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.