There’s a great, new online news article by Science’s Richard Kerr about the role of the Zipingpu Dam in last year’s Wenchuan earthquake. A new article in Geophysical Research Letters (which I haven’t read – my library doesn’t have access to GRL) tests the plausibility of water as a trigger for the Wenchuan quake, and concludes that the weight of the water, combined with its penetration into the fault zone, might have made the difference.
There have been a number of studies in the past decade or so that suggest that earthquakes can be triggered by little things, such as the passage of seismic waves. The studies are fascinating, in part because the triggers seem so small in comparison to any other force (like the weight of the rock). How could such a little thing unleash an earthquake?
The answer, I suspect, is that many faults may exist in a precarious balance – in a state of stress that’s on the brink of causing slip. It might not take much to set off an earthquake – a tiny change in the balance, and that’s it. The water might be the proverbial straw that breaks the camel’s back.
But although the water might have made the earthquake occur last year, rather than in a hundred years, I wouldn’t say that the dam caused the earthquake. The ultimate cause of the earthquake was the collision of India with Asia, and the resultant tectonic mess in the continent of Asia.
In reading Dave Petley’s blog posts about landslides, I’ve noticed something similar. Landslides are ultimately caused by gravity acting on a steep slope. But as Dave points out, again and again, there are two big triggers to the most destructive landslides: heavy rainfall, and earthquakes. Gravity makes rock and soil move downhill, but water or shaking controls when the landslide occurs.
The difference between causes and triggers is important for thinking about potential disasters, I think. Causes (like steep slopes, or tectonic plate boundaries) can help identify places that are likely to be dangerous. Triggers are shorter-term phenomena. (Maybe understanding triggers could be helpful for disaster planning, though Hurricane Katrina and the Boxing Day tsunami were sobering reminders of how difficult it is to save lives, even when scientists know what’s likely to happen.) I’m not sure the distinction between causes and triggers is obvious to most people, though. I noticed that my students listed “steep slopes” and “addition of water” as causes of landslides, and the commenters on Kerr’s piece discuss the Three Gorges Dam and carbon sequestration (anywhere) as problems that should be considered in light of the Wenchuan earthquake.
I think it’s important to think about those other situations, because humans mess with fluids in Earth’s crust all the time. (Water, oil, methane, carbon dioxide…) Yes, changes in fluids can trigger earthquakes, in places where the stresses are delicately balanced. But the big question, to me, is which places are actually in that delicate balance. Where are there faults that are close to that tipping point, not just on plate boundaries, but also in the far-worse-understood interiors of plates?
If there’s not a cause, the trigger isn’t going to matter. But if the stresses are in the right balance… watch out.