Absolute Strength of Megathrust Faults: Constraints From Stress Drop of the 2011 Great Tohoku Earthquake

Abstract:

The state of stress in forearc regions depends on two competing factors: stress coupling along the plate interface that generates margin-normal compression, and gravity which generates margin-normal tension. Studies of focal mechanisms of small earthquakes indicate that the 2011 Tohoku earthquake changed the stress state over much of the Japan Trench forearc from margin-normal compression to tension. That is, the plate coupling effect was dominant in controlling forearc stresses before the earthquake, but the gravitational effect has become dominant after the earthquake. This change requires either (1) a nearly complete stress drop on a strong megathrust (e.g., from 50 MPa to 10 MPa) or (2) a partial stress drop of a very weak megathrust (e.g., from 15 MPa to 10 MPa). Option 1 appears to be consistent with dramatic dynamic weakening observed in high-slip-rate rock friction experiments. Option 2 is consistent with seismological observations that large earthquakes are characterized by low average stress drop values. In this work, we demonstrate that the second option is correct. We show that a weak fault, represented by an apparent coefficient of friction of ~0.03 can put the Japan Trench forearc into margin-normal compression, and lowering this value by ~0.01 can switch the forearc into tension. For the Tohoku earthquake, we determine an average stress drop value from each of the 20 published coseismic slip distributions models that were constrained by seismic, geodetic, and tsunami data. Our results reconcile seismic observations that average stress drops for large megathrust events are quite low with laboratory work on high-rate weakening which predicts very high or complete stress drop. We find that, in all the rupture models, regions of high stress drop (20 – 80 MPa) corresponding to high-rate weakening can occur, but are surrounded by regions of stress increase, so that the average stress drop remains low (< 5 MPa). The very low average stress drop during the Tohoku earthquake, consistent with values reported for other large earthquakes, makes it unambiguous that the Japan Trench megathrust is very weak. Investigating the heterogeneity of coseismic stress change also helps us understand rupture mechanics of the megathrust.