The 2012 Emilia, Italy, Quasi-Consecutive Triggered Mainshocks: Implications for Seismic Hazard

The 2012 Emilia seismic sequence in central Italy represents an illustrative example of quasi-consecutive triggering with several mainshocks occurring within a few hours or a few days. The sequence was characterized by seven earthquakes of moment magnitude Mw >5, rupturing adjacent fault segments of the buried fold arc of the northern Apennines, for a total length of about 50 km (Scognamiglio et al., 2012; Fig. 1). The occurrence of so many large earthquakes, in such a short time window, suggests a possible interpretation in terms of mutual, static (Ganas et al., 2012), or dynamic (Convertito et al., 2013) triggering. In this article, we discuss the consequences of a consecutive succession of earthquakes, that is, a succession of close
earthquakes that occur almost simultaneously.
Multiple triggering—with variable time delays—has been observed in several damaging earthquake sequences of the Italian Apennine belt. In the central Apennines, the 2009 L’Aquila Mw 6.3 earthquake was followed, the day after, by an Mw 5.6 event occurred at the southeast edge of the main fault. This seismic sequence has been the deadliest one in Italy, with almost 300 victims, since 1980. The seismicity of the whole sequence spread over a 40 km long fault system (Chiarabba et al., 2009). The 2002 Molise seismic sequence is remembered for two major shocks Mw 5.8 and 5.7 that occurred within a few hours causing severe damage and destroying a school populated by children (Vallèe and Di Luccio, 2005). The 1997–1998
Umbria–Marche seismic sequence was characterized by two mainshocks breaking adjacent parts of two faults and occurring a few hours from each other (Pino and Mazza, 2000). Overall, this sequence numbered six shocks of moderate magnitude (5:2 ≤ Mw ≤ 6:0), occurring along a 40 km line. The 1980Irpinia earthquake, with surface-wave magnitude Ms 6.9, was indeed composed by the sequence of three distinct subevents,
nucleating on different faults at intervals of about 20 s, resulting in a total seismic moment M0 2:6 × 1019 N·m (Pingue et al., 1993; Troise et al., 1998). This event caused about 3000 deaths and huge damage. In all the above cited sequences, the total length of the fractured fault zone was approximately the
same, that is, about 50 km.
In this article, we simulate a consecutive seismic sequence using the seismograms recorded during the Emilia sequence. Our hypothesis is that the design spectrum may be exceeded when the seven major (Mw >5) earthquakes were to occur one immediately after the other. As our results show, structures can be stressed by consecutive wavetrains, possibly characterized by an increase of acceleration and displacement amplitude
due to resonance.
The obtained results suggest that at least for specific design applications, such as for strategic structures (i.e., power plants, nuclear waste disposals, bridges, hospitals, etc.) seismic-hazard analyses and design spectra should take into account the possibility that several mainshocks can be triggered consecutively, resulting in a ground shaking highly increased in amplitude and duration.

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