PHIVOLCS recorded 8,403 aftershocks, and within 17 days of the earthquake, 348 aftershocks above M w 4.5 were recorded by the USGS the aftershock sequence occurred southeast of the mainshock. Maximum slip was concentrated around the epicenter, although slightly up-dip, which was estimated at up to 1.8 m (5 ft 11 in). A finite fault model suggests rupture occurred around an elliptical area measuring 80 km × 80 km (50 mi × 50 mi). ![]() At this location, the Philippine Sea Plate moves west-northwest at a rate of about 103 mm (4.1 in) per year with respect to the Sunda Plate. The earthquake occurred as a result of shallow oblique-thrust faulting likely along the subduction interface of the Philippine Trench. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) said it had a magnitude of 7.4 and that it measured VII ( Destructive) on the PHIVOLCS Earthquake Intensity Scale in Tandag. The United States Geological Survey reported the earthquake had a magnitude of 7.6 and a maximum Modified Mercalli intensity of VII ( Very strong). The segment involved in the Decemearthquake was associated with a magnitude 7.5 earthquake in 1921 which produced a tsunami. Some large intraslab earthquakes have been associated with the trench including the M w 7.6 normal-faulting earthquake of 1975 and a reverse-faulting M w 7.6 shock in 2012. Large earthquakes occurring along the Philippine Trench are limited in records although two large earthquakes were recorded in 1897 (M s 7.3) and 1924 (M s 8.2) with little seismological characteristics known. The convergence rate along the trench varies from 3.2–5.4 cm (1.3–2.1 in) per year. At its southern portion, it runs north–south from the eastern coast of Mindanao to the northern part of Halmahera Island. ![]() The Philippine Trench which runs east of the Philippines islands represents a major subduction zone where the Philippine Sea Plate subducts westwards. See also: Subduction tectonics of the Philippines
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