Earthquake Swarm Region Research Articles

On the trail of fluids in the northernmost intracontinental earthquake swarm areas of the Leipzig-Regensburg fault zone, Germany

The Leipzig-Regensburg fault zone is documented as a band of seismic activity extending northwards from the earthquake swarm region NW-Bohemia/Vogtland at the Czech-German border area and is intersected by several Hercynian fault zones. Along the fault zone, there are several earthquake swarm areas, the northernmost of which are Schöneck and Werdau. In this study, we investigate the presumably fluid-induced earthquake swarm activity of the Schöneck and Werdau area. For this purpose, we apply two methods: local earthquake tomography and receiver functions to identify the structural composition of the crust, the areas affected by fluids and the origin of the fluids. We detected potential fluid paths characterised by high Vp/Vs ratios and granite intrusions nearby the swarms characterised by low Vp/Vs anomalies. Receiver function analysis yields the Moho at 25 to 33 km depth and two seismic discontinuities at 55 km and 68 km depth.

3D imaging of the subsurface electrical resistivity structure in West Bohemia/Upper Palatinate covering mofettes and Quaternary volcanic structures by using Magnetotellurics

The region of West Bohemia and Upper Palatinate belongs to the West Bohemian Massif. The study area is situated at the junction of three different Variscan tectonic units and hosts the ENE-WSW trending Ohře Rift as well as many different fault systems. The entire region is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of mantle derived CO2 in form of mineral springs and mofettes. Ongoing active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures. All these phenomena make the Ohře Rift a unique target area for European intra-continental geo-scientific research. With magnetotelluric (MT) measurements we image the subsurface distribution of the electrical resistivity and map possible fluid pathways. Two-dimensional (2D) inversion results by Muñoz et al. (2018) reveal a conductive channel in the vicinity of the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of their model; however, their 2D inversions allow ambiguous interpretations of this feature. Therefore, we conducted a large 3D MT field experiment extending the study area towards the south. The 3D inversion result matches well with the known geology imaging different fluid/magma reservoirs at crust-mantle depth and mapping possible fluid pathways from the reservoirs to the surface feeding known mofettes and spas. A comparison of 3D and 2D inversion results suggests that the 2D inversion results are considerably characterized by 3D and off-profile structures. In this context, the new results advocate for the swarm earthquakes being located in the resistive host rock surrounding the conductive channels; a finding in line with observations e.g. at the San Andreas Fault, California.

Single Layer Recurrent Neural Network for detection of local swarm-like earthquakes—the application

SUMMARYWe present results of applying a local event detector based on artificial neural networks (ANNs) to two seismically active regions. The concept of ANNs enables us to recognize earthquake-like signals in seismograms because well-trained neural networks are characterized by the ability to generalize to unseen examples. This means that once the ANN is trained, in our case by few tens to hundreds of examples of local event seismograms, the algorithm can then recognize similar features in unknown records. The detailed description of the single-station detection, design and training of the ANN has been described in our previous paper. Here we show the practical application of our ANN to the same seismoactive region we used for its training, West Bohemia/Vogtland (border area Czechia-Saxony, local seismic network WEBNET), and to different seismogenic area, Reykjanes Peninsula (South-West Iceland, local seismic network REYKJANET). The training process requires carefully prepared data set which is preferably achieved by manual processing. Such data were available for the West Bohemia/Vogtland earthquake-swarm region, so we used them to train the ANN and test its performance. Due to the absence of completely manually processed activity for the Reykjanes Peninsula, we use the trained ANN for swarm-like activity in such a different tectonic setting. The application of a coincidence of the single-station detections helps to reduce significantly the number of undetected events as well as the number of false alarms. Setting up the minimum number of stations which are required to confirm an event detection enables us to choose the balance between minimum magnitude threshold and a number of false alarms. The ANN detection results for the Reykjanes Peninsula are compared to manual readings on the stations of the REYKJANET network, manual processing from Icelandic regional network SIL (the SIL catalogues by the Icelandic Meteorological Office) and two tested automatic location algorithms. The neural network shows persuasively better detection results in terms of completeness than the SIL catalogues and automatic location algorithms. Subsequently, we show that our ANN is capable of detecting events from various focal zones in West Bohemia/Vogtland although mainly the focal zone of Nový Kostel was used for training. The performance of our detector is comparable to an expert manual processing and we can state that no important event is missed this way even in case of complicated multiple events during the earthquake swarms.

