Static stress changes and fault interaction during the 1997 Umbria-Marche earthquake sequence |
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| Authors: | Massimo Cocco Concetta Nostro G Ekström |
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| Institution: | (1) Istituto Nazionale di Geofisica, Rome, Italy;(2) Istituto Nazionale di Geofisica, Rome, Italy;(3) Harvard University, USA |
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| Abstract: | We study the static stress changes caused by moderatemagnitude earthquakes that occurred in Umbria-Marcheduring a seismic sequence which started on September3, 1997, with a ML 4.7 foreshock and consisted ofeight earthquakes whose magnitudes range between 5.0and 6.0. The earthquakes occurred on normal faultsstriking in the Apennine direction and dipping at lowangles towards the SW. The goal is to verify if stresschanges induced by each mainshock can explain theoccurrence of subsequent events. Our results show thatthe foreshock slightly increased the Coulomb stress onthe first mainshock fault plane. The distribution ofseismicity that followed the foreshock is clustered inthe area of Coulomb stress increase comprised betweenthe two faults which ruptured in opposite directionsduring the two largest shocks of September 26. Thelocations and the geometry of the three largestearthquakes agree well with the pattern of Coulombstress changes suggesting elastic interaction betweenthese faults. However, we were not able to model thewhole sequence of ML  5.0 events in terms ofCoulomb stress changes. The difficulties are due tothe similarity of fault plane solutions for eventslocated very close to each other and in the hangingwall of the mainshock rupture planes. Our results showthat normal stress changes agree better with thespatial pattern of the whole sequence of moderatemagnitude events. If previous ruptures unclamp thefault planes of subsequent earthquakes, fluid flow canplay a dominant role in promoting earthquakes duringthe seismic sequence. |
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| Keywords: | Coulomb stress changes fault interaction pore pressurechanges spatio-temporal seismicity pattern |
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