225 Brunswick Avenue, Toronto, ON, Canada, M5S 2M6, Tel. (416) 964-9223, Fax: (416) 964-8239 Email: [email protected] Web: http://journal.probeinternational.org Once more, impoundment of two mega-dams on China’s Jinsha River triggers another earthquake in Ludian By Fan Xiao According to the China Earthquake Networks Centre (CENC), a 6.5-magnitude earthquake occurred at 16:30 on August 3, 2014, in Longtoushan Town, Ludian County, in Yunnan Province. The epicenter was located at 27.1N, 103.3E with a focal depth of 12 km. According to the official statistics, as of 15:00, August 8, 2014, the Ludian earthquake had killed 617 people; left 3,143 injured, caused the collapse of more than 80,000 homes and necessitated the emergency relocation of 229,700 people. Overall, more than 1,088,400 people were adversely affected in Ludian, Qiaojia and Yongshan counties, Zhaoyang District in Zhaotong City, and Huize County in Qujing City, in Yunnan Province. A study by the Institute of Geology, of the China Earthquake Administration, and other institutions, has confirmed that the 6.5M Ludian earthquake was a left-lateral, strike-slip quake, which occurred on the northwest Baogunao-Xiaohe fault, a branch of the northeast trending Zhaotong-Ludian fault. According to the region’s seismic conditions, however, the Zhaotong-Ludian fault is part of the Mabian-Zhaotong fault. As the historic records show, the Mabian-Zhaotong fault is an active seismic zone: at least 50 events with a destructive impact have been recorded in this zone since 1900, seven of which were greater than 6M, including a 7.1M quake which occurred in Yongshan County, Yunnan Province, in 1974. In fact, Lake Mahu (known as Lake Horse in English), one of the biggest lakes in Leibo County in Yunnan, is actually a quake lake produced by an earthquake pre-dating 1900. In the wake of the recent 6.5M Ludian earthquake, questions have arisen over whether the region’s mega-dams – in particular the Xiluodu and Xiangjiaba projects on the Jinsha River, the main channel of the upper Yangtze – had something to do with an increase in the frequency of seismic events like this one. Given that the Mabian-Zhaotong fault passes through the Jinsha Valley, significant seismic events 1 have been induced following the impoundment of the Xiluodu and Xiangjiaba reservoirs, and more significantly, the Xiluodu reservoir is only 40 km away from the epicenter of the Ludian earthquake.i This paper examines the relationship between the time and location of earthquakes in the Sichuan-Yunnan region during a four-and-a-half year period (January 1, 2012 - July 31, 2014) and the times the reservoirs at Xiangjiaba and Xiluodu were being filled. When and how the Xiangjiaba and Xiluodu reservoirs were filled In order to gain a clearer picture of the relationship between the Ludian earthquake and the impoundment of these two giant reservoirs, the author drew a sketch (see Figure 1). In the Study Area A (with locations ranging from 101.25 ~ 101.75E to 27.75 ~ 28.25N, defined by the author for the convenience of this study), Figure 1 shows 401 earthquakes ≥ 2M were recorded in this area between January 1, 2010 and July 31, 2014, based on the data provided by the websites of China Seismic Information (CSI) and Yunnan Earthquake Prevention and Disaster Reduction (YEPDR). Figure 1: The relationship between large reservoirs and seismic activity in Sichuan-Yunnan region 2 Click here to access the full-sized image of this sketch In this sketch, epicenters of ≥ 2M earthquakes are marked in the following colours: Before filling of the Xiangjiaba and Xiluodu reservoirs: 2010, purple; 2011, sky blue; January 2012 to August 2012, green. The period of rising water levels before formal impoundment of the Xiangjiaba and Xiluodu reservoirs is marked: September 2012 in yellow. After the filling of the Xiangjiaba and Xiluodu reservoirs is completed: October 2012 to May 2013 is marked in purplish red; June 2013 to August 2013, orange; after September 2013, red. The red rectangular boxes show maximum earthquakes and the times they occurred in various seismic clusters. Please note, the data used in this article does not include seismic events recorded by the specially designated seismic monitoring network around the Xiluodu and Xiangjiaba reservoir areas (because it is not publicly available, and largely draws on data recorded by regional monitoring stations and networks. Thus, many minor seismic events, which might better reflect the scope and intensity of the reservoir-induced-seismicity (RIS) in this area, are not included. Despite the incomplete data, however, the data that is available is still sufficient to show the significant change in seismic activities before and after the impoundment of the two large dams, and provides crucial information relating to RIS in this region. Xiangjiaba Dam: With a height of 162 meters and a storage capacity