Migration of Events During The January 1997 Earthquake Swarm (The West Bohemia/Vogtland Region)

A detailed analysis of the time-space pattern of seismic activity during the January 1997 swarm demonstrates the gradual and ordered migration of earthquake foci, suggesting a step-by-step penetration of crustal fluids into a remarkably small fractured volume.     

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 Novy 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 Novy 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.     

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.     

A preliminary seismic model for the region of the west-Bohemian earthquake swarms

Summary A layered model of the crustal and upper mantle structure for the region of the West-Bohemian earthquake swarms is proposed. This model represents a compilation of the results of deep seismic soundings, surface-wave investigations, and some local seismic measurements in the western regions of the Bohemian Massif.     

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.     

The Western Bohemia/Vogtland Region in the Light of the Webnet Network

The local network of digital seismic stations WEBNET monitors the seismic activitv of practically the whole region of Western Bohemia/Vogtland swarms. The network consists of ten short-period stations and one very broadband station. The paper describes the configuration of the network, instrumental equipment and the basic parameters of the stations. The method of and formula for computing the local magnitudes from the WEBNET and KRASLICE seismograms are also given. Based on continuous WEBNET observations in the period 1995-1999, we were able to improve the model of temporal and spatial energy release in the region, the principal characteristics of which are summarised in the paper. Apart from direct P and S waves, the WEBNET seismograms also contain other significant P- and S-type waves, provisionally interpreted as reflected PxP, SxS and SxP waves. The fundamental characteristics of these waves are given in the paper, and tentative mechanisms of their origination are discussed. The large residua in the travel times of the P and S waves, and the discrepancies in the seismograms recorded at stations located east of the principal focal zone are pointed out.     

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.     

S-wave splitting from records of local micro-earthquakes in West Bohemia/Vogtland: An indicator of complex crustal anisotropy

Most S-wave particle motions of local micro-earthquakes in the West Bohemia/Vogtland region display S-wave splitting. The split S waves are usually well defined, being separated in time and polarized in roughly perpendicular directions in the horizontal projection. In most cases, the polarization of the fast S wave is aligned NW-SE (referred to as “normal splitting”), which is close to the direction of the maximum horizontal compression in the region. However, for some ray directions, the polarization of the fast S wave is aligned NE-SW (referred to as “reverse splitting”). The pattern of normal/reverse splitting on a focal sphere is station-dependent, indicating the presence of inhomogeneities in anisotropy. For some stations, the normal/reverse splitting pattern is asymmetric with respect to the vertical axis, indicating the symmetry axes of anisotropy are probably inclined. The presence of inclined anisotropy is confirmed by observations of directionally dependent delay times between split S waves. A complex and station-dependent anisotropy pattern is probably the result of a complicated anisotropic crust characterized by diverse geological structures. The spatial variation of anisotropy probably reflects the presence of a variety of different types of anisotropic rocks in the region.     

