Derived gravity field of the seismogenic upper crust of SE Germany and West Bohemia and its...

Stud. Geophys. Geod., 52 (2008), 567588 567 © 2008 Inst. Geophys. AS CR, Prague

DERIVED GRAVITY FIELD OF THE SEISMOGENIC UPPER CRUST OF SE GERMANY AND WEST BOHEMIA AND ITS COMPARISON WITH SEISMICITY

J. ŠVANCARA1, J. HAVÍŘ1 AND W. CONRAD2

1 Institute of Physics of the Earth, Masaryk University, Brno, Czech Republic

([email protected], [email protected]) 2 Haydn Str. 8/912, Leipzig, Germany

Received: January 9, 2008; Revised: June 10, 2008; Accepted: September 9, 2008

ABSTRACT

The gravity field of the seismogenic upper crust was derived from the Bouguer gravity map by applying the Butterworth high-pass filter in the wave-number domain. The cutoff wavelength of the filter was 110 km, to pass the gravity signals of structures within the 18 km thick seismogenic layer. The derived residual gravity map reveals potential stress concentrating structures, which may cause seismicity provided they lie within the existing zones of weakness. Furthermore we derived a shaded relief map of the horizontal gravity gradient, which highlighted the tectonic lines accompanied by density contrast. The directional analysis of this map shows three dominant strike directions. The most prominent one is “the Hercynian” NW-SE strike direction represented by the Franconian Line, the Gera-Jáchymov Fault Zone and the Elbe Zone. The second dominant strike is the Rhenisch NNE-SSW trending represented by the Upper Rhine Graben Zone, Rheinsberg-Heldburg Line and several Proterozoic volcanic belts in the Teplá-Barrandien Unit. The third pronounced trending of the ENE-WSW direction is represented by the Erzgebirge and Eger Graben gravity low. The N-S trending Rostock-Leipzig-Regensburg Zone (Pritzwalk-Naab Lineament) is not distinctly reflected in the derived gravity maps, although many fault segments have a meridian direction.

The relative reactivation potential of some pre-existing fault systems identified in the gravity map was studied with respect to the wide range of the recent stress configuration determined in the West Bohemia/Vogtland region. The resulting diagrams show that the steep NNW-SSE to N-S faults (represented by some segments of the Mariánské Lázně Fault Zone) are oriented favourably for reactivation. On the contrary, the orientation of the ENE-WSW faults limiting the Eger Graben (Litoměřice Fault, etc.) is unfavourable for reactivation for all dip values.

Ke y wo rd s : Bouguer gravity, seismicity, reduced stress tensor, fault reactivation

potential, Mariánské Lázně Fault Zone, Gera-Jáchymov Fault Zone, Eger Graben, Rostock-Leipzig-Regensburg Zone

J. Švancara et al.

568 Stud. Geophys. Geod., 52 (2008)

1. INTRODUCTION

In continental areas of moderate to high heat flow, the thickness of the seismogenic zone is the top 1020 km of the Earth’s crust (Sibson, 2002). Bouguer anomaly maps yield information about the distribution of density inhomogeneities in the whole crust and upper mantle. The gravitational effect of the seismogenic upper crust can be derived by applying of suitable high-pass filter in the wave-number domain. Since the cut-off wavelength of this filter is typically several times greater than the lower bound of the crust structures in question it is necessary to use input gravity data set with very large overlap. Therefore, the unique Bouguer gravity map of Germany, West Bohemia and adjacent regions, compiled by Behr et al. (2002), was used. The rationale for the correlation of the derived gravity field with seismicity is based on the fact that seismicity occurs in the vicinity of stress concentrating structures within pre-existing zones of weakness (Gangopandhyay and Talwani, 2003; Talwani, 1999), which are both associated with density inhomogeneities. Typical stress concentrating structures, well reflected in gravity anomalies, are intersecting faults and buried plutons (Švancara et al., 2000; Špaček et al., 2006; Lenhardt et al., 2007).

The horizontal gradient of gravity is a derived field suitable for highlighting structural information in gravity data. To analyse trending, we have constructed the grey shaded relief map of the horizontal gravity gradient. Based on this map we have defined the striking of selected, well-pronounced, regional faults and fault zones, i.e. Gera-Jáchymov, Mariánské Lázně, Litoměřice and Rheinsberg-Heldburg, and performed a fault reactivation analysis. The subsequent analysis was carried out to verify whether the N-S Rostock-Leipzig-Regensburg Zone (Bankwitz et al., 2003) is reflected in the derived gravity maps.

