How many crustal plates are near mexico city

But in southern Mexico, because the subducting slab flattens out, it does not reach km in depth until about km from the trench, thus forming volcanoes km from the trench. Since this flat slab region is bounded on both sides by normal subduction zones, the volcanic arc bends inland in this region.

What were the conditions that caused this subducting slab to flatten out? And what then caused it to end abruptly at km depth? To answer such questions, TO scientists are developing numerical models to simulate the forces inside the earth over the last 22 million years. With these models, they are able to predict the evolution of normal subduction changing into flat subduction over this time span. They find that 30 million years ago there had been normal subduction here, with active volcanoes along the Pacific coast.

But about 22 million years ago those volcanoes shut off because the subduction went horizontal. New volcanoes started to erupt, on the Gulf coast, indicating that the flat slab extended nearly all the way to that coast. Since then, the line of active volcanoes has been rolling back toward the Pacific coast, as the extent of the slab has been decreasing.

This million-year march of the volcanoes from the Gulf coast back toward the Pacific coast is the cause of the high elevation of Mexico City, which sits on the remnants of old volcanoes.

One way such a change from normal to flat subduction could have occurred is if there had been a layer of low-viscosity easily flowing material right below the crust, which then gave way as the subducting slab pressed against it from below.

One possible source of a low-viscosity mantle layer could be the release of water by the subducting plate as it reaches high pressures and temperatures, as discussed in the previous section. The model results Figure 6 predict a slab geometry similar to that obtained by the seismic data Figure 5. Another unusual aspect about the Mexican subduction zone is the occurrence of silent earthquakes aseismic slip events that are so slow that we cannot feel them. In fact, these slow slip events are only visible in continuous GPS Global Positioning System time series measurements, which show the position of a GPS station on the continental crust of the upper plate yellow points in Figure 7 as a function of time.

The gradual motion of the GPS station landward, away from the trench the rise of the yellow line is due to the upper plate being "stuck" to the subducting plate beneath, and thus being squeezed along with it.

The reversal of the direction of the GPS station, back toward the sea the fall of the yellow line , indicates that the upper plate has become partially unstuck, and is slipping back toward the trench. In most subduction zones, it snaps back in a matter of seconds, causing a large earthquake. But here it takes months to slip back. These slow slip events amount to about 2 to 4 cm of displacement, and recur about every 5 years.

We cannot feel these slow slip events. Seismometers cannot even detect them, at least not directly. It was only by comparing seismographs from different instruments, using the technique of cross correlation or looking for patterns in the signals that TO scientists detected the weak seismic signals, called tremors, and located their sources called hypocenters.

Such an analysis is shown in Figure 8. Panel A shows the weak signals on the left. The blue arrows on the right show that the seismometers are picking up the same nearly imperceptible signals, though at different times.

Panel B shows that the sources of the seismic signals red and yellow stars occur mainly at and above the plate boundary, indicating that stress in the upper plate causes the tremors.

The frequency of these tremors is compared with the GPS signal in Figure 7. Two time periods, early and mid, show heightened seismic activity, as indicated by the density of green and red lines. These times correlate with slow slip events detected by the GPS signal yellow. Thus thanks to the MASE seismic data, we now know that these slow slip events are accompanied by swarms of seismic activity called tremors. Tremors have been detected in other subduction zones Japan, Cascadia and their origin remains poorly understood.

Scientists think that tremors may be produced by fluids circulating in the highly strained and deformed medium of the subducting plate. A detailed analysis of seismic waves crossing the interface between the subducting and overlying plates has revealed the existence of an anomalous thin layer, sandwiched in between the two plates.

Scientists have found that this layer coincides with the locations of slow earthquakes, as shown schematically in Figure 9, and that adjacent to these regions lie the hypocenters of tremor. By studying this unusual, flat-slab subduction zone, TO scientists are gaining insight into the workings of subduction zones in general, and into the history of flat subduction zones in particular.

These insights may help unravel the history of subduction zones that have occurred long ago. One example of this is the Farallon plate, believed to have subducted under the western continental United States several hundreds of millions of years ago. Like the blade of a shovel gliding into the ground at a steep angle, the diving slab in normal subduction zones does not usually affect the level of the ground above it.

But when we push down on the handle of the shovel, thus lifting and flattening out the blade, the ground above it lifts up.

Similarly, the slab's flattening may have raised the great plains of the continental United States from sea level to their current mile-high elevation. Moreover, that flat subduction zone may also have been responsible for moving the coastal volcanoes inland, all the way to Kansas.

Thus understanding the current flat slab subduction in Mexico may shed light on how the western United States came to be. A paper published shortly after the area's earthquake found that the shaking had been amplified by as much as percent in regions near the epicentre where the soil was the softest. Since then, the city has taken some steps to manage the risk from future quakes - such as updating building codes near the capital and launching an earthquake early warning system - but many parts of the country still suffer from a lack of safe infrastructure.

This article was originally published by Business Insider. This final kink in the Cocos plate is located right where the 7. This warping of the Cocos plate caused a large earthquake to occur close to Mexico city at a relatively shallow depth.

The two earthquakes occurred within just 11 days of each other and both were within the sinking Cocos plate. So there is no immediate indication that these two quakes are directly linked. Although these types of earthquake are rare, the amount of damage caused by them is not globally unprecedented. This quake in Chile occurred at a similar depth, with a similar faulting mechanism, and beneath a highly populated area in a sedimentary basin, similar to the two recent Mexico earthquakes.

We know very little about the geometry of these faults within plates, how they behave from one earthquake to another, and the frictional properties of the rocks that fracture. Our detailed earthquake records from instruments only go back a hundred years or so, and these deep quakes do not leave any noticeable imprint in the geological record from surface ruptures or large tsunamis.

Normal faults can only rupture where the slab is being extended within the shallower segments. The Tehuantepec quake rupture, however, spread to even deeper parts of the slab that should be compressed. This is potentially solvable. The paper suggests that the slab is being pulled down by its own weight so effectively that gravity is creating a major extensional force. This trumps the expected compressional forces, thereby allowing normal faulting to take place.

A quake like Tehauntepec requires rock to be cooler and therefore harder, so it can break in a more brittle way. Powerful normal fault earthquakes can take place in deep-ish parts of slabs, says study coauthor Emmanuel Garcia , a tectonics expert at Kyoto University.

However, this only really applies to truly ancient tectonic plates that have had many millions of years to cool down, which makes them more prone to break in a brittle fashion. The Tehauntepec quake involved the Cocos plate, which is a relatively young 25 million years old and is somewhat warmer than plenty of other tectonic plates. As the Cocos slab heads into the subduction zone under the North American plate, it bends and cracks. This creates normal faults, which take in seawater.

As the slab passes into and through the subduction zone into the lower mantle, it warms up and dehydrates. This dehydration creates mechanical weaknesses and can cause brittle fracturing, creating small quakes or, perhaps, a huge one.

The same theory was applied to the Iran and Chilean quakes. Melgar adds that when the oceanic Cocos plate first formed at a fiery mid-ocean trench, its cooling pattern created little hills and valleys in its rock. These imperfections may have eventually formed zones of weaknesses that could have generated the Tehuantepec earthquake, making this a story of destruction tens of millions of years in the making.

However, he notes, it still seems curious that brittle fracturing could take place so spectacularly at such hellishly hot depths.