Offshore Landward Motion Shortly After a Subduction Earthquake Implies Rapid Relocking of the Shallow Megathrust

Abstract

Geodetic observations after large subduction earthquakes reflect multiple postseismic processes, including megathrust relocking. The timing of relocking and the observational constraints on it are unclear. Relocking was inferred to explain some observed landward motion that occurs within months. It was also considered unable to explain other, greater landward motion, including that off the coast of Japan beginning weeks after the 2011 Tohoku earthquake, attributed to postseismic relaxation. We use generic, 3D numerical models to show that relocking, particularly of the shallow interface, is needed for postseismic relaxation to produce landward motion on the tip of the overriding plate. We argue that this finding is consistent with previous simulations that implicitly relock the megathrust where afterslip is not included. We conclude that the Tohoku megathrust relocked within less than 2 months of the earthquake. This suggests that the shallow megathrust probably behaves as a true, unstably sliding asperity.

How Long Can the Postseismic and Interseismic Phases of Great Subduction Earthquake Sustain? Toward an Integrated Earthquake‐Cycle Perspective

“…Because of the coincidence of the surge of great megathrust earthquakes (Hayes, 2017; Lay, 2015) and the maturity of space geodesy (e.g., GNSS) in the early 21st century (Bürgmann et al., 2013), most of the recent modeling efforts are dedicated to the deformation through the earthquakes (i.e., late‐interseismic, coseismic, and early‐postseismic phases), which is hence best studied in the above‐described cycle (e.g., Bedford et al., 2016; D'Acquisto & Govers, 2023; Fukuda & Johnson, 2021; Govers et al., 2018; S. Li et al., 2015; Sobolev & Muldashev, 2017; Sun et al., 2014, 2018; Trubienko et al., 2013; Wang et al., 2012). In particular, based on the study of Wang et al.…”