Three-dimensional gravimetric modelling to detect the deep structure of the region Vogtland/NW-Bohemia

The occurrence of swarm earthquakes in the Vogtland/NW-Bohemia area results probably from the physical interactions of fluids, the stress field and the geometry of the geological units. Therefore the present study aims at the development of a 3-D density model of the region with a vertical range of 35 km. A new Bouguer anomaly map is presented containing about 17 000 gravity data points. Prominent Bouguer anomalies are produced by the granites of Eibenstock and Karlovy Vary (low with −75 mGal), the metabasites near Mariánzké Lázně (high with 5 mGal) and the Münchberg Gneiss Massif (gravity high of Hof with 10 mGal). The geometry of the internal model structures correspond to geological units and, thus, the modelled gravity fits well the observed Bouguer anomaly. The 3-D gravimetric modellings indicate detailed geometries of the geological settings. With regard to the periodic occurrence of swarm earthquakes in the Vogtland region the existence of an upwelling mantle or a magmatic body is investigated. Precise information only can be given, if the vertical extension of the near surface bodies is known.     

Geodynamic modelling of the recent stress and strain field in the Vogtland swarm earthquake area using the finite-element-method

The study of several swarm earthquake regions led us to the assumption that swarm quakes mostly occur in combination with fluid migration and often in the neighbourhood of volcanoes. Observations of the strong carbon dioxide flux and investigations of Weinlich et al. (1999) about recent volcanism in the Vogtland/ Western Bohemia are the linking part to global observations of swarm earthquakes. This idea and the theoretical modelling of Yamashita (1999) were the starting point to develop a model using the finite-element-method with the software package ABAQUS concerning stress field, pore pressure, temperature and deformation in the Vogtland swarm quake area. The model is intended to reveal the interaction between fluid migration, temperature changes and deformation in this area. Its horizontal dimensions are 55 km×60 km and 30 km in depth. The main geological units include the Marianske Lazne Fault Zone and the Eger Rift. We focus on that part of the Vogtland/Western Bohemia region where these two fault zones cross. The model takes into consideration the stress field, Mohr–Coulomb failure, thermal stresses as well as creep and poro-elastoplastic rheologies. The results show that the rate of deformation and the stress accumulation caused only by the regional stress field is not high enough to generate the swarm earthquakes. But periodic change of pore pressure (which includes fluid migration) in combination with temperature changes has a severe influence on the rate of deformation and is in the dimension to cause swarm quakes.     

One-Dimensional qP-Wave Velocity Model of the Upper Crust for the West Bohemia/Vogtland Earthquake Swarm Region

The western part of the Bohemian Massif (West Bohemia/Vogtland region) is characteristic in the relatively frequent recurrence of intraplate earthquake swarms and in other manifestations of past-to-recent geodynamic activity. In this study we derived 1D anisotropic qP-wave model of the upper crust in the seismogenic West Bohemia/Vogtland region by means of joint inversion of two independent data sets - travel times from controlled shots and arrival times from local earthquakes extracted from the WEBNET seismograms. We derived also simple 1-D P-wave and S-wave isotropic models. Reasons for deriving these models were: (a) only simplified crustal velocity models, homogeneous half-space or 1D isotropic layered models of this region, have been derived up to now and (b) a significant effective anisotropy of the upper crust in the region which was indicated recently by S-wave splitting. Both our anisotropic qP-wave and isotropic P-and S-wave velocity models are constrained by four layers with the constant velocity gradient. Weak anisotropy for P-waves is assumed. The isotropic model is represented by 9 parameters and the anisotropic one is represented by 24 parameters. A new robust and effective optimization algorithm - isometric algorithm - was used for the joint inversion. A two-step inversion algorithm was used. During the first step the isotropic P- and S-wave velocity model was derived. In the second step, it was used as a background model and the parameters of anisotropy were sought. Our 1D models are adequate for the upper crust in the West Bohemia/Vogtland swarm region up to a depth of 15 km. The qP-wave velocity model shows 5% anisotropy, the minimum velocity in the horizontal direction corresponds to an azimuth of 170°. The isotropic model indicates the V_P/V_S ratio variation with depth. The difference between the hypocentre locations based on the derived isotropic and anisotropic models was found to be several hundreds of meters.     

