Plate geometry model and seismicity in the northern Ryukyu subduction zone, Japan, deduced from amphibious seismic observations

Abstract

In this study, the geometry of the subducted Philippine Sea Plate interface beneath the northern Ryukyu Trench subduction zone was estimated based on seismic tomography and focal mechanism estimations using passive data obtained from a temporary amphibious seismic network and permanent land stations. Using relocated low-angle thrust-type earthquakes and information obtained from previous studies, we constrained the plate geometry from the trench axis to a depth of 60 km with uncertainties of <2 km between the depths of 15 and 30 km. The estimated plate geometry model appears to be smooth; there is no clear evidence of the existence of large subducted seamounts or ridges in the forearc of the study area. The active earthquake region near the plate boundary was primarily limited to the depth range of 15–30 km. These upper and lower limits may be controlled by the temperature along the plate boundary and the existence of serpentinized mantle in the hanging-wall plate, respectively. Moreover, we determined that the active region of interplate earthquakes is clearly separated from the area dominated by short-term slow slip events and low-frequency tremors, suggesting that suitable frictional conditions along the plate boundary for regular and slow earthquakes are different. There are two areas that were nearly seismically quiet, indicating either full interplate coupling or full creeping conditions. On comparing the historical record in the seismic catalog, tsunami records, and the very-low frequency earthquake distribution, one of the areas has a plausible tsunamigenic potential for an M∼8 event. The spatial pattern of seismicity shows obvious variations between the northern and southern halves of the study area; intraslab earthquakes were dominant in the northern half, whereas interplate earthquakes were dominant in the southern half. Furthermore, the lower limit of the seismogenic zone in the northern half was ∼5–10 km deeper than that in the southern half. These regionalities indicate that differences in age and/or temperature or degree of hydration in the north–south direction exist in the slab.