Geologic causes vs geologic triggers

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.

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Nice post. I noticed a similar confusion in media discussions over whether the Samoan earthquake triggered all those other large earthquakes around the edges of the Australian plate in late September/early October. 'Cause' was used a little too often, when passing seismic waves would - possibly - only tip a fault already predisposed to rupture over the failure threshold.

KIm, Yes, cause and effect can definitely be another one of those misconceptions and difficult concepts. I work with lots of "kids" and developmentally they are still working on cause and effect. But sometimes, I also see it in their teachers when they don't stop for a second and really think the scenario out.

Enjoyable post. The CNN website has a really spectacular video of a rockfall wiping out a highway in Tennessee.…

I agree cause and effect can be a difficult concept for some people to sort out.

A somewhat related concept to the discussion in the post is one of my favorite bug-bears, namely the Pathetic Fallacy. Which is when people ascribe human feeling or emotions to inanimate objects, for example saying âthe water wanted to flow downhillâ. Iâm glad to see that nowhere in this post or the linked article did it happen but all too easily it could have. Itâs a little thing but implies a lack of critical thinking or maybe just laziness in the thought process.

In this case itâs a bit of a looser connection but in essence the maybe the writers stating that water potentially âcausedâ rather than âtriggeredâ the earthquake might be a case of them mentally ascribing that the fault zone âwanted to slipâ and therefore the introduction of the water âcaused it to fulfill it wantsâ.

By Phyllograptus (not verified) on 12 Nov 2009 #permalink

Probably the biggest issue with this is hot dry rock geothermal. At least one project has been abandoned because of a "triggered" earthquake.

I suspect that cause and effect here can get pretty muddled, even to a rational observer. If something moves up an inevitable event by a small amount of time, like triggering an earthquake at week early, then it is clearly just a trigger. If the event is still inevitable, but probably wouldn't have ocurred within a human lifespan, then our trigger has released some sort of stored energy/catastrophe, and on a human timescale can be thought of as a "cause".

Then I can think of instances, where the basic nonlinear instability might not ever cross the threshold for catastrophic failure, but for the triggering event. An example would be a skier induced avalanche, perhaps if left alone the snowpack would have stabilized. That sort of thing is even possible with earthquakes -if there exists a slower creep mode of stress release, then perhaps triggers can affect the balance of how much energy goes to the catastrophic failure mode versus to the slow creep mode.

I suspect we will be witnessing events of this type triggered by climate change. Slope failure triggered by the thawing of formerly frozen ground for sure. Or perhaps even volcanic eruptions triggered by the reduction in hydrostatic pressure as ice caps thin.

By Omega Centauri (not verified) on 13 Nov 2009 #permalink

Phyllograptus: "one of my favorite bug-bears, namely the Pathetic Fallacy. Which is when people ascribe human feeling or emotions to inanimate objects, for example saying âthe water wanted to flow downhillâ."

Be glad you're not a chemist, then. In my experience at several of the top chemistry departments in the country, chemists teach using anthropomorphic terms: its all about "wanting electrons", "back-side attacks", "unhappy strained molecules", and so forth. However, it didn't bother me at all - I actually found it useful in terms of developing and passing on intuition: humans are designed to think in terms of emotions, and therefore using reasonable emotional analogies is, in my opinion, acceptable - as long as the math and underlying theory gets taught as well.