Seismohydrological effects related to the NW Bohemia earthquake swarms of 2000 and 2008: Similarities and distinctions

For more than 20 years, seismohydrological investigations have been undertaken at the mineral aquifer system of Bad Brambach (Vogtland, Germany). Two strong swarm earthquake series in 2000–2001 and 2008–2009 at the Nový Kostel epicentre (Czech Republic, 10 km E of BB) have enabled for the first time a comparison of seismological and groundwater hydraulic features in a semi-quantitative way. In spite of their similar spatial distribution in 2001 and 2008, the earthquake foci of each swarm migrated differently through time, horizontally as well as in depth. The seismic energy of the 2008–2009 events was released predominantly within 1 month, in contrast to 2000–2001 when it occurred over 3 months. The main distinctive features of each are seen in the hydraulic pressure anomalies which accompanied the earthquake swarms: number, shape, and progression (duration) of the anomalies. The comprehensive hydraulic data, with high temporal resolution, suggest that fluid triggering dominated not only the earthquake initiating phases. In particular, the long-lasting seismicity of the 2008–2009 swarm can be attributed to a continued triggering of weak earthquakes by over-pressured deep fluids. Here, the remaining static strain was obviously not sufficient to generate strong earthquakes as at the beginning of the earthquake swarm periods. Furthermore, the enduring high fluid pressure in 2009 could also indicate a continuation of the long-term gas flow increase observed at several gas outlets in the Vogtland/NW Bohemia region between 1998 and 2008. However, it is not possible at present to derive a systematic relationship between anomaly occurrence and seismic activity, as generally proposed in the context of earthquake prediction discussion.     

Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)

We summarise the results of seismological studies related to triggering mechanisms, driving forces and source processes of the West Bohemia/Vogtland earthquake swarms with the aim to disclose the role of crustal fluids in the preparation, triggering and governing of the swarms. We present basic characteristics distinguishing earthquake swarms from tectonic mainshock-aftershock sequences and introduce existing earthquakes swarm models. From the statistical characteristics and time-space distribution of the foci we infer that self-organization is a peculiarity of West Bohemia/Vogtland swarms. We discuss possible causes of the foci migration in these swarms from the viewpoint of co-seismic and/or post-seismic stress changes and diffusion of the pressurized fluids, and we summarize hitherto published models of triggering the 2000-swarm. Attention is paid to the source mechanisms, particularly to their non-shear components. We consider possible causes of different source mechanisms of the 1997-and 2000-swarms and infer that pure shear processes controlled solely by the regional tectonic stress prevail in them, and that additional tensile forces may appear only at unfavourably oriented faults. On data from the fluid injection experiment at the HDR site Soultz (Alsace), we also show that earthquakes triggered by fluids can represent purely shear processes. Thus we conclude that increased pore pressure of crustal fluids in the region plays a key role in bringing the faults from the subcritical to critical state. The swarm activities are mainly driven by stress changes due to co-seismic and post-seismic slips, which considerably depend on the frictional conditions at the fault; crustal fluids keep the fault in a critical state. An open question still remains the cause of the repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones in a wider area of West Bohemia/Vogtland. The analysis of the space-time relations of seismicity in the area between 1991 and 2007 revealed that during a significant part of this time span the seismicity was switching among distant focal zones. This indicates a common triggering force which might be the effect of an increase of crustal-fluid pore-pressure affecting a wider epicentral region.     

