(April 1, 2014) A magnitude-4.7 earthquake hit Zigui county in central China’s Hubei Province last Sunday, around 23 kilometres from the Three Gorges Dam site location, several days after a magnitude-4.3 tremor was felt early Thursday morning about 30 kilometres from the dam. Authorities say the dam was not affected but they are monitoring the situation. There have been no reports of casualties or property damage, although news coverage has noted an increase in Chinese experts who support the speculation that the project itself is the cause of local seismic activity.
A magnitude-4.7 earthquake shook Zigui county on Sunday morning, March 30, where the Three Gorges Dam is located in central China’s Hubei Province, following a jolt three days earlier in the same vicinity.
The Beijing-based Global Times reports the epicenter of the Sunday quake was recorded with a depth of 5 kilometers and struck an area around 23 kilometers from the dam, saying the shake could be felt clearly. According to the Hubei Earthquake Bureau, no casualties have been reported so far in affected neighbouring counties and cities.
The Sunday quake followed a magnitude-4.3 tremor early Thursday morning on March 27 about 30 kilometers from the dam.
The China Three Gorges Corporation announced on its official website that the quake had not affected dam operations and that they were closely monitoring the dam complex. The water level and the navigation of ships in the Three Gorges reservoir were normal, it said, and there had been no reports of any large landslides as a result of the recent seismic activity.
In its coverage, the Global Times notes that “Chinese experts increasingly support the speculation that the project itself causes local earthquakes.”
It was perfectly normal for a large reservoir to cause earthquakes, Liu Shukun, a professor at the China Institute of Water Resources and Hydropower Research, told the Global Times.
“The water is 100 meters deep,” Liu said. “Its huge weight exerts massive pressure on the land below. It causes the geological structure to transform or break.”
Earthquakes were usually around magnitude 4, he said, with stronger ones rarely recorded. He warned authorities to closely monitor the issue.
Earthquakes happen more frequently during the initial stage of storing water, but decrease or even disappear as time goes by, the China Three Gorges Corporation told China Science Daily.
Chinese Academy of Engineering member Chen Houqun said in January that 100 earthquakes were recorded as caused by dams or reservoirs around the world, a “relatively small number” compared to the tens of thousands of dams globally.
However, a 2010 study by seismologists at the China Earthquake Administration (formerly known as the China Seismological Bureau) revealed that in the Three Gorges Dam region alone, seismic monitors around the reservoir and in Hubei Province registered 3,429 earthquakes between June of 2003 (when inundation of the reservoir began) and December 31, 2009: a 30-fold increase in seismic frequency over the pre-dam period.
Large reservoirs are known to trigger earthquakes in a phenomenon called “Reservoir-induced Seismicity (RIS)”. Interest in RIS has grown since geoscientists began suggesting that the Zipingpu dam in Sichuan province may have triggered the deadly 2008 earthquake that killed close to 90,000 people. Researchers and citizens alike are now alert to the risk of Three Gorges, the world’s largest dam, triggering an earthquake that could topple buildings. In a worst-case scenario, an earthquake could also damage the dam itself, with catastrophic consequence for the millions of people who live downstream of the project.
In December 2013, a 5.1-magnitude earthquake struck a mountainous and populous area of China’s Hubei Province, 100 kilometres from the Three Gorges Dam site. Described as “moderate but dangerous,” the quake injured several people and took a fair toll on a number of houses in affected areas, including around 16 million yuan (2.6 million U.S. dollars) in damage to property in rural areas.
Although that tremblor, with its relatively shallow depth of around 3 miles below the surface, was considered moderate, it was thought by some experts to portend further risk to the region.
According to John Jackson, a geologist with a detailed knowledge of western China, shallow earthquakes (less than 10 km deep) indicate active faults that could be reactivated by routine practices, such as the filling of a reservoir to accommodate flood waters and its drawdown to generate power. The location of large dams near clusters of recorded earthquakes with magnitudes greater than 4.9, and especially when the earthquake focal points are also close to the surface, “is cause for grave concern,” he said. [See Feverish Chinese dam building could trigger tsunami].
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Categories: China's Dams, Dams and Earthquakes, RIS, Three Gorges, Three Gorges Probe
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