
Deadly Mexico earthquake had unusual cause - mzs
http://www.nature.com/news/deadly-mexico-earthquake-had-unusual-cause-1.22586
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njarboe
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.

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devb
Same with the 2011 Virginia earthquake!

[https://en.wikipedia.org/wiki/2011_Virginia_earthquake](https://en.wikipedia.org/wiki/2011_Virginia_earthquake)

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

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sandworm101
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.

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EwanG
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?

~~~
dmix
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.

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cossatot
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.

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cossatot
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
                  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).

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cbogie
> _“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 ???.

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King-Aaron
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.

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

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

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hyperpallium
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?

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madaxe_again
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.

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hyperpallium
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...]

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

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mosselman
Click baity title.

