Seismotectonic and macroseismic characteristics of the earthquake of Bovec (NW Slovenia: April 12, 1998)

On the 12th Apri1 1998, a wide area in the middle valley of the Soča/Isonzo River near Bovec (NW Slovenia) was affected by an earthquake of MD = 5.6. It was followed by a long seismic sequence from which the 40 aftershocks, with MD ≥3.0 recorded until the 30th June 1998, have been analysed. The study area belongs to the External Dinarides.: it is characterised by a set of dextral NW-SE strike-slip faults that act as kinematic release for the Eastern Southalpine Chain. The main shock presents a classical focal mechanism of strike-slip fault (P = 179/02; T= 269/02) with vertical planes of motion, NW-SE (dextral) and NE-SW (sinistral), and a hypocentre depth of about 15 km. The main aftershock of the 6th May 1998 (MD = 4.6) shows the same focal mechanism, with the hypocentre at about 15 km and the epicentre shifted 6 km towards SE. Also the aftershocks with 3.5 ≤ MD ≤ 4.0 show a prevailing strike-slip focal mechanism. On the contrary, the focal mechanism of the events with 3.0 ≤ MD < 3.5 are more diversified: the earthquakes with transtensional or extensional focal mechanism with planes ranging from NW-SE to N-S prevail over the earthquakes which present a fault plane solution typical of low-angle NW-SE to NE-SW trending reverse faults. Both the main shock and the aftershocks are kinematically compatible with the actual strike-slip tectonic framework of this region. The Čez Potoče fault, one of the main dextral NW-SE strike-slip faults, along which the main shock and aftershock hypocentres are distributed, is considered to be the seismogenetic fault. The temporal analysis suggests a migration of the events towards SE along this fault. The analysis of the distribution and the type of damages indicated that there is no correspondence between the instrumental epicentre and the macroseismic one. The intensity of the latter has been valued equal to the VIII-IX MCS, with heavy structural settlings and noteworthy phenomena of site amplification. The earthquake caused some effects also in the Friuli-Venezia Giulia region (macroseismic intensity surveyed equal to V-VI MCS) where the anomalous distribution of the damage appears to be connected with the partial or full retrofitting of the buildings after the earthquake of 1976. Such a fact once more shows the fundamental importance of strengthening the buildings, particularly the old ones, in order to mitigate the seismic risk.