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.