Date August 31, 2012 Depth 34.9 km | Magnitude Mw 7.6 Areas affected Philippines | |
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Tsunami Yes (highest 20 inches) Aftershocks 298 (as of September 2, 2012) | ||
An earthquake off the coast of Samar occurred on August 31, 2012, at 20:47 local time (12:47 UTC) in the Philippines. The populated islands of Visayas were struck by an earthquake of magnitude Mw 7.6. The earthquake has a depth of 34.9 km (21.7 miles). A tsunami warning was announced within the Pacific area and was later lifted after two hours. The Philippine archipelago is located in the Pacific Ring of Fire, where earthquakes and volcanic activity are common.
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Geology
The Philippine Mobile Belt is between the Eurasia Plate and the Philippine Sea Plate. The Philippine Sea Plate is subducting along the Philippine Trench in this region. At the latitude of the earthquake, the Philippine Sea Plate moves in the WNW direction at a velocity of approximately 100 mm/yr with respect to the Sunda Plate.
This earthquake was an intraplate earthquake greater than 50 kilometers to the east of the boundary of the Philippine Sea Plate. This earthquake was unusual in that it occurred as a result of reverse faulting within the oceanic lithosphere of the Philippine Sea Plate. There have been approximately 40 events of M6 and above over the past 40 years, within 250 km of this earthquake. Most of those earthquakes were the result of normal faulting within the shallower oceanic lithosphere rather than deeper reverse faulting like this event.
Earthquake
After the initial earthquake, the power supply was cut off in affected areas after power lines were damaged. Power plants in Visayas tripped or shut down following the earthquake, although no major damage was sustained in transmission facilities.
A house in the southern city of Cagayan de Oro, Mindanao, was involved in a landslide along with several other homes, killing one of its inhabitants and hospitalizing another.
In General MacArthur, Eastern Samar, 77 homes were damaged. There were also 6 houses damaged in Barangay Casoroy, San Julian. A wall from an old building collapsed in Butuan City.
The NDRRMC reported on Saturday noon that cracks had occurred in some structures such as bridges and roads. The damage was not severe.
Tsunami
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) gave a level three tsunami alert in the Philippines and Other institutes gave a level three tsunami alert in Japan, Indonesia, Taiwan, Palau indicating that the public should be on watch for "unusual waves", but did not call for any evacuation. The Pacific Tsunami Warning Center rapidly issued tsunami warning in the Pacific together with its adjacent islands. However, it was lifted after two hours as no tsunami was recorded. In Japan, the Japan Meteorological Agency issued tsunami warning throughout the coastal areas of the country. The Hong Kong Observatory also issued tsunami warning on Hong Kong on that same time.
About an hour after the quake, sea level readings from gauges in the epicentral region confirmed that a tsunami had been generated. A small 3 cm (1.2 in) wave was recorded at Legazpi in Albay province shortly thereafter, as well as further south near Davao City; slight sea level anomalies were observed in several other locations.
Response
Based on preliminary estimates of the magnitude, specialists noted that a significant tsunami could have been generated. A tsunami warning was issued within 8 minutes after the tremor for coastal regions of the Philippines, Indonesia, Taiwan, Guam, and Japan, as well as for the island nations of Palau, Yap, and Northern Mariana. A tsunami watch was at the time in effect for much of Micronesia, the Solomon Islands, Hawaii, and the Pacific coast of Russia. Most of the warnings and watches were quickly cancelled following revisions to the earthquake's intensity, although the warnings for the Philippines, Indonesia and Palau remained in effect.
The tremor caused widespread panic in Eastern Visayas, where many inhabitants exited their homes. At the risk of destructive waves, officials in the Philippines urged remaining residents of eastern coastal areas to evacuate to higher ground.