Deadly Mexico earthquake had unusual cause
Quick explainer about intraslab&#x2F;intraplate subduction zone events (a slab is a subducting plate):<p>There are two major classifications of crust on earth: oceanic and continental. Oceanic crust (mostly basalt&#x2F;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.<p>When an oceanic tectonic plate converges with a continental plate, the denser oceanic plate slides underneath, at a subduction zone. (<i>edit:</i> I tried some ascii art to illustrate but HN deleted the spaces and ruined it; I fixed it in a comment below.)<p>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 &#x27;megathrust&#x27; 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.<p>The second type is an &#x27;outer rise&#x27; 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&#x27;re asking about).<p>These types of earthquakes are easily observed because they&#x27;re shallow and relatively well understood.<p>The third type of earthquake is what the Mexican event was; it&#x27;s an earthquake that happens within the subducting (downgoing) slab&#x2F;plate at depth (see diagram in comment below).  It&#x27;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 <i>why</i> 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.<p>But (unlike at the surface) we can&#x27;t directly test this hypothesis because we don&#x27;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.