Magnitude 7.6 Earthquake in Philippines; Triggers Tsunami Warnings

Earthquake in Philippines

BOSTON, Aug. 31, 2012 – According to catastrophe modeling firm AIR Worldwide, a major earthquake struck off the town of Guiuan (population 44,000) in Samar province at 12:47 UTC today. The USGS had initially issued a magnitude of 7.9 for the event but later downgraded the estimate to 7.6. Depth is issued at 34.9 kilometers. Because today’s event occurred off the coast of Philippines and take-up rates in the affected region are low, insured losses resulting from this event are expected to be minimal.

According to AIR, Immediately after the earthquake, the Pacific Tsunami Warning Center issued a tsunami warning for several countries including Japan and for Pacific islands as far away as the Northern Marianas. Since then, many warnings have been lifted, leaving only ones for the Philippines, Indonesia, and Palau.

According to local reportsEarthquake in Philippines, there have been widespread power outages and damage to infrastructure, with reports still surfacing.

“Today’s earthquake occurred in the subduction zone of the Philippine trench, where the Philippine Sea plate subducts under the Philippine archipelago eastward at a rate of 3-5 centimeters per year,” said Dr. Bingming Shen-Tu, senior principal scientist at AIR Worldwide. “The 1,000 kilometer-long subduction zone is very active in respect to both seismicity and volcanism. Unlike its neighboring subduction zone-where the Philippine Sea plate subducts beneath southwest Japan and has led to several earthquakes of magnitude 8-8.5 in the past 200 to 300 years-some seismologists have postulated that this subduction zone system may not be as efficient in producing large ( >8.0 ) earthquakes.”

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“This event occurred in a segment where no earthquakes larger than 7.5 have occurred in the known history. Large earthquakes (magnitude 7.8-7.9) have ruptured the subduction zone segments both to the north and south of this segment in 1951-1952 and 1975. Therefore, today’s earthquake appears to fill a gap where tectonic energy in the subduction zone has not been released for a long time.”

Dr. Shen-Tu concluded, “In 1990, a powerful 7.8 earthquake struck the densely populated island of Luzon. This was the most catastrophic earthquake to impact the Philippines in recent history.”

Since that event, the Philippines has made significant improvements in their building standards during the past several decades. In coastal areas, houses are usually elevated to prevent damage from tidal waves. Such houses often have pitched and overhanging roofs covered in nipa palm or galvanized iron sheets. Although these houses do not qualify as engineered structures, they generally perform well and usually do not collapse during an earthquake. In 2001, the code was updated to align with the Uniform Building Code, and dynamic analysis is now required for structures that are greater than 75 meters (20 stories in height).

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