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

Abstract

The northern part of the West Philippine Basin is presently subducting at the Ryukyu Trench. 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. There are several anomalous bathymetry highs in the West Philippine Basin which imply complex tectonic history of the region. They are the Amami Plateau, Daito Ridge, Oki Daito Ridge, and Kyushu-Palau Ridge (Fig. 1). Dredge hauls and drill cores indicate the Daito Ridge to be a remnant island arc (e.g., KLEIN and KOBAYASHI, 1980). Seismic refraction profiling data over the Oki Daito Ridge and the Kyushu Palau Ridge show thick crust with velocities similar to those of continents (MURAUCHI et al., 1968; LUDWIG et al., 1973). It is not known whether these anomalous regions are seismically active or not except the Central Basin Fault. Further, the topography suggests that some of these bathymetry highs are interacting with another plate at the subduction zone. Thus, seismic investigations at these regions to clarify local seismicity and structures may provide important clues to the understanding of their tectonic roles, which are not yet well known. We deployed a tripartite seismic array of OBSs on the Amami Plateau to in-