TECTONICS OF THE KYUSHU-RYUKYU ARC AS EVIDENCED FROM SEISMICITY AND FOCAL MECHANISM OF SHALLOW TO INTERMEDIATE-DEPTH EARTHQUAKES

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Abstract
Tectonic features of the Kyushu-Ryukyu arc have been investigated in detail, on the basis of seismicity and focal mechanism of shallow to intermediate-depth 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 100km reaches 70° in the northern arc, while it decreases to 40-50° in the southern section. Focal mechanisms of intermediate-depth earthquakes show down-dip tension and down-dip compression within the northern and southern parts of the lithosphere, respectively. Two interpretations may be possible of the above differential subduction, both of which are attributed to the difference in shear resistance to the subduction; one is due to a difference in the convergence plate velocities, and the other to that in the physical properties such as viscosity and density in the surrounding mantle in relation to volcanism. For lower resistance, the lithosphere will sink rather smoothly into the asthenosphere, which is dragged down by a gravitational pull with high dip angles and will be in a tensional state, whereas the subducting lithosphere will be subjected to compressional stress and have low dip angles if it receives higher resistance

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