(February 24, 2012) Reservoir-induced seismic events in dam-mad China are a growing problem requiring urgent attention.
By Patricia Adams
A magnitude 4.8 earthquake that rattled Hong Kong last week, thought to have been triggered by the filling of the Xinfengjiang dam reservoir on China’s mainland, is the aftershock of a serious earthquake that occurred 50 years ago, for the same reason, claims a researcher at the Institute of Geology, China Seismological Bureau.
Ma Wentao, speaking to Caixin Online, a Beijing-based business news outlet, said a 1962 magnitude 6.1 earthquake—that still ranks as one of the world’s largest and most famous reservoir-induced seismic events—caused ruptures to the underlying structure of the Xinfengjiang dam’s surrounding area in the northern part of Guangdong Province, leaving it prone to seismic activity, like a “wound” that leaves a scar.
Any increase in stress, for example, changes in the dam’s reservoir level, can trouble this scar and easily cause it to “fester,” he said. Mr. Ma was unable to determine a direct relationship between last week’s tremor and the dam’s reservoir level, however, because data showing water level changes in the reservoir are still unavailable. The aftershock event on Thursday, February 16, was felt in Guangzhou, Shenzhen and Hong Kong.
Mr. Ma pointed out that back in the 1950s, when the dam was constructed, no earthquakes had been recorded historically in the region so engineers failed to consider the effect a reservoir would have on a fault zone. In fact, according to Xia Qifa, a senior engineer at the China Academy of Hydropower and Water Conservancy, who witnessed the March 19, 1962 quake, “at that time we had no idea that filling a reservoir could trigger earthquakes, so we treated the Xinfengjiang tremor as a natural occurrence.”
Now, scientists know better.
Reservoir-induced seismic events occur when a full reservoir creates extra pressure in the micro-cracks and fissures in the ground under and near the reservoir, in essence lubricating them. When the reservoir is drawn down, the friction caused by the mass of the reservoir relaxes, allowing slippage to occur.
Though experts estimated back in 1998 that China had experienced 19 cases of RIS ranging from the M6.1 RIS event at Xinfengjiang to a 2.2M event at the Dengjiaqiao Dam, that number seems to have increased.
According to data from the Guangdong Provincial Seismological Bureau, at the Xinfengjiang Dam alone, six earthquakes with magnitudes greater than 5 (including the largest M6.1 quake in 1962) have occurred, and small and medium earthquakes have been relatively active, with 44 greater than a magnitude of 4 on the Richter scale, occurring since impoundment began.
In addition, Mr. Ma, who calls RIS a “normal phenomenon,” estimates that more than 40 reservoirs have induced seismic (RIS) events in different provinces of China.
The Xinfengjiang Dam-induced earthquake in 1962 was a wake-up call for Chinese scientists who, Mr. Ma says, started paying attention to the risk of RIS events in seismic-prone regions when building dams. But in recent years, he says, this caution has given way. Now, more and more dam projects are slated for construction, or have already been completed in China’s southwest, where the geology is very complicated and the need to consider the risk of RIS vulnerability, greatest.
Meanwhile, Chinese authorities largely dismiss RIS concerns, saying that a low-level seismic response to the filling of large dam reservoirs is to be expected and that dams can be designed to withstand severe shaking.
But a study obtained by the Toronto-based watchdog Probe International, in June of last year, disclosed a 30-fold increase in seismic activity in China’s massive Three Gorges dam reservoir area, as a result of the dam’s construction, which also helped to explain numerous landslides that have caused havoc in the region since.
Evidence is growing that China’s massively destructive 2008 earthquake in Sichuan was triggered by the Zipingpu dam. The dam cracked and, though it withstood the 7.9 earthquake, it subsided up to one metre and was displaced downstream up to 60 cm. An estimated 90,000 people were killed by landslides and collapsed buildings caused by the earthquake.