The Gravity Field of the Vogtland and NW Bohemia: Presentation of a Project

The Vogtland and NW Bohemia are characterized geoscientifically by periodically occurrence of swarm earthquakes. The basic geophysical mechanism is not yet sufficiently clarified, just like detail questions to geology in especially the deeper underground. Complex geophysical investigations in the seismoactive region indicate geodynamic phenomena like mass redistribution or stress accumulation and release (Spiák et al., 1998). According to Grünthal (1989) a weakness zone is suggested in the region of the swarm earthquakes. This zone can be caused by fluid-tectonics (Kämpf et al., 1992), a mantle plume (pers. com. J. Svancara, 1999) and/or by the geometry of the geological structures (Neunhöfer & Güth, 1988). A three-dimensional gravimetric model can clear up the underground situation. By means of high-resolution gravimetry a three-dimensional model will be developped for the Vogtland and NW Bohemia region. In the first step a homogeneous Bouguer map of the Vogtland and NW Bohemia was created (fig. 1) containing gravity structures analysed by Ibrmajer & Suk (1989) and Blízkovsky et al. (1985). The used gravimetric data were made available by the Saxonian National Office for Environment and Geology, by the Czech Geological Survey, Prague and by the GGA Hannover. In the context with the interpretation of the deep-seismic profile MVE 90 a two-dimensional gravimetric modeling was carried out (Behr et al., 1994), too. Anomaly-producing source bodies apparently do not offer themselves in a two-dimensional model, because after Jung (1961) the length of a gravimetric source structure must be about four times larger than it's width. The technique of the three-dimensional gravimetric modeling by means of any polyhedrons was developed by Götze (1976, 1984). Gravimetry is a potential method and supplies an infinite number of solutions, so the model has to be developed close to other geoscientific results. The aim is to construct a high-resolution three-dimensional underground model, which includes the upper earth's crust and the deep-seated structures of the middle and lower crust, too. The determination of the mass distribution in the underground supplies contradicting or supporting facts for geodynamic views in the Vogtland and NW Bohemia for example of Bankwitz et al. (1993). The interpretation of the Bouguer map of the Vogtland and a three-dimensional gravimetric model ought to contribute a substantial, also geodynamic part to understand the origin and the emergence of the swarm earthquakes in this region.     

Scenario of the January 1997 West Bohemia Earthquake Swarm

In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland earthquake swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the swarm. Two of them, types A and B in our denotation, comprised all M _L 1.3 events and predominated in the swarm. Type A were pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For class B events, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. Type A events predominated in the southern subcluster of the swarm, whereas most of type B events occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the swarm. The swarm essentially developed in four phases: in the first, type A events prevailed and the southern plane was active; during the second, characterised by the occurrence of both type A and B events (the former in the southern, the latter predominantly in the northern subcluster), the activity of the swarm culminated; in the third and fourth, the occurrence of type B events in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of types AB , C , D , E , F and G , which were typical for M _L 1.2 events, occurred randomly throughout the swarm. Type AB events were identified in both the southern and northern clusters, type C , E , F and G mechanisms only southwards from NKC. Type D events exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 earthquake swarm and may, therefore, be typical for the region under study.     

The crust-mantle transition and the Moho beneath the Vogtland/West Bohemian region in the light of different seismic methods

The structure of the crust and the crust-mantle boundary in the Vogtland/West Bohemian region have been a target of several seismic measurements for the last 25 years, beginning with the steep-angle reflection seismic studies (DEKORP-4/KTB, MVE-90, 9HR), the refraction and wide-angle experiments (GRANU’95, CELEBRATION 2000, SUDETES 2003), and followed by passive seismic studies (receiver functions, teleseismic tomography). The steep-angle reflection studies imaged a highly reflective lower crust (4 to 6 km thick) with the Moho interpreted in a depth between 30 and 32 km and a thinner crust beneath the Eger Rift. The refraction and wide-angle reflection seismic studies (CELEBRATION 2000) revealed strong wide-angle reflections in a depth of 26–28 km interpreted as the top of the lower crust. Long coda of these reflections indicates strong reflectivity in the lower crustal layer, a phenomenon frequently observed in the Caledonian and Variscan areas. The receiver function studies detected one strong conversion from the base of the crust interpreted as the Moho discontinuity at a depth between 27 and 37 km (average at about 31 km). The discrepancies in the Moho depth determination could be partly attributed to different background of the methods and their resolution, but could not fully explain them. So that new receivers function modelling was provided. It revealed that, instead of a first-order Moho discontinuity, the observations can be explained with a lower crustal layer or a crust-mantle transition zone with a maximum thickness of 5 km. The consequent synthetic ray-tracing modelling resulted in the model with the top of the lower crust at 28 km, where highly reflective lower crustal layer can obscure the Moho reflection at a depth of 32–33 km.     