The 2008 West Bohemia earthquake swarm in the light of the WEBNET network

A swarm of earthquakes of magnitudes up to M _L = 3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Novy Kostel focal zone, where also all recent earthquake swarms (1985/1986, 1997, and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET (West Bohemia seismic network), which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicenters. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s². A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/1986 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms that occurred in the West Bohemia/ Vogtland region since the M _L = 5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (4 weeks) and rapidity and, consequently, the fastest seismic moment release compared to previous swarms. Up to 25,000 events in the magnitude range of 0.5 < M _L < 3.8 were detected using an automatic picker. A total of nine swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude–frequency distribution of the complete 2008 swarm activity shows a b value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Novy Kostel focal zone that was activated by the 2000 swarm (M _L ≤ 3.2) in a depth interval from 6 to 11 km and also by the 1985/1986 swarm (M _L ≤ 4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Furthermore, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously, it would represent an earthquake of M _L ~5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past.     

Focal Mechanisms and Stress Field in The Region Vogtland/Western Bohemia

We calculated focal mechanisms for 30 of the strongest events (1.5 M _L 3.3) in distinct subregions of Vogtland/Western Bohemia between 1990 and 1998. Our investigations are concerned with events of the swarms near Bad Elster (1991), Haingrün (1991), Nový Kostel (1994 and 1997) and Zwickau (1998), two events from a group of earthquakes near Klingenthal (1997) and eight single events. Seismograms were provided by the digital station networks of the Geophysical Observatory of the University of Munich, the Technical University of Freiberg, the Academy of Sciences of the Czech Republic in Prague, the Masaryk University in Brno and some nearby stations of the German Regional Seismic Network (GRSN). To calculate focal mechanisms two inversion methods were applied. The inferred focal mechanisms do not show a simple, uniform pattern of seismic dislocation. All possible dislocation types – strike-slip, normal and thrust faulting - are represented. The prevailing mechanisms are normal and strike-slip faulting. Considerable differences in the fault plane solutions are noted for the individual subregions as well as in some cases among the individual events of a single swarm. For the Nový Kostel area we succeeded to resolve a change in the orientations of the nodal planes for the two successive swarms of December 1994 and January 1997. Besides this we also observe a change in the mechanisms, namely from strike-slip and normal faulting (December 1994) to strike-slip and thrust faulting (January 1997). Based on the inferred focal mechanisms the stress field was estimated. It turned out, that the dominating stress field in the region Vogtland/Western Bohemia does not substantially differ from the known stress field of West and Central Europe, being characterized by a SE-NW direction of the maximum compressive horizontal stress. We conclude that the seismicity in the Vogtland/Western Bohemia region is not predominantly caused by an independent local stress field, but rather controlled by the dominating stress regime in Central Europe.     

The August–December 2000 earthquake swarm in NW Bohemia: the first results based on automatic processing of seismograms

Main features of the August–December 2000 earthquake swarm which occurred in the major focal area of the North-West Bohemia / Vogtland swarm region are presented. Seismograms from four stations of WEBNET were automatically processed to get arrival times, first motion amplitudes and hypocentre coordinates of a representative set of events. Altogether 7017 microearthquakes in the magnitude range of M_L=0–3.3 were identified. It is shown the decay of activity of individual swarm phases followed the modified Omori law, which points to a partial similarity with aftershock sequences of tectonic earthquakes. The space-time distribution of a subset of 2913 events with low location residuals shows a strong space clustering of the earthquake hypocentres and their pronounced migration between individual swarm phases. Most of the activity took place along an elliptical, nearly vertically dipping, 6 km long N-S oriented fault plane in depths ranging from 10.5 to 6.5 km. The P and T axes were estimated by FOCMEC for the 782 strong events and three groups of earthquakes with similar faulting type were distinguished. In contrast to the normal and strike-slip faulting events that created the prevailing portion of the swarm and were distributed uniformly within the focal area, the reverse events were clustered in time and space.     

The Catalogue of the Vogtland/Western Bohemia Earthquakes and Vocatus as an Auxiliary Code

The catalogue of earthquakes from the Vogtland/Western Bohemia region joins data which have been observed with local seismic stations since 1962. Up to now, data on more than 17.000 earthquakes were compiled to the catalogue. A special code VOCATUS supports its use and explains the collected data. Both, catalogue and code VOCATUS are described. From the catalogue, there follows a recurrence of the earthquake occurrence which is discussed in detail.     

