Katy Yan
International Rivers
May 1, 2013
On the morning of April 20th, a deadly earthquake struck Ya’an city in Sichuan province, killing at least 193 people and injuring 12,211 more. Recorded as magnitude 6.6 by the US Geological Survey, the Ya’an earthquake has revived debates around reservoir-induced seismicity (RIS) and where the next big earthquake might hit in China’s earthquake-prone and increasingly dammed southwest region.
According to Fan Xiao, a geologist and chief engineer of the Regional Geological Survey Team of the Sichuan Geology and Mineral Bureau, the Ya’an earthquake may have been an aftershock of the magnitude-7.9 Wenchuan earthquake that collapsed schools and killed around 80,000 people in May 2008. Scientists have also linked the Wenchuan quake to the filling of the Zipingpu Dam nearby.
The two quakes occurred on the same fault – the Longmenshan fault – with the Wenchuan quake occurring as a result of a rupture in the northern end, while the Ya’an quake resulted from a rupture in the southern end. As Fan Xiao told Global Times immediately after the earthquake, “After the Wenchuan quake, the fault line became active as its stress wasn’t completely released. So it was possible for a 7.0 magnitude aftershock even five years later,” Fan said, adding that it was the Longmenshan fault line adjusting its stress.
After the Wenchuan earthquake, China’s Ministry of Water Resources report that as many as 2,380 dams were damaged by the earthquake. While the Ya’an quake was smaller in scale, two medium and 52 small dams were damaged, and residents downstream of five dams have been evacuated. Experts fear that further evacuations may occur as a result of secondary disasters such as mudslides and dam failures.
Experts Debate Cause of China Quakes
The most widely accepted explanation of how dams cause earthquakes is related to the extra water pressure created in the micro-cracks and fissures in the ground under and near a reservoir. When the pressure of the water in the rocks increases, it acts to lubricate faults that are already under tectonic strain, but are prevented from slipping by the friction of the rock surfaces. In deep and large reservoirs, the extra load of the water over an existing fault line can also lead to movement along a fault. Thus the rapid filling of a large reservoir may trigger an earthquake that would have otherwise naturally occurred several if not hundreds of years later. It is well established (although little known to the general public) that large dams can trigger earthquakes. Experts in China and the US have for years researched the possible link between the Wenchuan quake and the filing of the Zipingpu Reservoir – just 5.5 kilometers away from the epicenter. While the results are not conclusive, the phenomenon known as RIS has been studied for decades, with global estimates identifying over 100 cases of earthquakes that scientists believe were triggered by reservoirs.
According to a report by Fan Xiao published just before the Ya’an earthquake, a recent string of strong earthquakes measured between 4.2 and 5.7 have already occurred in the Sichuan-Yunnan region since June 2012. A magnitude-5.5 earthquake in Dali on March 3, 2013, for instance, led to the relocation of 21,000 people. While there is no evidence that these quakes were induced by reservoirs, such frequent seismic activity, both moderate and strong, indicates that the region is highly seismic and more earthquakes can be expected. Thus, Fan Xiao warns,
“Given this geological condition, the unchecked construction of a large number of giant, even super-sized, hydropower projects in the region is cause for heightened concern about reservoir-induced seismicity.”
When and Where Will the Next Big One Strike?
Not all scientists agree with Fan Xiao that the Ya’an earthquake was an aftershock of Wenchuan, or that reservoirs can induce them in the first place. Zhou Bengang, a researcher with the China Earthquake Networks Center, claimed that the Ya’an quake in Lushan county was an isolated case, noting that “the quake occurred on the southern part of the fault line, while the Wenchuan earthquake occurred in the middle part.”
Regardless of the connection, the risk that more earthquakes might hit this dam building hotspot is real – it’s just a matter of where and when. Liu Qiyuan, a geophysicist at the China Earthquake Administration’s Institute of Geology, for instance, predicts that the faults that intersect the southern end of the Longmensha fault – Xianshuihe and Anninghe – now pose the greatest seismic hazards. By comparison, Mian Liu a geophysicist at the University of Missouri in Columbia thinks that the Xianshuihe and Anninghe faults are less risky because, whereas the Longmenshan fault itself still has a lot of accumulated energy that could lead to further magnitude 6 or 7 earthquakes.
In either case, reports by Fan Xiao and others offer a sobering warning to government and dam planners. A number of dams exist or are being planned for both the Xianshuihe and Anninghe faults, for instance, including the 186-meter high Pubugou Dam (already completed) on the Dadu River and the 305-meter high Lianghekou Dam on the Yalong (under construction). Most of the strongest cases of RIS have been observed for dams over 100 meters high, though dams just half this height are also believed to have induced quakes. Many of the dams that the government is planning on the Jinsha, Nu, Lancang, Dadu and Yalong and their major tributaries are 200 meters in height or more, with water storage capacities in the billions of cubic meters.
When asked what key lessons can be drawn from the Ya’an experience, Fan Xiao told us that damaged dams should be repaired especially in anticipation of the summer flood season, buildings should be strengthened, and earthquake standards for new buildings should be strictly followed. But above all else, he says,
“Large hydropower stations should not be built on earthquake fault lines. Hydropower development that turns entire rivers into a cascade of reservoirs should be abandoned. At geological and ecological sensitive areas, there should be protected areas that exclude hydropower dams and other large engineering projects.”
Whether the government will pay attention to these warnings will be a critical test for its commitment to safe, scientific and sustainable development.
