THE 23 NOVEMBER 1980 SOUTHERN ITALY EARTHQUAKE

The seismic activity associated with the catastrophic southern Italy earth- quake was monitored by 11 seismic stations operating before this event, within an epicentral distance of 200 km, and by 32 additional short-period seismom- eters installed soon after the main shock. The hypocenter of this event was located at 40°46'N and 15°18'E, at 16 km depth. The fault-plane solution reveals normal faulting, with tensile axis dipping 18 ° and oriented orthogonal to the axis of the Apennines chain. This mechanism is in good agreement with the stress pattern inferred from some previous earthquakes and the local seismotectonics.
The hypocenter locations of more than 600 aftershocks, with local magnitudes greater than 2.4, show a pronounced alignment extending for about 70 km, oriented north 120 ° and scattered laterally less than 15 km. These events are mostly concentrated between 8 and 16 km depth. A cluster of aftershocks occurred close to the hypocenter of the main shock covering a region elongated 25 km which corresponds also to the highly damaged area. No significant spreading of the aftershock area with time is observed, but one of the events with higher magnitude (M, = 4.8, 14 February 1981) is displaced 20 km NW from the tip of the aftershock region. The time evolution of the number of aftershocks fits well Omorrs hyperbolic law with a decay coeffcient of 1.07 __. 0.06.
The possibility of a future delayed multiple sequence of large events, as already observed in the past along the central and southern Apennines, is discussed. In particular, a relatively high seismic potential seems to exist along the northern boundary of the 1980 rupture segment.