The Perugia (Italy) earthquake of 29, April 1984: A microearthquake survey

A field study after the Perugia earthquake of 29 April 1984 provided more than 300 well-recorded events concentrated within two parallel clusters separated by 2 km and trending along the Apenninic direction. The length of the aftershock area is 14 km, focal depths being shallower than 8 km. Relocation of the main event places the epicenter at the southern end of the aftershock zone, suggesting a rupture propagation from SE to NW. Most focal mechanisms are consistent with normal faulting. The spatial distribution of seismicity suggests that the Gubbio normal fault was activated during the main shock. This earthquake, together with the Norcia 1979 and the Abruzzi 1984 shocks, is typical of the extension in the high Apennines generated by the flexure of the mountain chain in response to regional compression. The Parma 1983 event, a thrust, belongs to the compres- sion zone at the eastern flank of the chain. These results are consistent with the EW continental collision along the Apennines.

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