Abstract:
In September–October 1997 a seismic sequence struck the southern part of the Umbria-Marche region in central Italy. The three largest earthquakes took place on 26 September (0033 UT, Mw 5.7; 09:40 GU, Mw 6.0) near Colfiorito, and on 14 October
(Mw 5.6) near Sellano. We present a slip model for the main sequence over two separate fault planes. We use previously published Global Positioning System (GPS) and differential interferometric synthetic aperture radar (DInSAR) data as well as new DInSAR data covering the Sellano event as input to a simulated annealing solution for the distribution of fault slip. The combination of both types of geodetic data as well as the combination of SAR interferograms from both ascending and descending satellite tracks allows us to make a more complete geodetically based assessment of the fault slip for these earthquakes. For the solutions presented here we required the slip distribution to match the observed surface displacements while maintaining a total scalar seismic moment approximately equal to the seismologically inferred moments for this sequence. We find the slip was concentrated in a zone at 4–5.5 km depth with a shallower tail extending to near the surface at the northern end of the Colfiorito sequence. For both the second Colfiorito event and the Sellano event, individual patches were calculated with maximum slip of 2 m, implying stress drops as high as 20 MPa. Coulomb stress calculations show that the coseismic slip distribution for the Colfiorito events facilitated slip of the adjacent Sellano earthquake.
