Static stress changes in extensional regimes: An application to southern Apennines (Italy)

The analysis of static stress changes due to co-seismic dislocations has been widely applied in recent years to study the variation in failure stress on well-known segmented seismogenic faults. These studies assert that earthquakes induce changes in static stress on neighboring faults that may delay, advance, or trigger impending earthquakes. So far, most of the investigations have focused on California strike-slip faults and, more recently, on thrust faults. In this work, we study the static stress changes in an extensional tectonic setting. Following Okada (1992), we calculate static stress changes caused by normal-faulting earthquakes in the southern Apennines (Italy) using a 3D dislocation model in an elastic half-space. We first studied the static stress changes produced by the 1980 Irpinia earthquake. This normal-faulting event consisted of four fault segments that ruptured together within 40 sec. The last two subevents occurred 20 and 40 sec after the nucleation on two normal faults parallel to the main one. Because the rupture of these subvents are separated by a sufficiently long time interval, we assume that, before each subevent, the dynamic stress has reached its static configuration. Therefore, we analyze the static stress changes caused by each subevent in order to determine the variations of the initial conditions for subsequent ruptures. Moreover, we applied our procedure to study the static stress changes after the 1980 Irpinia earthquake on the fault zone where the 1990 and 1991 Potenza earthquakes ( M ≈ 5) occurred. Our calculations indicate that, even if the amount of stress change depends on the fault geometry assumed for the 20-sec subevent of the Irpinia fault, the static stress increased on the Potenza fault after the 1980 Irpinia earthquake. This event also increased the static stress on seismogenic faults that are believed to have ruptured in the 1732 and 1857 earthquakes ( M ≈ 6.5). In these calculations, we have considered the effect of a regional extensional stress field whose minimum principal axis is horizontal and oriented 216° N. The results show that the 1980 Irpinia earthquake increased the static stress by roughly 1 bar on the 1732 fault zone. Right lateral strike-slip faults oriented nearly E-W, such as the Potenza fault, are favorably oriented with respect to the total stress resulting from the extensional regional stress field and the coseismic stress induced by the Irpinia earthquake. Finally, we investigate the static stress changes caused by several historical earthquakes that occurred along the southern Apennines seismogenic belt. Most of the large historical earthquakes occurred in areas of static stress increase, yielding evidence for fault interaction.