The reactivation potential of the pre-existing fault plane depends on the orientation of this fault with respect to the stress field. If the stress tensor and fault strength are known, the fault reactivation tendency can be calculated for any orientation of the fault plane (Morris et al., 1996; van Ruth et al., 2006). Even if we know only the reduced stress tensor (Angelier, 1990) and approximate value of the friction coefficient, we can determine orientations of the theoretical planes with higher or lower reactivation potential (Melichar, unpublished results). Using this method we estimated fault orientations that are favourable and unfavourable for reactivation in the stress regime determined in the West Bohemia/Vogtland region. These results are compared with the orientations of some selected fault zones detected in the map of the horizontal gravity gradient and discussed with a view to their potential and/or observed seismic activity.

2. GRAVITY DATA AND GEOLOGICAL OUTLINE

2 . 1 . B o u g u e r G r a v i t y M a p

In this paper, we use the 1 1 km gravity grid, which results from the Bouguer gravity map of Germany and adjacent regions on a scale of 1:1 000 000 (Behr et al., 2002) which was the first large-scale compilation of gravity data in Central Europe. The compilation was based on existing contour gravity maps, which were digitized on a scale of 1:500 000

Derived Gravity Field of the Seismogenic Upper Crust of SE Germany and West Bohemia

Stud. Geophys. Geod., 52 (2008) 569

mainly with 1 105 m/s2 (1 mGal) contour intervals. In some adjacent regions (North Sea, France, Austria and Switzerland) only maps with 2 105 m/s2 or even 5 105 m/s2 contour interval were available. The observed gravity is tied to the IGSN 71 and the GRS 80 has been used to calculate the normal gravity. The Bouguer correction density varies between 2000 and 2700 kg/m3. The Bouguer gravity map shown in Fig. 1 was constructed using Lambert’s conic conformal projection with parallels 4840 and 5340. The rms error of the Bouguer gravity map ranges from 0.5 to 2.5 105 m/s2 for land. The geological interpretation of the Bouguer gravity anomaly field using mainly the Linsser filtering technique was described by Behr et al. (2002). The geological interpretation of the Bouguer gravity map of Thuringia, Saxony, Saxony-Anhalt, Brandenburg and Mecklenburg-West Pomerania was summarized by Conrad (1996).

2 . 2 . S t r u c t u r a l G e o l o g i c a l O u t l i n e

The Variscan basement, covered by Permian to Cenozoic sediments, forms the main part of Germany and adjacent regions (Fig. 2). In the uplifted regions (Bohemian Massif, Rhenish Massif, Harz, Vosges, Black Forest, etc.), the Variscan units crop out from the Permian to Cenozoic sedimentary cover. Their Cenozoic uplift was controlled by various processes (Cloetingh et al., 2006). At the southern margin of the studied region, the Variscan basement dips under the Alpine nappes.

The European Cenozoic rift system (ECRS) originated during the Late Eocene (Dèzes et al., 2004). Its Cenozoic evolution was accompanied by volcanic activity concentrated above all in the Massif Central, the Rhenish Massif and the Bohemian Massif (Ziegler, 2005). Tectonic activity of ECRS continues up to the present. The seismically active NNE-SSW Upper Rhine Graben represents the most distinctive part of the ECRS, the seismicity is also linked to other structures which belong to the ECRS (e.g. the Lower Rhine Graben and the Hessian Grabens).

2 . 3 . H o r i z o n t a l G r a v i t y G r a d i e n t

The horizontal gravity gradient was used to emphasize high-frequency information contained in the gravity data, because its maxima indicate abrupt lateral density contrasts which occur close to faults and steeply dipping contacts. The magnitude of the horizontal gravity gradient HGM(x, y) is defined by the following formula:

2 2, ,

,g x y g x y

HGM x yx y

, (1)

where the partial derivatives of the Bouguer gravity g in the x- and y-directions can be derived either by using an approximate differential formula, or by a derivative filter in the wave-number domain.

