EARTHQUAKE OF JULY 12, 2004 IN POSOČJE AND SEISMIC HAZARD MAP - THE INFLUENCE OF SEISMIC RESISTANCE REDUCTION AND RUPTURE DIRECTIVITY
Abstract
In 1976, 1998 and 2004 earthquakes strongly affected the Upper Soča Valley in Slovenia. The magnitude of the earthquake in 2004 was the smallest (4.9) of the four earthquakes (6.5 in May 1976 and 6.0 in September 1976, and 5.6 in 1998). Nevertheless, it causes some striking effects. Some houses, which were apparently undamaged in the previous events, suffered serious damage in the last earthquake. In addition, two accelerometers out of three installed after the 1998 earthquake in the broader epicentral region, registered high peak ground accelerations that considerably exceeded design ground acceleration in the official seismic hazard map for the return period of 475 years. The damage to the aforementioned houses can be simply explained by the assumption that the seismic resistance of those, as well as other houses, was significantly diminished in the previous earthquakes. Analyses of strong motion registration showed that high values can only be partly explained by local soil conditions. The proximity of the seismic source, and especially the rupture directivity, are probably the main causes of the extremely high peak ground accelerations, velocities and spectral values.
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