March 4, 2010 Taiwan Earthquake

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Posting Date: March 4, 2010, 10:00:00 AM

A magnitude 6.4 earthquake struck south central Taiwan this morning at 8:20 a.m. local time (just after midnight GMT). The epicenter was located about 40 km (25 miles) west northwest of the city of Taitung (est. pop. 200,000) on Taiwan's east coast, and 60 km (40 miles) east southeast of the city of Tainan (est. pop. 800,000) on the island's west coast. Focal depth has been estimated at 23 km (14.3 miles) by the U.S. Geological Survey, but at just 5 km (3 miles) by Taiwan's Central Weather Bureau (CWB). It is still in the early aftermath of this event and these estimates may well be revised as additional information comes in from seismic networks worldwide. The quake was centered in Kaohsiung County, a relatively sparsely-populated mountainous region that last year experienced devastating flooding and mudslides from Typhoon Morakot. No deaths have been reported, though there have been dozens of injuries from falling debris. Power was interrupted to more than 500,000 households on several parts of the island, but has since been restored. High speed rail service came to a halt and will remain suspended for at least 24 hours while tracks are inspected. The quake was felt as far away as the capital Taipei, some 250 kilometers from the epicenter, where skyscrapers swayed for several minutes. Nearly two dozen aftershocks have occurred, the largest measuring M5.7. In Shanshang, Tainan County, a textile warehouse went up in flames within minutes of the quake. Company officials have estimated damage at NT$100 million (USD 3 million). Spokespeople for Taiwan's seimiconductor industry have indicated that highly sensitive equipment was unaffected. The Ministry of Education has said that 99 schools were damaged, though the available information suggests that the nature of the damage is largely non-structural. In the epicentral region, temporary housing built for people made homeless from last year’s typhoon reportedly collapsed; it is not clear whether the structures were still inhabited. Elsewhere, ground-shaking toppled goods from supermarket shelves and pictures from walls. In Taiwan, the majority of low- to mid-rise buildings are constructed with reinforced concrete frames and brick infill walls. Current Taiwan Building Codes (TBC) require ductile detailing of reinforced concrete frames, similar to the requirements of the American Concrete Institute and the Uniform Building Code (UBC) of 1982. Tall buildings are dominated by construction using reinforced concrete frames and shear walls. At the level of reported shaking, AIR expects damage to unreinforced masonry construction near the earthquake’s epicenter. The degree of damage will depend, in part, on whether focal depth is closer to the current estimate issued by the USGS or that issued by the CWB. In general, the shallower the earthquake, the more damaging it will be. In the population centers of Tainan and (to a lesser extent) Taitung, AIR expects damage to be largely limited to nonstructural elements such as glazing, cladding, suspended ceilings and interior walls, as well as to contents. Well-engineered high-rise buildings should be unaffected by today’s quake. According to the Taiwan Residential Earthquake Insurance Program (TREIP), residential take-up rates are today roughly 26%, up from just 6% in 2002, when the insurance pool was formed. That year, earthquake insurance became a mandatory element of homeowners insurance for newly bought residential properties that are financed through bank loans. Last December, a M6.4 earthquake struck just off the sparsely populated east coast of Taiwan, near the city of Hualien, but it was a significantly deeper event and caused no significant damage. In 1999, the M7.6 Chi-Chi earthquake in central Taiwan killed more than 2,000 people and caused property losses estimated at USD 11 billion. Taiwan is located in the collision zone between the Philippine Sea plate and the Eurasian Plate. The most important tectonic features in Taiwan are the Longitudinal Valley Fault Zone (LVF) in eastern Taiwan and the Deformation Front Fault Zone (DFZ), a fold-thrust fault zone in western Taiwan. The DFZ is composed mainly of a series of active crustal faults along its central and northern segments. Thus, most historical earthquakes in central and northern Taiwan have been shallow. Toward the south, the DFZ gradually makes a transition to becoming a subduction zone. Along this segment, earthquakes are relatively deep. There is some disagreement as to the source of today’s quake. The CWB has suggested the source as the southern section of the Chaochou Fault, but this has been dismissed by other seismologists because it has long been thought that the Chaochou fault in this region is inactive. However, today’s earthquake occurred at the intersection of several active faults that border the southwestern Central Mountain range—including the Chishan Fault as well as the Chaochou Fault. While in fact there have been few historic earthquakes in southern Taiwan compared to central Taiwan in the last century, the deformation rate in this area actually is as high as the rate in central Taiwan, as shown by recent GPS surveys. Thus, the possible occurrence of an earthquake on the Chaochou Fault or the Chishan Fault is not necessarily surprising. Two earthquakes of similar magnitude occurred in 1964 and 1993 less than 30 km (less than 20 miles) northwest of today’s reported epicenter. The AIR earthquake team is compiling and analyzing the available information on the parameters of this event, around which there is still considerable uncertainty. At this time, damage appears to be quite limited in scope. However, if the results of AIR’s analysis show otherwise, additional information will be made available.

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Posting Date: March 4, 2010, 10:00:00 AM