanations: 1 - Quarternary sediment, 2 - Tertiary sediments, 3 - Tertiary volcanics, 4 - Variscan granites: SmrCiny (Fichtelgebirge) Pluton (W), Eibenstock - Nejdek (Karlovy Vary) Pluton (NE), Zandov granite body (SE), 5 - metabasites and metasediments of the Kladska Unit and the Mariansk6 LaznS Complex, 6 - serpentinite of the Marianske LaznS Complex, 7 - phyllites and mica-schists of the KruSne hory - SmrCiny and Cheb - Dyleft Crystalline Units together with the Vogtland and Saxony Paleozoic, 8 - orthogneiss, 9 - Horni Slavkov Crystalline Unit.

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Fig. 2. Elevation color contour map, contour interval 100 m. The black dots indicate epicentres of micro-earthquakes for the period 1991 - 1998. Legend: the black box symbols mark positions of the seismological stations of the KRASLICE network. The deep reflection profile 9HR is marked by a black line with 1 km ticks.

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Fig. 3. Bouguer anomaly contour map. The contour interval is 2 mGal = 20 pim/s2. Gravity anomalies were calculated using a Bouguer reduction density of 2670 kg/m3. For legend see Fig. 2.

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Fig. 4. Gray-shaded relief map of the gravity horizontal gradient illuminated from the north. The red dots indicate epicentres of micro-earthquakes for the period 1991 - 1998. The thick yellow lines mark selected fault systems. For legend see Fig. 2.

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Fig. 5. Colour-shaded relief map of total magnetic field illuminated from the north. The map was generated from airborne data obtained along flight profiles 250 m appart. The sensor height was 80 - 100 m. The red dots indicate epicentres of micro-earthquakes for the period 1991 - 1998. For legend see Fig. 2.

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Fig. 6. Total radioactivity map based on the total count channel of the airborne gamma-ray spectrometry. For legend see Fig. 2.

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Fig. 7. Potassium content colour map obtained by airborne gamma-ray spectrometry. For legend see Fig. 2.

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Fig. 8. North-western part of the crustal-scale density balanced cross-section along deep seismic reflection profile 9HR (Klingenthal - Sokolov - Tepla). Black circles indicate the projection of hypocentres. Integer numbers show the density of the individual bodies in kgm .

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Fig. 9. Migrated time section of the northwestern most part of the deep seismic reflection profile 9HR (Klingenthal - Sokolov basin margin). Red ovals show the envelopes of hypocentres projected onto the seismic time section. Yellow colour is used to mark important groups of reflections. The horizontal axis shows the distance in kilometres and the vertical axis displays the two-way reflection time in milliseconds.

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Fig. 10. Sketch of geodynamic model of the area. The green lines delineate the main fault systems, i.e. the Marianske Lazn£ regional fault (MLF) and the Kraslice- Rotava - Chodov fault (KChF) system, both well expressed in geophysical data. The brown arrows show the direction of regional compression, and the blue arrows the direction of regional extension. The sinistral displacement along the young 'en-echelon' fault coinciding with the main Novy Kostel - PoCatky epicentral line should be accompanied by a dextral displacement along the Marianskd Lazne fault.

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Geophysical Field Pattern in the West Bohemian Geodynamic Active Area Manuscript received: 2 December 1999

Revisions accepted:

28 March 2000

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