Magnitude 6.4 Earthquake Shakes Taiwan; AIR, EQECAT Analysis

March 5, 2010

Catastrophe risk modeling firms AIR Worldwide and EQECAT have both issued bulletins analyzing the magnitude 6.4 earthquake that struck south central Taiwan Thursday morning at 8:20 a.m. local time.

AIR noted that 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.”

EQECAT said the earthquake “was perceptible throughout the island, and moderate shaking intensity occurred in the epicentral region, which is mountainous and relatively remote. However, most portions of Taiwan, including the capital of Taipei, 250 km {156 miles] north of the epicenter, experienced shaking intensity below the threshold typically associated with damage. The earthquake also initiated a limited number of damaging fires and landslides.”

EQECAT’s early estimates put “total economic damages from this event not to exceed $1 billion, including losses from secondary fires and landslides.” Insured losses are expected to be only a “small fraction” of that figure.

Dr. Bingming Shen-Tu, principal scientist at AIR Worldwide explained: “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 tall and high-rise buildings should be unaffected by today’s quake.”

AIR also noted that “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.”

Dr. Shen-Tu described Taiwan’s geological terrain. He noted that the island 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.”

In addition AIR said; “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.

Dr. Shen-Tu explained, “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. Therefore, 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.”

According to the Taiwan Residential Earthquake Insurance Program (TREIP), 26 percent of residential property is insured against earthquakes, up from just 6 percent in 2002, when the insurance pool was formed.

Source: AIR World wide – and EQECAT –

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