Overlapping post-seismic deformation processes: afterslip and viscoelastic relaxation following the 2011 Mw 9.0 Tohoku (Japan) earthquake

Abstract

Large tectonic earthquakes lead to significant deformations in the months and years thereafter. These so-called post-seismic deformations include contributions mainly from afterslip and viscoelastic relaxation, quantification of their relative influence is of importance for understanding the evolution of post-seismic crustal stress, strain and aftershocks. Here, we investigate the post-seismic deformation processes following the 2011 M w 9.0 Tohoku earthquake using surface displacement data as observed by the onshore global positioning system network in the first ∼1.5 yr following the main shock. We explore two different inversion modelling strategies: (i) we simulate pure afterslip and (ii) we simulate the combined effect of afterslip and viscoelastic relaxation. By assuming that the afterslip is solely responsible for the observed post-seismic deformation, we find most afterslip activities to be located close to the downdip area of the coseismic rupture at 20-80 km depth with a maximum cumulative slip of ∼3.8 m and a seismic moment of 2.3 × 10 22 Nm, equivalent in moment to an M w 8.84 earthquake. By assuming a combination of afterslip and viscoelastic components, the best data fit is found for an afterslip portion that is spatially consistent with the pure afterslip model, but reveals a decreased seismic moment of 2.1 × 10 22 Nm, or M w 8.82. In addition, the combined model suggests an effective thickness of the elastic crust of ∼50 km overlying an asthenosphere with a Maxwell viscosity of 2 × 10 19 Pa s. Temporal analysis of our model inversions suggests that the rate of afterslip rapidly decreases with time, consistent with the state-and rate-strengthening frictional law. The spatial pattern of afterslip coincides with the locations of aftershocks, and also with the area of coseismically increased Coulomb failure stress (CFS). Only a small part of the coseismically increased CFS was released by the afterslip in 564 d after the event. The effect of the viscoelastic relaxation within this initial stage only plays a secondary role, but it shows an increasing tendency, that is, the contribution of viscoelastic relaxation increases with time. Further geodetic observations are needed for a robust quantification of the role of the viscoelastic relaxation in the post-seismic deformation.

Interpretation of Offshore Crustal Movements Following the 2011 Tohoku-Oki Earthquake by the Combined Effect of Afterslip and Viscoelastic Stress Relaxation

“…That is to say, the afterslip has mainly proceeded at the downdip extension of the main rupture. This is a characteristic common to most of the preceding studies (Ozawa et al 2012;Diao et al 2014;Yamagiwa et al 2015) except Perfettini and Avouac (2014). The point at issue will be whether significant afterslip has occurred or not in a shallow part of the plate interface, which gives a key to understanding the frictional property of plate interfaces in subduction zones.…”

“…The thickness of elastic surface layer and the viscosity of viscoelastic substratum are almost the same as those used in the preceding studies: e.g., 50 km and 2 9 10 19 Pa s for Diao et al (2014); 50 km and 9 9 10 18 Pa s for Yamagiwa et al (2015).…”

“…First, using an elastic half-space model, namely ignoring the effect of viscoelastic stress relaxation in the asthenosphere, Ozawa et al (2012) and Perfettini and Avouac (2014) estimated afterslip distribution from the postseismic GPS data on land for almost the same period (the first 7 and 9 months, respectively), though the results are totally different. Then, Hashima et al (2014) and Diao et al (2014) demonstrated the importance of the contribution of viscoelastic stress relaxation to the postseismic crustal movements through a forward numerical simulation of co-and post-seismic internal deformation and a comparison of the inversion results of postseismic GPS data with a simple afterslip model and a combined afterslip-viscoelastic relaxation model, respectively. Using the combined afterslipviscoelastic relaxation model, Yamagiwa et al (2015) estimated coseismic slip and afterslip distributions simultaneously from the whole data set of GPS and GPS/Acoustic observations at and after the Tohoku-oki earthquake.…”

Analysis of the Far-Field Co-seismic and Post-seismic Responses Caused by the 2011 M W 9.0 Tohoku-Oki Earthquake

“…5a); the total moment released by the afterslip is equivalent to that of a M W 8.77 earthquake. DIAO et al (2013) found that the relative proportion of the crustal deformation caused by the viscoelastic relaxation in the post-seismic deformation increased with time after the event. Using an inversion method with the post-seismic GPS crustal deformation data from Japan, they estimated the optimal values of elastic crust thickness and lithosphere viscosity to be 40 km and 8 9 10 18 Pa s, respectively, both of which are close to the inversion results from the post-seismic crustal deformation data after the Hokkaido Nansei-Oki M W 7.8 earthquake in 1993 (UEDA et al 2003).…”

“…Based on the GPS-based post-seismic crustal deformation data from Japan GEONET, DIAO et al (2013) analyzed the temporal and spatial evolution of the aseismic fault slip near the Japan Trench after the earthquake. The same authors also estimated the effective viscosity coefficient of the upper mantle along the Japan Trench subduction, which can be used to explain the post-seismic crustal deformation observations in Japan.…”