Fig. 3 shows the grey shaded relief map of the horizontal gravity gradient magnitude. The map indicates four major regional structural trends:

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1. NW-SE (“Herzynian”) trending is represented by the Franconian Line, the Gera-Jáchymov Fault Zone, the Elbe Zone, the Bavarian Pfahl and the Danube Fault;

2. NNE-SSW (Rhenisch) trending is represented by the Upper Rhine Graben and its continuation to the north, the Rheinsberg-Heldburg Line, the Blanice Graben and several Proterozoic belts in the Teplá-Barrandian Unit;

3. ENE-WSW (Erzgebirge) trending is represented by the Litoměřice Fault in the Eger Graben, the Central Bohemian Suture Zone and Klatovy Fault;

4. E-W to ENE-WSW trending at the edge of the Northern and Eastern Alps.

In the uppermost NE part of the maps in Figs. 1 and 3 we see NW-SE trending associated with the Trans-European Suture Zone. Similar analysis of recently compiled large scale potential field data shows that many anomalies and fault zones in Central Europe turn into parallelism with this suture zone of “Hercynian” trending (Bielik et al., 2006).

2 . 4 . G r a v i t y F i e l d o f t h e S e i s m o g e n i c U p p e r C r u s t

The thickness of the continental seismogenic zone depends among others on the temperature, lithology and the regional strain rate. In continental areas, where the heat flow ranges from 60 to 100 mW/m2, the seismic activity is confined to the top 1020 km of the crust (Sibson, 2002). It is assumed that large earthquakes with M > 6 rupture the whole thickness of the seismogenic zone, whereas small earthquakes are confined within the seismogenic zone (Scholz, 1982, 1997; Pacheco et al., 1992). To estimate the thickness of the seismogenic zone, we have used the earthquake catalogue of Germany and adjacent regions for the period from 800 to 2004 (Leydecker, 2005). Analyzing the histogram of 11418 hypocentral depths, we found that the median hypocentral depth is hmedian = 7.0 km, nevertheless the data form three groups-clusters. Nearly 50% of the events are clustered around the depth of 7.3 km, whereas the other two, each accounting for 13% of events, cluster around 1.0 km and 10.0 km. Under the conservative requirement that 98% of the events have to lie within the seismogenic depth interval, we have H = 18 km for the thickness of the seismogenic zone in Central Europe.

The gravitational effect of the seismogenic upper crust can be derived from the Bouguer gravity anomaly grid by applying a suitable high-pass filter in the wave-number domain. The Butterworth high-pass filter was selected, because it is easy to control the degree of filter roll-off while the central wave-number/wavelength is fixed. The high-pass Butterworth filter is described by the following formula:

0

111 n

L rr r

, (2)

where r is the wave-number, r0 is the central wave-number of the filter, and n is the degree of the filter (Whitehead and Musselman, 2007).

Fig. 1. (Facing page) Bouguer gravity anomaly map of Germany, West Bohemia and adjacent regions. Referenced to IGSN 71, normal gravity formula GRS 80, Bouguer correction density varies between 2000 and 2700 kg/m3. Contour interval is 10 105 m/s2 (10 mGal). Red symbols show the epicentres of damage earthquakes (epicentral intensity greater than 6.5 MSK 1964) for the period from 800 to 2004 based on the Leydecker (2005) catalogue.

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Fig. 2. Simplified geological sketch of Germany, West Bohemia and adjacent regions.



To estimate the central wavelength 0, we have used the formula defining the peak in the spectrum of a prismatic body (Bhattacharya, 1966):

0 02 2 lnbot top bot topr h h h h , (3)

where htop is the depth to the top and hbot is the depth to the bottom of the prism. The central wavelength of a filter, which should pass the gravity signals of structures within the 18-km thick seismogenic layer, was set to be 110 km, which equals the peak wavelength in the spectrum of an elementary prism at the depth of 18 km. The degree of the filter was n = 8. The grid filtering was carried out in three separate steps. First the 3rd order regional term was removed from the Bouguer gravity data and the grid was expanded by 10% to dimensions acceptable by the Winograd FFT algorithm. After converting the space domain gravity grid to the Fourier domain, the Butterworth filter, described above, was applied by simple multiplication of the gravity transform by the filter on an element-by-element basis. Finally the filtered wave-number grid was transformed back to the space domain and the extended parts of the grid were masked back to the original grid size and shape. In general, the separation of the residual gravity field is a non unique problem because of spectral overlapping with the regional field component.

