Precise hypocenter distribution of deep low-frequency earthquakes and its relationship to the local geometry of the subducting plate in the Nankai subduction zone, Japan

Abstract

[1] We determine the precise hypocenter distribution of deep low-frequency earthquakes (LFEs) in the Nankai subduction zone and compare it with the local geometry of the subducting Philippine Sea plate. We apply a new hypocenter determination method utilizing the summed cross correlation coefficient over many stations, termed a network correlation coefficient (NCC), to 112 LFEs in the western Shikoku and 1566 LFEs in the whole Nankai subduction zone. While the catalog depths are widely distributed in some regions, the relocated hypocenters in every region construct a plane surface several km above the oceanic Moho interface and quite consistent with the geometry of the oceanic Moho. This result strongly supports the hypothesis that LFEs in the Nankai subduction zone occur on the subducting plate boundary and are directly generated by shear slips. If LFEs are indeed direct indicators of the locations of the plate interface, they might be useful to investigate the minute structure of the plate interface. The thin distributions of LFEs indicate that the interface between the subducting and the overriding plates is a distinct very thin boundary, and not a distributed shear zone.