Strong M7.1 Earthquake Strikes the Central Philippines: AIR

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BOSTON, Oct. 16, 2013 – According to catastrophe modeling firm AIR Worldwide, at 8:12:32 Tuesday morning (UTC), a magnitude 7.1 earthquake struck the central Philippines. The epicenter was located about 619 kilometers south southeast of Manila near the city of Catigbian on Bohol Island. The quake struck at a depth of 20 kilometers and caused widespread damage.

The early morning timing of Tuesday’s temblor combined with the closure of schools and offices due to a national holiday likely mitigated the quake’s deadly impact. However, there are still many reports of damage, injuries, and loss of life throughout the impacted region. Cebu province, home to more than 2.6 million people, and nearby Bohol province, with a population of 1.2 million people apparently
sustained the worst damage.

According to AIR, on Bohol Island, there are extensive reports of collapsed buildings, cracked roads, and fallen bridges. Government officials declared a state of calamity for both the islands of Cebu and Bohol authorizing additional national government assistance for those areas.

According to AIR, the Philippine Mobile Belt (PMB) is a region of rapid deformation and intense volcanism, where multiple tectonic blocks are confined between two, opposing subduction zone systems produced by the convergence of the Philippine Sea Plate and the Sundaland (part of Eurasia) Plate. Subduction occurs along the west-verging Philippine Trench-East Luzon Trough and the east-verging Manila-Negros-Cotabato Trench. The entire PMB is a region of intense earthquakes due to these different subduction zones, and also due to internal strike-slip faulting occurring mainly along the Philippine Fault System and its numerous splays, which traverses the entire Philippine archipelago from South to North.

Tuesday’s earthquake occurred in the South-Central Philippines, a region which absorbs the east-westEarthquake Insurance compression due to the Negros Trench. It is bounded on the east by crustal faults: the Philippine Fault, on the north by the Verde Passage-Sibuyan Sea Fault System, on the west by the Negros Trench, and on the northwest by the Mindoro-Palawan collision zone (situated in between the Manila Trench and Negros Trench). Further east of the Philippine Fault, westward subduction along the Philippine Trench occurs along this region. The immediate region is pervaded by shallow strike-slip earthquakes on the east due to the Philippine Fault, and thrust earthquakes on the west, due to the Negros Trench and related thrust faults. Further east, strong thrust earthquakes due to subduction along the Philippine Trench are also common.

According to AIR, this particular M7.1 earthquake is characterized by northwest-southeast thrust motion, originating from a shallow crustal source at 20 km deep which may be associated with thrust faults related to the Negros Trench. The last major earthquake to occur in this immediate area was an M6.7 event occurring in Negros Island on February 6, 2012. This event also struck at a depth of 20 km and
was associated with a nearly similar thrust mechanism orientation. In 1948, an M8.1 earthquake occurred west of this area, along the shallow subduction interface of the Negros Trench. For the entire PMB, among the most recent destructive earthquakes include the M7.8 1990 Luzon Earthquake, a shallow strike-slip event which occurred along a segment of the Philippine Fault, approximately 700 km north northwest of this region. In 1976, a destructive M8.0 tsunamigenic earthquake occurred along the
Cotabato Trench, about 370 km south of the region. The latter two events caused thousands of fatalities (>1,600 and >5,000 respectively) and extensive property damage.

According to AIR, in the Cebu and Bohol areas, as in most of the Philippines, the prevalent construction types are generally unreinforced masonry, reinforced masonry, reinforced concrete, and wood. Reinforced masonry performs much better than unreinforced masonry under small to moderate amounts of shaking. However, they are both prone to collapse under strong shaking. Many reinforced concrete buildings in
the region are also very vulnerable due to not being built to standard because of a lack of code enforcement and code compliance practitioners. Among the prevalent building types in the region, those constructed without engineering consideration are most vulnerable. In the main urban cities such as Cebu, buildings are constructed relatively well, but in coastal areas there are many non-engineered and
poorly constructed buildings.

Inadequate foundation in addition to poor construction practices have also resultedin building failures in the region. For instance, a magnitude 7.9 earthquake centered in the Celebes Sea, between the Islands of Mindanao and Borneo produced extensive damage to buildings constructed on alluvial deposits without adequate pile support.

The seismic code in the Philippines has since been updated in 2001. However, the significant damage caused by three moderate earthquakes since 2001 (as many as 800 buildings in the M6.8 Palimbang earthquake in 2002) is evidence that the codes have only begun to affect construction practices.

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