By Dai Miao, Yao Yunsheng, Chen Junhua, Qin Xiaojun, Wang Qiuliang

Institute of Seismology, China Seismological Bureau, Wuhan

Originally Published in Chinese in Renmin Changjiang^[i]

Journal of the Changjiang Water Resources Commission

Volume 41, No 17

September 2010

Probe International Translation

May 2011

To read the full report, see here.

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Translation by Probe International scientists, English Editor: Patricia Adams

Table of Contents

Introduction

1. An overview of the process in which water levels were altered and seismic activity occurred in the reservoir area

Table 1 Distribution of earthquakes in different periods of filling the Three Gorges reservoir

2. The relationship between water levels and seismic activity

Translator and Editor’s Notes

Introduction

This paper deals with the relationship between water levels in the Three Gorges reservoir and seismic activity. It is based on recent data provided by seismic monitors around the reservoir and in Hubei Province. The purpose is to determine the cause of seismic activity for different sections of the river during the different stages of reservoir impoundment.

1. An overview of the process in which water levels were altered and seismic activity occurred in the reservoir area

From June 1, 2003, when the Three Gorges reservoir began its first stage of impoundment up to the 135 metre mark above sea level, seismic activity increased significantly as the water level rose, much more than before inundation began. The data shows that seismic activity was concentrated along the banks of the Xiangxi River in Zigui County (30 km upstream of the dam), the banks of the Shenlongxi River in Badong County (78 km upstream of the dam), and in a 5 km wide area on both the north and south banks of the Xietan section of the Yangtze River in Zigui County (about 43 km upstream of the dam).

Then, beginning on September 20, 2006, during the initial period of the second stage of the reservoir’s impoundment to 156 metres, an upsurge of microseismic activity occurred in Dongrangkou in Badong County, and in Xietan Town, Quyuan Town and Luoquanhuang Village in Zigui County. The cause was a rapidly rising water level brought about by heavy rainfall upstream. Throughout the period of filling to 156 metres (including the lowering of water levels in April 2007 and the reservoir’s re-impounding in September 2007), the region affected by seismic activity was slightly larger than the area impacted by the 2003 filling of the reservoir to 135 metres. But the region affected by this seismic activity, although slightly larger, was still concentrated within a 5 km wide area (on both banks) of the river in the main channel and tributaries of the Yangtze. This slightly larger area made up 43.9% of the total area in which seismic activity was recorded, which included the area from the dam site to the middle section of the entire reservoir (see Table 1).

Table 1 Distribution of earthquakes in different periods of filling the Three Gorges reservoir

* Dates of monitoring periods

1) Before impoundment: January 1, 2000 – May 31, 2003

2) Impoundment to 135 metres: June 1, 2003 – September 20, 2006

3) Impoundment to 156 metres: September 20, 2006 – September 27, 2008

4) Trials to impound to 175 metres: September 28, 2008 – December 31, 2009

On September 28, 2008, a trial filling of the reservoir to 175 metres began. On November 10, 2008, the reservoir reached 172.80 metres, the highest water level for this trial. Water levels rose rapidly during this trial period: the daily average rose as high as 2.93 metres, followed by a relatively slower drop in the water level. The second trial filling of the reservoir to 175 metres began on September 15, 2009 and reached 171.43 metres on November 24, 2009. The process was similar to that of the first trial: the water level rose rapidly but the drawdown occurred relatively more slowly.

During this period of time – in which two attempts were made to raise the reservoir to 175 metres above sea level in 2008 and 2009 respectively – 22 earthquakes above M2.0 were recorded. The strongest was recorded as M4.1. The frequency of this seismic activity was significantly higher than before inundation began (i.e. in the January 1, 2000 to May 31, 2003 period), during the first stage of impoundment to 135 metres, and the second stage when the reservoir was filled to 156 metres above sea level. The earthquakes occurred mainly on both sides of the middle of the Badong Gaoqiao Fault, the north section of the Xiannushan Fault, and the west side of the Jiuwanxi Fault. Meanwhile, both north and south sections of the Badong Gaoqiao Fault, the south section of the Xiannushan Fault, and the Tianyangping Fault areas were relatively quiet, with no earthquakes registering more than M2.0.

2. The relationship between water levels and seismic activity

On June 1, 2003, the Three Gorges reservoir began filling to 135 metres and reached 139.03 metres on December 30, 2003. During this process, the river surface became slightly wider than in its natural state, and shallow microseismic activity occurred in several river sections such as Xietan, Quyuan Town and Guojiaba Town, among others. Site investigations later confirmed that the earthquakes were mainly induced when the rising reservoir water infiltrated mines in the Xiangxi and Xietan areas, causing them to collapse. Earthquakes caused by collapsing mine pits and tunnels also became more frequent in the Huoyanshi, Baotahe and Jiziyan mining areas in Badong County due to their close proximity to the reservoir shore. Earthquakes associated with collapsing karst caves occurred in river sections between Nanmuyuan in Badong County and Peishi in Wushan County, and for 5 km of riverbanks along the Wu Gorge, where limestone predominates. Landslide activity also significantly increased in the Leijiaping area of Badong along the reservoir because the rising waters caused deformation of the riverbanks and slope instability.

