Combining stress transfer and source directivity: the case of the 2012 Emilia seismic sequence

Abstract

The Emilia seismic sequence (Northern Italy) started on May 2012 and caused 17 casualties, severe damage to dwellings and forced the closure of several factories. The total number of events recorded in one month was about 2100, with local magnitude ranging between 1.0 and 5.9. We investigate potential mechanisms (static and dynamic triggering) that may describe the evolution of the sequence. We consider rupture directivity in the dynamic strain field and observe that, for each main earthquake, its aftershocks and the subsequent large event occurred in an area characterized by higher dynamic strains and corresponding to the dominant rupture direction. We find that static stress redistribution alone is not capable of explaining the locations of subsequent events. We conclude that dynamic triggering played a significant role in driving the sequence. This triggering was also associated with a variation in permeability and a pore pressure increase in an area characterized by a massive presence of fluids.

The role of fluid migration and static stress transfer in searching connections between the May 2012 Emilia earthquakes through a fully 3D finite element modeling

“…Previous works have reached contrasting results on the role of coseismic and postseismic (aseismic) stress transfer in promoting the May 29 earthquake (Ganas et al 2012;Convertito et al 2013;Pezzo et al 2013). Our outcomes clearly reveal that the hypocentral area of the May 29 event was not loaded by the previous rupture, meaning that coseismic stress associated with the May 20 main shock did not act towards the rupture.…”

“…Ganas et al (2012), computing the stress transfer from a coseismic slip distribution model for the May 20 rupture, predicted that the May 29 earthquake could have been triggered by slip on the fault caused by the preceding main shock, as its hypocenter is found to lie in the crustal region where static stress was transferred, even if the amount of the loading stress is small. Differently, Convertito et al (2013), evaluating cumulative changes in the static stress field, reported that static Coulomb stress changes did not significantly contribute to the triggering process, while dynamic triggering, caused by passing seismic waves and enhanced by source directivity, played a primary role in driving the sequence. Cesca et al (2013), by regional waveform inversion, inferred that the static stress perturbation associated with the May 20 earthquake can not be responsible for the rupture of the Mirandola fault, but it can have sped it up, being the event mature on that fault.…”

“…Definitely, the issue concerning the triggering between the main events of the May 2012 Emilia sequence remains still unsolved. In fact, previous analyses based on okadastyle modeling of fault interactions (Ganas et al 2012;Pezzo et al 2013) have not reached conclusive evidence on the effect of static stress transfer, while dynamic triggering as suggested by Convertito et al (2013) could be not a robust explanation for two failures separated by a time window of 9 days, since the dynamic effect is likely to vanish few hours after the rupture.…”

Conditions for the occurrence of seismic sequences in a fault system

“…However, in some cases pore fluid diffusion may have a role in the evolution of a seismic sequence (e.g. Convertito et al, 2013).…”

“…The Emilia sequence has been studied in detail by several authors, who determined the locations and seismic moments of the events (Scognamiglio et al, 2012), the source func- tions and seismic spectra (Castro et al, 2013) and the coseismic deformation (Pezzo et al, 2013). Convertito et al (2013) suggested that dynamic triggering caused by seismic waves might be the primary factor to explain the evolution of the Emilia sequence, in addition to the variation in permeability and pore-pressure effects due to a massive presence of fluids in the Po Plain basin. As stated in Sect.…”