Large Earthquakes and Crustal Deformation Near Taiwan

Source parameters of 15 large (mb -> 6) earthquakes which occurred near Taiwan during the last 25 years are determined by synthetic seismogram analysis of teleseismic P and S waveforms. Source depths range from 13 to 88 km. Three earthquakes of intermediate depth are located within the southernmost limit of the Ryukyu arc and show thrust-type mechanisms with T axes subparallel to a northward dipping Wadati-Benioff zone. Shallow earthquakes (<30 km) are distributed over a broad region along and off the east coast of Taiwan and show both high-angle reverse and strike-slip mechanisms. The P axes of the shallow events located off the east coast of northern Taiwan are oriented near horizontal, trend south to southwest, and may reflect relative motion between the Okinawa platelet and Taiwan or complex deformation at the southwest limit of the Ryukyu Trench. In contrast, P axes of shallow earthquakes both onshore and off the east coast of central and southern Taiwan consistently trend northwest and are interpreted to reflect relative convergence between the Philippine Sea and Eurasian plates. Summation of the seismic moment tensors of eight large shallow earthquakes that occurred along and off the central and southern coast of Taiwan shows that relative plate motion between the Eurasian and Philippine Sea plates, when averaged over the last 25 years, has been accommodated by crustal contraction at a rate of 2.6-5.4 cm yr-• along an azimuth of about 290 ø .