Moment tensor analysis of the central Italy earthquake sequence of September-October 1997

Abstract

The larger earthquakes in the Umbria-Marche (central Italy) seismic sequence of September-October 1997 are analyzed using long-period seismograms from the Mediterranean seismographic network (MEDNET) and additional data from the global seismographic network (GSN). We modify the Harvard centroid-moment tensor (CMT) algorithm to allow moment tensor inversion of long-period waveforms, primarily Rayleigh and Love waves, for small earthquakes at local to regional distances (A<15°). For the three largest earthquakes (Mw>5.5) in the sequence, moment tensors have previously been determined using teleseismic wave-forms and standard methods of analysis; our results agree well with those of earlier studies. We determine additional moment tensors for the largest foreshock and 10 aftershocks with Mw>4.2. The earthquakes are characterized by normal faulting mechanisms, with a NE-SW tension axis, and the presumed fault plane dips towards the SW. Only one of the fourteen events studied has a different faulting geometry, indicating instead right-lateral strike-slip faulting on a plane oriented approximately E-W, or left-lateral faulting on a plane oriented N-S. The September 26 mainshock (09:40 UT) accounts for only approximately ∼50% of the total moment release in the sequence.