Moment tensor analysis of the Central Italy Earthquake Sequence of September–October 1997

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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 (Δ5.5) in the sequence, moment tensors have previously been determined using teleseismic waveforms 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.