Source Parameter Variability of Intermediate-Depth Earthquakes in Japanese Subduction Zones

The physical mechanism of intermediate-depth earthquakes is still uncertain. Dehydration embrittlement and thermal shear heating mechanisms are the leading hypotheses, and each has been supported both by observations and experiments. Slab character is likely to affect either mechanism. We apply uniform processing to data sets from the two main subduction zones in Japan: the older, colder, and faster-subducting Pacific plate and the younger, warmer, and slower-subducting Philippine Sea plate. We compare the stress drops and radiated efficiencies of intermediate-depth earthquakes in these settings and find no significant differences between the scaling of source properties. In particular, we find both an increase of stress drop and apparent stress with increasing moment for the Pacific Plate subduction in Hokkaido and for the Philippine Sea Plate subduction in Kyushu. We suggest that this, along with apparent invariance of radiated efficiency, suggests that an embrittlement process is more important in these regions than a thermal shear mechanism.