Alteration as possible cause for transition from brittle failure to aseismic slip: the case of the NW-Bohemia / Vogtland earthquake swarm region

The geodynamic activity of the NW-Bohemia / Vogtland region with its earthquake swarm activity is always studied in relation to the local emission of juvenile fluids, in particular of CO2. Based on a 3D interpretation of the geological setting, geophysical results, and new evaluation of existing data, our approach suggests a spatial separation of the upper crust into areas with differing pore fluids. The first area comprises a mixture of juvenile and meteoric pore fluids, where juvenile CO2 diffuses towards the surface. The second area exclusively contains meteoric and no juvenile pore fluids. It is in this area where the earthquake swarm activity occurs.This separation provides an important constraint which influences shear processes as the contrasting fluid chemistry induces different interaction of wall rock with pore fluids. Long-lasting chemical degradation of the wall-rock with clay as a typical alteration product in crustal volumes with pervasive CO2 discharge reduces the friction coefficients resulting in the weakening of the fault zone and allowing an aseismic slip according to the stress field.The presented studies suggest that the junction of two regional geological features of crustal scale, the N–S trending seismically active Leipzig-Regensburg-zone and the NW-SE trending fluid emission zone of magmatic volatiles, can be interpreted as a transition from frictional slip in the recent focal zones to aseismic behavior towards the southwest caused by clay coatings on the shear planes induced by a CO2 containing pore fluid.

Geophysical evidence of the Eastern Marginal Fault of the Cheb Basin (Czech Republic)

The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Ohře/Eger Graben and the NNW-SSE trending Marianske Lazně Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of ML 3+ earthquake swarms. The Eastern Marginal Fault of the Cheb Basin (northern segment of the Marianske Lazně Fault) separates the basin sediments and underlying granites in the SW from the Krusne Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth. The survey was conducted at the Kopanina site near the Nový Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction. We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60° towards SW. At greater depths, the slope turns to subvertical with dip angle of about 80°. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys.

Regional two-dimensional magnetotelluric profile in West Bohemia/Vogtland reveals deep conductive channel into the earthquake swarm region

The West Bohemia/Vogtland region, characterized by the intersection of the Eger (Ohře) Rift and the Mariánské Lázně fault, is a geodynamically active area exhibiting repeated occurrence of earthquake swarms, massive CO2 emanations and mid Pleistocene volcanism. The Eger Rift is the only known intra-continental region in Europe where such deep seated, active lithospheric processes currently take place. We present an image of electrical resistivity obtained from two-dimensional inversion of magnetotelluric (MT) data acquired along a regional profile crossing the Eger Rift. At the near surface, the Cheb basin and the aquifer feeding the mofette fields of Bublák and Hartoušov have been imaged as part of a region of very low resistivity. The most striking resistivity feature, however, is a deep reaching conductive channel which extends from the surface into the lower crust spatially correlated with the hypocentres of the seismic events of the Nový Kostel Focal Zone. This channel has been interpreted as imaging a pathway from a possible mid-crustal fluid reservoir to the surface. The resistivity model reinforces the relation between the fluid circulation along deep-reaching faults and the generation of the earthquakes. Additionally, a further conductive channel has been revealed to the south of the profile. This other feature could be associated to fossil hydrothermal alteration related to Mýtina and/or Neualbenreuth Maar structures or alternatively could be the signature of a structure associated to the suture between the Saxo-Thuringian and Teplá-Barrandian zones, whose surface expression is located only a few kilometres away.