尾波干涉技术在2017年重庆武隆MS 5.0地震后震源区地壳介质变化的应用

2017年11月23日,重庆武隆发生M_S 5.0地震,震后1个月内发生107次M_L≥1.0地震,双差定位结果显示,主震震源深度约10 km,破裂面呈近SW向单侧破裂。选取震中附近布设的3个流动台地震波形及观测资料记录,利用波形互相关技术,挑选武隆M_S 5.0地震后符合条件的重复地震事件,利用尾波干涉技术,分析震源区震后地壳介质变化。结果显示：①S波早期尾波部分呈明显的线性变化,可能由震源区附近地壳介质的波速变化所引起;②由震中距最小的重复地震计算的相对波速变化最大;③在P波尾波发现走时延迟和不相关系数均存在1个短周期“尖峰”变化,可能与震后地下介质中散射体的运移有关;④震后局部地区的地震波速度存在上升—下降的恢复过程。     

地下流体对地震孕育发生过程的影响研究综述

地下流体对地震的孕育、发生过程起着重要作用.本文简述了近年来国内外关于地下流体影响地震的研究进展,对流体诱发地震的物理机制进行了深入介绍,主要有孔隙压的增加对有效应力的降低、流体受热膨胀扩容以及对岩石的润滑和软化作用;同时介绍了一些典型的流体触发地震模式,包括地震泵、断层阀、膨胀—扩散模式等,并讨论了目前存在的主要问题...     

Static stress changes as a triggering mechanism of a shallow earthquake: case study of the 1999 Chi-Chi (Taiwan) earthquake

The role of static stress changes in triggering an earthquake has long been debated in the fields of geophysics and fault mechanics. Valuable data sets for the study of static triggering were provided within the 1-year period following the devastating 1999 Chi-Chi (Taiwan) earthquake (M_W=7.6), during which more than 20,000 aftershocks occurred. In this study, stress waves generated by the Chi-Chi earthquake were calculated using a source rupture model in conjunction with a layered elastic model. Static (permanent) stress changes were extracted from the long-period offsets in the stressgrams. Correlations between the calculated stress changes and seismicity were analyzed at different depths and over varying time intervals to ascertain the impact effects of stress changes on triggering aftershocks. Correlations between prior seismicity rates and static stress changes imposed by the Chi-Chi event were low, while correlations between late seismicity rates and static stress changes were much higher. This indicates that static stress changes did affect the occurrence of the Chi-Chi aftershock sequence. The percentage of early aftershocks at shallow depths (0-10 km) in static stress-enhanced areas within 2 weeks of the main shock was high but decreased considerably at greater depths (>10 km) and over longer time periods. It is concluded that static stress changes at depths of 0-10 km played a major role in triggering crustal aftershocks, especially those that occurred within 2 weeks of the main shock. In the deeper crust, static stress changes may have been modified by viscous flow, and at later times, perturbed by earlier, larger aftershocks. Although the correlations between seismicity rate changes and static stress changes are imperfect, a region that was anti-triggered is detected when these two results are compared. Static stress changes are presumably not the only aftershock triggering mechanism, but they definitively play a major role in triggering shallow aftershocks.     

The 1708 Manosque earthquake (France): A reading of its archaeological traces as a contribution to estimate the effects on buildings

Archaeological techniques and methodology are used to identify seismic traces and disorders in ancient buildings. Clear evidence could be identified as direct seismic consequences or as communities’ technical answers for repair or reinforcement of buildings in order to reduce their vulnerability. This methodology is called “archaeological reading of buildings.” It is based on the identification of different construction phases, modifications and past events (human actions or natural phenomena) suffered by buildings during their life. The data read from the buildings are successively observed, identified, described, and recorded, following the principal of archaeological stratigraphy, in order to explain the buildings history. The seismic pathologies are identified according to detailed engineering knowledge of the behaviour of buildings during seismic motion. In this case study, our approach is applied to the historical city of Manosque, located in a seismic area along the “Moyenne–Durance” active fault. As a result of historical researches (Quenet G, Baumont D, Scotti O, Levret A, Ann Geophys, 47(2/3):583–595, 2004), many historical documents gave evidence about this earthquake’s effects. Among these documents, was an exceptional one: the record of a survey made in Manosque by bricklayers a few days after the 14th August 1708 shock. This gave us specific information concerning the seismic damage caused in the town of Manosque and was the starting point to validate the method. In the present paper, the archaeological reading of buildings method is illustrated by two specific cases: the Charité building in Manosque and the Sainte-Agathe chapel in Saint-Maime village. The buildings suffered various modifications during many centuries. This complicates the application of the method, however the observations made from the buildings correlate well with the indications deduced from written sources, validating our approach. The study highlights the necessity to cross correlate different field data in the frame of a multidisciplinary approach in order to obtain valuable results concerning details of seismic damage, its approximate dating by architectural chronology and the communities’ reaction in terms of repairs and reinforcement techniques.     

