(December 1, 2008)

Abstract

To investigate the mechanism of seismicity due to impoundment of a reservoir, we propose a method for integrated study on regional/local seismicity and stress by both surface loading and pore pressure diffusion. As an example, the possible role of the Zipingpu reservoir on nearby seismicity was studied in detail. The epicenter of the M8.0 Wenchuan earthquake on May 12 of 2008 is very close to the Zipingpu Reservoir. On one hand, several statistical properties included event rate (n), cumulative energy release (E), seismic b-value in the magnitude-frequency relation, and spatial correlation length (SCL) of earthquakes occurring in Zipingpu area from 2004/8/1 to 2008/5/11 were estimated in detail. On the other hand, we quantitatively examined the change in Coulomb Stress (‚ñ∫CFS) due to the impoundment of the reservoir. Both weight loading and pore pressure diffusion resulted in significant ‚ñ∫CFS on the underlying Yingshu-Beichuan and Guanxian-Mianzhu Faults, which were considered as the source faults of the Wenchuan earthquake. Some clear correlations were verified between the local seismicity and stress change, thus we concluded that the impoundment of the Zipingpu Reservoir clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake.

Introduction

As a growing number of studies have shown, even minor changes in stress as small as 0.1 bar would have an obvious impact on seismicity under certain conditions” (King et al., 1994). For example, changes in stress caused by surface loading and earth tides are likely to trigger seismic activities, even earthquakes. In certain conditions, the surface loading or the redistribution of surface loading caused by rains or melting snow would have an impact on natural seismicity to certain extent (Wolf et al., 1997; Muco, 1999). Filling water or changes in water levels in many medium and large reservoirs would likely induce seismicity and even hazardous earthquakes. For example, after the Xinfengjiang Reservoir in China was filled, an earthquake with a magnitude of 6.1 occurred. A bigger earthquake, with a magnitude of 6.3, occurred in the Koyana Reservoir in India, which was the biggest recorded example of RIS (Guha, 2000). However, no significant seismic activities were recorded in many other big reservoirs in the world, though minor tremors were reported in the early period of filling the reservoirs.

The key to whether the construction and operation of reservoirs would induce significant seismic activities is dependent upon the status of stress as well as whether there are fault belts of a certain scale near the reservoirs. The weight of a reservoir and its fluid pressure alone are not enough to break complete rocks beneath the reservoir, let alone to induce significant earthquakes. How the surface loading of a reservoir impacts the fault belts nearby is associated with the location and attitude of the fault belts, as well as its sliding direction which is controlled by the field stress.

On May 12, 2008, a powerful earthquake with magnitude of 8 occurred in the Longmenshan Fault Belt in northwest Sichuan, resulting in a huge and devastating disaster. Thus a debate over whether the incident was related to the filling of a reservoir started because Zipingpu, a large-scale reservoir, is close to the epicenter of the earthquake.

With a height of 156 metres (the elevation of the dam site is 728 metres above sea level), a Normal Pool Level of 877 metres and storage capacity of 1 billion of cubic metres (at the Normal Pool Level), the Zipingpu dam was built on the main channel of the Min River. The dam project officially started on March 29, 2001, dammed the Min in November 2002, started filling the reservoir on December 1, 2004, and completed fillingin December 2006. Given that the Zipingpu Reservoir strides across the Yingxiu-Beichun Fault Belt on which the Wenchun Earthquake occurred, and the nearest distance was only 6 kilometres between the epicenter and the reservoir, it’s an important and unavoidable scientific issue that filling the reservoir had impacts on the reservoir area and seismic activity in the nearby region.

Taking Zipingpu as an example, this article deals with how filling the reservoir played a role in the fault belts and seismic activity through quantitative analyses, preliminarily explores the relationship between the Zipingpu reservoir and seismic activity in the Longmenshan Fault Belt area, and other issues related to the Wenchuan earthquake occurrence mechanism.[6]

1 A comprehensive statistical analysis on the temporal and spatial distribution of earthquakes
2 Changes in stress caused by the surface loading
3 The impact of the Zipingpu on the Longmenshan Fault Belt
4 Discussions and conclusions

If a reservoir strides across a broken belt of a fault line, the propagation of pore pressure would produce significant effects. In the particular cases of dams with a height of 100 metres and more, the pore pressure on the top of the fault belt tends to diffuse gradually and slowly to deep beneath the reservoir, producing a Coulomb stress level of several bars.

Thus, in areas with a high risk of large magnitude earthquakes, the weight of dam reservoirs and the diffusion of fluid pressure would produce different additional Coulomb stress to the reservoir area and fault belts nearby, resulting in changes in the potential for seismic activity in the region.  First, in terms of time, it’s possible to enable earthquakes to occur earlier. Secondly, from a spatial perspective, it’s also possible to force potential earthquakes closer to the reservoir area.

Many medium and large hydro dams have been and will be built in southwest China, where water resources for developing hydro power electricity are plentiful but at the same time, this area is a seismicity prone zone, with a number of disastrous earthquakes having occurred in the region’s history. Therefore, it is quite important to monitor seismic activity in the reservoir areas of large dam projects and re-examine the plans to build big dams in these areas, paying particular attention to the issue of dam safety.

Based on a comprehensive analysis of seismic activities that have occurred in the vicinity of the Zipingpu reservoir and the stress in the main fault belts produced by filling the reservoir, our preliminary view is that the filling of the Zipingpu Reservoir apparently affected the Longmenshan Fault Belt[7] and fault belts before the mountain beneath the reservoir. As a preliminary calculation showed, when the Wenchuan Earthquake with a magnitude of 8 occurred, the Coulomb stress reached 0.5 bar and even above in the 10 kilometre area of the central fault[8] beneath the Zipingpu reservoir, and was as high as several bar in shallower layers below the reservoir. Such a change in the Coulomb stress was much bigger than the change in stress caused by Earth tides. Thus, an important scientific issue is whether the Wenchuan Earthquake occurred earlier because of the above process. The issue shouldn’t be avoided and is worthy of further study.

In general, the increase in pore pressure tends to help increase the Coulomb stress in fault belts, no matter what the attitudes of fault belts are, and this is more important and evident for deep reservoirs whose dams have a height of 100 metres or more. So there is a need to conduct more detailed research on the issue. As experimental studies have shown, there is a positive feedback effect between the pore pressure propagation and the destruction of rocks.[9] On the one hand, the pore pressure propagation tends to help the destruction of rocks, and on the other hand, the destruction of rocks helps to increase the pore pressure propagation in return (Masuda et al, 1990).

Seismic activities, even low magnitude seismic activities, tend to greatly enhance the permeability of fault belts. Moreover, between main fault belts, more sub-fault belts with different scales are likely to force the fluid pressure to diffuse to other fault belts that are not directly below the reservoir, but close by. With a series of medium and large reservoirs being built or put into operation in west Sichuan in general and along the Dadu and Yalong rivers[10] in particular, where many of the reservoirs stride near to major fault belts, the research methods in this article can be widely applied to those areas.

LEI Xing-lin, MA Sheng-li, WEN Xue-ze, SU Jin-rong, DU Fang, Chinese scientific journal of Dizhen dizhi (Geology and Seismology), Vol. 30, No.4, 1046-1064, December 1, 2008

Edited by Probe International, March 2009

Editors: This is a partial translation of the piece that appeared in Geology and Seismology.