For displaying the full content of the amplitude and gradient information in the derived gravity data, we prepared a composite map (Fig. 4) showing the gravitational effect of the seismogenic upper crust in colour and the shaded relief of the horizontal gravity gradient magnitude. The major difference between this map and the Bouguer gravity map shown in Fig. 1 is clearly visible in the area of the Northern and Eastern Alps and the Alpine Molasse basin. The extensive negative anomaly in the Bouguer gravity map is caused primarily by the increased depth of the crust-mantle boundary beneath the Alps and secondarily by the effect of low-density sedimentary rocks. Less significant changes between the gravitational effect of the upper crust seismogenic layer (Fig. 4) and the Bouguer gravity map (Fig. 1) are evidenced by the most extensive positive anomalies, e.g., the Rhenish Massif or the East Elbe Massif in the North German Basin. The existence of those differences may indicate the contribution of lower-crust and perhaps upper-mantle sources to major gravity anomalies.

The major positive gravity anomalies in Fig. 4 are the Kreichgau-Tauber, Black Forest, Northern Vosges, Rhenish Massif (Schiefergebirge) and Bramsche gravity high in the western part of the map followed by the Teplá-Barandian Unit, Altenburg, Halle (Schafstädt), Harz, Müncheberger Gneiss Complex and ZEV (Zone Erbendorf -Vohenstrauss) gravity maxima in the eastern part of the map. The extensive positive anomalies in the NE part of Fig. 4 are the Magdeburg, Lausitz and East Elbe Massifs. The positive gravity anomaly between Salzburg and Vaduz is associated with high density rocks of the Northern Calcareous Alps.

Major gravity minima are associated with the Upper Rhine Graben and their northward continuation and with the Erzgebirge - Eger Graben, whose linear anomaly apparently continues SSW to the Black Forest. Other gravity minima are mainly associated with the light granites of the Central Bohemian Pluton, with similar bodies in the Thuringian and Franconian area and also with the Odenwald-Spessart-Rhön granite belt (Conrad et al., 1996).

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3. THE N-S ZONE ROSTOCK-LEIPZIG-REGENSBURG (RLR ZONE)

The N-S 1230 zone was originally described by Lauterbach (1952) as the Pritzwalk-Naab Line based on geomagnetic data and later discussed by Behr (1992) and Behr et al. (2002). Bankwitz et al. (2003) describe the RLR Zone, 700 km in length and 40 km in width, as a set of subparallel N-S faults composed of en echelon segments. The middle part of the RLR Zone between Leipzig and Cheb (Eger) is seismically active with the strongest events occurring near Gera, Nový Kostel and Klingenthal.

We have analyzed whether the RLR Zone is reflected in the Bouguer gravity and derived gravity fields shown in Figs. 1, 3 and 4. The result of our verification shows that the RLR Zone is not correlatable as a coherent anomaly in the analyzed gravity fields based on the 1 1 km grid spacing with approximately 0.5 mGal gravity anomaly accuracy. There are only isolated elements of N-S direction between Altenburg and Nový Kostel. This conclusion is further supported by the performed directional filtering of the gravity field. Our result might indicate that the RLR Zone in Saxony is not a deep-seated steep lineament accompanied by a significant density contrast. The analysis of higher resolution potential field data may resolve the fine structure of the RLR Zone.

4. POSSIBLE NORTHWARD CONTINUATION OF THE MARIÁNSKÉ LÁZNĚ FAULT ZONE (MLFZ)

The MLFZ is considered to have a large impact on the seismicity in the West Bohemia/Vogtland area, especially because the main epicentral area at Nový Kostel is situated at the interpreted intersection of the MLFZ with the nearly N-S striking, steeply dipping, seismically active fault plane (Počátky-Plesná Zone) confining most of the hypocentres between depths of 6 and 12 km (Bankwitz et al., 2003; Babuška et al., 2007). Recently, Pohl et al. (2006) presented a plausible northward continuation of the MLFZ based on a comprehensive interpretation of remote sensing data, topographic relief and geological data on a scale of 1:50 000. Pohl’s et al. (2006) interpretation is shown in Fig. 5 superimposed on the gravity field of the seismogenic upper crust. Similar results were published by Behr (1992). The assumed northward continuation of the MLFZ, which is composed of NW-SE and N-S elements, intersects the Gera–Jáchymov Fault near Gera, where a number of strong events including the Central German Earthquake of March 1872, I0 = 78, occurred (Leydecker, 2005). This area is known in geology as the

