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Comment by stephengillie | Hacker News Reader

> But this quake was different: it occurred within the Cocos plate as it warped or bent, not at the boundary with the North American plate, according to the US Geological Survey.

“The type of faulting that occurred here does not usually produce earthquakes of this magnitude,” says Polet. “There have been others in the past 50 years of similar type and location, but none that was even close to this size.” It is still too early to say why the earthquake was so massive, she adds, but “it is sure to inspire much future research”.

Cause isn't too clear but the scenario sure appears unusual. What vector of pressure would cause a tectonic plate to flex or bend like a drum?

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The flooding in Houston did that, albeit 2 centimeters.

https://www.theatlantic.com/technology/archive/2017/09/hurri...

(I'm not suggesting a link between the flooding and the quake. It would be fascinating if there were a link though)

Continents are basically rafts of granite. Like any raft, if you put more stuff on it, it sinks a bit.

Thank you for that comment. I've been taught that Northern Europe was depressed by the weight of the ice sheets during the Ice Age and is still rebounding now that the ice sheets have gone and the weight is off.

I was imagining this as like compressing a sponge and never quite believed that re-expanding would take thousands of years.

But an enormous raft of granite, floating on an extremely viscous sea of even denser hot rock? At least I can see clearly that I have no useful intuitions about this. Does the raft bob up and down on a ten thousand year time scale? (Probably too heavily damped to oscillate) But the raft metaphor implies some lateral flow of the "sea" as the raft rises, so that is something to think about.

I don't see it. It's not even the same plate. You'd need the North American Plate to distort from Houston (definitely happened, but as you said, 2 cm). You'd need that to make the edges of the North American Plate move (maybe, but how much?). You'd need that to shove on the edge of the Cocos Plate in a way that adds to the stress in the center of the plate rather than on the edge.

I'm not a geologist, but as I said, I don't see it.

Right. I don't see it either, I wrote that stuff to try to clarify that I was responding to the question about what might bend a plate, that I wasn't trying to tie the events together.

Compression stress due to insufficient slipping at the boundary with the North American plate?

This also suggests that the sea floor may now be shallower (or deeper?) in that area.

I'm curious, how viable would a controlled underground atomic blast be in this scenario?

You might be underestimating the size of even a small tectonic plate. Large explosions will cause seismic waves, of course, but to actually move anything, I believe you'd need a spectacularly big one - bigger by far than anything we've detonated before.

My understanding is that the thousands of tiny detectable tremors each day worldwide involve as much energy as several atomic bombs exploding every day. The energy to move hundreds of cubic miles of rock even a nanometer is difficult to imagine.

Which suggests finding a place with constant tremors might be worthwhile to try to harvest energy from? Possibly just by having a material that generates a current when flexed?

It's nearly impossible to predict earthquakes so I don't see how this could be feasible at all. Scientists have been trying hard for decades with no luck. It's been one of the most impenetrable natural phenomenons. At most you get decade+ long likelihoods of the number of earthquakes of x strength, across a very large geographic area.

Even in places with a high number of them it's very challenging to give probabilities of when, where, or how often they'll hit.

There's a good reason the predictions for the upcoming US Westcoast major earthquake is always given within the span of centuries.

To harvest energy you need a device on either side of an energy differential. To pull substantial energy from shifting plates you would need something as long, and deep, as those plates. Point devices, ropes being pulled across a slowly separating plate, wouldn't generate much of anything. You would need millions of ropes attached to billions of anchors, all several KM deep within the earth before you could hope to pull any fraction of the energy.

It would be similar to trying to move a truck by hitting it very very hard with a hammer.

The total seismic moment energy for a 8.1 magnitude quake is roughly 1.7 * 10^21 Joules.

For comparison, Tsar Bomba (the largest nuke ever built) as tested was around 50 megatons of TNT or 210 Petajoules 2.1 * 10^17.

So you would roughly need 8000 Tsar Bombas triggered in just the right pattern to make their force additive in order to release that much energy into the plate.

I have a sneaking suspicion that if you could mine all the material for those Tsar Bombas in the same place, isostasy would give you equal or greater response from the plate, but I'm not going to do those equations right now.

Do you need to put as much energy into an already-loaded system as will be released, though? Compare the energy released by a stack of blocks falling to the amount it takes to topple it. I think it's unlikely we will ever be able to trigger quakes intentionally, but there's some implication fracking to extract natural gas [1] is causing seismic activity, for example.

1: http://time.com/84225/fracking-and-earthquake-link/

hot spot below? magma on it's way up?

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