The 1998 and 2004 Bovec-Krn (Slovenia) seismic sequences: aftershock pattern, focal mechanisms and static stress changes

Abstract

SUMMARY We investigate the main seismotectonic aspects of the Bovec-Krn sequences (northwestern Slovenia), caused by the 1998 April 12 MD 5.6 and the 12 July 2004 MD 5.1 earthquakes. The seismicity occurred along an extended right-lateral strike-slip fault zone, trending NW–SE. The analysis involves the aftershock pattern, the focal mechanism of the largest events and the stress changes induced by the 1998 main shock. The study is performed with high-quality earthquake locations obtained with tomographic techniques that provided also velocity images of the fault zone, related to the mechanical properties of the medium and to the geometry of the fault. The tomographic model shows in the fault zone sharp lateral P-wave velocity contrasts and marked lateral Vp/Vs variations and images an extended high rigidity body along the fault, where the 1998 and 2004 main shocks are located. The aftershock activity is intense along highly heterogeneous zones characterized by sharp variation of Vp and Vp/Vs values. The spatial distribution of the 1998 aftershocks defines sharp vertical planar features consistent with the orientation of the main strike-slip fault while the 2004 aftershock pattern is quite diffuse. The tomographic images and the spatial distribution of the aftershocks indicate a fault zone characterized by branches. The fault plane solutions of the 1998 and 2004 main shocks show evidence of dextral strike-slip motion concordant with the kinematics of the Bovec-Krn fault. The largest aftershocks of both sequences are characterized by different types of focal mechanisms and variable plane orientation. The different focal mechanisms in the fault zone suggest that the mode of fracture of the largest aftershocks can be influenced by the fault strength heterogeneities imaged by the tomographic inversion. The Coulomb stress changes caused by the 1998 main shock was calculated to explore the stress transfer mechanism and the earthquake interactions. A positive correlation of the Coulomb stress increase is found with the largest aftershock (MD 4.6) and with a part of the aftershocks. Our modelling shows also that the Coulomb stress changes caused by the 1998 main shock and its largest aftershock were not sufficient to trigger the 2004 main shock. The locations of the 1998 and 2004 sequences correlate well with areas of increased Coulomb stress changes when the regional tectonic loading is considered.