2014 Mainshock-Aftershock Activity Versus Earthquake Swarms in West Bohemia, Czech Republic

A singular sequence of three episodes of $$M_{L}$$ 3.5, 4.4 and 3.6 mainshock-aftershock occurred in the West Bohemia/Vogtland earthquake-swarm region during 2014. We analysed this activity using the WEBNET data and compared it with the swarms of 1997, 2000, 2008 and 2011 from the perspective of cumulative seismic moment, statistical characteristics, space-time distribution of events, and prevailing focal mechanisms. For this purpose, we improved the scaling relation between seismic moment $$M_{0}$$ and local magnitude $$M_{L}$$ by WEBNET. The total seismic moment released during 2014 episodes ( $$M_{0\mathrm{tot}}\approx 1.58\times 10^{15}$$ Nm) corresponded to a single $$M_{L}4.6+$$ event and was comparable to $$M_{0\mathrm{tot}}$$ of the swarms of 2000, 2008 and 2011. We inferred that the $$M_{L}4.8$$ earthquake is the maximum expected event in Nový Kostel (NK), the main focal zone. Despite the different character of the 2014 sequence and the earthquake swarms, the magnitude-frequency distributions (MFDs) show the b-values $$\approx$$ 1 and probability density functions (PDFs) of the interevent times indicate the similar event rate of the individual swarms and 2014 activity. Only the a-value (event-productivity) in the MFD of the 2014 sequence is significantly lower than those of the swarms. A notable finding is a significant acceleration of the seismic moment release in each subsequent activity starting from the 2000 swarm to the 2014 sequence, which may indicate an alteration from the swarm-like to the mainshocks-aftershock character of the seismicity. The three mainshocks are located on a newly activated fault segment/asperity (D in out notation) of the NK zone situated in the transition area among fault segments A, B, C, which hosted the 2000, 2008 and 2011 swarms. The segment D appears to be predisposed to an oblique-thrust faulting while strike-slip faulting is typical of segments A, B and C. In conclusion, we propose a basic segment scheme of the NK zone which should be improved gradually.

Lithium and strontium isotopic systematics of waters around Ontake volcano, Japan: Implications for deep-seated fluids and earthquake swarms

Since 1976, earthquake swarms have occurred beneath the southeast flank of Ontake volcano in central Japan. Electrical conductivity surveys have shown that these earthquake swarms are associated with the upwelling of deep-seated fluid. To investigate the nature of the deep-seated fluid, we analyzed 7Li/ 6Li and 87Sr/ 86Sr ratios of water samples collected from springs and wells around Ontake volcano in 2000, 2003, 2005, 2007, and 2009. The Li and Sr isotopic compositions of these water samples are largely explained by binary component mixing between near-surface meteoric water and non-surface fluid at each sampling site. On the basis of their Cl/Li and Cl/Sr ratios, we singled out water samples whose Li and Sr isotopic ratios were minimally affected by meteoric water contamination to represent non-surface fluids. The Li and Sr isotopic compositions of most Ontake non-surface fluids, except for samples from the earthquake swarm region, can be explained as the result of volcanic fluids reacting with basement rocks, where they acquired upper crustal signatures. We attribute the fluid associated with the region of earthquake swarms to the lower crust beneath the study area.

Infrared remote sensing of Earth degassing - Ground study

Geodynamical processes e.g., volcanoes, often cause degassing at the Earth surface. The geogas emanates via mineral springs, water mofettes, or dry mofettes. It is assumed that the emerging gas influences the temperature of the spring or mofette water, respectively and the surface temperature of the soil at and around the dry gas vents. This causes a thermal anomaly in comparison to the close vicinity. Under specific conditions this effect should be extractable from remotely acquired infrared images allowing detection, mapping and monitoring of gas vents/springs within large areas and short times. This article describes preparatory investigations for which emanating Earth gas was simulated by leading compressed air into the ground and releasing it in some depth via a metal lance. The thermal effect at the surface was observed from a nearby thermovision camera in summer and winter under varying meteorological conditions. A procedure was developed to reliably identify gas release areas within the recorded thermal images of the scene. The investigations are aiming at studies to be performed later in the Western Bohemia (Czech Republic) earthquake swarm region where especially CO2 of magmatic origin from European SubContinental Mantle (ESCM) emanates.

