According to catastrophe modeling firm AIR Worldwide, a magnitude 6.2 earthquake struck central Taiwan Sunday night, June 2, 2013, at 9:43 p.m. local time (13:43 UTC), in a sparsely populated region of Nantou County just seven kilometers (4.3 miles) northeast of a similar rupture only two months ago (M6.0 on March 27). The epicenter of the Sunday night quake was located at 23.794oN, 121.082oE, by the United States Geological Survey and estimated to be at a depth of 20 km. AIR does not expect significant insured losses from this event because yesterday’s earthquake occurred in a rural area and take-up rates in the region are relatively low.
“The likely cause of the earthquake is ‘reverse faulting,’ which is when the two sides of a fault push toward each other,” said Dr. Bingming Shen-Tu, senior principal scientist at AIR Worldwide. “It was the largest earthquake in Taiwan so far this year.”
The event took place only 22 km south-southeast of the city of Buli (population 86,400). Taiwan’s third-largest city, T’ai-chung (population 2,630,000 in 2010), lies only 38 km (23.6 miles) to the northwest of the epicenter. Thus, although the immediate area around the quake’s epicenter is rural, as many as 35 million people live in the region. The event also caused shaking as far away as the island’s capital, Taipei (located about 250 km to the north [155 miles] to the north), where early reports indicate structural damage has been limited. Falling rock and rockslides, however, have been common in the epicentral region.
The Central Taiwan High Speed Rail service has been suspended. Similarly, cable car service has been suspended in many places, and power outages have been reported. Building damage to many schools has been reported, which includes cracked exterior walls, fallen tiles, and broken windows. Contents damage has also been reported. The most severe damage—and the cause of the deaths and serious injuries so far reported—has been the widespread rockslides and falling rocks.
Dr. Shen-Tu continued, “Earthquakes are not uncommon in Taiwan; it is located where the Philippine Sea Plate is colliding with and subducting (pushing under) the Eurasian Plate along a zone that extends from Taiwan itself to the southern part of Japan’s Honshu island. The island’s most important tectonic features 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 mostly made up of a series of active crustal faults along its central and northern segments. Most earthquakes occurring here rattle the countryside, but generally are minor and cause little or no damage.”
However, Nantou County, where Sunday’s earthquake took place, is the epicenter of the M7.6 Chi-Chi earthquake of 1999, which killed more than 2,400 people and caused property losses estimated at USD 11 billion. “Yesterday’s event occurred about 30-40 km east of the Chelungpu fault, and first indications are that it occurred further down the dip (slope) of the 1999 rupture,” commented Dr. Shen-Tu. “Compared to the Chi-Chi earthquake, however, this event is much smaller and farther away from the more populated west coastal areas. Thus, its impact is expected to be considerably smaller.”
According to AIR, the majority of low- to mid-rise buildings in Taiwan are constructed with reinforced concrete frames and brick infill walls. Tall buildings are dominated by construction using reinforced concrete frames and shear walls. Also, current Taiwan Building Codes require ductile detailing of reinforced concrete frames, similar to the requirements of the American Concrete Institute and the Uniform Building Code (UBC) of 1982. As a result of these advanced codes, modern buildings in Taiwan generally meet stringent seismic design requirements and are expected to perform well in earthquake events of this size.
At the level of reported shaking (V-VI/MMI), AIR expects some damage to unreinforced masonry construction near the earthquake’s epicenter. In population centers closest to the epicenter (within 15 km), AIR expects some building damage, with the majority of damage limited chiefly 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 the earthquake.