Non-seismological indications of recent tectonic activity in the West Bohemia earthquake swarm region

The West Bohemia seismoactive region is characterised by repeated occurrence of earthquake swarms with the strongest event of magnitude M=4.5 recorded during the 1985/1986 swarm. In the 1990s, complex observations of some geodynamic parameters have been introduced. Special attention was paid to geodetic measurements, using the techniques of precise levelling and GPS. In the most active focal zone of the region, vertical movements were distributed with an organised pattern during swarm periods but randomly during quiet periods. The GPS data have not shown any systematic trend of horizontal displacements yet. Consistent compression-like indications were obtained between 1998 and 1999 campaigns, followed by a reverse displacement in the subsequent 2000 swarm period. Temporal gravity changes were analysed from a set of data recorded twice a year since 1993. During the last three swarm periods in December 1994, January 1997 and Autumn 2000, significant temporal variations were observed in the most active focal zone of the region. On the contrary, during seismically quiet periods only weak signals of gravity changes were recorded. The variations probably reflected strain evolution in the upper crust with possible effect of fluid migration and mass displacement on gravity. The groundwater level has been recorded in 2 wells. The water level changes differ significantly in their characteristics. Water level record of the first well provides a smooth curve with a noticeable minimum coinciding exactly with the December 1994 swarm and with some changes before the January 1997 and after the Autumn 2000 swarms. The water level record of the other well reflects significant earth tide and air pressure effects. After a complex analysis, sharp residual signals indicated the first strong, macroseismically recorded, events of the Autumn 2000 swarm.