GNGTS 2013 - Atti del 32° Convegno Nazionale

the May 20 shock, which produce an increase in the strain rate from about 1-1.5 yr -1 (Fig. 2b) to 2.5-3.5 yr -1 (Fig. 2c). In order to identify a possible quiescence period (often named as temporal gap or gap of 2 nd kind) preceding the May 20 earthquake, we have analyzed the variation of the seismic moment release rate with time. More specifically, we plot 50-year running averages of the (smoothed) seismic moment release rate as a function of time (Fig. 3a) accounting for the contribution of earthquakes with magnitude M w ≥ 5.0 since 1825 [which corresponds to the year of catalog completeness for M5+ events as determined by Barani et al. (2010)]. In such a way, quiescence periods are identified by minima in the curve which are attributable to seismic inactivity (particularly concerning the occurrence of moderate to large earthquakes). The temporal variation of the cumulative seismic moment (again calculated using the smoothed seismicity approach) is also shown (Fig. 3b). In this latter plot, quiescence periods are identified by “plateau” in the cumulative curve. In this study, we compare the curves for two sites, one (site S1) located close to the May 20 event and one (site S2) between the two larger aftershocks occurred on May 29 (see Fig. 2c). Similarly to the case study of the 2009 L’Aquila earthquake, Fig. 3a evidences that the occurrence of the main shock is preceded by a minimum in the running aver- age of . The minimum in the curve, which presents a rise in the latter stages (last 15 years) before the occurrence of the main shock, corresponds to a period of time during which the seismic ac- tivity in the region surrounding the seismic gap is sporadic (i.e., earth- quakes of M w ≥ 5.0 are almost ab- sent). Only five events of M w great- er than or equal to 5.0 occurred in the study region (dashed square in Fig. 1) from year 1923 to 2012, at distances greater than 30 km from the gap area. Fig. 3 – Evolution of seismic activity ( M w ≥ 5.0) in the area shocked by the Emilia 2012 seismic sequence from 1825 to 2013: 50- year running averages of the seismic mo- ment release rate versus time (data points in the running average curve are plotted at the end of each time interval) (a); cu- mulative seismic moment versus time (b); distribution of earthquake magnitude ver- sus time (c). The horizontal dashed line in Fig. 3a indicates the average moment rate calculated in the seismicity gap area. Blue circles in Fig. 3c indicate the earthquakes belonging to the 2012 sequence; the May 20 main shock is displayed by a red star. 213 GNGTS 2013 S essione 1.2