Controls on Fore‐Arc Deformation and Stress Switching After the Great 2011 Tohoku‐Oki Earthquake From Discrete Numerical Simulations

Abstract

The large magnitude of the 2011 Mw 9.0 Tohoku‐Oki earthquake, which occurred off the east coast of Japan, was not expected or predicted by any previous studies. One surprising observation was the sudden change in stress state; local earthquakes confirmed a compressional stress state before the main shock, whereas an extensional stress state was evident after the main shock. Using discrete element method modeling, this project attempts to reproduce the stress change after the main shock, and explores the conditions that can cause stress switching both onshore and offshore Tohoku. Our simulations demonstrate that rapid fault weakening can produce stress change and predominant normal‐fault earthquake mechanisms in the upper plate of Tohoku‐Oki. Several specific conditions seem to favor such stress switching; the megathrust fault must have been frictionally strong before the main shock, and comparable values of internal (μinternal) and basal friction (μbasal) are necessary to cause the formation of widespread normal faults within the wedge. Furthermore, dynamic extension during elastic unloading appears to play an important role in accommodating stress changes and wedge deformation in the Tohoku area; these cannot be explained solely based on Critical Coulomb Wedge models, but instead require dynamic unloading processes.

Relationships Among Forearc Structure, Fault Slip, and Earthquake Magnitude: Numerical Simulations With Applications to the Central Chilean Margin

“…Building on recent modeling efforts investigating the controls on extensional deformation in the Japan Trench forearc following the Tohoku earthquake (Wang & Morgan, 2019), we use numerical simulations to estimate the megathrust slip distributions that would arise from different ratios of inner and outer wedges for a reference subduction zone, and then compare the magnitudes of megathrust slip to published slip distributions for the Valdivia and Maule earthquakes. The well‐documented Tohoku earthquake also provides a valuable point of comparison, as it also experienced trench breaking rupture (Ito et al., 2011; Sun et al., 2017; Wang & Tréhu, 2016) and thus is a useful analog for the less well‐constrained Valdivia event.…”

The state of pore fluid pressure and 3D megathrust earthquake dynamics

“…The larger rotations in these scenarios appear to scale with fault slip and stress drop, both of which are larger than in scenarios 4 and 6. Wang and Morgan (2019) attribute observed changes in stress orientations following the 2011 Tohoku earthquake to rapid weakening of a statically strong fault with µ s in the range of 0.3 -0.6. This is supported by the scenarios presented here with high P f , where the megathrust is statically strong in terms of its moderate value of µ s =0.4, but dynamically weak, in terms of its dynamic friction coe cient of µ d =0.1.…”

“…It has been suggested that principal stress rotations are promoted by complete or near-complete stress drops that permit principal stresses to swap orientations (Brodsky et al, 2017(Brodsky et al, , 2020Wang & Morgan, 2019). However, by connecting 2D stress rotations to the ratio of stress drop over pre-earthquake deviatoric stress magnitude, Hardebeck (2012) shows that partial stress release may generate moderate rotations.…”

Structural Features and Evolution of the Northwestern Sichuan Basin: Insights From Discrete Numerical Simulations

“…Geologists have applied DEM mainly to gain insight into continuum fracture problems. Large-scale fold and thrust belt deformation, and shallow crustal rocks subject to Coulomb behavior (Burbidge and Braun, 2002;Naylor, 2005;Hardy et al, 2009;Miyakawa et al, 2010;Dean et al, 2013;Morgan, 2015;Morgan and Bangs, 2017;Liu and Konietzky, 2018;Vora and Morgan, 2019;Wang and Morgan, 2019). One of the attractions of DEM is that discrete particles under the contact mechanism can localize strain and yield emergent behavior in the system.…”