HR: 1340h
AN: T33B-0533    [Abstracts]
TI: Focal depth distribution using sP depth phase and implications for plate coupling in the Hyuganada region, Japan
AU: * Tahara, M
EM: tahara@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Shimizu, H
EM: shimizu@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 2-5643-29 Shin'yama, Shimabara, 855-0843 Japan
AU: Nakada, M
EM: mnakada@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AB: The recurrence interval and magnitude of great interplate earthquakes and characteristics of crustal uplift in the Late Pleistocene indicate clear lateral variations of the interplate coupling on the convergent boundary along the Nankai Trough, i.e. strong for off the Shikoku and Kii Peninsula and relatively weak around the Hyuganada region. 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 causes the observed tensile events and weakens the interplate coupling on the convergent boundary. This may be related to the weak interplate coupling around the Hyuganada region.
DE: 8122 Dynamics: gravity and tectonics
DE: 8123 Dynamics: seismotectonics
DE: 8138 Lithospheric flexure
DE: 8150 Plate boundary: general (3040)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005