Dams and Earthquakes

Can’t rule out possibility that dams induced earthquake, says expert

Three Gorges Probe

May 28, 2008

Chief engineer of the Regional Geology Investigation Team of the Sichuan Geology and Mineral Bureau argues that the now damaged Zipingpu dam may have induced the May 12 earthquake.

The following exclusive interview with Fan Xiao, chief engineer of the Regional Geology Investigation Team of the Sichuan Geology and Mineral Bureau appeared on May 22 in South Urban Daily (Nanfang dushibao). (The South Urban Daily is a sister newspaper to South Weekend, both published by China’s outstanding investigative Southern Media Group based in Guangzhou.)

As early as 2003, Fan Xiao voiced his opposition to the plan to build dams on the Min River, near the epicenter of China’s recent earthquake. Fan Xiao argues that the now damaged Zipingpu dam may have induced the May 12 earthquake and the possibility must be investigated. He is joined by U.S. engineer Dr. Philip Williams who thinks that Zipingpu’s possible contribution to China’s May 12 earthquake should be investigated.

Following is Three Gorges Probe‘s translation of the article.

TGP Background: The Min River, a major tributary of the Yangtze, runs along the 400 km long and 70 km wide Longmenshen fault, dividing the Tibetan Plateau and the Chengdu Plain in an area known to be seismically active. Near the epicentre of the Wenchuan earthquake, the Min River runs through the heavily damaged city of Dujiangyan and a 2,200-year-old, still functioning dam and irrigation system (a UNESCO World Heritage Site), through Chengdu, the capital of Sichuan province (with a population of 10 million), and into the Yangtze, some 300 km upstream of Chongqing.

A cascade of dams have been built on the Min river in recent decades in the following order beginning upstream: Tianlonghu, Jinglongtan, Tongzhong, Jiangsheba, Caopo, Futang,Taipingyi, Yingxiu, and Zipingpu, and the Guanyingyan ( to be built upstream of Tianlonghu)—with the two-year-old Zipingpu dam built just seven kilometres upstream of the city of Dujiangyan. It appears that all the dams are earthquake damaged, though to different degrees.

Fan Xiao makes an important distinction in the interview between the two scales measuring the status of an earthquake: the Richter Magnitude scale of the earthquake, and the seismic intensity of the earthquake. The latter – seismic intensity -is a way of measuring or rating the effects of an earthquake at different sites – for example the degree of damage on the earth’s surface and to man-made structures, such as dams and buildings.

The Modified Mercalli Intensity scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end. The Intensity scale differs from the Richter Magnitude scale in that the effects of any one earthquake vary greatly from place to place, so there may be many intensity values measured from a single earthquake. Each earthquake, on the other hand, should have just one magnitude, although the several methods of estimating it will yield slightly different values (e.g.: 6.1, 6.3). The two scales, though different, are related, with the greater magnitude usually being associated with greater seismic intensity.

The Zipingpu dam, which has been damaged by the May 12 earthquake, was designed to withstand earthquakes with seismic intensity of 8 (VIII), but the recent earthquake produced levels of seismic intensity reaching as high as 11(XI).

It is believed that the much larger Three Gorges dam is designed to withstand a magnitude scale 7 earthquake, and possibly 7(VII) or 8 (VIII) seismic intensity earthquake, but more research is needed to determine this conclusively.

For more information on Richter Magnitude scale and seismic intensity scales see:

http://earthquake.usgs.gov/learning/topics/richter.php [PDF]

http://pubs.usgs.gov/gip/earthq4/severitygip.html [PDF]

http://earthquake.usgs.gov/learning/topics/mag_vs_int.php [PDF]

South Urban Daily Interview:

South Urban Daily: If the Taipingyi and other dams upstream of the Zipingpu dam (which is itself just seven km upstream of the cities of Dujiangyan and Chengdu) collapse, would the Zipingpu dam be in danger?

