Seismicity and mean magnitude variations correlated to the strongest earthquakes of the 1997 Umbria-Marche sequence (central Italy)

Abstract

Nearby faults can interact, affecting the timing of a future earthquake.Alarge earthquake can alter the static stress surrounding faults, possibly activating an aftershock sequence. Here we test the hypothesis of earthquake triggering by examining the earthquakes that affected the Umbria-Marche region (central Italy) during September 1997 to April 1998. The analysis was performed combining information contained in parameters a and b of the Gutenberg-Richter relationship, before and after the strongest main shocks, using a catalog of 6211 earthquakes (M 1.5). By analyzing the changes in space of the a parameter,we found that Colfiorito main shocks (Mw 5.7 and 6.0) on 26 September 1997 increased the seismicity rate within 40–50 km of the epicenter. We interpret this behavior as the effect of stress redistribution. After analyzing the b value changes in space and time we found the strongest increase in mean magnitude in the Sellano area, between the Colfiorito shocks and the two strong events (Mw 5.2 and 5.6, respectively) which hit this region on 12 and 14 October 1997. This area was also the location of the strongest increase in change of the local occurrence rate density. On the basis of these observations, we postulate that temporal variations in both a and b values indicate a probable causal connection between the largest earthquakes in the sequence.