A study on the relationship between water levels and seismic activity in the Three Gorges reservoir

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Translation of A study on the relationship between water levels and seismic activity in the Three Gorges reservoir By Dai Miao, Yao Yunsheng, Chen Junhua, Qin Xiaojun, Wang Qiuliang Institute of

1 Translation of A study on the relationship between water levels and seismic activity in the Three Gorges reservoir By Dai Miao, Yao Yunsheng, Chen Junhua, Qin Xiaojun, Wang Qiuliang Institute of Seismology, China Earthquake Administration¹, Wuhan Originally Published in Chinese² in Renmin Changjiang³ Journal of the Changjiang Water Resources Commission Volume 41, No. 17 September 2010 June 2011 TRANSLATION BY PROBE INTERNATIONAL, ENGLISH EDITOR: PATRICIA ADAMS

2 Abstract The M4.1 earthquake which occurred in Zigui along the Xiannushan Fault on November 22, 2008 was a reservoirinduced earthquake, and the result of the combined effect of the reservoir water loading and reservoir water penetration. This paper reviews the seismic activity in the Three Gorges reservoir area since inundation began in June 2003, it describes features about the permeability structure of major faults, and it discusses in detail the relationship between changes in water levels and seismic activity. The statistical results of seismic activity in the Three Gorges reservoir area show: earthquakes were more likely to be triggered as the water level changed sharply; as the water level rose, the epicenters of earthquakes concentrated along the reservoir shore of the Yangtze River and its major tributaries; seismic activity in the reservoir area was mainly affected by the rise and fall of the reservoir rather than by changes in the geological structure of the area. In addition, because of differences in the permeability structures of different segments of the reservoir area, with the rise of the water level, there were more earthquakes greater than M2.0 in the Gaoqiao Fault (on the western end of the dam site area) in the early stage of filling, and more seismic activity in the Xiannushan and Jiuwanxi faults (on the eastern side of the dam site area) in the later stage of filling. Key words: reservoir earthquakes; permeability structure; dam water level; the Three Gorges reservoir area 1 Introduction Water plays an integral role in seismic activity in reservoirs. As early as the 1970s, Howells and Bell discussed how reservoir water affects rocks in a reservoir area and, by using a one-dimensional homogeneous diffusion model and a two-dimensional non-homogeneous diffusion model respectively, how it plays an important role in earthquake preparation and occurrence through penetration. In 1983, as a result of their study of features of the process of seismic activity in combination with characteristics of the reservoir fault permeability structure in the Xinfengjiang Reservoir in China, Zang Shaoxian et al proposed that the infiltration of reservoir water is the main cause of earthquakes in a reservoir area. In 2006, through statistical studies of cases of reservoir earthquakes at home and abroad, Wang Rushu and others suggested that the timing of the main shock is closely related to the filling of the reservoir, that there is a good correlation between seismic activity and changes in water levels in the early stage of filling, and that most relatively strong earthquakes occur in periods when water levels are high, when they are falling, or when they are low in the first and second stages of the filling of the reservoir. In 2009, based on seismic and geological data, and a site investigation in the Three Gorges reservoir area, Che Yongtai et al concluded that the M4.1 earthquake which occurred in Zigui along the Xiannushan Fault on November 22, 2008 was a reservoir-induced earthquake, and the result of the combined effect of the reservoir water loading and reservoir water penetration. 2

3 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. 2 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). 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. On September 28, 2008, a trial filling of the reservoir to 175 metres began. On November 10, 2008, the reservoir reached 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 metres on November 24, The process was similar to that of the first trial: the water level rose rapidly but the drawdown occurred relatively more slowly (see Figure 1). 3

4 Table 1 Distribution of earthquakes in different periods of filling the Three Gorges reservoir Period* 1) Before impoundment Distance of earthquake epicenter from the reservoir shore Number of earthquakes, by magnitude (km) > % of Earthquakes During this period of time - in which two attempts were made to raise the reservoir to 175 metres above sea level - the frequency of this seismic activity was significantly higher than before inundation began. 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. 4 2) Impoundment to 135 metres 3) Impoundment to 156 metres 4) Trials to impound to 175 metres * Dates of monitoring periods 1) Before impoundment: January 1, May 31, ) Impoundment to 135 metres: June 1, September 20, ) Impoundment to 156 metres: September 20, September 27, ) Trials to impound to 175 metres: September 28, December 31, 2009 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 M The study of the relationship between water levels and seismic activity

5 3.1 Fault permeability structure and RIS Based on the previous studies, different permeability structures in the faults would have different influences on reservoir-induced earthquakes after the reservoir is filled. The permeability structures of faults include four types: partially-impermeable, partially-conductive, compound, and diffuse-conductive. It s unlikely that the first two will cause RIS while it is likely that the last two will cause RIS. There are several main faults including the Xiannushan Fault, Jiuwanxi Fault and Gaoqiao Fault in the Three Gorges reservoir area. Composed of schistosity lens belt, cataclastic rock belt, and breccia or mylonite belt, the Gaoqiao Fault is a typical diffuse-conductive type: the fault experienced not only strong compression but subsequent tensional activity, so it is poorly cemented, the fractured rock belt is well developed, and the fault core is wide. Both Xiannushan and Jiuwanxi faults belong to the compound type fault, with well-developed tectonic fracture belts, the nucleus of the faults are well-cemented, have poor permeability, with a permeability structure that is impermeable in the middle but conductive on both sides of the fault. The mechanical properties of the faults include a pressure-resistant or compresso-shear character, complex lithology, and depths ranging from the subcrust fault, basement fault, even to the lithosphere fault. Generally in such a fault, the neotectonic activity is weak, and the latest events can be considered as early as the Early and Middle Pleistocene. 3.2 The relationship between water levels and seismic activity On June 1, 2003, the Three Gorges reservoir began filling to 135 metres and reached metres on December 30, 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. 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. 5

