Effects of layered crust on the coseismic slip inversion and related CFF variations: Hints from the 2012 Emilia Romagna earthquake

Abstract

The 2012 Emilia Romagna (Italy) seismic sequence has been extensively studied given the occurrence of two mainshocks, both temporally and spatially close to each other. The recent literature accounts for several fault models, obtained with different inversion methods and different datasets. Several authors investigated the possibility that the second event was triggered by the first mainshock with elusive results. In this work, we consider all the available InSAR and GPS datasets and two planar fault geometries, which are based on both seismological and geological constraints. We account for a layered, elastic half-space hosting the dislocation and compare the slip distribution resulting from the inversion and the related changes in Coulomb Failure Function (CFF) obtained with both a homogeneous and layered half-space. Finally, we focus on the interaction between the two main events, discriminating the contributions of coseismic and early postseismic slip of the mainshock on the generation of the second event and discuss the spatio-temporal distribution of the seismic sequence. When accounting for both InSAR and GPS geodetic data we are able to reproduce a detailed coseismic slip distribution for the two mainshocks that is in accordance with the overall aftershock seismicity distribution. Furthermore, we see that an elastic medium with depth dependent rigidity better accounts for the lack of the shallow seismicity, amplifying, with respect to the homogeneous case, the mechanical interaction of the two mainshocks.

Poroelasticity and Fluid Flow Modeling for the 2012 Emilia-Romagna Earthquakes: Hints from GPS and InSAR Data

“…Both the aftershock distribution [1] and the InSAR deformation measurements [3,6,7] clearly suggest that the overall seismic sequence involved two different, partially overlapping, fault planes: the May 20 mainshock occurred on the Ferrara fault (the easternmost one), while the May 29 mainshock occurred on the Mirandola fault (the westernmost one). Both faults are mainly east-west oriented, dipping towards south.…”

“…This allows us to focus on the effect of pure poroelastic readjustment, excluding concurrent afterslip processes in the model. In our simulations for both the May 20 and May 29 events, we use the coseismic slip distribution models, the faults geometries, and the elastic layering (Table 1 and Figure 3) proposed by Nespoli et al [7]. The conversion from diffusivity to permeability is not trivial, but we can estimate the order of magnitude of the permeabilities as k ≅ ηDS s /ρ w g, where η is the dynamic viscosity of water, ρ w is the water density, and g is the gravity acceleration.…”

“…Several fault geometries, based on different geodetic data inversions, are proposed in the literature (e.g., [3,6,8]). According to the model proposed by Pezzo et al [3] and by Nespoli et al [7], most of the slip of the May 20 event occurred along the Ferrara fault plane, even though some minor, aseismic slip on the Mirandola fault before the May 27 event is required to explain a tail-shaped deformation detected by the SAR satellites in the western part of the interferogram (Figure 2b in [7]) within the time window May12-27, 2012. Nespoli et al [7] also show both the ground displacements and the Coulomb failure function (CFF) variations generated by the two mainshocks using a layered half-space.…”

“…According to the model proposed by Pezzo et al [3] and by Nespoli et al [7], most of the slip of the May 20 event occurred along the Ferrara fault plane, even though some minor, aseismic slip on the Mirandola fault before the May 27 event is required to explain a tail-shaped deformation detected by the SAR satellites in the western part of the interferogram (Figure 2b in [7]) within the time window May12-27, 2012. Nespoli et al [7] also show both the ground displacements and the Coulomb failure function (CFF) variations generated by the two mainshocks using a layered half-space. They concluded that the second mainshock has been mechanically promoted by the first event, especially accounting for the CFF change induced by the aseismic slip on the Mirandola fault.…”

InSAR-Based Mapping to Support Decision-Making after an Earthquake

“…The static Coulomb stress criterion, although a simplistic approach, is a powerful tool to explain many fault and volcanic interactions and is issued as a tool for earthquake forecasting (e.g., [7,19,67]). Different scientific software can be used to carry out these models (e.g., Coulomb 3.3 [48], RELAX [68]).…”