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

Abstract

[1] Nearby faults can interact, affecting the timing of a future earthquake. A large 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 (M w 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 (M w 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.

Stress interaction effect on the occurrence probability of characteristic earthquakes in Central Apennines

“…In several cases a clear correlation has been noted between the pattern of positive Coulomb stress change and the increase of the local occurrence rate density, both in normal and in strike fault systems [ King et al , 1994; Harris and Simpson , 1998; King and Cocco , 2001]. Focusing our attention on the geographical region considered in this study, a clear example of such correlation has been put in evidence by Murru et al [2004] and Nostro et al [2005], for the Colfiorito earthquake sequence that hit the Umbria‐Marche Central Apennines from September 1997 up to April 1998. The correlation appeared significant for Coulomb stress changes larger than 0.02–0.1 MPa.…”

Magnitude‐dependent Omori law: Theory and empirical study

“…There are many indications that the space‐time organization of earthquakes is consistent with the idea that a single physical triggering mechanism is responsible for the occurrence of aftershocks, main shocks, foreshocks, and multiplets, leading to the more encompassing concept of earthquake triggering [see, e.g., Lin and Stein , 2004; Felzer et al , 2004; Murru et al , 2004; Huc and Main , 2003; Marsan , 2003; Helmstetter and Sornette , 2003a; Papazachos et al , 2000].…”

Modeling seismicity rate changes during the 1997 Umbria‐Marche sequence (central Italy) through a rate‐ and state‐dependent model

“…Figure 3 reveals that the inferred reference seismicity rate is quite spatially uniform in the selected area and rather low (≈4 ·10 −5 events d −1 km −2 ). A likely explanation for this relatively small seismicity rate is the quiescence that has been detected in this area before the beginning of the 1997 seismic sequence [ Console and Murru , 2001; Murru et al , 2004]. This suggests that the rate of earthquake production in the study area is not stationary before the 1997 sequence, making the measure of a reference seismicity rate a quite difficult task [ Marsan , 2003].…”

“…The quiescence observed in this region [ Murru et al , 2004] before the 1997 sequence might justify these high values of the temporal characteristic decay of aftershocks. This emphasizes the practical difficulties to estimate a suitable reference seismicity rate, which strongly influences the parameters r and t a .…”