Fig. 3. (Facing page) Grey shaded relief map of horizontal gravity gradient magnitude of Germany, West Bohemia and adjacent regions. Geological names of regional structural elements: BG - Blanice Graben, BP - Bavarian Pfahl, CBS - Central Bohemian Suture, CO - Cottbus Fault Zone, DF - Danube Fault, FL - Franconian Line, GF - Gardelegen Fault, GJ - Gera - Jáchymov Fault Zone, HN - Harz Northern Boundary Fault, KF - Klatovy Fault, LM - Lausitz Main Fault, MLFZ - Mariánské Lázně Fault Zone, RH - Rheinsberg - Heldburg Line, OF - Allertal - Oschatz - Freital Fault Zone, PC - Prettin - Calauer Fault Zone, TTZ - Tornquist - Teisseyre Zone, URG - Upper Rhine Graben main boarder faults. Red symbols show the epicentres of damage earthquakes (epicentral intensity greater than 6.5 MSK 1964) for the period from 800 to 2004 based on the Leydecker (2005) catalogue.

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so- called “Gera Vorsprung”, a promontory of the Variscan basement in the Thuringian Basin along elements of the Gera-Jáchymov Fault Zone. The remote sensing data show here the NNW-SSE trending along the Weiße Elster valley. Fig. 5 also shows the distribution of epicentres based on the KRASNET data for the period from 1991 to 2007 (Nehybka and Tilšarová, 2007). The possible northward continuation of the MLFZ passes places with three different types of seismicity (Hemmann and Kämpf, 2002): namely, earthquake swarms, sequences of events and single events. The spatial proximity of those places makes it easier to study processes, which might be responsible for the different types of observed seismicity: particularly, interaction of mantle fluids with crustal structures, fault forming processes and relative movements of density-anomalous bodies. Seismicity is very probably affected by active fluid processes in the Nový Kostel area. En echelon arrangement of epicentres of earthquakes, already described by Nehybka and Skácelová (1997) and Nehybka and Tilšarová (2007), may be interpreted as manifestation of fault forming processes.

5. FAULT REACTIVATION POTENTIAL ANALYSIS

5 . 1 . D e t e r m i n a t i o n o f t h e R e l a t i v e R e a c t i v a t i o n P o t e n t i a l

On the basis of Cauchy’s rule:

σs n , (4)

where is the stress tensor, n is the normal vector of the plane and s is the stress vector acting to this plane, the actual value of shear stress t, acting to the given fault plane can be calculated. The fault is reactivated when the shear stress reaches the critical value defined by the Coulomb criterion:

t critical n C , (5)

where t critical is the critical shear stress, n is the normal stress to the plane, is the coefficient of friction and C is cohesion. Thus, the fault reactivation potential depends on the difference t between the actual and critical shear stresses (Fig. 6):

Fig. 4. (Facing page) Composite coloured map showing the gravitational effect of the seismogenic upper crust and in grey the shaded relief of the horizontal gravity gradient magnitude of Germany, West Bohemia and adjacent regions. Contour interval is 5 105 m/s2 (5 mGal). Gravity highs: AL - Altenburg, BF - Black Forest, BR - Bramsche, EE - East Elbe Massif, HA - Halle, HE - Eastharz, HW - Westharz, LA - Lausitz, KT - Kreichgau - Tauber, MA - Magdeburg, MG - Müncheberger Gneiss Complex, ML - Mariánské Lázně Complex, NCA - Northern Calcareous Alps, RM - Rhenish Massif, TB - Teplá - Barrandian Unit, VO - Northern Vosges, ZEV - Zone Erbendorf - Vohnenstrauß. Gravity lows: CBP - Central Bohemian Pluton, ER - Erzgebirge, Eger Rift, HT - Hohe Tauern Window, KVP - Karlovy Vary Pluton, OSR - Odenwald - Spessart - Rhön, TF - Thuringian - Franconian, URG - Upper Rhine Graben. Red symbols show the epicentres of damage earthquakes (epicentral intensity greater than 6.5 MSK 1964) for the period from 800 to 2004 based on the Leydecker (2005) catalogue.

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t t critical t actual . (6)

The reactivation potential is highest in the cases of faults with minimum value of the shear stress difference t .

