Tectonics of the Kyushu-Ryukyu Arc as Evidenced From Seismicity and Focal Mechanism of Shallow to Intermediate-Depth Earthquakes

Abstract

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

Source mechanisms of subcrustal and upper mantle earthquakes around the northeastern Kyushu region, southwestern Japan, and their tectonic implications.

“…It is obvious that the focal depths of the deepest earthquakes increase from the northern to southern region. It may also be seen that the dip of the Wadati-Benioff zone becomes abruptly steeper at depths around 50-70 km, particularly in sections C, D, and E, and that the seismicity at these depths is quite low, as have been pointed out by SHIONO et al (1980). Figure 3C (cross section C) includes the hypocenter of the August 26, 1983 Kunisaki peninsula earthquake (M=6.8, h=116 km), with a dotted circle, while the Suonada earthquake of July 13, 1979 is included in Fig.…”

“…These features imply that the subcrust and upper mantle beneath the northern to central Kyushu region are strongly subjected to tensional stress. It is evident from cross-sectional seismicity maps along profiles A-E given in Figs. 3A-E that the focal depths of the deepest earthquakes increase steeply from the northeast to southwest profile, as has been pointed out by SHIONO et al (1980), indicating that the Philippine Sea plate is subducting deeper in southwestern Kyushu (about 150 km) than northeastern Kyushu (about 50 km). The difference in the penetration depth of the subducting oceanic plate will yield a southwestdownward pull due to negative buoyancy acting on the excess mass of the slab in the southwest region, and hence could explain the systematic alignment of the T-axes.…”

“…The stress state at subcrustal depths over an extensive region from the southern Chubu region to the Iyonada, northwestern Shikoku are characterized by systematic alignments of the tensional axis parallel to the leading edge of the subducting Philippine Sea plate (parallel extension). SHIONO (1977) and SHIONO et al (1980) also found that the tensional axis dips westwards and is approximately parallel to the dip of an inclined seismic zone under the Kyushu Island and the northern Ryukyu arc. In the inland Kyushu region, on the other hand, tensile stress working in an N-S direction appear to prevail at a shallow crust, in view of the focal mechanism with normal faulting type of relatively large earthquakes in central Kyushu (YAMASHINA and MURAI, 1975).…”

Petrology and geochemistry of volcanic rocks dredged from the Okinawa Trough, an active back-arc basin.

“…The outer islands form the main island chain extending from Kyushu to Taiwan, and are the prolonga tion of the geological zones of southwest Japan. The inner islands correspond to an active volcanic arc (called Ryukyu island arc, hereafter) associated with a subduction of the Philippine Sea plate along the Ryukyu trench, and are fixed relative to the 100-km focal depth of the Wadati-Benioff zone dipping northwest from the Ryukyu trench beneath the Ryukyu island arc and the Okinawa Trough (Shiono et al, 1980). Submarine topographic data, seismic reflec tion profiles and seismic refraction records revealed the principal features of the extensional processes taking place in the Okinawa Trough (Lee et al, 1980;Letouzey and Kimura, 1986;Nagumo et al, 1986).…”

Microearthquake activity in the West Philippine Basin and northern Ryukyu Trench region as observed by ocean bottom seismometers at the Amami Plateau.

“…Routine land network observation of seismicity shows that the West Philippine Basin is aseismic and that the activity is associated with the trencharc subduction zone (e.g., KATSUMATA and SYKES, 1969;SHIONO et al, 1980). SHIMAMURA et al (1975) deployed an ocean bottom seismometer (OBS) at the axis of the Central Basin Fault and recorded no earthquakes in its vicinity during an observation period of 60hr.…”