Modelling coseismic displacements resulting from SAR interferometry and GPS measurements during the 1997 Umbria-Marche seismic sequence

In this study we analyse coseismic GPS displacements and DInSAR data to constrain a dislocation model for the
three largest earthquakes of the 1997 Umbria-Marche seismic sequence. The first two events, which occurred on
September 26 at 00:33 GMT (Mw 5.7) and 09:40 GMT (Mw 6.0) respectively, are investigated using both GPS displacements
and DInSAR interferograms.We discuss and compare the results of previous studies which separately
modeled a smaller 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 results of seismological and geological investigations. The
first event consists of a unilateral rupture towards the southeast with a uniform dislocation. The strike, rake and dip
angles are those resulting from the CMT solution. The second event consists of an unilateral rupture towards the
northwest and a variable slip distribution on the fault plane. The strike and the rake are consistent with the CMT
solution, but the dip angle has been slightly modified to improve the simultaneous fit of GPS and DInSAR data.
While the 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), despite the more evident surface geological effects. The analysis of new
SAR interferograms allows the identification of a 5–6 cm additional displacement caused by the October 3 (Mw
5.2) and 6 (Mw 5.4) seismic events.We use data from a new DInSAR interferogram to model the displacement field
of the Sellano earthquake of October 14, 1997. For this event significant GPS measurements were not available.
We tested two different fault plane geometries: a blind, planar fault (top depth = 2.4 km), and a curved (listric)
fault reaching 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 –45 ) fault surface. They also show
that a unilateral rupture does not allow fitting the interferometric fringes since there is evident surface deformation
to the northwest of the hypocenter. Moreover, we suggest that the concentration of high residuals in the southern
part of our uniform slip model may in fact indicate a certain slip variability in this area. We conclude that, despite
the moderate magnitudes and the lack of significant surface faulting, the space geodetic data allowed to constrain
dislocation models giving new insights in the rupture process of the three largest events of the sequence.