China's Dams

Aftershock from 2008 Wenchuan quake hits Sichuan

(December 4, 2012) Experts say the tremor that jolted Sichuan Province last weekend is an aftershock of the killer quake that struck the province in 2008, linked to the Zipingpu Dam.

by Lisa Peryman for Probe International

A 4.3 magnitude earthquake that shook southwest China’s Sichuan Province on Saturday, Dec. 1,  is an aftershock of the devastating May 12, 2008 Wenchuan earthquake that claimed more than 80,000 lives, say experts.

Striking at 23:16 Beijing time on Saturday in Beichuan county (the worst-hit area in the Wenchuan disaster), the tremor was also felt in Chengdu, the capital of Sichuan, as well as other cities. According to news coverage from China, the epicentre of the quake registered a depth of 18 km. The U.S. Geological Survey, meanwhile, lists the earthquake as magnitude 4.8 and 40.4 km in depth.

The Sichuan Seismological Bureau reports the tremor occurred along the Longmenshan Central Fault zone, the same fault belt involved in the deadly 2008 Wenchuan earthquake located at the eastern margin of the Tibetan Plateau and is an aftershock of that event. No casualties or damage from Saturday’s quake have been reported, so far.

A growing body of evidence suggests the 2008 M7.9 Wenchuan earthquake was triggered by the nearby Zipingpu Dam, after its reservoir began filling in 2004 near to where the earthquake’s epicentre occurred four years later.

Human-caused earthquakes, such as “reservoir-induced seismicity” (RIS), can rupture after a decade and create seismic activity that lasts for decades, says Christian D. Klose, of Think Geohazards in New York. In a recent paper on human-made earthquakes, published by the Journal of Seismology, Mr. Klose cites a disastrous earthquake that struck scenic Koynanagar town, and surrounding areas, in Maharashtra, India in 1967, near the site of the Koyna Dam. The earthquake measuring 6.3 on the Richter scale, occurred after the reservoir behind the dam had been filled. Tremors continue to be felt in the region decades after the original event and as recently as this year. The area was believed to have been seismically stable before the earthquake.

Reservoir filling and seismic activity have long been considered linked. When a reservoir is impounded (filled), it can cause mass shifts and water seepage can also penetrate micro-cracks and fissures in the depths beneath and near to the reservoir which, in essence, lubricates them. When the reservoir’s water level is lowered, the friction that keeps rock masses stable is relaxed, permitting slippage to occur. Earlier this year, a magnitude 4.8 earthquake in Hong Kong was thought to have been triggered by the filling of the Xinfengjiang dam on China’s mainland, 50 years after it first caused a M6.1 quake in 1962.

Just prior to Saturday’s earthquake in Sichuan, Chinese and U.S. military officers had joined together for a two-day simulated disaster exercise, to practice humanitarian rescue and disaster relief. State-run news agency, Xinhua, reports the  simulation was based on a 7.8-magnitude earthquake scenario for an unspecific Southeast Asian country, causing huge damages, injuries and leaks of chemical and radiative materials, according to a statement by the Chinese delegation. After completion of the drill, the U.S. delegation was also scheduled to visit sites affected by the 2008 Wenchuan earthquake.

Further Reading:

Mega-dams in China’s earthquake zones could have “disastrous consequences”

Public asked to report earthquake omens

Three Gorges Dam failing: Chinese dam increases risk of earthquakes

Feverish Chinese dam building could trigger tsunami

Chinese dam triggers earthquake, rattles Hong Kong

Earthquakes, dams and responsibility

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