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

Abstract

The Emilia-Romagna seismic sequence in May 2012 was characterized by two mainshocks which were close in time and space. Several authors already modeled the geodetic data in terms of the mechanical interaction of the events in the seismic sequence. Liquefaction has been extensively observed, suggesting an important role of fluids in the sequence. In this work, we focus on the poroelastic effects induced by the two mainshocks. In particular, the target of this work is to model the influence of fluids and pore-pressure changes on surface displacements and on the Coulomb failure function (CFF). The fluid flow and poroelastic modeling was performed in a 3D half-space whose elastic and hydraulic parameters are depth dependent, in accordance with the geology of the Emilia-Romagna subsoil. The model provides both the poroelastic displacements and the pore-pressure changes induced coseismically by the two mainshocks at subsequent periods and their evolution over time. Modeling results are then compared with postseismic InSAR and GPS displacement time series: the InSAR data consist of two SBAS series presented in previous works, while the GPS signal was detected adopting a variational Bayesian independent component analysis (vbICA) method. Thanks to the vbICA, we are able to separate the contribution of afterslip and poroelasticity on the horizontal surface displacements recorded by the GPS stations. The poroelastic GPS component is then compared to the modeled displacements and shown to be mainly due to drainage of the shallowest layers. Our results offer an estimation of the poroelastic effect magnitude that is small but not negligible and mostly confined in the near field of the two mainshocks. We also show that accounting for a 3D fault representation with a nonuniform slip distribution and the elastic-hydraulic layering of the half-space has an important role in the simulation results.

Modeling the Contribution of Poroelastic Deformation to Postseismic Geodetic Signals

“…Meanwhile, large vertical deformation is more directly related to the inverse of the coseismic head change. These relationships are consistent with those observed in other postseismic poroelastic studies (Albano et al, 2017;Nespoli et al, 2018) The overall pattern of the horizontal postseismic poroelastic displacement, shown in Figure 1b, is in the opposite direction of the fluid flow during poroelastic relaxation. This is contrary to the behavior observed around pumping wells, where horizontal displacements are generally in the direction of fluid flow and hence towards minima in head (Wang, 2000).…”

“…Poroelastic deformation is thought to occur on timescales of days to weeks, whereas longer timescales on the order of months to years are associated with afterslip and viscous relaxation (Barbot & Fialko, 2010;Helmstetter & Shaw, 2009;Freed et al, 2017;Perfettini & Avouac, 2004). However, postseismic poroelastic deformation can occur on multiple timescales that overlap with those currently linked to afterslip and viscous relaxation (Hughes et al, 2010;McCormack & Hesse, 2018;Nespoli et al, 2018). Thus, attributing surface deformation to these different processes may be more complicated than previously thought.…”

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

“…Differently, fluid diffusion‐related phenomena should propagate with the square of time (e.g., Ross et al, ). It has also been shown that most of the pore fluid dynamics occur in the shallowest layers in the Emilia seismic sequence (Nespoli et al, ) and that, in general, the permeability should considerably decrease with depth (e.g., Ingebritsen & Manning, ). All these considerations led to exclude the fluid diffusion as a dominant triggering mechanism for the aftershocks migration.…”

“…Nevertheless, this aspect is still controversial. Nespoli et al () show that the pore fluid dynamics occur in the shallowest layers (i.e., less than 3.3‐km deep); at greater depths, the permeability is lower and the fluid flow is not sufficient to drain the rock. They model the influence of fluids and pore pressure changes on surface displacements and on the CFF in a 3‐D half‐space and obtain small postseismic pore pressure and CFF changes on the Mirandola fault plane that exclude fluid migration as a primary triggering mechanism.…”

Aftershock Rate and Pore Fluid Diffusion: Insights From the Amatrice‐Visso‐Norcia (Italy) 2016 Seismic Sequence

“…Fluid diffusion after large earthquakes alters crustal stresses and can facilitate subsequent events. This phenomenon has been described by a posteriori complex models that can quantify the stress perturbations and assess the time required for pore fluids to return to hydrostatic conditions (Albano et al, ; Antonioli et al, ; Jónsson et al, ; Malagnini et al, ; Nespoli et al, ; Tung & Masterlark, ; Tung et al, ). The extent of the pore pressure perturbation and the flow recovery time depend on the modeling approach and parameters.…”