Fan Xiao: There would a chain-reaction for sure. We don’t know how much water is in the Taipingyi reservoir at the moment. If it is full of water, a collapse (of the Taipingyi dam) would hit the downstream Yingxiu reservoir with extraordinary force. The problem is that Yingxiu has itself been damaged by the earthquake, so if both the Taipingyi and the Yingxiu collapse, the rushing flood water from the two reservoirs would hit the Zipingpu dam downstream. Currently, Zipingpu is capable of receiving a lot of water because (an outlet has been opened and) it can discharge the water out of the reservoir. What worries me most is that Zipingpu has itself been damaged, so I am concerned about whether or not Zipingpu can deal with a torrent of water from upstream.

SUD: We know you have voiced your opposition to the construction of these dams. Why did you do so?

FX: I opposed the idea of building dams, not only Zipingpu, but all dams on the Min River. There are 10 major dams on the main channel, 19 on the tributaries, and that figure would be larger if we included the smaller dams which have been built too. The river, as a whole, is blocked by the cascade of dams, creating a really high risk geologically.

SUD: Why?

FX: Because this region is a seismically active zone. This earthquake had a magnitude 8. But, as a matter of fact, earthquakes around 7 on the Richter scale have occurred very frequently in the area in general and in Yingxiu Town area (the epicentre) in particular.

SUD: So you were aware of this situation before.

FX: Yes, from the point of view of professional geologists, we were aware of this risk. As early as 1933, a powerful earthquake occurred at Diexi area in the Min valley, and a landslide-choked-lake formed as a result (referred to now as “quake lakes” or “barrier lakes”). The Zipingpu dam for instance, was designed to withstand earthquakes with a seismic intensity of 8 (VIII), but these recent quakes are so powerful, with seismic intensities ranging from 9 (IX) to 10 (X) at the epicentre and the extreme reaching 11(XI) in some regions. Under such force the dams could be damaged and the facilities might be unable to operate normally. If a dam structure isn’t damaged, then it would be ok, but dams can collapse if the dam structure is damaged and the water has no way to get out. See How dams fail

SUD: Therefore, it appears we’ve been lucky with Zipingpu: no big problem has occurred so far, despite the fact that seismic intensity of these earthquakes was greater than Zipingpu’s capability to withstand.

FX: It sounded fine for the Zipingpu dam to be built to withstand earthquakes with a seismic intensity of 8 (VIII) since the greatest recorded earthquake in the Longmenshan (Dragon Gate Mountain) area, where Zipingpu is located, was around magnitude 6.5 (on the Richter scale). But in the world of nature, there are so many things beyond our prediction. My question is: will we be so lucky next time?

SUD: What was your first reaction after the earthquake?

FX: Worry. I worried so much. Actually, I didn’t know where the epicentre was at that time. But my worry grew after I learned that the epicentre was so close to Zipingpu. I have been concerned about the safety of these dams and reservoirs in the region since they were built. I think we cannot rule out the possibility that building the Zipingpu dam induced the earthquake because the epicentre is so close to the dam. This phenomenon is called “reservoir-induced earthquake.”

SUD: Do you think this is a possibility?

FX: I haven’t yet made such a conclusion, but there may be a possibility.

SUD: Why do you say that?

FX: Because the reservoir is built in the fault zone. With the height of the dam being 156 metres, and the water depth more than 100 metres, the reservoir exerts hydraulic pressure, crushing the fault belt below it and causing a disturbance in the fault belt as a result. Certainly, it would be fine if the reservoir was not built in a fault zone. But it is built in a fault zone and the water seeping from the reservoir would work like a lubricant. Moreover, there are so many cases of reservoir-induced earthquakes, and Zipingpu has all conditions that provoke reservoir-induced earthquakes.

SUD: Do you think it is safe to say that?

FX: Yes, but I’d put a question mark on whether the Zipingpu reservoir can induce an earthquake as powerful as 8 on the Richter scale. So further evidence is needed to prove that. Generally speaking, regarding reservoir induced earthquakes, the distance between the epicenter and the reservoir should be within a dozen kilometres. In any case, we need more data and further studies on it.

SUD: How can we determine a conclusion on that? What kind of research or analysis is needed?

FX: An earthquake monitoring network was established before building the Zipingpu dam, so I believe the detailed data on the reservoir should be available before and after the incident. With careful analysis and a comparison of the data before and after May 12, we should be able to determine an answer. In my opinion, the work should be carried out by the earthquake authority.

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