Imaging Crustal Heterogeneity in Jiashi Strong Earthquake Swarm Region by Coda Envelope Inversion Analysis

AbstractSynthetic envelopes are calculated on the basis of the single scattering theory, assuming spherical radiation sources and a one‐dimensional depth‐dependent velocity structure. An observation equation, which connects synthetic and observed energy densities, is obtained employing coda normalization method. Finally, we obtain the spatial distribution of scattering coefficient of Jiashi strong earthquake swarm region using a standard least squares technique iteratively. The results show that there is obvious inhomogeneous structure in middle and upper crust. A low scattering coefficient zone exists at depth of 10~25 km in the central part of research area. This low scattering coefficient zone is surrounded by large scattering coefficient zone at depth of 10 km and 15 km, but only east large scattering coefficient zone remains at depth 20 km. Seven MS ≥ 6.0 earthquakes in 1997 occurred at the center or edge of the low scattering coefficient zone.

Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis

We present the pattern of seismic activity in the period between 2001 and 2007 for the Nový Kostel focal zone, which is recently the most active zone of the West-Bohemia/Vogtland earthquake swarm region. While the year 2001 was characterized by dying out of the 2000-swarm activity in the form of a few microswarms, almost no seismicity occurred in the period between 2002 and 2003. Since 2004 an elevated seismic activity occurs in the form of repeating microearthquake swarms. We used a relative location method to relate the hypocenter positions of the post-swarm activity to the geometry of the 2000-swarm cluster. We found that the activity has concentrated in several clusters, which have been repeatedly activated. Some clusters coincide with the position of the previous activity; the others have activated so far inactive deep segments at the southern edge of the Nový Kostel fault. Besides the shift of the hypocenters to the edges of the previously active area we observe a southward migration of the activity and an increase of maximum depths of earthquakes from 10 to 13 km. The waveform similarity analysis disclosed that some fault patches consist of only a single, repeatedly activated fault plane, while the others consist of multiple, differently oriented fault planes activated almost simultaneously. Most of the focal mechanisms are consistent with the geometry of hypocenters showing NNW-SSE trending steep fault planes with left-lateral strike-slip mechanisms and varying dip-slip component.

Relation of surface movements in West Bohemia to earthquake swarms

GPS observations in the Western Bohemia/Vogtland earthquake swarm region revealed indications of horizontal displacements of low amplitude, and no clear long-term trend in 1993–2007. On the other hand, in 1998–2001 there was relatively significant active movement along NNE-SSW oriented line that we called the “Cheb-Kraslice GPS Boundary” (ChKB), identical with an important limitation of earthquake activity. The most impressive were dextral (right-lateral) movements in the 1998–1999 period followed by reverse sinistral (left-lateral) movements in 1999–2000 that correlate with prevailing motion defined by fault plane solutions of the Autumn 2000 earthquake swarm. Before the February 2004 micro-swarm, two points located on opposite sides of the Marianske Lazně fault showed extension in the order of about 7 mm in the same NNE-SSW direction of ChKB. The new NOKO permanent GPS station in Nový Kostel showed the peak-to-peak vertical changes up to 10 mm before and during the February 2007 micro-swarm.

Tomographic Determination of the Upper Crustal Structure in Jiashi Strong Earthquake Swarm Region‐Joint Inversion of Explosion and Earthquake Data

AbstractThe Jiashi strong earthquake swarm region is located at the juncture area of three tectonic units, i.e. the Tianshan fold zone, Pamir tectonic arc and Tarim block. Many strong earthquakes occurred recently in this area. In order to obtain the fine crustal velocity structures of this area, the data from a 3‐D seismic transmission experiment and a portable seismograph network acquired in the Jiashi strong earthquake swarm region in 1997 are processed. Joint inversion of explosion and earthquake data is carried out to reconstruct the 3‐D images of velocity perturbations of the upper crust under the seismic array. The temporary seismic array was used to record the seismic waves generated by 8 shots fired at different azimaths, and the additional 20 seismic stations were deployed in this area to record local earthquakes. The results show that the structure of the upper crust in the study area is characterized by obvious inhomogeneities both laterally and vertically. The low P wave velocity zone centered at Saruxi beneath the Jiashi earthquake swarm area is gradually replaced by high P wave velocity with the increase of depth. There is a high P wave velocity anomaly striking NNW at the depth of 12 km under the earthquake swarm area. It extends downwards to the depth of 16 km at least, and is surrounded by relatively low velocities. VP/VS ratios are also characterized by high values in this region. This kind of structural differences may be related to the occurrence of the Jiashi strong earthquake swarm. It can be found from the P wave velocity images at the depth of 16 km that the Jiashi strong earthquake swarm generally occurred in the regions with comparatively high P wave velocity perturbations or in the transition zones between high and low velocity perturbations. The existence of the high P wave velocity anomaly in the crust may play an important role in the seismogenic process.