As water levels rose and the flooded area expanded during the initial stage of filling the reservoir to 156 metres (begun on September 21, 2006), more earthquakes occurred as a result of collapsing mines and karst caves, especially in the river section of Badong County. At the same time, the frequency of earthquakes caused by mine collapses increased in the mining areas of Xietan, Luoquanhuang, Quyuan Town in Zigui County, as well. Overall, the level of seismic activity was not that high from the beginning of inundation until the reservoir reached 156 metres, with mechanical corrosion playing a dominant role, and changes in the geological structure being secondary.

On September 28, 2008, the first trial to fill the reservoir to 175 metres began. During the initial stage, the level of seismic activity was relatively low. The main reason might have been that most of the coal mines and karst caves along the river had already collapsed in the previous stages of reservoir impoundment. When the water level rose above 156 metres on October 20, 2008, however, earthquakes of a non-tectonic nature became active once again, in the Nanmuyuan, Huoyanshi, Baotahe and Jiziyan areas in Badong (County) and the Xietan area in Zigui (County), in particular.

As the water level continued to rise, the load of the reservoir increased, and water penetration continued to gradually and steadily increase. Thus, as a result, 22 earthquakes above M2.0 were recorded – with the strongest at M4.1 occurring in Zigui (County). Eighteen of the 22 earthquakes were located along active faults (of which nine occurred in the riverside region of the Gaoqiao Fault and seven near the Xiannushan Fault). The focal depth (or the epicenter depth) was generally shallow. Some of the seismic activity was likely the result of the dislocation of faults caused by an increase in the pore pressure due to the infiltration of water in those faults.

By studying the relationship between water levels and seismic activity in the Three Gorges reservoir, this paper has discovered that the sharp rise and fall of the Three Gorges reservoir level is likely to trigger Reservoir-Induced Seismicity (RIS) in the early stages of reservoir impoundment.

Generally, two phenomena occurred here:

1) The water level rose rapidly before reaching the targeted high for each stage of impoundment of the reservoir. Under this circumstance, as the water level rose, the lower part of the slope of the riverbank was flooded first, creating a buoyancy effect. This buoyancy counteracted the downward pressure caused by the weight of the landslide, decreasing the effective weight of the lower part of the bank slope, thereby reducing resistance in the landslide, and causing instability in the landslide. The faster the water level rose, the greater the destabilizing effect on the reservoir bank slope. This destabilizing effect caused by a rapid rise of the reservoir took place in October 2007, September 2008, November 2008, and October 2009 respectively: the daily average difference of water levels was relatively big, and earthquakes above M1.0 were more active during these periods of the rapidly rising reservoir level as well.

2) The water level dropped relatively quickly after reaching the high level for each stage of impoundment of the reservoir. When the water level in the reservoir dropped after remaining at the highest level in each of the impoundment stages for a fairly long period of time, the groundwater table in the reservoir area also dropped, but more slowly. The water table during the impoundment trial periods was higher than the water table before the reservoir was filled (i.e. pre-impoundment) because water infiltrated the banks of the reservoir. This infiltration caused increased water pressure in the permeated soil, causing instability in the slopes of the riverbank which, in turn, was likely to trigger landslides. This effect is even more apparent because the permeability of the riverbank slope along the Three Gorges reservoir is low.^[ii] For example, in July 2007, August 2007, and between November 2008 and February 2009, when water levels dropped quite rapidly, the frequency of seismic activity above M1.0 was significantly higher than under normal conditions (i.e. before the dam was constructed and the reservoir filled). Compared to the period when an attempt was made to fill the reservoir in 2008, both the frequency and intensity of earthquakes during the filling of the reservoir in 2009 was less, perhaps because the reservoir was filled more slowly.

TRANSLATOR AND EDITOR’S NOTES

[i] The journal title translates as The Peoples’ Yangtze.

[ii] This process involves the lithology and nature of rocks of the riverbanks. The groundwater table rises as the reservoir rises and then drains more slowly than the rate at which the reservoir is drawn down. The less permeable the lithology of the riverbank, the more slowly the water will drain through cracks and pores in the rocks along the reservoir shore. Under these circumstances, the pressure created by the presence of water in the rocks and cracks of the geology surrounding the Three Gorges reservoir will remain high for longer periods of time, and will cause greater instability in the slopes of the riverbank. In other words, the less permeable the riverbank, the greater the pressure, which in turn causes greater instability in the riverbank slopes.