Seismological and field observations of the 1984 Lazio‐Abruzzo earthquakes: implications for the active tectonics of Italy

Abstract

Seismological and field studies of the 1984 Lazio‐Abruzzo earthquakes provide new constraints on the pattern of Quaternary extension in the Apennine mountains of central Italy. Waveform modelling of the mainshock (Ms= 5.8; Mo= 0.59 X 1018N m) at 17:49 on May 7 indicates centroid strike 123°, dip 48°, rake ‐96° and depth 10 km. The largest aftershock (Ms= 5.2; Mo= 0.21 X 1018N m), at 10:41 on May 11, had strike 170°, dip 46°, rake ‐67° and centroid depth also 10 km. The mainshock fault plane was part of the Sangro fault, one of three major SW‐dipping normal faults that take up much of the extension of this part of central Italy in Lazio and Abruzzo. A much larger earthquake occurred on 1915 January 13 on an in‐line continuation of this fault ˜30 km further NW. Scarps ˜10 m high suggest ˜1 mm yr‐1 Holocene slip rate on the Sangro fault, and imply ˜2 mm yr‐1 average Holocene extension rate across central Italy in good agreement with rates calculated from the seismicity of the last few decades. Topography and present‐day dips of active normal faults in Abruzzo suggest that extension has been occurring locally for no more than ˜2 Myr. Slip vector azimuths for the Lazio‐Abruzzo earthquakes were ˜45°, approximately perpendicular to the overall strike of the actively‐extending zone and major active normal faults inside it. They thus imply that extension in this zone is accompanied by negligible local rotation of fault‐bounded blocks around vertical axes. On a larger scale, the instantaneous present‐day extension occurring across Italy between northern Campania and Tuscany can be regarded as anticlockwise rotation of the Adriatic Sea region to the NE relative to the Tyrrhenian Sea region to the SW around a pole. We suggest this pole is at present near 44°30′N, 9°30yE. Our 2 mm yr‐1 extension rate in Abruzzo implies ˜0.3° Myr‐1 angular velocity around this pole, giving ˜4 mm yr‐1 extension rate in Campania. Moving further southeast, some major normal faults in Campania and Calabria show substantial left‐lateral strike‐slip, suggesting blocks there may be rotating around vertical axes. The southernmost block of Calabria has rotated 36° clockwise since ˜4 Myr ago, approximately the time when extension began locally. The leading edge of the extending zone appears to have propagated northwestward during the course of extension at ˜200 mm yr‐1 from Calabria to its present‐day position. Copyright © 1989, Wiley Blackwell. All rights reserved