The Philippines lies within the zone of complex interaction between several tectonic plates, involving multiple subduction zones and one large zone of strike-slip, all of which are associated with major earthquakes. Many intraplate earthquakes of smaller magnitude also occur very regularly due to the interaction between the major tectonic plates in the region. The largest historical earthquake in the Philippines was the 1918 Celebes Sea earthquake with M_w8.3.

Much of the Philippines lie within the area of strongly tectonised blocks of mainly island arc origin, known as the Philippine Mobile Belt. To the east, Philippine Sea Plate is subducting beneath the mobile belt along the line of the Philippine Trench and the East Luzon Trench at the northern end of the belt. The convergence across this boundary is strongly oblique and the strike-slip component is accommodated by movement on the left lateral Philippine Fault System. To the south of the Philippines lies the Molucca Sea Collision Zone, which involves opposite facing subduction zones to either side of the Molucca Sea Plate. To the west of the mobile belt the Sunda Plate is subducting eastwards beneath the belt along the lines of the Manila, Negros and Cotabato trenches. Within the Sunda Plate, the oceanic crust of the Sulu Sea is subducting beneath the Sulu Ridge along the Sulu Trench.^[1] The Sunda Plate carries with it parts of the Palawan Microcontinental Block, which has collided with the mobile belt at the Negros and Cotabato trenches.^[2]

The continuing movement of the tectonic plates leads to active faulting within the mobile belt, such as on the left lateral Cotabato Fault System that cuts across Mindanao and the right lateral Marikina Valley Fault System on Luzon.

Given the presence of major fault zones throughout the archipelago, any part of the Philippines may be affected by earthquakes, apart from parts of Palawan, where the seismic hazard risk is comparatively low. The greatest shaking hazard comes from shallow crustal faulting close to the Manila, Davao and Cebu metropolitan areas. Active reverse faults have >20 km wide zones of peak ground acceleration (PGA) >0.6g (acceleration due to gravity) for a 10% probability of exceedance (PoE) in a 50 year period, while active strike-slip faults have narrower zones centered around the fault traces at a similar level. All areas close to active subduction zones show increased hazard.^[9]

In Metro Manila the estimated hazard has a mean PGA of 0.32 g for a PoE of 10% in 50 years. The main hazard comes from shallow fault sources, such as the Marikina Valley Fault System, but there is an important contribution to the overall hazard from the Manila subduction zone to the west and the potential for strong shaking from earthquakes originating the Philippines Trench to the east. In Metro Cebu, the mean PGA is also 0.32 g for the same PoE and period. The hazard is dominated by shallow crustal fault zones from this area of ongoing compressional tectonics. Using the same parameters Metro Davao has the higher value of 0.45 g. The metropolitan area sits close to shallow faults of left lateral strike-slip and oblique reverse type, and these generate the greatest hazard, although a significant contribution comes from sources in the Halmahera and Philippine subduction zones.^[9]

• Geology of the Philippines