Stress–strain ‘paradox’, plate coupling, and forearc seismicity at the Cascadia and Nankai subduction zones

Abstract

In the continental forearcs of the Cascadia and Nankai subduction zones, geodetic strain measurements indicate crustal contraction in the direction of plate convergence (nearly ‘margin-normal’), but earthquake focal mechanisms and other stress indicators show maximum compression along strike (‘margin-parallel’). It is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles, not the absolute stresses. The absolute margin-normal stress is not only much less than the margin-parallel stress but also no greater than the lithostatic value, indicating a very weak subduction thrust fault. Weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults. Great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime. Because these small elastic stress perturbations are relatively fast, they give large strain rates that are detected by geodetic measurements. The present nearly margin-normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence. With the margin-parallel compression being dominant, the small increase in margin-normal stress may change the forearc seismicity in Cascadia from a mixture of thrust and strike–slip types into mainly thrust before the next great subduction earthquake. In Nankai, the change should be from strike–slip to quiescence, as observed in the Nankai forearc prior to the 1944–1946 great subduction earthquakes.