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

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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.

Key Points

Stress reversal in offshore forearc due to the Tohoku‐Oki earthquake indicates weak subduction megathrust with effective friction ~0.032
Most of the forearc is under very low differential stress and is stable throughout subduction earthquake cycles except the outer wedge
Earthquakes and active faulting in a strong and stable forearc reflect heterogeneities in structure, stress, and/or pore fluid pressure

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Fault lubrication during earthquakes.

G. Di Toro, R Han, T Hirose … (2011)

The determination of rock friction at seismic slip rates (about 1 m s(-1)) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip. Given the difficulty in determining friction by seismological methods, elucidating constraints are derived from experimental studies. Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1-2.6 m s(-1). The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built, quartz-built and carbonate-built) rocks and non-cohesive rocks (clay-rich, anhydrite, gypsum and dolomite gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7-15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved.