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

Abstract

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 1Danisotropic 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. K e y w o r d s : West Bohemia/Vogtland region, earthquake swarm, anisotropy, 1D velocity model, inverse problem, earthquake location Stud. Geophys. Geod., 49 (2005), 501−524 501 © 2005 StudiaGeo s.r.o., Prague J. Málek et al.

Retrieval of source parameters of an event of the 2000 West Bohemia earthquake swarm assuming an anisotropic crust

“…Existence of such volume changes follows from non-double-couple components of the retrieved moment tensors (Vavryčuk, 2002). Let us note that seismic anisotropy observed in this region (Vavryčuk, 1993;Růžek et al, 2003;Málek et al, 2005) was neither accounted for during interpretations nor during inversions for moment tensors.…”

“…Three anisotropic/isotropic models used in this study and referred to as Models I-III, see Fig. 2 and Tables 2−4, were derived by Málek et al (2005) and Vavryčuk (1993). Málek et al (2005), see Fig.…”

“…2 and Tables 2−4, were derived by Málek et al (2005) and Vavryčuk (1993). Málek et al (2005), see Fig. 2 and Table 2.…”

Bottom to top lithosphere structure and evolution of western Eger Rift (Central Europe)

“…The crustal structure of this region has been investigated using several refraction and reflection profiles (e.g. Tomek et al 1997;Málek et al 2005;Hrubcová et al 2005). The importance of multidisciplinary approach to the research of tectonic evolution of the region can be seen from the results of the KTB deep-drilling program that revealed great complexity of crustal structures and demonstrated the need to consider 3-D aspects in seismic imaging (Emmermann and Lauterjung 1997).…”

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

“…First, we have improved the 1-D velocity model by a number of Velest runs using different subsets of earthquakes and initial velocity models. The reason for this step was to check if the resulting velocity model is sensitive to observations from stations in the northern part of the region, which were not included in the existing crustal models (e.g., Málek et al, 2005). A final optimum model differs only slightly in the shallowest parts from the isotropic version of model of Málek et al (2005) which they derived from observations not including the northern stations.…”