of 4.977 billion cubic meters, the Xiangjiaba Dam on the main stream of the lower Jinsha is China’s third-largest hydropower station. On October 10, 2012, the Xiangjiaba began filling. The reservoir’s water levels rose from 280 meters to 354 meters (above sea level) between October 10 – 16, 2012, with a total increase of 74 meters; the second phase of impoundment began on June 26, 2013. The reservoir was filled to 370 meters on July 5, 2013, with a total increase of 16 meters in 9 days; the water level rose further to 372 meters on the completion of the second phase of impoundment; the third phase of impoundment began on September 7, 2013. The water level rose to 380 meters on September 12, 2013, with a total increase of 8 meters. It is noteworthy, and of concern, that the reservoir water level was raised from 280 meters to 380 meters above sea level, with a total increase of 100 meters and a total storage capacity of 4.9 billion cubic meters in just 11 months; namely from October 2012 to September 2013. Then the water levels vary between 370 meters and 380 meters (normal pool level) above sea level. The fluctuating water levels between 370 m and 380 m above sea level is equivalent to the loading or unloading of about 1 billion cubic meters, or about 1 billion tonnes in this time period. Generally speaking, the water level remained at more than 378 meters above sea level during the period 3 between September 2013 and June 2014. The water level dropped to 372 meters in early July of this year and then remained at around this level until the event of the Ludian earthquake. Xiluodu Dam: On May 4, 2012, the Xiluodu Dam, which is located upstream of the Xiangjiaba Dam, began filling. The reservoir’s water levels rose from 440 meters to 540 meters (above sea level) on June 23, 2013, with a total increase of 100 meters. The second phase of impoundment began on October 19, 2013. The reservoir was filled to 560 meters on December 8, 2013, with a total increase of 20 meters; the water level further rose to 563 meters in March 2014. The water levels dropped from April 7, 2014 to below 541 meters on May 17, although the water level rose again to 560 meters on July 3, 2012, following completion of the second phase of impoundment; the third phase of impoundment began on July 3, 2013. The water level rose to 570 meters on July 23, 2013, and increased to 571.7 meters on August 1, 2014, two days before the Ludian earthquake. Prior to the Ludian earthquake, the Xiluodu reservoir’s water level was raised from 440 meters to 571.7 meters above sea level, with a total increase of up to 132 meters and a total storage capacity of 7 billion cubic meters in 15 months, or from June 2013 to August 2014. Seismic activity significantly increased after filling of the two reservoirs To show the changes to seismic activity in spatial distribution before and after the impoundment of the Xiangjiaba and Xiluodu reservoirs, the author uses different colors to present epicenters of the 401 ≥ 2M earthquakes recorded in the Study Area A between January 1, 2010 and July 31, 2014 (see Figure 1). In the Study Area A, scattered ≥ 2M earthquakes were recorded along the Yanjin-Daguan-Yiliang-Zhaotong-Ludian-Qiaojia line, before the impoundment of the Xiangjiaba and Xiluodu reservoirs from January 2010 to August 2012, while almost no ≥ 2M earthquakes occurred along the Jinsha valley or Shuifu-Suijiang- Leibo-Yongshan line. On September 7, 2012, a row of two 5.7M and 5.6M earthquakes occurred in Yiliang (County), but the Xiangjiaba and Xiluodu dams appeared to have no impact on these two seismic events because, at that time, only the Xiluodu had commenced filling and no substantial increase of water levels were recorded around that time. After October 2012, however, it was a totally different story: seismic activity surged dramatically on both sides of the two reservoirs and the surrounding area with the rapid and substantial rise of water levels at both the Xiangjiaba and Xiluodu reservoirs. As Figure 1 shows, ≥ 2M earthquakes not only dramatically increased and became widespread in this region but four dense seismic clusters appeared as a result: including the Suijiang-Mabian seismic clusters at the tail of the Xiangjiaba reservoir, 4 which is associated with the northwest trending Mabian-Yanjin fault zone; the Leibo-Yongshan seismic cluster near the Xiluodu dam, which is connected with the northeast trending Yongshan fault zone; the Ludian-Qiaojia seismic cluster southeast of the tail of the Xiluodu reservoir, which is mainly controlled by an intersection zone where several north-west, north-east, and south-north fault belts meet; and the Yiliang seismic cluster, near the epicenter of the Yiliang earthquake which occurred on September 7, 2012, and is mainly controlled by an intersection zone where several north-west, north-east, and south-north fault belts criss-cross.