Modelling coseismic displacements during the 1997 Umbria-Marche earthquake(central Italy)

We propose a dislocation model for the two normal faulting earthquakes that struck the central Apennines (Umbria-Marche, Italy) on 1997 September 26 a t 00:33 (M-w 5.7) and 09:40 GMT (M-w 6.0). We fit coseismic horizontal and vertical displacements resulting from GPS measurements at several monuments of the IGMI (Istituto Geografico Militare Italiano) by means of a dislocat ion model in an elastic, homogeneous, isotropic half-space. Our best-fittin g model consists of two normal faults whose mechanisms and seismic moments have been taken from CMT solutions; it is consistent with other seismologic al and geophysical Observations. The first fault, which is 6 km long and 7 km wide, ruptured during the 00:33 event with a unilateral rupture towards the SE and an average slip of 27 cm, The second fault is 12 km long and 10 km wide, and ruptured during the 09:40 event with a nearly unilateral ruptu re towards the NW. Slip distribution on this second fault is non-uniform an d is concentrated in its SE portion (maximum slip is 65 cm), where rupture initiated. The 00:33 fault is deeper than the 09:40 one: the top of the fir st rupture is deeper than 1.7 km; the top of the second is 0.6 km deep. In order to interpret the observed epicentral subsidence we have also consider ed the contributions: of two further moderate-magnitude earthquakes that oc curred on 1997 October 3 (M-w 5.2) and 6 (M-w 5.4), immediately before the GPS survey, and were located very close to the 09:40 event of September 26, We compare the pattern of vertical displacements resulting from our forwar d modelling of GPS data with that derived from SAR interferograms: the fit to SAR data is very good, confirming the reliability of the proposed disloc ation model.