Tectonics of the Kyushu-ryukyu Arc as Evidenced from Seismicity and Focal Mechanism of Shallow to Inter- Mediate-depth Earthquakes

Tectonic features of the Kyushu-Ryukyu arc have been investigated in detail, on the basis of seismicity and focal mechanism of shallow to intermediatedepth earthquakes with magnitudes around and greater than 5.0. All shallow earthquakes in the Hyuga-nada region east of Kyushu are characterized by low-angled thrust faulting, which may be directly related to the underthrusting of the Philippine Sea plate beneath southwest Japan. Their frequent occurrence, dissected fault regions and somewhat large stress drop, in comparison with great earthquakes along the Nankai trough, may be attributed either to local stress concentration due to the contortion of the subducting plate resulting from southwestward warping of the trough, or to heterogeneous structure extending northward from the Kyushu-Palau ridge. The shape of the Wadati-Benioff zone and the stress state within the descending lithosphere show marked differences between the northern and southern Ryukyu arcs bounded by the Tokara channel; the dip of the zone at depths below