Velocity structure and the role of fluids in the West Bohemia Seismic Zone

In this study, we apply the double-difference tomography to investigate the detailed 3-D structure within and around the Novy Kostel Seismic Zone, an area in the Czech Republic known for frequent occurrences of earthquake swarms. We use data from the 2008 swarm since it has already been analysed in terms of earthquake focal mechanisms, principal faults, tectonic stress and foci migration. We selected about 500 microearthquakes recorded at 22 local seismic stations of the West Bohemia seismic monitoring network (WEBNET). Applying double-difference tomography, combined with weighted average model (WAM) post-processing to correct for parameter dependence effects, we produce and interpret 3-D models of the Vp-to-Vs ratio (Vp / Vs) in and around the focal zone. The modelled Vp / Vs ratio shows several distinct structures, namely an area of high Vp / Vs ratio correlating with the foci of the microearthquakes, and a layer of low values directly above it. These structures may reflect changes in lithology and/or fluid concentration. The overlaying low Vp / Vs ratio layer coincides with the base of the Fichtelgebirge (Smrciny) granitic intrusion. It is possible that the base of the layer acts as a fluid trap and an upper limit to the seismicity, resulting in observed periodic swarms.