Kinematics of the Philippine Sea Plate

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Abstract
In this study new instantaneous rotation vectors describing the motion of the Philippine Sea plate relative to surrounding plates are determined that most nearly satisfy all available geological, geophysical and seismological observations along the plate margins. Pacific‐Philippine and Caroline‐Pacific, poles are generated from a set of trial Eurasia‐Philippine poles of rotation, using published determinations of the Eurasia‐Pacific and Caroline‐Philippine poles. The Pacific‐Philippine, Caroline‐Pacific, and Eurasia‐Philippine poles are then checked for agreement with: (1) slip vectors determined from focal mechanisms of earthquakes along the Ryukyu trench and the Nankai trough, (2) convergence at the Sagami trough, (3) extension in the Mariana trough, (4) extension behind the Bonin trench, (5) extension in the western Sorol trough, and (6) convergence at the Mussau trench. Trial Eurasia‐Philippine poles are eliminated if predicted plate motions do not match observed plate motions. The locations and rotation rates of the three best fitting poles are Eurasia‐Philippine: 37.0°N, 141.0°E, 1.60°/Ma; Pacific‐Philippine: 3.8°N, 130.4°E, 1.68°/Ma; Caroline‐Pacific: 16.9°N, 141.4°E, 0.54°/Ma. The plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the Philippine, Pacific, and Caroline plates intersect. Intermediate and deep focus earthquake hypocenters along the southern Mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the Mariana trough disappears. The absence of volcanism and intermediate‐depth seismicity along the southernmost Mariana, Yap, and Palau sections of the plate boundary is attributed to very low convergence rates. A new focal mechanism solution along the trench south of Guam is the first clear underthrusting solution found along the Mariana trench and agrees with the convergence direction implied by the Pacific‐Philippine pole of rotation. No clear underthrusting focal mechanism solutions were found along the Yap and Palau trenches, although three high‐angle thrust solutions demonstrate that compression is occurring in the upper plate, perpendicular to the plate boundary.