Seismicity and tectonics of the western Pacific: Izu-Mariana-Caroline and Ryukyu-Taiwan regions

Abstract

The hypocenters of about 1000 earthquakes in the Izu-Bonin, Mariana, Palau, Caroline, and Ryukyu-Taiwan regions were relocated by computer, and earthquake mechanism solutions based on the first motions of P, pP, and S were determined for 26 earthquakes. The spatial distribution and the mechanisms were compared with major tectonic features such as volcanic zones, island arcs, and trenches. Although a nearly continuous zone of shallow earthquakes can be traced from Honshu to Palau, deep shocks were not detected south of 16°N; intermediate-depth events were not found south of the Marianas. A region of low seismicity for shallow earthquakes and a discontinuity for deeper events occurs in the Volcano Islands adjacent to a prominent gap in the trench where the Marcus-Necker ridge intersects the arc. In some profiles of this series of arcs, the deep seismic zone is nearly planar, but in other cases it is more contorted and the thickness of the zone appears to vary from place to place. Maxima and minima in seismic activity occur for deep and intermediate depths, and these features migrate in depth similar to that found in the Tonga arc. Activity as deep as about 680 km occurs in the northern Marianas where the deep focal zone is nearly vertical for depths greater than about 200 km. This nearly vertical distribution suggests that gravitational tectonics may play an important role as a motive force for global tectonics. Seismicity in the Ryukyu arc is concentrated in a thin planar zone that dips 35° to 45° northwest to a depth of about 280 km. Near the northwest coast of Taiwan all tectonic elements change strike abruptly, and the mechanisms change from thrusting of the Philippine Sea plate beneath the Ryukyu arc to strike slip near the east coast of Taiwan. South of this point, most of the events are shallower than 100 km, and the focal zone is not well defined in vertical sections. No events were detected from the Kyushu-Palau ridge, from the Central Basin fault, or from the rest of the interior of the Phillipine Sea. Thus, this region appears to be a single lithospheric plate. Normal faulting is found for some earthquakes either in the deepest parts of the trench or along the seaward wall. A zone of shallow earthquakes also can be identified on many of the profiles near the volcanic axis. Mechanism solutions for two events near this zone are also of the normal-fault type. Mechanism solutions for shallow earthquakes beneath the islandward margins of the Izu-Bonin and Mariana trenches indicate that the Pacific plate is underthrusting the Philippine Sea plate in a westerly direction. The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W. Washington, D.C. 20037. Document J69-002; $1.00. Payment must accompany order.