southern MEXICO... 8.0 EARTHQUAKE, 9-7-2017

To describe the workings of these huge temblors seismologists have coined the phrase "megathrust earthquake." They work like this: While the oceanic plate - in our two cases the Nazca and the Cocos plates - dives into the Earth's mantle, the friction of the downwards movement deforms the leading edge of the opposing plate severely. The diving plate even tries to drag the upper plate along for the ride, into the depths of the mantle. But instead of following the downward movement, the plate on the continental side resists this move. As a consequence it accumulates an enormous amount of mechanical stress. At one point - and nobody can say exactly when - this stress overcomes the friction between the two plates and the deformed leading edge of the plate thrusts upward in a giant earthquake. This sudden thrust movement is very big, hence the name megathrust quake.

...it danced to a very different beat. There, the leading edge of the plate on the continental side of the subduction zone is not the only place where deformation and stress accumulate.The diving plate itself is bent severely on its way down. This bending also causes a large amount of internal stress. At one point the internal forces which hold the subducting plate together are overcome by this stress and the plate breaks. The resulting movement is not a thrusting snap upward, but an internal slip downward in the direction of the earth's mantle. It is extremely rare that this kind of so called "normal faulting" within a downgoing plate gets so large that it generates a quake of a magnitude of more than 8, leading to the strongest earthquake which has struck Mexico in at least a century.

It is extremely rare that this kind of so called "normal faulting" within a downgoing plate gets so large that it generates a quake of a magnitude of more than 8, leading to the strongest earthquake which has struck Mexico in at least a century.

The United States Geological Survey and a coalition of partners has been developing a similar alert system for the past decade called ShakeAlert, but it won’t be operational until there’s funding to install the full network of sensors.

In 2016, Congress allocated $10 million for the early warning system, with the state of California kicking in an additional $10 million. But earlier this year, the $38 million needed to get the system up and running was slashed from the federal budget. (It now seems like funding may be allocated as planned—pending budget approval.)