(December 20, 2013) High-profile Chinese geologist Fan Xiao — and the author of several reports for Probe International — notes with interest the rush by China’s state media, and the country’s official seismological agency, to dismiss a link between the 5.1-magnitude Badong County earthquake on Monday and the Three Gorges Dam reservoir. A dismissal that runs contrary to common sense and the basic facts of seismic analysis, says Mr. Fan, who believes reservoir-induced-seismicity (RIS), triggered by impoundment of the massive dam, was likely behind the recent quake and could induce stronger earthquakes in the region.
Experts drawn on by Chinese media in the days following the earthquake earlier this week, have either dismissed the event as having nothing to do with the Three Gorges Dam reservoir, because years have passed since the reservoir was filled to its normal pool level of 175 metres, or have said mining activities in the region, along with cave collapses, are to blame. Fan Xiao, a Chinese geologist and retired chief engineer of the Regional Geological Survey Team of the Sichuan Geology and Mineral Bureau, rejects both claims. According to Mr. Fan, earthquakes caused by mining and cave collapses normally occur in the very beginning period of reservoir impoundment rather than years afterwards. He also points to a number of RIS cases globally that prove strong earthquakes tend to occur years after large reservoirs are filled. The delay, he says, is caused by the time it takes for reservoir water to infiltrate existing fault lines. In light of these findings, Mr. Fan says the present time represents the most dangerous period posed by impoundment of the Three Gorges Dam reservoir and that the reservoir region is at risk from even stronger RIS-triggered earthquakes. Impoundment (following the region’s flooding season) is likely to have directly impacted seismic vulnerability in the area, he says, and is likely to have also caused the Badong earthquake – a result of reservoir water penetration along existing fault lines in a gradual and steady build-up that has continued to increase since impoundment of the Three Gorges reservoir began in June 2003.
Analysis of 5.1M earthquake in Badong County, Hubei, December 16, 2013
By Fan Xiao, for Probe International, Retired Chief Engineer of the Regional Geology Investigation Team of the Sichuan Geology and Mineral Bureau
According to the China Earthquake Networks Centre (CENC), a 5.1-magnitude earthquake occurred at 13:04:52 on December 16, 2013, in Dongrangkou Town, Badong County, in Hubei Province (31.1N, 110.4E), with a focal depth of 5 km.
Residents in Ankang City of Shaanxi Province (more than 200 km away from the quake’s epicenter), along with Hubei residents in northwestern Xiangyang, central Zhongxiang, western Yichang, and Jingzhou (located on the banks of the Yangtze River), report having clearly felt the earthquake, said China News Service.
State-run Xinhua news agency reported that by 14:00 the next day, Dec. 17, 71 aftershocks were registered following the 5.1M quake, destroying 96 houses and severely damaging 2,556. The affected area impacts a population of 27,286 scattered across 208 villages. Multiple bridges were damaged by the quake and its aftershocks, including the cable-stayed Badong Bridge — the closest upstream bridge to Three Gorges Dam in the reservoir area.
Fears over whether the Three Gorges Dam reservoir induced the earthquake are running high given that the event’s epicenter was located along the north bank of the Badong section of the Yangtze River, around 66 km upstream of the dam site.
An unusual point of note here is that on the afternoon of the day the earthquake occurred, the Seismological Bureau of Hubei Province quickly released a “modified” magnitude estimate for the quake of 4.8, rather than 5.1. That ranks as unusual because it is the first time a provincial-level seismological bureau has moved to “correct” the magnitude of an earthquake officially reported by the state-level CENC. Furthermore, both the CENC and the state media continued to report the quake’s magnitude as 5.1 the following day (Dec. 17). China Seismic Information (CSI) even recorded it as magnitude 5.5 in its earthquake catalog.
Even more unusual were the number of news reports that quickly followed dismissing the possibility that the Three Gorges Dam reservoir had anything to do with the earthquake. Xu Kaixiang, former chief engineer of the Three Gorges Geological Disaster Control Headquarters, under China’s Ministry of Land and Resources, told Caixin Net that the earthquake that occurred in Badong County [Editor’s note: henceforth called the “Badong earthquake”] was not induced by the Three Gorges Dam project. Mr. Xu insisted that if the earthquake were induced by “Reservoir-induced Seismicity” (RIS), it would have been much stronger and would have occurred after the filling (impoundment) of the dam’s reservoir. But impoundment of the reservoir began in 2008, he said [Editor’s note: in fact, the date was June 2006], and five or six years have passed since then. As such, the Badong earthquake would have little connection with the impounding of the Three Gorges reservoir, said Mr. Xu.
Other factors were to blame, some argued. According to its preliminary analysis, Xing Canfei, deputy director of the Hubei Seismological Bureau, told China National Radio that mining activities and cave collapses were the likely causes of earthquakes in the area. Mr. Xing said the earthquake occurred along the Hubei’s famed Enshi cave tectonic belt, where mining for minerals, such as coal and gypsum, is an active undertaking. Thus, a rise in water levels, as well as cave collapses, were likely to have induced the seismic events, but further investigation was needed, he said.
