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Deadly Mexico earthquake had unusual cause (nature.com)
103 points by mzs 10 months ago | hide | past | web | favorite | 34 comments

Similar to the New Madrid earthquakes (1811-1812) in the US. These intraplate earthquakes also cause more damage than other earthquakes of the same size because they travel greater distances than the more common ones near plate boundaries. The less fractured bedrock in the middle of plates propagates the energy with less loss.

Same with the 2011 Virginia earthquake!


> 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?

The flooding in Houston did that, albeit 2 centimeters.


(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?

Quick explainer about intraslab/intraplate subduction zone events (a slab is a subducting plate):

There are two major classifications of crust on earth: oceanic and continental. Oceanic crust (mostly basalt/gabro) is quite a bit more dense and thin than continental crust (mostly granite). Both float on the more dense mantle like big rafts, but the much thinner and denser oceanic crust sits ~3 km lower on average, creating ocean basins. In fact, the cold oceanic crust (and its attached subcrustal lithosphere) is in some places more dense than warmer mantle at depth; this density contrast is part of the convection cells in the mantle that drive plate tectonics.

When an oceanic tectonic plate converges with a continental plate, the denser oceanic plate slides underneath, at a subduction zone. (edit: I tried some ascii art to illustrate but HN deleted the spaces and ruined it; I fixed it in a comment below.)

There are three types of earthquakes that happen in oceanic plates in subduction zone environments. The first, most familiar and strongest are subduction zone or 'megathrust' earthquakes, like Tohoku (Japan 2011). This is an earthquake caused by the release of stored energy due to frictional locking between the downgoing oceanic plate and the upper continental plate.

The second type is an 'outer rise' event. These are almost always small (M~6 max) and not very damaging. This is caused by the flexing of the oceanic plate as it gets ready to dive down (@stephengillie, this is the stress you're asking about).

These types of earthquakes are easily observed because they're shallow and relatively well understood.

The third type of earthquake is what the Mexican event was; it's an earthquake that happens within the subducting (downgoing) slab/plate at depth (see diagram in comment below). It's not clear exactly why these happen but the pattern of energy release is consistent with vertical(ish) contraction and horizontal(ish) extension, i.e. normal faulting. The best explanation for why a downgoing slab would be contracting vertically and extending horizontally is that it is slowing down as it descends, probably due to a decreasing density contrast between the cold (but warming) slab and the mantle around it. So basically the downgoing slab is buckling like a train would if the front train cars slammed on the brakes but not the back train cars. Actually the best way to visualize this is to think about dropping a flexible rubber sheet (that is more dense than water) into a pool. It will continue to sink in the pool but more slowly than in the air above, and will buckle at the interface due to the buoyancy contrast.

But (unlike at the surface) we can't directly test this hypothesis because we don't really have any data on the velocity field to see if there is a deceleration of the downgoing slab (plate). Many of these deep intraslab events, particularly at 100-700 km depth, are thought to be associated with density changes in the downgoing slab as it undergoes mineralogical transformations to thermodynamically restabilize in vastly different P-T conditions.

Ok thanks to @temporal here is the diagram. It's a cross-section of a subduction zone, i.e. it's in a vertical plane (not map view).

              0       depth (km)
  o->____====Δ== <-c  -0
  (fast)  * <-e       -50 
        d-> \         -100
  (slow)     \          
The 'o' is the oceanic plate (thin line), the 'c' is the continental plate (double lines), 'd' points to the downgoing slab, 'e' points to where the earthquake occurred. O and C are the surface of the earth. 'fast' and 'slow' indicate that the slab is sinking/subducting fast at shallow depths and slower at deep depths. The Δ is an arc volcano that is smoking (like Mt. St. Helens or Fuji) that are always present above subduction zones because at ~100 km depths, the subducting slab loses some water which rises and then melts the upper mantle and lower crust of the continental crust above (these rocks are very hot and adding water lowers their melting temperature without changing the actual temperature much).

> (edit: I tried some ascii art to illustrate but HN deleted the spaces and ruined it.)

Prefix each line with two spaces to enter monospaced code block mode.

  Like this.

And people ask me why I read HN comments. Thank you for that explanations!

Thanks for your explanation! The bit about the front of the train stopping and the back of the wagons really helped visualize this for me. (together with the rest of your explanation).

> “I said ‘this is it’ and even thought about jumping out the window. I got very scared."

I've never considered this as a survival option during a quake. I suppose the consideration is between getting crushed should the building collapse vs surviving a 23 story fall onto ???.

I think it's more of a 'quick and painless exit' option rather than the potential of being crushed alive over the course of hours.

This person seems not to have much survival instinct or knows that he can't get out. How fast can you get out from the 23rd floor? He had 84 seconds right?

So earthquakes can happen anywhere now? Sounds kinda scary. Hope they will find better methode to find out about this before they happend.

They always could. An earth quake does not need to come from a fault line.

A bit far-fetched, but: I read one theory that artic icebergs melting did not produce a rise in sea-level was because the crust may have moved slightly vertically with the change in ice-mass weighing it down.

If the crust is getting slightly redistributed in one place, might there not be some pings and bongs elsewhere?

I think you may have some slightly crossed wires...

Land ice, such as that which covers Greenland and Antarctica, does indeed depress the local crust. Exactly how much we're not quite sure, but ice penetrating and non-penetrating radar observations over both places has revealed that as the ice mass decreases, the crust uplifts, very slowly.

This actually amplifies sea level rise, as you have both the melt water and the additional displaced volume from the uplifted land to account for.

As to pings and bongs, totally. A lateral movement in one place or expansion or uplift could all increase tension elsewhere, which would then be released as an earthquake. The most readily observed example of this is aftershocks, which are often hypocentred near but not at the original hypocentre.

Thanks! Yeah, icebergs didn't seem to quite make sense.

But if the basic idea is sound, is it a possible explanation for this Mexico quake, despite the artic being so far away?

[Though it seems more likely to me that 50 years of data isn't enough to tell whether a geological event is unusual or not...]

I wouldn't say it's an explanation, but there's certainly a causal link between all major geological activity on Earth.

Click baity title.

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