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 earthqua kes 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 ma inshock can explain the occurrence of subsequent events. Our results show t hat the foreshock slightly increased the Coulomb stress on the first mainsh ock 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 sho cks of September 26. The locations and the geometry of the three largest ea rthquakes agree well with the pattern of Coulomb stress changes suggesting elastic interaction between these faults. However, we were not able to mode l 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 faul t plane solutions for events located very close to each other and in the ha nging wall of the mainshock rupture planes. Our results show that normal st ress changes agree better with the spatial pattern of the whole sequence of moderate magnitude events. If previous ruptures unclamp the fault planes o f subsequent earthquakes, fluid flow can play a dominant role in promoting earthquakes during the seismic sequence.