Seismotectonics of the Philippine and Taiwan Subduction Systems and Implications for Seismic Hazards

The seismicity of the Philippines and Taiwan provides insight into the tectonics and seismic hazards of a region characterized by subduction and collision. We summarize the seismotectonics of the Philippines and Taiwan by documenting the distribution of hypocenters for earthquakes of magnitude M >= 4.6 and focal mechanisms for earthquakes of magnitude M >= 5.0 over similar to 21 years. We quantify seismicity rates (earthquake frequency) and compare seismicity distributions with proposed tectonic and faulting models. 6,187 earthquakes of magnitude M >= 4.6 occurred between 1 January 2000 and 31 March 2021, 79% at depths <70 km and 70% having magnitudes M < 5.0. Approximately 88 earthquakes of magnitude M >= 5.0 occur per year, with 12 events of magnitude M >= 7.0 occurring since January 2000. Seismic activity decreases exponentially between 50 and 210 km depth at a rate similar to 10% faster than the global average. Intermediate and deep earthquakes at depths >70 km trace the Wadati-Benioff zones of subducting slabs, most of which are only seismically active to depths of similar to 250 km. The distribution of earthquakes at depths >70 km is likely influenced by the subduction of young lithosphere, slab tearing, and phase boundary interactions between depths of 410 and 660 km. Shallow earthquakes at depths <= 70 km are generated by megathrust, crustal, and intraslab faulting. Crustal thrust and strike-slip faulting are the most abundant and prevalent sources of damaging earthquakes. The Philippines and Taiwan are subject to high seismic risk, similar to nearby Indonesia.