Stable Forearc Stressed by a Weak Megathrust: Mechanical and Geodynamic Implications of Stress Changes Caused by the M = 9 Tohoku-Oki Earthquake

Kelin Wang, Lonn Brown, Yan Hu, Keisuke Yoshida, Jiangheng He, Tianhaozhe Sun

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

Rupture-zone averaged static stress drop in the 2011 M=9 Tohoku-Oki earthquake was less than 5 MPa, but it caused a stress reversal in most of the offshore forearc, although the reversal is less well constrained far offshore by earthquake mechanisms because of 20- to 30-km errors in event depths. Using a finite element model of force balance, we demonstrate that the stress reversal unambiguously indicates (1) a very weak subduction megathrust and (2) very low differential stresses in the forearc. Prior to the reversal, the upper limit of megathrust strength could not be determined from forearc stresses. In the forearc, effects of megathrust friction and gravity are in a fragile balance, and stresses fluctuate around a neutral state in earthquake cycles. If most of the offshore forearc is to be compressive before but extensional after the earthquake, the effective friction coefficient of the megathrust must be ~0.032. Under low differential stresses associated with megathrust weakness, the forearc is generally well below yielding. Applying the concepts of dynamic Coulomb wedge, we show that the inner wedge, and by inference farther landward, stays stable throughout earthquake cycles. The outer wedge is stable most of the time but may occasionally enter a critical state during great earthquakes; its geometry suggests that complete stress drop of the underlying shallow megathrust is unlikely to have happened. We reason that the occurrence of earthquakes and active faulting under low stress in the stable forearc is due to heterogeneities in structure, stress, and/or pore fluid pressure.

Original languageEnglish
Pages (from-to)6179-6194
Number of pages16
JournalJournal of Geophysical Research: Solid Earth
Volume124
Issue number6
DOIs
Publication statusPublished - 2019 Jun

Keywords

  • 2011 Tohoku-Oki earthquake
  • dynamic Coulomb wedge
  • stable forearc
  • strength of subduction fault
  • stress change in earthquake cycles

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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