Focal depth distribution using sP depth phase and implications for plate coupling in the Hyuganada region, Japan

The recurrence interval and magnitude of great interplate earthquakes and characteristics of crustal uplift in the Late Pleistocene indicate clear lateral variations of the plate interaction on the convergent boundary along the Nankai Trough, i.e. relatively less cohesive for the Hyuganada region than for off the Shikoku and Kii Peninsula. The Hyuganada region is characterized by both significant aseismic crustal uplift in the Late Pleistocene and negative free-air gravity anomalies with the maximum magnitude of ∼-130 mgal. In order to examine the relationship for these observations, we investigated the focal depth distribution in the Hyuganada region by using sP depth phases. The focal depth distribution obtained by sP depth phases indicates different seismicity patterns in the southern and northern regions of the forearc. That is, most events for the southern and northern areas are thrust type and normal fault type, respectively. In the central part of the Hyuganada region located in the central area of negative gravity anomalies, however, each type event occurs in clusters and there is a seismicity gap between each cluster. The hypocenters for thrust type events generally coincide with the plate boundary as usual cases. However, most normal fault type events, with hypocenters in the landward side for the thrust type events and T-axes parallel to the plate boundary, occur in the crust or around the plate boundary of the forearc and the epicenters coincide with the peak position of negative gravity anomalies. The correlation between the seismicity and gravity anomaly suggests that the buoyancy inferred from negative gravity anomalies may cause the observed tensile events and a few relatively small asperities of the observed M_w 7 class earthquakes on the convergent boundary. This may be related to relatively less cohesive coupling around the Hyuganada region compared with that for off the Shikoku.