Dams and Landslides

Minor tremors rattle Three Gorges during reservoir filling

21st Century Economic Report

June 26, 2003

‘The real danger comes from active faults in the vicinity of the dam site,’ which is located near six fault lines, a senior engineer warns in a recent Chinese newspaper report.

The following article appeared in 21st Century Economic Report (Ershiyi shiji jingji baodao) on June 16, 2003.

“As many as 1,000 micro-earthquakes have occurred in the Three Gorges reservoir area since June 7, with the biggest recorded at 2.1 on the Richter scale,” Xu Guangbin, director of the Hubei Seismological Monitoring and Prevention Centre, told a reporter on June 12. “These minor tremors have had no significant impact on the dam or reservoir, and have caused no damage.”

Mr. Xu said the seismic activity was concentrated in the Badong area, about 80 kilometres upstream of the Three Gorges dam. “It’s normal and to be expected because of the filling of the reservoir,” he said. The situation is being closely monitored, but no action is needed at this stage, the newspaper said.

Construction of the dam was likely to trigger seismic activity, Mr. Xu said, but there had been no serious problems with the first phase of filling the reservoir to the 135-metre level. As the reservoir is filled higher in the future, more and bigger shocks are anticipated.

The greatest magnitude associated with the first phase of filling the reservoir is expected to be 4 on the Richter scale. Tremors of this size could be felt, but would cause no major damage, the newspaper said.

Bigger quakes, in the range of 6 to 6.5 on the Richter scale, are to be expected once the dam is completed in 2009. However, these will pose no threat to the dam structures, which are designed to withstand earthquakes of magnitude 7, the newspaper said.

China has experienced earthquakes triggered by reservoir-filling before. On March 19, 1962, an earthquake measuring 6.1 on the Richter scale shook the Xinfengjiang reservoir in Guangdong province. The epicentre of the tremor was just 1,100 metres beneath the dam, and the structure sustained severe damage: A powerhouse was destroyed and an 82-metre-long crack developed on the upper part of the dam. The quake claimed six lives, injured 80 others, and damaged 1,800 houses. Several years earlier, in 1959, the Guangzhou Seismological Station had recorded increased seismic activity a month after the reservoir filling began. Later, from May to July 1960, a series of earthquakes, ranging in magnitude from 3.1 to 4.3, were also recorded.

Xia Qifa, a senior engineer at the China Academy of Hydropower and Water Conservancy, said: “At that time we had no idea that filling a reservoir could trigger earthquakes, so we treated the Xinfengjiang tremor as a natural occurrence.” According to Prof. Xia’s research, the impounding of water has caused 57 per cent of earthquakes associated with reservoirs. In India, 90 per cent of the earthquakes linked to the filling of reservoirs have occurred in areas that had experienced little or no seismic activity in the past.

Thus, there is an obvious correlation between the filling of a reservoir and increased seismicity. The November 1967 filling of the reservoir at the Danjiangkou dam on the Han River in Hubei province, for example, caused increased seismic activity.

However, there is no direct correlation between a particular water level in a reservoir and reservoir-induced earthquakes. In Xinfengjiang, the main shock took place when the reservoir was filled to a height of 88 metres above sea level. When the main shock occurred at the Huangshi reservoir, however, the water level at the epicentre was only five metres.

How exactly the filling of a reservoir induces seismic activity is poorly understood at this stage and accurate forecasts are difficult, Prof. Xia said. “Globally, the study of this issue remains at the stage of data collection and theoretical research,” he said.

Historical records show two main periods of seismic activity in the Three Gorges area. From 1407 to 1631, four medium and strong earthquakes were recorded, with the biggest believed to have been magnitude 6.5. From 1855 to the present, six medium and strong earthquakes have been recorded, the biggest having a magnitude of 5.5. At the peak of these active periods, four or five earthquakes occurred every eight to 10 years in the Three Gorges area.

“The real danger comes from active faults in the vicinity of the dam site, which was the consensus reached during the feasibility study conducted in the late 1980s and early 1990s,” Prof. Xia said.

The Three Gorges project authority later conducted an extensive survey, and found that the dam is situated near six fault lines, with the Jiuwanxi Fault located just 10 kilometres upstream. Two fault lines – the Jiuwanxi-Xiannushan fault and the Zigui-Badong fault – are considered likely to produce earthquakes that could have an impact on the dam site.

Li Zongji, a member of the expert group of geologists and seismologists who took part in the feasibility study, argues that landslides and riverbank collapses caused by impounding water actually pose a greater threat to the dam and reservoir area than reservoir-induced seismic activity. In 1992, the expert group identified 260 landslides and collapses containing at least 100,000 cubic metres of rock and earth; 140 of these had a volume of 1 million cubic metres or more. At least 14 landslides are considered likely to be activated by the filling of the reservoir.

The most recent major landslide in the area occurred at Xintan, in Zigui county, Hubei province, in June 1985. The landslide caused a huge 36-metre-high wave that claimed 10 lives, damaged 77 boats and led to a two-day interruption of shipping on the Yangtze.

Cracks were found 20 years ago in a landslide that has been developing near the county seat of Badong. “The new town of Badong is actually located on an old landslide,” Prof. Xia said.

Large-scale landslides and riverbank collapses
in the Three Gorges reservoir area
(from the dam upstream to Yunyang)
Location Category Distance from
the dam
Estimated volume
(1,000 cubic metres)
Yemaomian Collapse 16 7,300 Unstable
Guangjiaya-Jiangjiapo Collapse 26.5 13,000 Developing
Lianziyan Collapse 27 2,500 Unstable
Fanjiaping Landslide 55 10,000 Unstable
Daping Landslide 61 8,750 Unstable
Xirangkou Landslide 72 16,800 Unstable
Huoyanshi Collapse 77 1,000 Unstable
Zuoyituo Collapse +
80 15,000-
Likely to activate
after filling
Xiangjiawan Collapse +
113 5,000-
Likely to activate
after filling
Caojiawan Collapse +
117 7,000 Likely to activate
after filling
Yaqiantan Collapse 119 20,000 Likely to activate
after filling
Shuizhuyuan Collapse +
137 5,000-
Likely to activate
after filling
Liyutuo Landslide 140 10,000 Unstable
Liujiawuchang Collapse +
142 21,000 Likely to activate
after filling
Tudiya Landslide 168 5,000 Unstable
Cicaotuo Landslide 170 7,000 Unstable
Sandengzi Landslide 180 15,000 Unstable
Xinxiao Landslide 185 10,000 Unstable
Jipazi Landslide 220 13,000 Unstable
Huangguacao Collapse +
240 >10,000 Unstable

Translated by Three Gorges Probe

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