By David Jacobson, Temblor
Just before 1:30 p.m. local time, a deep M=6.2 earthquake struck the northern Philippines, near the capital city of Manila. The quake occurred underneath the southwestern portion of the island of Luzon, which is home to over 53 million people, making it the most populous island in the Philippines. Because of the depth of the earthquake (168 km according to the USGS) no damage has been reported, though because of its proximity to the city of Manila, buildings swayed, leading to brief evacuations. Based on the USGS ShakeMap, only light shaking was experienced, and it is estimated that no damage should have been caused by the quake. Residents of Manila told ABC reported that the shaking lasted for only a few seconds and that other than the brief evacuations, there were no power or communications disruptions.
The Philippines is one of the most seismically active countries on earth as it is flanked to the east and west by two subduction zones. Additionally, cutting through the country is the Philippine Fault, 1,200 km-long left-lateral strike-slip fault which has produced historic, large and damaging earthquakes. Based on the focal mechanism produced by the USGS, today’s M=6.2 earthquake was compressional in nature, meaning it likely occurred on the subduction zone off the western coast of the country. This subduction zone is where the Sunda Plate subducts beneath the Philippine Sea Plate at a rate of approximately 96 mm/yr. At the southern end of this subduction zone, it begins trending to the southeast, matching the strike of today’s earthquake, which is why it is believed to be the source of the quake.
While deep earthquakes are not extremely common, they are not unheard of in this part of the Philippines. In 2004, two M=6+ earthquakes less than a month apart struck this part of the Philippines. Both of these quakes occurred at depths greater than 100 km, and had compressional focal mechanisms. Therefore, today’s quake should not be considered an extremely surprising event. Unlike most of the earthquakes we cover, we cannot use the Global Earthquake Activity Rate (GEAR) model for this quake because the GEAR model does not forecast earthquake likelihood for events at greater than 70 km depth.
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