Petrological and seismic studies of the lithosphere in the earthquake swarm region Vogtland/NW Bohemia, central Europe

New petrological and geochemical data of upper mantle and lower crustal xenoliths from a Quaternary tephra deposit in Mýtina, Czech Republic, are discussed in the frame of previous geophysical results (receiver functions, reflection seismology) of the western Eger/Ohře Rift area. The Vogtland/NW Bohemia region is well known for intraplate earthquake swarms, which are usually associated with volcanic activity. As previously reported, 3He/ 4He data of CO 2 emissions in mofettes and mineral-water springs point at ongoing magmatic processes in this area. Using teleseismic P receiver functions, an approximately 40-km-wide Moho updoming (from 31 to 27 km) and indications for a seismic discontinuity at 50 to 60 km depth were observed beneath the active CO 2-degassing field. The studied xenolith suite probes a lithospheric profile within the structural and gas geochemical anomaly field of the western Eger Rift. With regard to texture, composition, p– T estimates and origin, five xenolith groups can be discriminated. Upper crustal xenoliths (quartzites, phyllites, mica schists) resemble crystalline country rocks at surface. One noritic xenolith (6 kbar, 800 °C) could represent a sample of the lower crust. Clinopyroxenites and hornblendites probably represent cumulates of the nephelinitic magma or fragments of magmatic veins. Porous wehrlites and one hornblende peridotite xenolith reflect a metasomatied upper mantle. Megacrysts of Ti-rich amphibole, olivine, clinopyroxene, and phlogopite could be fragments of pegmatitic veins or high-pressure phenocrysts. Most of the ultramafic nodules (xenoliths and megacrysts) formed at pressures between 6 and 11 kbar (22 to 38 km depth), at temperatures well above regional geotherms of the Bohemian Massif calculated from surface heat flow studies. Orthopyroxene-bearing spinel-lherzolite xenoliths were not observed. Our petrographical, geochemical, and thermobarometric results indicate a lithospheric mantle strongly altered by magmatic processes. This metasomatism can cause slower than typical uppermost-mantle seismic velocities in a greater area and might help to explain observed seismic anomalies.

Fine upper crustal structure of Jiashi strong earthquake swarm region in Xinjiang inferred from high resolution seismic refraction profile data

The data obtained from a high resolution seismic refraction profile, which was carried out in Jiashi, Xinjiang, strong earthquake swarm area, were processed with both finite difference inversion and Hagedoorn refractor wavefront imaging technique and the fine upper crustal structure was determined. The results show that the upper crustal structure is relatively well-distributed in laterally and obviously by layers vertically. From surface to 11.0 km depth, there are about four layers. The P wave velocity of top two layers range from 1.65 to 4.5 km/s and their bottom boundaries, the buried depths of which are 0.4, 2.96–3.0 km respectively, are almost horizontal; The third layer is comparatively complicated and its P wave velocity presents inhomogeneous in both laterally and vertically. The bottom boundary of third layer is crystalline basement and shows a little uplift, which seemly suggest that the upper crust had been resisted while the hard Tarim block inserting into Tianshan Mountain; The forth layer is relatively even and its P wave velocity is about 6.3 km/s. There are a lateral velocity variation at the depth of about 4.0 km, and suggest that it has something to do with the hidden Meigaiti fault and Meigaiti-Xiasuhong fault but there are no the structure features about these faults stretching to the surface and passing through the crystalline basement. The seismogenic tectonic of Jiashi strong earthquake swarm at least lies in middle or lower crust beneath 11.0 km depth.

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.

