New constraints shed light on strike-slip faulting beneath the southern Apennines (Italy): The 21 August 1962 Irpinia multiple earthquake

Abstract

On 21 August 1962 an earthquake sequence set off near the city of Benevento, in Italy's southern Apennines. Three earthquakes, the largest having Mw 6.1, struck virtually the same area in less than 40 min (at 18:09, 18:19 and 18:44 UTC, respectively). Several historical earthquakes hit this region, and its seismic hazard is accordingly among the highest countrywide. Although poorly understood in the past, the seismotectonics of this region can be revealed by the 1962 sequence, being the only significant earthquake in the area forwhichmodern seismograms are available. We determine location, magnitude, and nodal planes of the first event (18:09 UTC) of the sequence. The focal mechanismexhibits dominant strike-slip rupture along a north-dipping, E-W striking plane or along a west-dipping, N-S striking plane. Either of these solutions is significantly different fromthe kinematics of the typical large earthquakes occurring along the crest of the Southern Apennines, such as the 23 November 1980 Irpinia earthquake (Mw 6.9), caused by predominant normal faulting along NW-SE-striking planes. The epicentre of the 21 August 1962, 18:09 event is located immediately east of the chain axis, near one of the three north-dipping, E-W striking oblique-slip sources thought to have caused one of the three main events of the December1456 sequence (Io XIMCS), the most destructive events in the southern Apennines known to date. Wemaintain that the 21August 1962, 18:09 earthquake occurred along the E-Wstriking fault systemresponsible for the southernmost event of the 1456 sequence and for two smaller but instrumentally documented events that occurred on 6May 1971 (Mw 5.0) and 27 September 2012(Mw 4.6), further suggesting that normal faulting is not the dominant tectonic style in this portion of the Italian peninsula.