6 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. As the water level continued to rise, the load of the reservoir increased, and water penetration continued to gradually and steadily increase. As a result, 22 earthquakes above M2.0 were recorded - with the strongest at M4.1 occurring in Zigui. 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. 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. As a result, 22 earthquakes above M2.0 were recorded (see Table 1) - 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. 3.3 Relationship between changes in water levels and seismic activity 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. 6 By studying corresponding water levels and seismic activity in the Three Gorges reservoir area, it can be seen that earthquakes are more likely to be triggered if water levels change sharply in the early stage of filling (see Figure 2). Generally, two phenomena occurred here: i) 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 (see Figure 2). ii) The water level dropped relatively quickly after reaching the high

7 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. 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. (See Figure 2 (b)). 3.4 Relationship between changes in water levels and distribution of the epicenters of earthquakes in the reservoir area Before the reservoir was filled, earthquakes were scattered in the area near the dam and the middle of the dam site area (see Figure 3 (a)). But with the rise of the water in the reservoir, the epicenters of earthquakes clustered in strips (see Figure 3 (b), 3 (c), 3 (d)) and, in particular, the epicenters of micro-earthquakes ranging from M0 ~ 0.9 concentrated along the Yangtze River and its main tributaries. This demonstrated a striking feature that the RIS was mainly affected by the reservoir water. At the same time, earthquakes above M2.0 became active and spread from west to east, but all focused on the major fault lines in the reservoir area - the Gaoqiao, Xinhua, Xiannushan and Jiuwanxi faults. This demonstrated that after the filling of the reservoir began, changes in crustal materials caused by the infiltration of groundwater and the water load of the reservoir occurred first in the Badong area, where limestone predominates and the permeability is greater. In the Badong area, the combination of rapid water infiltration in the riverside region of the fault belt, leading to a smaller friction coefficient on the fault plane, and the increased water load from the reservoir caused fault dislocation and earthquakes. This was then followed by earthquakes in Xietan of Zigui (County)where clayey sandstone predominates and permeability is relatively less. Finally earthquakes occurred in Qu Yuan Town area (of Zigui County), much closer to the dam site, where the permeability is lowest. Earthquakes above M2.0 became active and spread from west to east, but all focused on the major fault lines in the reservoir area. Figure 3 Earthquake epicenter distribution in different storage periods (a) Before filling the reservoir: ~ ; (b) During the period of filling to 135m: ~ ; (c) During the period of filling to 156m: ~ ; (d) During the period of filling to 175m: ~ Conclusions 7

8 Reservoir earthquakes are closely related to the filling of a reservoir. The frequency and intensity of seismic activity significantly increased in the Three Gorges reservoir area after the reservoir began to fill in June 2003, compared to the time period before the reservoir was filled. By studying the distribution of earthquakes, characteristics of seismic activity, and so forth, in different stages of filling the reservoir, we discovered that most of the earthquakes recorded by the monitoring network in the reservoir area were karst type, mine collapse type, and landslide type, that micro-earthquakes dominated, with shallow focal depths - the vast majority less than 5 km, and that they were mainly affected by the rise and fall of the reservoir water. We discovered that most of the earthquakes recorded by the monitoring network in the reservoir area were karst type, mine collapse type, and landslide type, that micro-earthquakes dominated, with shallow focal depths - the vast majority less than 5 km, and that they were mainly affected by the rise and fall of the reservoir water. In addition, we also found that because of the difference in the mechanism of different reservoir earthquakes, the tectonic earthquakes occurred after the non-tectonic-type earthquakes. We also found that the influence zone of tectonic-type earthquakes was very limited, was mainly concentrated along major faults and their surrounding area, and was much smaller than the nontectonic-type earthquake, such as the karst type. The rate of the rise and fall of the reservoir water had somewhat of an effect on the occurrence of earthquakes in the reservoir area. As the water level rose, microearthquakes in the reservoir area concentrated mainly in the Yangtze River and its major tributaries, and tectonic earthquakes demonstrated a pattern of spreading from west to east. Table and Figures Found in the Chinese Study: Table 1 Distribution of earthquakes in different periods of filling the Three Gorges reservoir Figure 1 Dam water level curve: Reservoir level in the Three Gorges dam by year, from January 1, 2000 until December 31, 2009 Figure 2 (a) Daily average change in the Three Gorges reservoir level, and absolute reservoir level, by month, beginning September 1, 2006 and ending December 31, 2007 Distribution of earthquakes by magnitude and month, from September 1, 2006 to December 31, 2007 Figure 2 (b) Daily average change in the Three Gorges reservoir level, and absolute reservoir level, by month, beginning September 1, 2008 and ending December 31, 2009 Distribution of earthquakes by magnitude and month, from September 1, 2008 to December 31, 2009 Figure 3 Earthquake epicenter distribution in different storage periods (a) Before filling the reservoir: ~ ; (b) During the period of filling to 135m: ~ ; (c) During the period of filling to 156m: ~ ; (d) During the period of filling to 175m: ~ TRANSLATOR AND EDITOR S NOTES 8 1 Formerly the China Seismelogical Bureau

9 2 Subsequent to publication, minor revisions were made to this study. This is a translation of the revised study. 3 The journal title translates as The Peoples Yangtze. 4 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. 9

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