Rupture mechanism and source parameters of Umbria-Marche mainshocks from strong motion data

A long sequence of earthquakes causing few casualties and considerable damage in a wide zone struck Central Italy starting on September 26, 1997. The earthquakes are characterized by normal faulting mechanism, with a NE-SW (anti-Apenninic direction) tension axis. In this paper we analyze the accelerometric recordings collected by the accelerograph stations belonging to the National Accelerograph Network. About 10 stations were triggered by the mainshocks of the sequence. In particular, a small size foreshock and the two mainshocks that occurred on September,26 (00:33(GMT) M-W = 5.7 and 09:40 M-W = 6.0) have been recorded by two digital 3-C accelerometers located at near source distances (within 30 km from the faults). These records are relevant to investigate the detail of the rupture kinematics, due to the close epicentral distance and azimuthal location relative to the fault orientation and geometry. Using a trial and error approach we modeled the source mechanism through the fit of the arrival times, the apparent source time duration, the main polarization features and the entire waveforms of the recorded signals, in order to get some insight on the rupture evolution, the location of the fracture origin point and the fault geometry. Based on this fault kinematic model, inferences on fault slip distribution are obtained by modeling the S acceleration waveform, comparing the ray theory synthetics with 1-5 Hz band filtered ground velocity records. The final model shows that the seismic ruptures occurred along two adjacent, sub-parallel, low angle dipping normal faults. Ruptures both nucleated from the fault bottom and propagated up-dip, showing different rupture velocity and length. The presence of a transfer zone ( barrier) can be suggested by the mainshocks rupture evolution. This transfer zone has probably controlled the amplitude increase of local stress released by the first rupture at its NW edge which triggered about 9 hours later the second rupture. The inferred model was used to compute the predicted ground acceleration in the near source range, using a hybrid statistical-deterministic approach. A similar trial and error method has been also applied to the October 14, 1997 15:23 earthquake (M-W = 5.6). The inferred kinematic model indicates a rupture nucleating from the fault bottom and propagating up-dip, toward the SE direction. Thus the three mainshocks ruptured distinct fault segments, adjacent and slightly offset from one to another.