There’s been a lot of discussion about the effects of the Sichaun earthquake, such as the Tangjiashan ‘quake lake’ which has now been drained in a safe, if not strictly controlled, manner (the BBC also has some cool photos). But aside from Kim’s excellent summary of the tectonic context, we haven’t talked much about the earthquake itself. According to this article in the Washington Post, although the Sichaun region was by no means seismically inactive, no-one was expecting such a large earthquake there:
many scientists were caught by surprise by the magnitude of the China earthquake, estimated at 7.9 by U.S. scientists. Sichuan province has a history of earthquakes, but none so devastating. It was not near the top of anyone’s list of the most likely locations for a great quake. The data from satellites, which can track the motion of vast plates of the Earth’s crust, suggested a relatively moderate amount of strain building up in the rugged mountain front along the edge of the Sichuan basin.
Of course, to a certain extent we are suffering from the limitations of the fact that just like volcanoes, the cycles of elastic strain build-up and earthquake rupture associated with large faults occur over timescales of hundreds or thousands of years. This is much longer than the interval over which we have complete and accurate records of where, how often and how strongly any particularly region has been shaken up (although it’s likely that China’s historical records are amongst the best anywhere), and thus any estimates of seismic risk can easily miss faults that may not have ruptured when people were around and able to record the event, but are still be accumulating strain nonetheless.
However, it seems that another theory is doing the rounds: that the weight of water in the reservoir behind the Zipingpu dam (which is only a few miles from the epicentre of the Sichaun quake, and was damaged by it), may have caused an unusually large rupture. Fan Xiao, the person being interviewed in the excerpts below, is the chief engineer of Sichuan’s Regional Geology Investigation Team.
Wang Yongchen: Now, two weeks after the powerful Wenchuan earthquake, what are your thoughts?
Fan Xiao: This earthquake was a Richter scale magnitude 8 and, strictly speaking, it is very unusual for this area. The historic records show that the highest recorded earthquake in this area was magnitude 6.5, and no seismic activities of more than magnitude 7 occurred along the Longmenshan seismic belt before. (see map here)
Wang Yongchen: Do you think this unusual quake was triggered by the reservoir?
Fan Xiao: Very likely, given its geological conditions, and the epicentre being so close to the reservoir.
Wang Yongchen: So, you are saying that the reservoir is likely to have induced the earthquake, but how? What are the conditions that trigger an earthquake?
Fan Xiao: There are many conditions, but from past experience and cases, both inside and outside China, at least three conditions are really important. First, the dam should be high enough, at least higher than one hundred metres. Second, the dam should be big enough, generally with a storage capacity of 1 billion cubic metres or more, because a small reservoir wouldn’t create significant threats. Third, and very important, the reservoir should be located in a fault zone. The case of the Zipingpu meets all three conditions: it has a height of 156 metres, a storage capacity of 1.126 billion cubic metres, and the reservoir is built in a fault zone. It wouldn’t be crucial if the reservoir was located in an inactive fault zone, but it does matter if a reservoir is located on an active fault belt. It’s unfortunate that Zipingpu is exactly this case: it is located on an active fault belt, with the tail of the reservoir really close to Yingxiu, where the quake occurred.
I’ve read before about small-scale seismic activity associated with reservoirs, but as I recall, it’s pretty low-level: stuff that would only register on nearby seismometers. Still, I can see how additional loading from a water-filled reservoir could cause a nearby fault to rupture earlier than it would have done naturally. What is more difficult for me to believe is the implication that the unusual magnitude of the earthquake is also the result of it being triggered in this manner: how could what is, in the grand tectonic scheme of things, a pretty small shove, boost the strength of an earthquake by more than an order of magnitude?
Going back to the Washington Post article, Ken Hudnut of the USGS is quoted explaining that the Sichaun earthquake looks like it may not be the result of a single fault rupture. Instead there was some sort of ‘cascade’ effect, where the original fault triggered a rupture on a separate fault nearby, which went on to trigger a rupture on yet another fault. The energy from these three near-simultaneous quakes then reinforced each other to produce the intense shaking which caused so much damage. This makes me wonder: if this proves to be the case, could loading from the Zipingpu reservoir have resulted in faults more finely poised to rupture, and more likely to be triggered sequentially in this fashion? If so, perhaps – but only perhaps – the dam was a factor, after all.