The instantaneous rotation vector of the Philippine sea plate relative to the Eurasian plate

Abstract

Using all the available fault-plane solutions of the inter-plate earthquakes along the northwestern and eastern boundaries of the Philippine Sea plate, the instantaneous rotation vector of the Philippine Sea plate relative to the Eurasian plate is determined (pole: 45.5°N, 150.2°E, angular velocity: 1.20 deg/m.y.). The solutions along the Taiwan—Philippine region are not used because of the tectonic complexity in this region. To overcome the difficulty in determining the rotation vector only from the northwestern boundary of the Philippine Sea plate, the general property that the three instantaneous rotation vectors, Pacific to Eurasia, Philippine Sea to Eurasia, and Pacific to Philippine Sea, are contained in the same plane, is used. The rotation vector of Pacific to Eurasia is assumed as in the model RM1 of Minster et al. (1974). The rotation vector of Pacific to Philippine Sea is determined at the same time. Using these results, historical earthquakes in the Sagami and Nankai trough region are studied on the basis of plate motion and fault models deduced from the geodetic data. In this region, the co-seismic slip is comparable to the rate of plate motion. The recent start of plate convergence (several million years ago) may be one reason for this strong coupling of the oceanic lithosphere with the continental one. The slip vectors at the triple junction which connects the Japan trench, Sagami trough, and Izu-Bonin trench, show that this junction is not stable and migrates to the northwest along the Sagami trough at the rate of 3 cm/yr. However, if the crustal extension exists behind the Izu-Bonin trench, it may stay stable. The slip vectors of the shallow earthquakes along the eastern coast of Luzon and the Philippine trench deviate from those computed from the rotation vector of the Philippine Sea plate relative to the Eurasian plate. To explain this, a minor plate which includes the entire Philippines is introduced on the evidence of seismicity and topography. The tectonic complexity in the Taiwan-Luzon region may be reduced to the collision of this block with Taiwan.