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

Abstract

On May 20 and 29, 2012, two earthquakes struck the Emilia Romagna region (Northern Italy), with similar mechanisms. The proximity in space and time between the two events required to investigate possible triggering effects in terms of stress transfer or fluid migration. Moreover, the debate concerning fluid extraction-injection activities brought to the appointment of an International Commission (ICHESE), for evaluating relationships between the hydrocarbon exploitation and the seismic activity near the focal zone: the ICHESE conclusive report did not exclude the possibility of a triggering effect associated with explorations, although this appears not supported by robust evidences. On the other hand, there is no agreement between published papers addressing classical triggering mechanisms and in this respect an improved analysis based on 3D numerical modeling is a valuable tool to provide new insights and outcomes. To this aim, we built up a 3D model honoring the complexities of the real Earth, to retrieve the slip patterns on both the rupture planes by implementing finite element computed Green's functions in a linear joint inversion scheme of geodetic data. Then, we inspected possible mechanisms which could have contributed to promote the second event, such as Coulomb stress transfer and natural fluid migration. Our findings suggest that both phenomena are not likely to be responsible for the May 29 earthquake, even if fluid migration was found to have a role in facilitating the rupture but without reaching hydrofracturing condition. Nevertheless a suprahydrostatic pore pressure is inferred which could have promoted the reactivation of the thrust fault.

New insights into fault activation and stress transfer between en echelon thrusts: The 2012 Emilia, Northern Italy, earthquake sequence

“…The aseismic slip in turn probably brought the Mirandola thrust closer to failure, suggesting that the subsequent 29 May M w 6.0 event has been triggered by this pulse of aseismic slip. This is different from the findings of Volpe and Piersanti [], which excluded a mechanism involving triggering due to an aseismic slip. Although we cannot precisely place the aseismic slip event in time, and thus we cannot rule out that it could have happened at the same time as the 29 May event or shortly after it, we can reason that it may have occurred in the time span between the two main shocks.…”

“…We calculated the static Coulomb stress changes from our final model to investigate the potential triggering relationships between the two main events, the pulse of aseismic slip, and the afterslip. Coulomb stress changes suggest that stress perturbation due to the 20 May M w 6.1 event does not appear to be responsible for the second rupture, in agreement with previous geodetic studies [ Pezzo et al , ; Volpe and Piersanti , ]. We find instead that the aseismic slip event on the Inner Ferrara thrust was likely triggered by the preceding 20 May M w 6.1 main shock.…”

“…On the Mirandola thrust, we find increased stresses only in the eastern sector of the fault plane (Figure S15a), whereas the 29 May M w 6.0 main shock occurred in an area where the Coulomb stress has remained unchanged or decreased by the previous event. A similar patter of Δ CFS was obtained also by Pezzo et al [] and Volpe and Piersanti [], showing that the stress variation near the 29 May hypocenter was definitely negative. In contrast, on the Inner Ferrara thrust, the shape of the aseismically slipped area corresponds well with the pattern of increased stresses (Figure a), suggesting that the aseismic event was likely triggered by the preceding 20 May M w 6.1 earthquake.…”

Seismic Evidence of an Early Afterslip During the 2012 Sequence in Emilia (Italy)

“…Previous geodetic studies discovered only negligible changes in static stress near the MEQ hypocenter due to the FEQ, suggesting that the FEQ had not triggered the MEQ (Cheloni et al, ; Pezzo et al, ). Similar results were obtained by Volpe and Piersanti (). Cheloni et al () noticed that a joint inversion of geodetic data using only the Ferrara and Mirandola thrusts generates high‐displacement residuals (up to 4–5 cm), hypothesizing that a pulse of aseismic slip occurred along a third contiguous thrust segment of the en echelon Ferrara arc fault system.…”

Pore Pressure Pulse Drove the 2012 Emilia (Italy) Series of Earthquakes

“…Aftershock rate suggests that a transient effect was superposed to the normal decay that is consistent with fluid overpressure along the fault system, well defined by high Vp/Vs anomalies (Figures 4a and 4b and 5). A suprahydrostatic regime acting on the 29 May rupture plane, which could have promoted the activation of the thrust fault, is also inferred by Volpe and Piersanti (2016).…”