Seismicity in the Vogtland/Western Bohemia Earthquake Region Between 1962 and 1998

The Vogtland/Western Bohemia region is part of the Saxothuringian Earthquake Province. It is an isolated area of active intraplate seismicity. Observations of the seismicity between 1962 and 1998 are summarized. More than 17000 earthquakes have been detected microseismically with M _L reaching from about –1.5 to 4.6. In the considered time interval, the catalogue of Vogtland events can be regarded as complete for magnitudes larger than 1.8. The region is well known for the occurrence of earthquakes clustered contemporarily in time and space. In this study, altogether 82 clusters are defined. Among them, clusters with swarm properties are distinguished from clusters with main shock accompanied by fore- and aftershocks, and from single events. 48 swarms are detected.The magnitude-frequency distribution of the maximum magnitudes of the clusters is studied. In the magnitude range 1.8  M _L  3.1, a bimodal character of the magnitude-frequency distribution is detected for both swarms and nonswarm-like events. The slope is greater for larger magnitudes than in the small-magnitude range. A gap in the magnitude-frequency distribution of clusters is observed for maximum magnitudes between 3.1 and 4.3. Furthermore, clusters themselves are characterized by the b-values of their magnitude-frequency distributions. Swarms show b-values greater than 0.7. Epicenters of swarms are confined to a few subregions. Epicenters of nonswarm-like events are distributed over a larger region than epicenters of swarms but hypocenters of swarms and nonswarm-like clusters may be located close to each other.The envelope of the distribution of magnitudes as a function of time is investigated. In the considered time interval, a statistically significant recurrence of strong events of about 72 months is discovered by a frequency analysis. Comparing the seismicity between 1897 and 1908 with the seismicity between 1962 and 1998 temporal variations in the recurrence become obvious. The Nový Kostel zone is discussed in more detail. The average hypocenters of swarms are located on a SW-dipping fault segment that intersects the Eger Rift in NNW-SSE direction.Discussing properties of the seismicity in the Vogtland/Western Bohemia region it is concluded that the increased seismicity may be explained by the presence of fluids on deep reaching faults. The occurrence of swarms, their variability as well as the small distances between hypocenters of swarms and nonswarm-like events point to strong lateral and possibly temporal changes of the properties of the fault system.     

First Results of Hydrogeochemical Measurements in the Soos Area,Czech Republic

For some years, the Saxon Academy of Sciences at Leipzig (SAW) is engaged in the research project Contributions to Environmental Research by means of Radiometric-Geochemical Methods in the Vogtland area (Germany). The main goal is to investigate the hydrogeological and hydrochemical parameters of mineral springs in that region and their interrelations. Here, the high CO ₂ degassing rates are of particular interest. The isotopic signature of these gases indicates an anomalous high mantle/crust contribution (Weinlich et al., 1999). A further geoscientifically important phenomenon of this region is the high seismicity, in particular the Vogtland/NW Bohemian earthquake swarms. Therefore, we have tried to use the mantle-originated fluids as transport media for information from the seismogenic horizons. The variations of the parameters recorded continuously at the springs for years show a connection to the seismic events of the epicentral area of Nový Kostel in the Czech Republic. In particular, numerous anomalous degassing intervals were recorded at the mineral spring Wettinquelle (formerly: Radonquelle), Bad Brambach, prior to earthquakes occurred in the region of Nový Kostel.     

