Seismic and volcanic activity in the Philippine Islands was examined in an attempt to decipher the tectonics of this region. Several new fault plane solutions for shallow, intermediate, and deep focus earthquakes were determined. This study has revealed the presence of a zone of eastward underthrusting in the western Philippines which is well developed near Negros Island. Fault plane solutions of several shallow earthquakes in the western Philippines show thrust faulting with slip vectors toward east or northeast. As active eastward subduction of the Eurasian plate is also taking place along the Manila trench near west central Luzon, it suggests that the underthrusting of the Eurasian plate may have occurred at one time along the western Philippine from Taiwan to Sulawes in the Molucca Sea. Subduction has ceased along sections where continental crust is present. This interpretation is consistent with the geology and gravity anomalies in the area. Near the eastern Philippines the westward subduction of the Philippine Sea plate occurs (1) along the Philippine trench and (2) in a localized zone near the western edge of the Benham rise. The Philippine Islands are therefore flanked in the east and west by active but disjointed subduction systems. Left lateral strike slip faulting has been deduced near one end of several of these active trench systems and suggests movement on transverse features. Seismic activity on the Philippine fault is concentrated in the zone between 10°N and 15°N and appears to be due to stresses generated by opposing movements of the Philippine and Eurasian plate which are not released in underthrusting. Fault plane solutions of shallow earthquakes associated with the Philippine fault show left lateral strike slip motion consistent with field observations. Our study
suggests that the extent and magnitude of earthquake activity on the Philippine fault forms one component of movement between the Eurasian plate and the Philippine Sea plate.