Seismic structure and location of a CO2 source in the upper mantle of the western Eger (Ohře) Rift, central Europe

P‐SV conversions provide new insights into the lithosphere of the western Eger (Ohře) Rift, a presently active CO2 emanation area, Quaternary volcanic field, and earthquake swarm region in central Europe. Gas and isotope (He and C) mapping of free gas phases in mineral springs and mofettes proved the origin of CO2‐ dominated gases from a subcrustal magmatic fluid reservoir. Analyzing teleseismic data from several seismic networks in the western Bohemian Massif, the source region of these gases was investigated. Moho Ps conversions have 3 to 4.5 s delay. Crustal thicknesses vary between 27 and 38 km; vp/vs ratios vary between 1.63 and 1.81. Beneath the western Eger Rift an approximately 40 km wide Moho updoming up to 27 km exists. Locally observed weak conversions indicate a complex Moho transition zone in this area. A local “6 s phase” possibly originates at a discontinuity in approximately 50 to 60 km depth or may represent multiples from velocity inversions at the base of the upper crust. Moho updoming and the distribution of the “6 s phase” coincide with the CO2 degassing fields and the positions of Quaternary volcanoes at the surface. We hypothesize the release of CO2‐dominated fluid/magma from isolated melt reservoirs in the depth range of 60 to 30 km, separation of CO2 from the melt at 29 to 21 km depths, and CO2 transport through the crust. The geophysical indications may point to presently active magmatic underplating beneath the study area, supporting the results of gas geochemical and isotope investigations. This is the first attempt that combines seismic and gas geochemical data for a tectonic model. Our model may be transferable to other continental rift areas worldwide.

Earthquake swarm examples and a look at the generation mechanism of the Vogtland/Western Bohemia earthquake swarms

Earthquake swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate earthquake swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered earthquake swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia earthquake swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of earthquake swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia earthquake swarm region.

Non-seismological indications of recent tectonic activity in the West Bohemia earthquake swarm region

The West Bohemia seismoactive region is characterised by repeated occurrence of earthquake swarms with the strongest event of magnitude M=4.5 recorded during the 1985/1986 swarm. In the 1990s, complex observations of some geodynamic parameters have been introduced. Special attention was paid to geodetic measurements, using the techniques of precise levelling and GPS. In the most active focal zone of the region, vertical movements were distributed with an organised pattern during swarm periods but randomly during quiet periods. The GPS data have not shown any systematic trend of horizontal displacements yet. Consistent compression-like indications were obtained between 1998 and 1999 campaigns, followed by a reverse displacement in the subsequent 2000 swarm period. Temporal gravity changes were analysed from a set of data recorded twice a year since 1993. During the last three swarm periods in December 1994, January 1997 and Autumn 2000, significant temporal variations were observed in the most active focal zone of the region. On the contrary, during seismically quiet periods only weak signals of gravity changes were recorded. The variations probably reflected strain evolution in the upper crust with possible effect of fluid migration and mass displacement on gravity. The groundwater level has been recorded in 2 wells. The water level changes differ significantly in their characteristics. Water level record of the first well provides a smooth curve with a noticeable minimum coinciding exactly with the December 1994 swarm and with some changes before the January 1997 and after the Autumn 2000 swarms. The water level record of the other well reflects significant earth tide and air pressure effects. After a complex analysis, sharp residual signals indicated the first strong, macroseismically recorded, events of the Autumn 2000 swarm.

Tomographic determination of the upper crustal structure in the Jiashi strong earthquake swarm region

A three-dimensional temporary seismic transmission array was arranged in a 50×60 km2 region around Jiashi strong earthquake swarm to receive seismic waves generated by 8 fires from different azimuths. With the inversion method without model blocks and using P and S reflections from Moho at critical distances, the 3-D images of P, S velocity perturbation and ratio vp/vs perturbation of the upper crust under the seismic array were reconstructed. Meanwhile, the seismicity of the Jiashi earthquake swarm was taken into consideration in the analysis of the seismogenesis. The results indicate that the upper crustal structure under the Jiashi strong earthquake swarm region is characterized by significant inhomogeneity both laterally and vertically. From 12 km depth, it is clear that there is an NNW-oriented high P-wave velocity anomalous body corresponding to the epicenter of the swarm with low-velocity anomaly around it, which is the direct cause of the strong earthquakes. High vp/vs is distributed in the same location, which may indicate the decline of shear strength of the source region owing to relative softness of the medium, this can be accounted as an explanation for the seismicity feature of the Jiashi strong earthquake swarm.