The 1997 Umbria-Marche (central Italy) earthquake sequence: Insights on the mainshock ruptures from near source strong motion records

A small size foreshock and the two mainshocks of the Umbria Marche earthquake sequence which occurred on September 26, 1997 have been recorded by two digital 3C accelerometers located at near source distances. The close epicentral distance and azimuthal location relative to the fault orientation and geometry make these records relevant to look at the detail of the rupture kinematics. S-wave polarizations, apparent source time duration and waveforms from strong motion records are used to constrain the location of the fracture origin point, the fault geometry, the final slip distribution, size and mechanism of the events. The final model shows that the seismic ruptures occurred along two adjacent, sub-parallel, low angle dipping normal faults. The relative timing, location and geometry of the mainshock faults suggest the presence of a transfer zone (barrier) which 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.