(April 21, 2013) The strong earthquake that struck China’s mountainous Sichuan province Saturday morning may have been an aftershock, says prominent Chinese geologist, Fan Xiao. The accumulation of stress had not yet been fully released, making this region a more dangerous area after the May 2008 Wenchuan earthquake.

My thoughts on the magnitude-7.0 earthquake in Lushan County, Sichuan Province

By Fan Xiao
Chengdu, China
April 21, 2013

At 8:02 am, Beijing time, on 20th April, 2013, a powerful earthquake with magnitude-7.0 hit Lushan County in Ya’an City of Sichuan Province. Numerous aftershocks followed, with magnitudes ranging from 3.6 to 5.1.

According to the data posted by the CENC (China Earthquake Network Centre), the earthquake measured magnitude-7.0, and the epicentre is located at 30.3 degrees latitude north and 103.0 degrees longitude east, with a focal depth of 13 km. The US Geological Survey says the earthquake occurred at 8:02:48 am Beijing time, measured magnitude-6.9, with the epicentre located at 30.314 degrees latitude north and 102.934 degrees longitude east, and a focal depth of 16.4 km.

When the earthquake occurred, I was having breakfast in my home on the fifth floor of an apartment building in Chengdu, Sichuan Province. I felt the building shake violently for about 20-30 seconds. Cups standing on the drinking water filter fell down. I believe the degree of shaking yesterday was far stronger than the aftershocks of the Wenchuan earthquake of May 12, 2008, which had magnitudes greater than magnitude-6.0. (I am unable to compare the degree of shaking of the April 20, 2013 earthquake and the May 12, 2008 magnitude-8 Wenchuan earthquake because I was not in my home when the latter occurred.)

The epicentre of this April 20 earthquake is located in the south section of the Longmenshan Fault Zone, and also located in the Front Mountain Fault (which is also known as the Jiangyou-Guanxian Fault, one of the three main faults of the Longmenshan Fault Zone). The other two main faults in the Longmenshan Fault Zone are the Longmenshan Central Fault or the Beichuan-Yingxiu Fault, and the Wenchuan–Maoxian Fault, which is also known as the fault zone of the back mountain [refer map]. The Front Mountain Fault extends south from Dujiangyan to Tianquan, Lushan and Ya’an area, where the geological conditions become complicated, not only dividing into multi-branches, but intersecting with the fault system of the NW-trending Xianshuihe Fault Belt, forming a tectonic transition zone.

For the 2008 Wenchuan earthquake, the initial rupture occurred in the central section of the Longmenshan Fault Zone near the Zipingpu reservoir, and then expanded in a northeast direction along the north section of the Longmenshan Fault Zone, extending up to 200 and 300 kilometres. It’s interesting to note that the rupture did not expand significantly in a southwest direction or along the south section of the Longmenshan Fault Zone. This was why experts named the Wenchuan earthquake a “unilateral rupture.” This also meant that the accumulation of stress in the south section of the Longmenshan Fault Zone had not yet been fully released, making this region a more dangerous area after the May 2008 Wenchuan earthquake. Considering both the epicentre location and the timing of the magnitude-7 Lushan earthquake that occurred on Saturday, I believe that the Lushan earthquake may be a strong aftershock of the Wenchuan earthquake in the stress adjustment process of the Longmenshan Fault Zone. In general, the probability of a magnitude-7 aftershock occurring after a magnitude-8 earthquake (such as the May 2008 Wenchuan earthquake) is very high.

I also believe that the impact of human engineering activity on seismic activity in southwest China, including the recent Lushan earthquake, must be closely monitored and further studied. With respect to the causes of the Wenchuan earthquake, many studies have investigated this massive seismic event and its close relationship to the Zipingpu reservoir (see my article: Did the Zipingpu Dam Trigger China’s 2008 Earthquake? The Scientific Case), especially the relationship to the dam’s impounding activities since 2008. Moreover, the Zipingpu reservoir, which was damaged by the Wenchuan earthquake, has been repaired and has now resumed its normal operations of filling and drawdown. It is, therefore, very important that more studies be done to determine the extent to which the filling and drawdown of Zipingpu’s reservoir has had an impact on the stress adjustment and seismic activity of the Longmenshan Fault Zone in general, and on the latest Lushan earthquake in particular.

In addition, the unchecked and rapid construction of a large number of giant, even super-sized, hydropower projects is under way in western China, an area of high seismicity, including the Longmenshan Fault Zone and Xianshuihe Fault Zone. More importantly, concerns are growing that the next 10 years will be especially dangerous as these high dams are completed and their large reservoirs filled.

The epicentre of this Lushan earthquake is only about 80 kilometres south of the newly-built Pubugou Dam on the Dadu River (a tributary of the Yangtze). The dam project is 186 metres high with a storage capacity of 5.39 billion cubic metres and an installed capacity of 3,300 MW. Impoundment of the Pubugou Dam reservoir began in 2009. From October 14, 2006, and December 31, 2011, some 1,834 small earthquakes were recorded in the region. According to analysis by experts in the Sichuan Seismological Bureau, these earthquakes have been concentrated in several places of the reservoir area ‒ in the central part of the reservoir, near the dam itself, and downstream of the dam.

Elsewhere, the Xiangjiaba Dam on the main stream of the lower Jinsha River, with a dam height of 162 metres, a storage capacity of 5.163 billion cubic metres and an installed capacity of 6,400 MW, will be China’s third-largest hydropower station if completed as scheduled. The first phase of reservoir impoundments at Xiangjiaba was completed in October 2012. It is noteworthy, and of concern, that the reservoir water level was raised from 278 metres to 354 metres above sea level in just six days, from October 10 to 16 in 2012. This extremely rapid rise of 76 metres is unprecedented in the history of large reservoir impoundments, both in China and abroad. Scientists have observed that reservoir-induced seismicity (RIS) is related to the filling and drawdown of reservoirs and the rate at which it happens. Therefore, this practice at Xiangjiaba undoubtedly increases the risk of reservoir induced seismicity and other reservoir induced geological disasters (such as landslides and mountain collapses). More is to come: later this year, from September to December, the Xiangjiaba reservoir will rise another 26 metres to its normal pool level of 380 metres above sea level.

Therefore, in the wake of the magnitude-7 Lushan earthquake, more attention must be paid to RIS which is most likely to occur in the Dadu, Yalong, and Jinsha River regions. Monitoring and analysis of seismic activity in the vicinity of the reservoirs along these rivers must be strengthened.

The Lushan earthquake once again reminds us that, in a region of high seismicity (such as southwest China), it is imperative that the construction of new housing and the strengthening of existing homes meet the standards of the national “Seismic Design Code” in an effort to reduce casualties and property losses and damages to the lowest possible level.

For more background to this issue see:

On alert: RIS risk amid rash of earthquakes in China’s Sichuan-Yunnan region

Fan Xiao: Did the Zipingpu Dam Trigger China’s 2008 Earthquake? —The Scientific Case

Feverish Chinese dam building could trigger tsunami