Since only reduced stress tensors, and not complete stress tensors, are used in the fault potential reactivation analysis presented in this article, we cannot determine the absolute values of the actual shear stresses acting on planes. The complete stress tensor has six

Fig. 5. Gravity map of seismogenic upper crust with superimposed generalized lineations of the Vogtland-Leipzig region according to Pohl et al. (2006). Thick black dashed line shows the interpreted continuation of the Mariánské Lázně Fault Zone (MLFZ). Selected gravity anomalies are marked by the same abbreviations as in the Fig. 4 and additionally the Počátky-Plesná Zone (PPZ) is plotted as a thick yellow line. The red symbols show the epicentres of damage earthquakes (epicentral intensity greater than 6.5 MSK 1964) for the period from 800 to 2004 based on the Leydecker (2005) catalogue. Small red symbols show the epicentres of earthquakes for the period from 1991 to 2007 based on KRASNET data (Nehybka and Tilšarová, 2007).



independent parameters, whereas the reduced stress tensor (Angelier, 1990) has only four independent parameters describing the orientations of the principal axes and the so-called “shape ratio”. The shape ratio R used in this article is defined as:

2 1

3 1R

, (7)

where 1, 2, 3 are the principal stresses. The independent parameters of the reduced stress tensor are not modified either by adding an isotropic stress, or by the multiplying the tensor by a positive constant. That is why both the radius and the position of the centre of the Mohr circle can be arbitrarily chosen for one and the same reduced stress tensor (see Fig. 6). In this way, the calculated values t represent not the absolute, but only the relative differences between the actual and critical shear stresses. However, since the Coulomb criterion is represented by a straight line, the transformation of t to t due to changing the radius and/or the position of the centre of the Mohr circle can be expressed by the formula: 1 2t tt t , (8)

where t1 and t2 are parameters (any constants, t1 is positive) depending only on the shift and/or scaling of the Mohr circles. Transformation formula (8) shows that the changing of the radius and/or the position of the centre of the Mohr circle only affects the range of the possible values of the relative shear stress difference t, and not the relation between the values t calculated for the individual orientations of the planes. Thus, these relations

Fig. 6. Scheme of calculation of the difference t between critical and actual shear stress. The sketches on the right show the possible variations of the Mohr circles (shifting on the upper scheme and scaling on the lower scheme) which do not modify the reduced stress tensor (see text for details).

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Fig. 7. Scheme of construction of summarized diagrams displaying minimum and maximum relative differences t against all theoretical orientations of fault plane pole (Lambert projection, lower hemisphere). Orientations of principal axes of accepted reduced stress tensors are illustrated in the diagram (Lambert projection, lower hemisphere) situated in the left upper part of the figure.



agree with the relations between the absolute differences t. Similarly, these relations between differences t do not depend on cohesion, because this influence is eliminated by the appropriate positioning of the Mohr circle. For simplicity, the radius and position of the Mohr circle (e.g., parameters t1 and t2) were chosen so that the value of the relative shear stress difference t varies between 0 and 1.

For a given reduced stress tensor and for a given coefficient of friction, the relative differences t can be illustrated in one diagram for all plane orientations (Fig. 7). Each calculated value t is plotted in Lambert projection to the point representing the orientation of the pole of the plane, and the plotted values are contoured.

5 . 2 . S t r e s s F i e l d D a t a a n d S u m m a r i z e d D i a g r a m o f t h e R e l a t i v e R e a c t i v a t i o n P o t e n t i a l

The fault reactivation potential analysis, discussed in this article, is focused on the West Bohemia/Vogtland seismically active region. The focal mechanisms of earthquakes that occurred in this region were used by several authors for stress analyses, whose results were published by Havíř (2000), Plenefisch and Klinge (2003), Slancová and Horálek (2000), Sonnleitner (1993), Vavryčuk (2000, 2001, 2002). Most of the analyzed focal mechanisms belong to events which occurred in the Nový Kostel epicentral area displaying the highest seismic activity in the West Bohemia region. Hydro-fracturing and breakout methods were carried out in the KTB deep borehole (Brudy et al., 1997) situated in Bavaria (see Fig. 5). Ibs-von Seht et al. (2006) have reported the results of stress analyses based on the focal mechanisms of events occurred in the NE Bavaria region. In the West Bohemia/Vogtland and NE Bavaria regions, the studies indicate NW-SE to NNW-SSE orientation of 1 (mostly plunging to the SE or SSE in the West Bohemia region) and NE-SW to ENE-WSW orientation of 3. These orientations correspond with the predominant orientation of the stress field in Central Europe (Müller et al., 1992).