Similarity of Waveforms at Stations Moxa and Plauen for the 1985/86 Swarm

A peculiarity of the Vogtland/West Bohemia earthquake region is the appearance of earthquake swarms. The strongest swarms in this century occurred in 1908 and 1985/86. The swarm in 1985/86 is the first one, for which digital data (about 220 events, 400 waveforms, sampling rate 100 Hz) are available for stations Moxa (MOX) and Plauen (PLN). The waveforms of the swarms in the Vogtland show a high similarity. The similarity is quantified by cross-correlation applied to approximately 400 waveforms. The results are shown in the form of similarity matrices. Four types of events have been detected using the results of the cross-correlation between the S-waveforms recorded at station PLN. Type IV occurs predominantly at the end of the swarm. The waveforms of type IV are clearly different from the waveforms of the other types. The differences between the waveforms of the first three types are less pronounced. Especially the first three types occur alternately.     

Mechanical intrusion models and their implications for the possibility of magma-driven swarms in NW Bohemia Region

Earthquake swarms are often assumed to be caused by magmatic or fluid intrusions, where the stress changes in the vicinity of the intrusion control the position, strength and rate of seismicity. Fracture mechanical models of natural intrusions or man-made hydrofractures pose constraints on orientation, magnitude, shape and growing rate of fractures and can be used to estimate stress changes in the vicinity of the intrusions. Although the idea of intrusion-induced seismicity is widely accepted, specific comparisons of seismicity patterns with fracture models of stress changes are rarely done. The goal of the study is to review patterns of intrusion-induced earthquake swarms in comparison to the observations of the swarm in NW Bohemia in 2000. We analyse and discuss the theoretical 3D shape of intrusions under mixed mode loading and apparent buoyancy. The aspect ratio and form of the intrusion is used to constrain parameters of the fluid, the surrounding rock and stress. We conclude that the 2000 NW Bohemia swarm could have been driven by a magmatic intrusion. The intrusion was, however, inclined to the maximal principal stress and caused shear displacement additional to opening. We estimate that the density diference between magma and rock was small. The feeding reservoir was possibly much larger than the area affected from earthquakes and may be a vertical dike beneath the swarm region.     

Magma Intrusions and Earthquake Swarm Occurrence in the Western Part of the Bohemian Massif

We are proposing a hypothesis that earthquake swarms in the West Bohemia/Vogtland seismoactive region are generated by magmatic activity currently transported to the upper crustal layers. We assume that the injection of magma and/or related fluids and gases causes hydraulic fracturing which is manifested as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the information from the neighbouring KTB deep drilling project and from the 9HR seismic reflection profile, and the detailed analysis of local seismological data. (1) Recent manifestations of geodynamic activity include Quaternary volcanism, rich CO ₂ emissions, anomalies of mantle-derived ³ He, mineral springs, moffets, etc. (2) The fluid injection experiment in the neighbouring KTB deep borehole at a depth of 9 km induced hundreds of micro-earthquakes. This indicates that the Earth's crust is near frictional failure in the western part of the Bohemian Massif and an addition of a small amount of energy to the tectonic stress is enough to induce an earthquake. Some pronounced reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by recent magmatic sills in the crust. (3) The local broadband seismological network WEBNET provides high quality data that enable precise localization of seismic events. The events of the January 1997 earthquake swarm are confined to an extremely narrow volume at depths of about 9 km. Their seismograms display pronounced reflections of P- and S-waves in the upper crust. The analysis of the process of faulting has disclosed a considerable variability of the source mechanism during the swarm. We conclude that the mechanism of intraplate earthquake swarms generated by magma intrusions is similar to that of induced seismicity. As the recent tectonic processes and manifestations of geodynamic activity are similar in European areas with repeated earthquake swarm occurrence (Bohemian Massif, French Massif Central, Rhine Graben), we assume that magma intrusions and related fluid and gas release at depths of about 10 km are the universal cause of intraplate earthquake swarm generation     

Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period 1985–2001

The NW Bohemia/Vogtland region situated at the western part of the Bohemian Massif is characteristic in a frequent reoccurrence of earthquake and micro-earthquake swarms. We present a comprehensive, integrated pattern of the space and time distribution of seismic energy release in the principal NK (Nový Kostel) focal zone for the period 1991–2001 and for the intensive 1985/1986 swarm. More than 3000 earthquakes, recorded by the WEBNET, the KRASLICE net and by temporary stations VAC, TIS and OLV operating during the 1985/1986 swarm, were located or re-located using the master event technique. Swarm-like sequences were identified and discriminated from solitary events by detecting local minima of the inter-event time using a standard short-time/long-time average (STA/LTA) detection algorithm. Most of the seismic energy in the NK zone was released during the two intensive 1985/1986 and 2000 swarms and in the course of the weaker January 1997 swarm. Further 27 swarm-like sequences (micro-swarms) and many solitary micro-earthquakes (background activity) were identified in the NK zone for the period 1991–2001 by the inter-event time analysis. Relative location revealed a pronounced planar character of the NK focal zone. Most of the events, including those of the intensive 1985/1986 and 2000 swarms, were located at the main focal plane (MFP) striking 169° N and dipping 80° westward at depths between 6 and 11 km. A singularity was the January 1997 swarm together with a micro-swarm that were both located across the MFP. The position and geometry of the MFP match quite well the Nový Kostel-Počátky-Zwota tectonic line. The space distribution patterns of larger events and of micro-swarms at the MFP differ: larger events predominantly grouped in planar clusters while the micro-swarms lined up along two parallel seismogenic lines. The temporal behaviour was examined from two aspects: (a) migration and (b) recurrence of the seismic activity. It was found that (a) the seismic activity in the time span 1991–2001 migrated in an area of about 12×4 km and (b) several segments of the MFP were liable to reactivation. The activity before, during and after the 2000 swarm took place in different parts of the MFP.     

Temporal variations of focal mechanisms in the Novy Kostel focal zone (Vogtland/NW-Bohemia)—comparison of the swarms of 1994, 1997 and 2000

The seismicity in the Vogtland/NW-Bohemia region is mainly characterized by the occurrence of earthquake swarms. A key to a better understanding of the reasons of earthquake swarms can be provided by focal mechanism investigations. Here we present focal mechanisms for 12 of the strongest events (M_L⩾3.0) for the new swarm of 2000. With more than 10,000 events and magnitudes up to 3.7 the new swarm is the most prominent one since the big swarm in 1985/1986. The focal mechanisms of the swarm 2000 show different styles of faulting, namely strike-slip, normal and reverse faulting. There are indications for systematic temporal variations in the dislocation type. A comparison with the mechanisms of the preceding swarms of 1985/1986, 1994 and 1997 which all took place at the same location shows similarities in the faulting types and orientations of the nodal planes for the swarms of 1985/1986, 1994 and 2000. However, the focal mechanisms of 1997 do not fit into the scheme of the others. The focal mechanisms have also been used to determine the regional stress field. It turned out that the stress field in the Vogtland/NW-Bohemia region does not substantially differ from the known stress field in West and Central Europe. It is a strike slip regime with a SE–NW directed σ₁-axis and a NE–SW directed σ₃-axis.     

Variations in discharge and temperature of mineral springs at the Františkovy Lázně spa,Czech Republic,during a nearby earthquake swarm in 1985/1986

The western part of the Bohemian Massif (West-Bohemia/Vogtland region at the Czech-German border) is characterized by relatively frequent intraplate earthquake swarms and by other manifestations of present-day geodynamic activity. During the strong earthquake swarm at the turn of the years 1985 and 1986, significant changes in mineral spring parameters were observed at the spa of Františkovy Lázně. In this study, we present all available data on the mineral springs parameters, and we discuss them in terms of relations to seismic activity. Some changes in discharge were very distinct, amounting up to 40%, and had a co-seismic character. The changes in temperature were less noticeable, but preceded the beginning of the swarm by several months. Some hydrological changes persisted for nearly two years after the earthquake swarm. The character of the observed changes seems to support the hypothesis on an injection of mantle fluids, in particular of CO₂, as the main triggering mechanism of the earthquake swarm and the main cause of discharge anomalies.     

