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| from Wikipedia/Wikiwand article, "Middle America Trench"; arrow, labels added |
The recent big earthquake in Mexico in the Middle America Trench subduction zone has left parts of southern Mexico and bordering Guatemala in a state of shock and recovery, with fallen buildings, interrupted and broken roads and utilities, and final searches for bodies still ongoing. The rate of cleanup and repair is picking up even as the hurt and homeless continue to be treated and helped.
The earthquake caused lots of damage. It knocked down, or partly knocked down, thousands of buildings, mainly in the states of Chiapas and Oaxaca at the south end of Mexico. Tragically, nearly 100 people died from the earthquake.
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| Juchitán de Zaragoza, Oaxaca (from Science Magazine) |
The earthquake has also been reverberating through the seismology community (the earthquake studies community). It was a deep earthquake, centered 70 km (45 mi) deep. There are a couple reasons why the depth may have been a bit lucky for people on the surface, despite all the damages and casualties that did occur.
First, the fault did not rupture all the way up to the ocean floor, which would have made a much larger tsunami. Instead, the seafloor shaking only generated a small tsunami of no more than 1 m (around 3 ft) high at maximum, which did not do significant damage to shores and harbors.
Second, the depth of the earthquake spread the energy out over a larger area by the time the waves hit the earth's surface than a shallower earthquake would have. This lowered the impact of seismic waves on the surface. The earthquake would probably have done worse damage, had it been a shallower earthquake.
It was a little bit of a surprise to read that the earthquake did not take place along the boundary between the downgoing Cocos Plate and the overriding North American plate. Instead, the earth broke, sheared, and slipped alongside itself within the Cocos Plate. Intraplate earthquakes (within the subducting plate) rather than interplate earthquakes (between the subducting slab and the plate above it) are a known entity in subduction zones, but not often of this big a magnitude.
Even more surprising to people like me, who assumed that this would check out as a megathrust-style subduction earthquake, this was not a thrust fault earthquake. It was a normal fault earthquake. The rock above the fault plane slid downward, the opposite of the thrust-faulting that characterizes most big subduction zone earthquakes.
(Reminder: Never assume. To assume "makes an ass out of u and me," or just me in this case.)
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| from earthscience.stackexchange.com: Why should this be a thrust fault? |
Seismologists have some explaining to do. In an article she posted in the Eureka (California) News online called "An 'abnormal' subduction zone earthquake," Lori Engler gives a good, thumbnail explanation of how the seemingly abnormal normal faulting in a subduction zone is kind of normal. Got it?
By the way, the whole Mexican subduction zone and volcanic arc, or subduction zones and volcanic arcs by some analyses, is fraught with oddities that have been a challenge for geologists and seismologists to explain.
Unusual things about subduction in Mexico include the misalignment between the inland volcanic arc and the subduction trench along the coast - why so out of whack? - and the lack of the zone of deepening earthquakes angling under the continent from the trench inward - the "Benioff zone," the hallmark of subduction, is missing!
See "The unusual case of the Mexican subduction zone" (California Institute of Technology Tectonics Observatory, May 4, 2009).
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| from Berger et al. (2011), "Seismic tomography at Popocatépetl volcano, Mexico," JVGR |
The next time you are in Mexico City, or one of the great colonial cities across the mountainous, volcano-studded belt of Mexico including Puebla, Cuernavaca, Morelia, and Colima, realize that you are in a volcanic arc, the culmination of Mexico's own, unique subduction zone, which has its special quirks, and is built into the culture of Mexico through such geological characteristics as the earthquakes, mountain-rimmed basins, steep slopes, high elevations, and volcanic soils. Think of the gifts and risks that subduction constantly presents to Mexico, from rough jagged rocks and deposits of gold and silver, to cities of eternal spring next to smoking volcanoes. Think of how the land, and how it behaves, affects the food and drink, and the history, lives and culture, of the people who live there.
Other References:
"Deadly Mexico earthquake had unusual cause" (Nature News online, 9/9/17)
"Unusual Mexico earthquake may have relieved stress in seismic gap" (Science Magazine online, 9/11/17)
"M 8.1 - 87km SW of Pijijiapan, Mexico" (USGS Earthquake Hazards Program online, 9/9/17)
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