Intra-plate seismicity in the subducting Philippine Sea Plate, southwest Japan: magnitude–depth correlations

Recent studies of intra-plate seismicity have focused on detailed earthquake locations and potential hypotheses for intermediate depth events. Three main competing hypotheses for intra-plate events are: (1) transformational faulting; (2) shear melt instabilities; and (3) dehydration reactions. Recent work, using precise hypocenters and thermal-petrological models, indicates that the spatial distribution of intra-plate events coincides with dehydration reactions. These studies have mainly focused on the location of events and the proposed reactions with little attention given to the magnitude distribution of events. Presented here is a magnitude–depth correlation associated with intra-plate events for southwest Japan. Earthquake hypocenters were obtained from relocating a data set consisting of Japan Meteorological Agency (JMA) seismicity data and Japan University Network Earthquake Catalog (JUNEC). Data from 1985 to 1996 were combined and relocated using a standard location routine (HypoInverse) that allows regional variations in velocity models and station corrections. HypoInverse results show a well defined plane of seismicity associated with the subducting Philippine Sea Plate. The data were further relocated using a double-difference relocation algorithm (HypoDD). HypoDD results show a magnitude–depth correlation for Kii peninsula and northern Kyushu. Histogram plots indicate that events near the plate interface were generally magnitude 3 or less, whereas most large events were located within the lower crust or possibly the subducting oceanic mantle. This observation also appears to hold true for intra-plate events beneath Tohoku (Japan trench) and Hokkaido (Kuriles), but is not clearly observed beneath the Tokai region. A dehydration reaction model can well explain the magnitude–depth correlation.