Static stress changes and fault interaction during the 1997 Umbria-Marche earthquake sequence

We study the static stress changes caused by moderate magnitude earthquakes that occurred in Umbria-Marche during a seismic sequence which started on September 3, 1997, with a M(L) 4.7 foreshock and consisted of eight earthquakes whose magnitudes range between 5.0 and 6.0. The earthquakes occurred on normal faults striking in the Apennine direction and dipping at low angles towards the SW. The goal is to verify if stress changes induced by each mainshock can explain the occurrence of subsequent events. Our results show that the foreshock slightly increased the Coulomb stress on the first mainshock fault plane. The distribution of seismicity that followed the foreshock is clustered in the area of Coulomb stress increase comprised between the two faults which ruptured in opposite directions during the two largest shocks of September 26. The locations and the geometry of the three largest earthquakes agree well with the pattern of Coulomb stress changes suggesting elastic interaction between these faults. However, we were not able to model the whole sequence of M(L) greater than or equal to 5.0 events in terms of Coulomb stress changes. The difficulties are due to the similarity of fault plane solutions for events located very close to each other and in the hanging wall of the mainshock rupture planes. Our results show that normal stress changes agree better with the spatial pattern of the whole sequence of moderate magnitude events. If previous ruptures unclamp the fault planes of subsequent earthquakes, fluid flow can play a dominant role in promoting earthquakes during the seismic sequence.