CHARACTERISTICS OF 1998 AND 2004 RAVNE FAULT EARTHQUAKE SEQUENCES AND THEIR SEISMOTECTONIC IMPLICATION

The area of Southern Alps in the NW part of Slovenia and NE part of Italy is the most seismically active region of the Alpine orogen. The seismic load in a geological recent N oriented stress regime, is released on E-W oriented thrust fault planes in the Friuli Alps of NE Italy and on NW-SE oriented strike-slip faults of Julian Alps in NW Slovenia. The 1976 Friuli, 1998 and 2004 Krn mountain seismic sequences occurred in the region of intersection between Alpine and Dinaric structures where both kinematic fault systems meet. All of the seismic events caused excessive damage both to natural and human environment and understanding of active faulting plays an important role in seismic hazard assessment. Both, 1998 and 2004, earthquakes happened along a Ravne Fault – a steep, segmented NE dipping fault. The earthquake cluster seized at the endpoint of a segment on the border of a transpressional overstep between two fault segments that was not breached during the seismic event, while a smaller stepover zone was breached during the same event. Within the same kinematic behaviour of movements on fault traces with the same fault geometry for a seismic event of a certain magnitude, the relationship between lengths of individual fault segments to overstep and separation distances in overstep zones play the deciding role in fault growth processes and propagation of an earthquake rupture. The 1998 and 2004 earthquake sequences affected the area of Zgornje Posocje in the Upper Soca Valley, NW Slovenia. Epicentres of the main earthquake shock occurred in the area of poor ground fault exposure. The hypocenters of both main earthquake shocks lie only a km in distance from eachother and both are confined to shallow crustal layers and focal mechanism solution for both events give an almost pure dextral strike-slip kinematics. The 1998 earthquake cluster displays a clear NW-SE orientation with the majority of stronger aftershocks exhibiting the same kinematics, while only the aftershocks occurring at the ending tips of earthquake rupture showing an oblique and thrust movements. The aftershocks of the 2004 earthquake are mostly distributed in the direction NW to WWN from those of the 1998 earthquake and do not display such an uniform spatial distribution. The majority of the aftershocks occurred in the area NW of the main shock and the whole earthquake cluster shows a less uniform distribution. A group of aftershocks which demonstrate a prevailed strike-slip type of deformation from their focal mechanisms continues in a NW – SE direction, while the aftershocks oriented in a WWN – EES to W – E direction show more of a thrust and reverse sense of deformation. Such temporal and spatial distribution of the aftershocks depicts a contemporary seismic activity on both NW – SE and WWN – EES to W – E oriented faults. The 2004 earthquake cluster therefore didn't activate just a NW-SE trending steep strikeslip faults, but it also caused the activity on a E-W trending thrust fault.