The Determination of Lithospheric Rheology and Long-Term Interplate Coupling in Japan: Finite Element Modeling
Northeast Japan experienced an approximately constant, compressional deformation during the last 5 million years resulting from the steady subduction of the Pacific plate. Because the direction of the maximum compression axis is approximately perpendicular to the strike of the island arc, 2-D finite-element modeling can be used to examine the deformation over time of the island-arc lithosphere.
The model geometry is based on geophysical and geological data, and each model run requires an assumed rheology and interplate coupling. Novel to our modeling is the ability to include erosion/deposition loading and the creation of strike-slip faults, based on a dynamically-applied fracture criterion. The criterion for acceptability is how well a model matches observed present-day topography, gravity, and seismicity patterns. Results given below are for models that satisfy this criterion.
The long-term effective elastic thickness is 10 km in the inner arc, increasing to about 50 km near the trench. The effective elastic thickness in the inner arc is therefore much smaller than the about 30 km short-term elastic thickness estimated from seismological data.
The viscosity of the lower crust is on the order of 1022 Pa s or less. The strength of interplate coupling off Sanriku is about two to four times greater than off Miyagi, and there is about twice as strong a coupling at greater depths. The relative strength of coupling correlates well with the observed interplate seismicity. Hence the inferred weaker coupling off Miyagi indicates a lack of seismogenic potential -- a low probability for large earthquakes in that region, not just a long return cycle.
The same modeling procedure was also applied to southwest Japan. The viscosity of the lower crust is not more than 1021 Pa s, and the elas tic thickness is about 10 km.
The calculated strength of interplate coupling for southwest Japan is about 1.5 times greater than for the off-Sanriku region in northeast Japan, which correlates well with the fact that there have been great (M>8) earthquakes in the Nankai Trough region, but none that large in the off-Sanriku region.