[Korn] Spatial and temporal seismic imaging of fluid migration through the crust in the W-Bohemia/Vogtland earthquake swarm area
German Title: Spatial and temporal seismic imaging of fluid migration through the crust in the W-Bohemia/Vogtland earthquake swarm area
Current Status: completed
Main Applicant:Prof. Dr. Michael Korn
Dr. Ulrich Wegler
Dr. Mohammad Javad Fallahi
Dr. Klaus Bauer
The project aimed at delineating spatial and temporal crustal structures within the NW-Bohemia/Vogtland swarm quake area in an effort to improve the knowledge of fluid-bearing structures, fluid-rock interactions and fluid pathways in the upper crust as a contribution to the preparation of an ICDP drilling program in that region. A variety of seismological methods has been applied to existing data from permanent and temporary seismic networks to obtain tomographic images of P- and S-velocity structure as well as attenuations structure and to test the hypothesis of velocity changes with time due to hidden magmatic activity and uprising fluids. Tomographic images were obtained by body wave travel time tomography for P-velocity and P/S velocity ratio, and by ambient noise surface wave tomography for S-velocity. Several previously unknown features were detected, e.g. an intrusive body close to and below the swarm quake hypocenters , a reduced S-velocity in a cone-shaped volume above the hypocenters, and indication of a fluid channel with low Vp/Vs ratio connecting the hypocentral area with the gas escapement centers in the Eger basin. Attenuation measurements have been undertaken by inversion of coda envelopes from local earthquakes into scattering and intrinsic attenuation parameters with the help of radiative transfer theory, and by fitting empirical attenuation laws to P-wave spectra. Both methods indicate an increase of attenuation in the vicinity of the epicentral area, which is mainly caused by intrinsic absorption. Also this result is in line with the assumption that fluids play a major role in the genesis of the earthquake swarms, and that fluid migration and increased fluid content and/or fluid pressure affects attenuation of seismic energy. As a third goal the hypothesis that temporal changes in fluid pressure and crack opening and closing lead to a change in seismic velocities with time within the hypocentral volume was tested by two approaches, Coda Wave Interferometry and Passive Image Interferometry. Both independent methods did not detect any temporal change that would be related to the occurrence of earthquake swarms. Therefore we conclude that either there is no temporal change, or that it happens only in a small volume that cannot be sensed from the surface.