(May 28, 2014) The events rank as significant according to Chinese geologist and environmentalist Fan Xiao, who says they are signals that the seismic threat posed by Three Gorges Dam is at its most critical stage now.
By Lisa Peryman for Probe International
Another earthquake has struck the Three Gorges Dam reservoir region in central China’s Zigui County. No casualties have been reported so far and officials say the dam is operating normally. The 3.4 magnitude tremor which hit early Monday morning, some 23 km from the dam, follows two earthquakes of magnitude above 4.0 and hundreds of aftershocks which shook the same region in late March of this year.
- Monday, May 26, 2014: A 3.4 magnitude tremor hits Zigui County in central China’s Hubei Province at 6:40 a.m., some 23 km from the Three Gorges Dam at a depth of 5 km, according to the China Earthquake Networks Center.
- Sunday, March 30, 2014: A magnitude-4.7 earthquake shook Zigui county on Sunday morning in the Three Gorges reservoir area, around 23 km from the dam, following a jolt three days earlier in the same vicinity: a magnitude-4.3 tremor recorded early Thursday morning on March 27 about 30 km from the dam.
Understanding the forces behind the recent quakes in the Three Gorges Dam reservoir region should sound warning bells, according to Fan Xiao, a geologist and former chief engineer of the Regional Geological Survey Team of the Sichuan Geology and Mineral Bureau.
Large reservoirs built on fault lines are known to trigger earthquakes in a phenomenon called “reservoir-induced seismicity” (RIS). According to Mr. Fan, RIS is most likely to occur within a few years, even a decade after initial filling of a dam reservoir to its highest level, due to the time it takes for reservoir water to penetrate deep into seismic faults and fissures before it triggers seismic activity.
A 2010 study revealed seismic monitors around the Three Gorges Dam 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.
Reservoir-induced seismic activity is typically at its highest a few years after the routine high water level of reservoirs. The Three Gorges Dam is at that most critical stage now.
Fan Xiao: “Since the Three Gorges Dam took up water in 2003, including the water level at the high level of 2008, the region is at the most dangerous time, especially with the fault lines underneath the dam. Historically, earthquakes above magnitude of 6.0 have taken place around the reservoir area sitting on top of the fault lines. Therefore, the Zigui area is prone to reservoir-induced earthquakes of catastrophic nature.” [For more on this, see: Three Gorges Dam triggers frequent seismic activities].
For a report on the most recent tremor, see: Three Gorges Dam area not affected by quake.
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