Intraplate seismicity in the western Bohemian Massif (central Europe): A possible correlation with a paleoplate junction

Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO₂ and ³He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Teplá-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of ³He- and CO₂-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.     

Possible role of fluid overpressure in the generation of earthquake swarms in active tectonic areas: The case of the Peloritani Mts. (Sicily,Italy)

The Peloritani Mts. (NE Sicily) are characterized by frequent seismicity. Between 1994 and 2006 more than 1000 earthquakes (1.0 ≤ M_L ≤ 3.3) occurred, mostly as highly clustered swarms located at shallow depth near the villages of Castroreale and Rodì Milici (western part of Peloritani Mts.). The same area is also characterized by some geothermal springs and gas vents. Using a multidisciplinary approach, data were collected on the tectonic setting, seismicity pattern and geochemical characteristics of fluid emissions, with the aim of understanding the process of earthquake swarm generation beneath the investigated area.     

The spatial temporal evolution characteristics of the load／unload response ratio（LURR） and its implications for predicting the three elements of earthquakes

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The middle-long term prediction of the February 3, 1996 Lijiang earthquake (MS=7) by the "criterion of activity in quiescence"

IntroductionIn the book Future CataS~ologr published in 1992, we proposed a viewpoiflt on using the"criterion of activity in quiescence" to predict big eathquake (MsZ7) (GUO, et al, 1992), and predicted in the book that in futore several years or in ten years a big earthquake (Ms27) will be possible to occur in the Zhongdian and nearby in Yunnan Province. In the 1994 nation-wide earthquake tendency consultation meeting we pointed out, once more, in the Zhongdian region of Yunnan Province…     

Defining a chronological framework for the Middle Stone Age in West Africa: Comparison of methods and models for OSL ages at Ounjougou (Mali)

Ounjougou is the name of a large complex of archaeological sites, that were extensively studied between 1997 and 2008 by the international team of the “Human settlement and palaeoenvironment in West Africa” project. This complex is important because well stratified Palaeolithic sites in West Africa are rare, and because it covers a long period of time (most of the Upper Pleistocene and Holocene) and exhibits a wide set of technical cultures. Therefore, for the first time it is possible to propose a chronological framework for the human settlement in this region that can be related to other technical cultures in Africa and to palaeoenvironmental data.The purpose of this paper is to present the OSL dating results obtained from 57 sediment samples that led to this framework. Measurements were first performed between 2004 and 2006 in Oxford (School of Geography), using micro-aliquots (2–10 grains) OSL and ICP-MS for determination of equivalent doses and beta-dose rates, respectively (gamma dose rates being deduced from field gamma spectrometry). More recently, new measurements were done on 28 samples of this former set at the IRAMAT-CRP2A laboratory in Bordeaux, where “true” single quartz grain OSL and high resolution gamma spectrometry measurements were performed. Both sets of results are, for all but two samples, statistically consistent with each other. A consistent chrono-stratigraphic framework can thus be deduced, covering the Upper Pleistocene. It suggests that the region was regularly visited during this time interval and more particularly during Marine Isotope Stage 3, when groups with different technical cultures followed each other relatively rapidly.