Seismicity patterns along the northern Manila Trench reflect crustal properties of the subducting plate

We examine the seismicity recorded by two temporary ocean bottom seismometer arrays deployed along the northeastern Manila Trench, covering the outer rise and part of the accretionary wedge. A total of 1437 events were determined during the 44-day recording period, showing a relatively more dynamic seismic activity than previously thought. Most events occurred in the subducting plate, oceanward of the trench, at depths between 25 and 50 km and revealed normal-fault type mechanisms. The relatively deeper focal depth and the lack of well-developed outer rise bulge near the seismic cluster suggest an untypical pattern of outer rise earthquakes. Seismicity with a high occurrence frequency and small magnitude is typical of earthquakes swarm, which is generally thought to be driven by fluid pressure variations. We suggest the infiltration of seawater through fractures into the rifted South China Sea margin is the main driver of the earthquakes detected during our deployments. Because earthquakes are also more abundant in the area where a significant change of crustal thickness is documented, we suggest the inherited crustal properties and tectonic structures strongly control the seismic behavior along the Manila subduction system.