“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?
(I'm not suggesting a link between the flooding and the quake. It would be fascinating if there were a link though)
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'm not a geologist, but as I said, I don't see it.
This also suggests that the sea floor may now be shallower (or deeper?) in that 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.
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.
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.
0 depth (km)
o->____====Δ== <-c -0
(fast) * <-e -50
d-> \ -100
Prefix each line with two spaces to enter monospaced code block mode.
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 ???.
If the crust is getting slightly redistributed in one place, might there not be some pings and bongs elsewhere?
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.
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...]