Dr. Wang Qiuliang, a senior researcher with the Hubei Seismological Bureau, told Xinhua News that the epicenter of the earthquake was located in the Three Gorges reservoir area, around only 66 km upstream of the dam site. He argues that a combination of factors contributed to the earthquake: the region is already prone to geological disasters and is under further pressure from complicated geological conditions, such as a relatively well-developed karst topography, and an active mining industry.
It is yet of further interest that China’s state media and the country’s official seismological agency were both eager to dismiss a link between the Badong earthquake and the Three Gorges reservoir, a dismissal that runs contrary to common sense and the basic facts of seismic analysis.
A number of RIS cases globally prove that strong earthquakes, or the main shocks induced by reservoir impoundments, tend to occur years after impoundment has occurred rather than immediately following the filling of a reservoir. The reason for this delay is that it takes time for the reservoir water to infiltrate existing fault lines and to affect its greatest impact on the fault or seismic belts. As such, it is particularly true to say that the most dangerous period for the Three Gorges Dam reservoir to trigger seismic events is several years after the trials and eventual filling of the reservoir to its maximum, or normal pool level (NPL) of 175 metres in 2010. Meanwhile, the majority of earthquakes caused by mining and cave collapses would normally occur in the very beginning period of reservoir impoundment, as opposed to several years after impoundment. [See Did the Zipingpu Dam Trigger China’s 2008 Earthquake? The Scientific Case].
Moreover, the earthquakes caused by mining and cave collapses are usually small ones with much lower magnitudes and shallow focal depths. The Badong earthquake’s focal depth measured 5 km, but there was no evidence or data to indicate that pits or caves with a depth of 5 km were located near to the earthquake’s epicenter. Furthermore, the Badong earthquake — a relatively large one in terms of magnitude — was followed by numerous aftershocks that were clearly felt across a broad area, resulting in serious widespread destruction. It is difficult to explain these seismic events as having been caused by mining activity or cave collapse.
Actually, concern about the location of the Three Gorges Dam project in a seismically active area — and fears that it might induce earthquakes — is not new. As early as the 1980s, experts working on a Three Gorges Dam feasibility study registered their concern about faults in the reservoir area: notably, the Gaoqiao Fault and Xiannushan-Jiuwanxi Fault. The Gaoqiao and Xiannushan-Jiuwanxi faults are located in the Qianjiang-Xingshan seismic belt and Zigui-Yuyangguan seismic belt respectively. As historic record shows, a 6.25-magnitude earthquake occurred in the Qianjiang-Xingshan seismic belt on June 10, 1856, and caused a massive landslide. The Badong earthquake on December 16, 2013, was actually located in the vicinity of the Gaoqiao Fault along the north county seat of Badong, or the north bank of the Yangtze River.
Data provided by monitoring networks in the Three Gorges reservoir region, along with studies conducted by a number of experts, show that the Three Gorges reservoir has induced earthquakes since the dam project began filling. For example, within a 30-km-wide area (that encompasses both banks of the Yangtze), and includes the river’s main channel and tributaries, only 96 earthquakes were recorded before inundation of the Three Gorges Dam began (between January 1, 2000 and May 31, 2003); 764 earthquakes were recorded during the first trial period of impoundment to 135 metres between June 1, 2003 and September 20, 2006; the earthquakes recorded increased to as many as 1,538 in the second phase of impoundment between September 21, 2006 and September 27, 2008, when the reservoir was filled to 156 metres. Earthquakes numbered 1,036 in the third trial filling of the reservoir to 172.8 metres, between September 22, 2008 and December 31, 2009.
On November 4, 2008, the reservoir reached 172 metres for the first time, the highest water level during this trial period. Water levels slowly dropped afterwards. Then, the strongest earthquake recorded in the area after the dam began filling on June 1, 2003 — a magnitude 4.1 quake in Zigui (County) — occurred on November 22, 2008.
The majority of these earthquakes were concentrated in the vicinity of both the Gaoqiao Fault and Jiuwanxi Fault, which indicates that most of them were tectonic earthquakes caused by fault activity, rather than earthquakes caused by cave collapses.
In terms of its background, the Badong earthquake (of December 16, 2013) was quite similar to the 4.1-magnitude event that occurred in Zigui County on November 22, 2008. The former also occurred when the reservoir water level had dropped to around 173.9 metres after reaching its NPL of 175 metres on November 11 of this year.
Given all of the factors discussed above, the Badong earthquake was most likely connected to seismic activity along the Gaoqiao Fault which was induced by the impoundment of the Three Gorges Dam reservoir. On the one hand, impoundment of the Three Gorges Dam reservoir after the flooding season had a direct impact on seismic activity in the area; on the other hand, the Badong earthquake may have also been the result of (reservoir) water penetration — a gradual and steady build-up that has continued to increase since impoundment of the Three Gorges reservoir began.
Therefore, it is imperative at the present time to strengthen monitoring, analysis and RIS prevention efforts in the dam reservoir region now that the Three Gorges reservoir has reached its most dangerous period and the likelihood of it inducing stronger earthquakes is greatest.
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