Modeling coseismic displacements resulting from SAR interferometry and GPSmeasurements during the 1997 Umbria-Marche seismic sequence

In this study we analyse coseismic GPS displacements and DInSAR data to con strain a dislocation model for the three largest earthquakes of the 1997 Um bria-Marche seismic sequence. The first two events, which occurred on Septe mber 26 at 00:33 GMT (Mw 5.7) and 09:40 GMT (Mw 6.0) respectively, are inve stigated using both GPS displacements and DInSAR interferograms. We discuss and compare the results of previous studies which separately modeled a sma ller subset of geodetic data. We provide a dislocation model for these two earthquakes which fits well both GPS and DInSAR data and agrees with the re sults of seismological and geological investigations. The first event consi sts of a unilateral rupture towards the southeast with a uniform dislocatio n. The strike, rake and dip angles are those resulting from the CMT solutio n. The second event consists of an unilateral rupture towards the northwest and a variable slip distribution on the fault plane. The strike and the ra ke are consistent with the CMT solution, but the dip angle has been slightl y modified to improve the simultaneous fit of GPS and DInSAR data. While th e second rupture (09:40 GMT) arrived very close to the surface, the fit to geodetic data shows that the first rupture (00:33 GMT) is deeper (2 km), de spite the more evident surface geological effects. The analysis of new SAR interferograms allows the identification of a 5-6 cm additional displacemen t caused by the October 3 (Mw 5.2) and 6 (Mw 5.4) seismic events. We use da ta from a new DInSAR interferogram to model the displacement field of the S ellano earthquake of October 14, 1997. For this event significant GPS measu rements were not available. We tested two different fault plane geometries: a blind, planar fault (top depth = 2.4 km), and a curved (listric) fault r eaching the surface. The two models provide a generally similar fit to the data, and show that most of the slip was released at depths greater than 2. 4 km along a gently dipping (40 degrees -45 degrees) fault surface. They al so show that a unilateral rupture does not allow fitting the interferometri c fringes since there is evident surface deformation to the northwest of th e hypocenter. Moreover, we suggest that the concentration of high residuals in the southern part of our uniform slip model may in fact indicate a cert ain slip variability in this area. We conclude that, despite the moderate m agnitudes and the lack of significant surface faulting, the space geodetic data allowed to constrain dislocation models giving new insights in the rup ture process of the three largest events of the sequence.