Modeling the Contribution of Poroelastic Deformation to Postseismic Geodetic Signals

Abstract

To constrain the poroelastic component of postseismic deformation, we model the subsurface hydrologic response to the M w 7.6 subduction zone earthquake that occurred on the plate interface beneath the Nicoya peninsula in Costa Rica on 5 September 2012. The model shows that poroelastic relaxation occurs on multiple time scales and the associated deformation can be up to 2 cm for the trench-perpendicular component. By modeling the time-dependent deformation associated with poroelastic relaxation, we can begin to remove its contribution from the observed geodetic signal. Inversions for after slip that ignore poroelastic deformation have errors of 10-20% overall and up to 50% locally. Poroelastic effects can both mute and amplify the inferred afterslip.Plain Language Summary Large earthquakes can affect regional groundwater pressures.This sets up large-scale groundwater movements that in turn lead to observable surface deformation. Accounting for these deformations is important during interpretation of GPS signals after large earthquakes. These measurements are often used to infer ongoing deformation on the fault. We show that if these calculations do not account for deformation due to groundwater flow, this can lead to errors of up to 40%. evaluated to time-dependent three-dimensional behavior of poroelastic rebound and its effect on afterslip estimates. To best capture the spatial and temporal extent of the poroelastic response, high-rate well level data across the study area is needed. Alternatively, we can use models based on well-known physical parameters to get a first-order approximation of this response.

Role of Poroelasticity During the Early Postseismic Deformation of the 2010 Maule Megathrust Earthquake

“…Moreover, poroelastic processes locally alter the estimated afterslip by up to ±40% near the region of maximum coseismic slip compared to the results of a purely elastic model. Similar patterns have been also reported for the 2012 Nicoya Costa Rica (K. McCormack et al., 2020) and the 2015 Illapel Chile (Yang et al., 2022) earthquakes. Nonetheless, in the work by K. McCormack et al.…”

“…Moreover, poroelastic processes locally alter the estimated afterslip by up to ±40% near the region of maximum coseismic slip compared to the results of a purely elastic model. Similar patterns have been also reported for the 2012 Nicoya Costa Rica (K. McCormack et al, 2020) , where A o is the cumulative afterslip calculated from the inversion approach, t is the time after the main shock, t r is the characteristic time of relaxation, and t c the critical time, which is introduced to avoid the singularity at t = 0 (Avouac et al, 2015).…”

“…Our results show that the predicted poroelastic vertical displacement is about two times higher than the horizontal displacement (Figure 3f), which is in good agreement with previous studies (Hu et at., 2014; Hughes et al., 2010; Masterlark et al., 2001; K. McCormack et al., 2020). Poroelastic vertical surface displacement patterns can also explain a major part of the observed uplift near the maximum coseismic slip region (Figure 3c).…”

“…The onset of the poroelastic and viscoelastic postseismic deformation is driven by the coseismically induced response (e.g., Hughes et al., 2010; K. McCormack et al., 2020; Masterlark et al., 2001). We prescribe the coseismic slip model of Moreno et al.…”

“…However, previous studies often neglect both poroelastic and viscoelastic relaxation, assuming that afterslip is the dominant process and that the crust and upper mantle respond in a purely elastic fashion (e.g., Aguirre et al, 2019;Rolandone et al, 2018;Tsang et al, 2019). Recently K. McCormack et al (2020) and Yang et al (2022) investigated the poroelastic effects on afterslip inversions during the first ∼1.5 months following the 2012 M w 7.8 Nicoya, Costa Rica, and 2015 M w 8.3 Illapel, Chile, earthquakes, using Global Navigation Satellite System (GNSS) data. They show that the resulting amplitude of afterslip may be affected by more than ±50% in regions of ∼40 × 40 km 2 when neglecting poroelasticity.…”