Transition from oblique subduction to collision: Earthquakes in the southernmost Ryukyu arc-Taiwan region
Tectonic characteristics of the region between Taiwan and the southernmost Ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5.5≦mb≦6.6. Five major seismogenic structures can be delineated: the Collision Seismic Zone (CSZ), the Interface Seismic Zone (ISZ), the Wadati-Benioff Seismic Zone (WBSZ), the Lateral Compression Seismic Zone (LCSZ), and the Okinawa Seismic Zone (OSZ). In the CSZ, located along the east coast of Taiwan and offshore, earthquake focal mechanisms show horizontal P axes distributed in two directions, 287°±10° and 333°±16°, possibly reflecting a strain partition associated with the relative plate convergence between the Eurasia plate and the Philippine Sea plate. The corresponding seismic strain tensor indicates a maximum compressive strain rater of 1.2x10-7 yr-1 along 293° and a comparable extension in vertical direction, presumably resulted from plate collision in the region. The geometry of the ISZ, which is distorted significantly at its northward with increasing dip. The seismogenic portion of the interface spans a short depth range form ~10 km to ~35 km. A clear pattern of earthquake slip partition is observed; the average slip vector residual is as large as 35°. Seismic strain patterns within the subducted Philippine Sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at ~270 km, different from the pattern of strain segmentation observed for the rest of the Ryukyu arc where the northern and southern portions are dominated by downdip extension and compression, respectively. Owing to the large convergence obliquity, the slab is descending at a rate significantly slower near Taiwan than in the southern Ryukyu. Thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab's negative buoyancy. A number of thrust of oblique strike-slip earthquakes between ISZ and WBSZ show a consistent pattern of lateral compression with P axes oriented roughly parallel to the local strike of the trench-arc system. They are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere. Shallow normal-faulting earthquakes show successive rotation of T axes from approximately N-S in the Okinawa trough to approximately E-W in northeast Taiwan, possibly as a result of interaction between the extension from the opening of the Okinawa trough and the compression form collision. One normal event (January 18, 1991 , mb=5.9) occurred in the Central Range with T axis roughly parallel to the structural trend of Taiwan, implying that the nature of the orogeny in Taiwan has changed from "thin-skinned" deformation to lithospheric collision involving the whole crust and uppermost mantle.