Seismicity and earthquake focal mechanisms in North-Western Croatia

Abstract

The seismicity of NW Croatia, seismically the most vulnerable part of the country is presented based on historical records and reanalysed and relocated earthquakes occurring after 1908. The improved picture of the distribution of seismicity shows consistent grouping of foci in space, mostly within eight epicentral areas. The database of fault-plane solutions has been considerably enlarged, and now lists 22 earthquakes. The earthquake mechanisms consistently reveal the subhorizontal to moderately dipping P-axis, predominantly N–S directed in the central part of studied area, to NW–SE and NE–SW directed in the western and eastern parts, respectively. They indicate the prevalence of compressional tectonics with reverse faulting in the central part versus strike-slip motions in the western and eastern sectors. These data are in agreement with stress calculations and kinematics of Quaternary structures obtained by geological studies. The completeness of the catalogue, estimated by comparing the cumulative activity rate for a large number of subcatalogues to the reference, contemporary rate, exhibits large spatial heterogeneity. Taking this into account, we have computed and mapped the parameters of the frequency-magnitude recurrence relation (the b-value and the activity rate).