The range of the stress parameters used for the fault reactivation potential analysis were selected so that the accepted ranges of these stress parameters (i.e. shape ratio R and orientations of principal stress axes) correspond to all results of the stress analyses carried out in the West Bohemia/Vogtland region, which are summarized in Table 1. The accepted ranges of shape ratio R (Eq.(7)) and orientations (azimuth and plunge) of 1 and 3 are summarized in Table 2 (see also Fig. 7). The set of all reduced tensors complying with these criteria, almost 20 000 tensors covering the whole accepted range of principal axes orientations at steps of 2 and the accepted range of the shape ratio at steps of 0.1, was used to calculate the relative difference t between the actual and critical shear stresses. Thus, for each theoretical orientation of the fault plane, a set of almost 20 000 values of t (one for each used reduced stress tensor) was obtained. The summarization of these data was based on the searching for the minimum and maximum values for each set of t values belonging to each individual orientation of the fault plane, which can be expressed by the formula:

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, ,

, , 1, 1, 3,, , ,

, ,

, , 1, 1, 3,, , ,

, ,

min , , , , , , ,

, ,

max , , , , , , ,

mint i plane j plane k

t i plane j plane k m n o pm n o p

maxt i plane j plane k

t i plane j plane k m n o pm n o p

a f r R

a f r R

(9)

where is the coefficient of friction, plane is the dip direction of the fault plane, plane

is the dip of the fault plane, a1 is the azimuth of 1 axis, f1 is the plunge of 1 axis, r3 is the rake of 3 axis in plane perpendicular to 1 and R is the shape factor. The calculated minimum and maximum values t were plotted in Lambert projection against the orientation of the fault pole. Diagrams, constructed in this manner, represent the summarized results of the relative fault reactivation potential analysis.

The variable value of the coefficient of friction was taken into account. The typical values of the coefficient of friction vary from 0.2 to 0.8 (Ramsay and Huber, 1987), therefore, the summarized diagrams of the relative differences t were constructed for the same range of friction coefficients.

Table 1. Results of several stress analyses carried out in the West Bohemia/Vogtland region.

Authors Time 1 (Azimuth/Plunge)

3 (Azimuth/Plunge)

R (Shape Ratio)

Focal Mechanisms - W Bohemia Region (Nový Kostel Area)

Havíř (2000) swarm 1985/86 142/34 51/1 0.40.6

swarm 1994 139/43 46/4 swarm 1997 152/26 259/31 0.80.9

Slancová and Horálek (2000) swarm 1997 135/30 239/23 0.4 Vavryčuk (2000) swarm 1997 145/30 245/17 0.58 Vavryčuk (2001) swarm 1997 160/35 265/20 0.76 Vavryčuk (2002) swarm 1997 156/33 262/23 0.69 Plenefisch and Klinge (2003) swarm 2000 142/18 48/13 0.25

Focal Mechanisms - W Bohemia/Vogtland Region (More Areas) Plenefisch and Klinge (2003) 19912000 132/14 38/18 0.35 Sonnleitner (1993) 19851992 130/27 225/8 0.5

Table 2. The accepted ranges of shape ratio R and orientations of principal stress axes 1 and 3 used for the fault reactivation potential analysis.

Parameter Range of Accepted Values (fromto) 1 (Azimuth/Plunge) 110160/2545 and 130145/1025 3 (Azimuth/Plunge) 3555/020 and 225245/025 and 245270/1530

R 0.30.8



5 . 3 . S t u d y o f t h e R e l a t i v e R e a c t i v a t i o n P o t e n t i a l o f S e l e c t e d F a u l t Z o n e s

The orientations of selected significant fault zones, situated in the West Bohemia/Vogtland region, were correlated with the summarized diagrams of the relative differences t between the actual and critical shear stresses (Fig. 8).

The strikes of the studied zones are determined from gravity and geological maps. In the case of some zones, the strikes of the individual segments significantly differ from one another. The correlation was carried out for various dips of the studied zones. However, high values of dip are more probable than low values in the case of the studied zones. The coloured lines plotted in the Fig. 8 show the theoretically possible orientations of the fault plane poles because the values in the diagrams are plotted against the orientations of plane poles. That is why the orientations of these coloured lines are perpendicular to the strikes of these faults.

The result of the correlation shows that steep NNW-SSE to N-S segments of the MLFZ (Fig. 5) are oriented favourably for reactivation. In the Nový Kostel epicentral area, the orientation of the seismically active Počátky-Plesná Zone (strike 169, dip 80 according to Fischer and Horálek, 2003) is significantly more favourable than the orientation of the Mariánské Lázně Fault, strictly speaking, which is represented by the eastern marginal fault of the Cheb Basin in this area.