Self-organization of earthquake swarms

The temporal clustering of swarm activity differs significantly from characteristics of aftershock sequences accompanying mainshocks. This is often assumed to be caused by crustal structure complexities and fluid migration. However, the underlying mechanism is not yet fully understood, especially, the processes and conditions which lead to the apparent differences between the swarm patterns and typical mainshock–aftershock sequences. In previous works, we have shown that the most conspicuous characteristics of tectonic earthquakes can be reproduced by stick-slip block models incorporating visco-elastic interactions. Now, the same model is shown to reproduce an almost periodical occurrence of earthquake swarms in the case of an enlarged postseismic response. The simulated swarms respect not only the Gutenberg-Richter law for the event sizes, they also reproduce several observations regarding their spatio-temporal patterns. In particular, the comparison with the January 1997 and the year 2000 swarm in Vogtland/NW-Bohemia shows a good agreement in the interevent-time distributions and the spatio-temporal spreading of the swarm activity. The simulated seismicity patterns result from self-organization within the swarm due to local stress transfers and viscous coupling. Consequently, the agreement with the Vogtland swarm activity do not allow any decision about the preparatory process of the swarms; in particular, the question whether the swarms are initially triggered by fluid intrusion or tectonic motion cannot be answered. However, the model investigations suggest that the process of self-organization is very important for understanding the activity patterns of earthquake swarms.     

Seismicity and seismotectonics of West Saxony, Germany — new insights from recent seismicity observed with the Saxonian seismic network

A new network of permanently recording seismic stations in West Saxony has considerably improved detection threshold, location accuracy and depth determination in this seismically active region. Between 2001 and 2007 more than 900 events have been located. Seismicity mainly occurred along a band stretching north-south between Leipzig and Vogtland/NW Bohemia area with local magnitudes ranging between −0.8 and 2.8. Seismicity clearly delineates the Leipzig-Regensburg (L-R) fault zone striking N-S, and the Gera-Jachymov (G-J) fault zone striking roughly NNW-SSE. The hypocentral depths can be divided into two depth ranges, one at depths below 10 km, and a second at less than 10 km depth that only extends S-N from the Vogtland until the crossing between L-R and G-J fault zones. A small earthquake sequence that occurred near Werdau/Zwickau in August 2006 at almost the same epicenters as an earlier sequence 1997/98 seems to confirm this finding: a relative localization of 15 events with the double-difference technique clearly reveals two distinct subclusters at about 6 and 12–14 km depth. With the improved station coverage 33 new fault plane solutions from events along the L-R fault zone north of the swarmquake area could be determined from P-polarities and P/S ratios. They do not differ significantly from solutions in the Vogtland/NW-Bohemia area and are mostly compatible with a N-S oriented fault plane. Strike slip mechanisms with or without a dip slip component dominate.     

Seismic regime of the west Bohemian earthquake swarm region: Preliminary results

Summary The western part of the Bohemian Massif located between two tectonic units, the Moldanubian and the Saxo-Thuringian, is characterized by the re-occurrence of earthquake swarms. The focal region for these swarms includes the territory of West Bohemia and the adjacent territory of SE Saxony and NE Bavaria. During the most recent swarm in December 1985 – January 1986, more than 8000 small earthquakes were recorded; the two largest earthquakes with local magnitudes (M_L) of 4·6 and 4·1. This paper presents a summary of the seismic energy release in space and time for the western part of the Bohemian Massif, based on seismic observations of permanent seismic stations established in West Bohemia since 1986. It was found out that microearthquake activity, mostly of a swarm-like character, persisted between two macroseismically observed swarms. The foci of the microearthquakes predominantly cluster in six main epicentral zones, four of which are located in West Bohemia or in its immediate vicinity in Saxony. The remaining two are in Saxony and in Bavaria. The four epicentral zones in West Bohemia were studied in detail. It was found that the individual zones differ in size, in depth of hypocentres, in geometry, as well as in temporal activity. Moreover, it was found that the seismicity in the most active epicentral zone is closely related to the system of principal tectonic faults referred to as the Kruné Hory fault and the Mariánské Lázn fault.