In the case of the ENE-WSW faults limiting the Eger Graben (the Litoměřice Fault, etc.), the orientation is unfavourable for reactivation for all dip values. This result corresponds with no seismic activity observed along these faults. However, there exists an exception to the rule. Ibs-von Seht et al. (2006) report reverse faulting along the NE-SW planes during the 2002/2003 swarm near Marktredwitz. Thus, some local activity along the NE-SW faults in the area of the SW continuation of the Eger Graben cannot be excluded.

The steep NW-SE segments of the Gera-Jáchymov Fault Zone seem to be oriented rather unfavourably for reactivation, especially in the case of the segment near Jáchymov. More favourable orientations are found for the NE dipping planes of the northern segment (near Gera). Skamletz and Korn (2000) have discussed the cluster of events distributed on a NE dipping plane near Zwickau. Thus, the distribution of weak earthquakes in West Saxony seems to indicate the recent activity of some structures forming this part of the Gera-Jáchymov Fault Zone. Nevertheless, the major seismicity in the West Saxony region is linked rather with the N-S than the NW-SE planes, as for instance reported by Hemmann et al. (2003).

J. Švancara et al.


6. CONCLUSIONS

Based on the first large-scale gravity data compilation of Germany, West Bohemia and adjacent regions (Behr at al., 2002), we have derived the gravity effect of the seismogenic zone confining the top 18 km of the crust. This residual gravity map shows potential stress

Fig. 8. Correlation of orientations of selected significant faults situated in the West Bohemia/Vogtland region with summarized diagrams of the relative difference t between actual and critical shear stresses (Lambert projection, lower hemisphere). The fault reactivation potential is high when the fault plane poles lie within the white areas and in reverse the resistance to reactivation is high when the fault plane poles lie within the green areas of the diagram. The coloured lines show the theoretically possible orientations of the fault plane poles (the continuous line shows the poles of steeply to moderately dipping planes, the dashed line shows the poles of moderately to gently dipping planes). The red asterisk indicates the orientation of the pole of the Počátky-Plesná Zone (PPZ) according to Fischer and Horálek (2003). The structural elements discussed in the figure: EG - ENE-WSW faults limiting the Eger Graben (Litoměřice Fault, etc.), GJ - Gera-Jáchymov Fault Zone (1 - southern segment near Jáchymov; 2 - northern segment near Gera), MLFZ - Mariánské Lázně Fault Zone and its northern prolongation (1 - southern segment along the Cheb-Domažlice Graben; 2 - eastern marginal fault of the Cheb Basin; 3 - faults in the northern prolongation of the MLFZ in Germany).



concentrating structures, which may cause seismicity provided they lie within the existing zones of weakness. The shaded relief map of the horizontal gravity gradient provides new essential information on the internal structure of sedimentary basins, the underlying basement and regional tectonic framework. The most prominent strike directions reflected in this map are NW-SE (“Hercynian”), NNE-SSW (Rhenisch) and ENE-WSW (Erzgebirge) trending. The N-S Rostock-Leipzig-Regensburg Zone is not reflected in the derived gravity maps as a coherent anomaly, which may indicate that this zone is either not a deep-seated steep lineament, or what is more probable, it is not accompanied by a significant density contrast.

To evaluate the reactivation potential of faults in the broad West Bohemia/Vogtland region, we have processed a huge set of relevant reduced stress tensors and constructed the diagrams of the relative difference t between the actual and critical shear stresses. The performed reactivation analysis of pre-existing faults shows relatively easy reactivation of steep NNW-SSE to N-S fault structures (e.g., some segments of the Mariánské Lázně Fault Zone). On the other hand, relatively higher resistance to sliding was found in the ENE-WSW planes (faults parallel with the Eger Graben). The orientation of the steep NW-SE fault planes (e.g., parallel with the Gera-Jáchymov Fault Zone) is rather unfavourable for reactivation in the recent stress field. These results correspond with the seismicity linked above all with the NNW-SSE to N-S zones in the West Bohemia/Vogtland region (the Počátky-Plesná Zone, etc.) and with no seismicity along the ENE-WSW faults limiting the Eger Graben NE of the Cheb Basin.

Acknowledgements: The authors thank D. Pohl for providing his results in the Vogtland-Leipzig

area as a shape file and V. Nehybka for providing the recent epicentre locations based on the KRASNET data. The authors are very grateful for the thoughtful reviews of the manuscript by H.J. Götze and T. Jahr.

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Derived gravity field of the seismogenic upper crust of SE Germany and West Bohemia and its...

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