{"id":98788,"date":"2017-09-18T12:00:25","date_gmt":"2017-09-18T11:00:25","guid":{"rendered":"https:\/\/www.transcend.org\/tms\/?p=98788"},"modified":"2017-09-17T12:31:58","modified_gmt":"2017-09-17T11:31:58","slug":"why-seismologists-didnt-see-mexicos-deadly-earthquake-coming","status":"publish","type":"post","link":"https:\/\/www.transcend.org\/tms\/2017\/09\/why-seismologists-didnt-see-mexicos-deadly-earthquake-coming\/","title":{"rendered":"Why Seismologists Didn\u2019t See Mexico\u2019s Deadly Earthquake Coming"},"content":{"rendered":"
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The hotel ‘Ane Centro’ was damaged after a 8.2 magnitude earthquake in Matias Romero, Oaxaca, Mexico.
Photograph by Angel Hernandez, EPA. National Geographic<\/p><\/div>\n

15 Sep 2017 – <\/em>Mexico has a long seismic history, so any given earthquake here does not necessarily come as a surprise. In the pre-Hispanic epoch, inhabitants of the country\u2019s central zone reported on earthquakes in their \u201cc\u00f3dices<\/a>,\u201d or indigenous records, attributing the shaking to the wrath of their gods.<\/p>\n

But the quake that convulsed the southeastern states of Oaxaca and Chiapas on Sept. 7, 2017, was a shock nonetheless.<\/p>\n

First, there was its magnitude<\/a>: at 8.2, it ties as Mexico\u2019s strongest earthquake since the invention of modern seismic-measurement tools, surpassing even the great Mexico City quake of Sept. 19, 1985<\/a>, which registered 8.1. This recent quake killed nearly 100 people<\/a>, most of them in Oaxaca, and the death toll is rising as locals continue to dig out from the rubble.<\/p>\n

Beyond the devastation<\/a>, it was the earthquake\u2019s location that took scientists by surprise. Until last week, seismologists believed that its epicentral area \u2013 near the old Zapotec city of Juchit\u00e1n, Oaxaca, in Mexico\u2019s poor southeastern region \u2013 was an \u201caseismic gap.\u201d In other words, we thought this zone, the Tehuantepec gap, was unlikely to cause an earthquake.<\/p>\n

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At least 96 people were killed and thousands left homeless in Oaxaca, Chiapas and Tabasco states.
Carlos Jasso\/Reuters<\/p><\/div>\n

Oaxaca\u2019s not-so-inactive fault<\/strong><\/p>\n

The Tehuantepec region is actually one of the few parts of Mexico\u2019s Pacific coast that had never suffered a major earthquake.<\/p>\n

Thus, most of the geophysical studies done in the area<\/a> have focused on the nearby Guerrero gap area, largely overlooking the seemingly inactive coastal zone off the Isthmus of Tehuantepec.<\/p>\n

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Major earthquakes along the Mexican Pacific coast, 1900-2017. The red star marks the epicenter of the Sept. 7, 2017, Tehuantepec quake. Adapted from Kostoglodov and Pacheco (1999), Author provided<\/p><\/div>\n

The tectonics of this oceanic area are not simple. A submarine mountain chain, the Tehuantepec Ridge, interacts with the North American continental plate, moving underneath it at a speed of roughly three inches per year. Scientists know remarkably little about how this subduction<\/a>, as it\u2019s called, affects seismicity<\/a> \u2013 not here, and not in other regions with similarly complex topography.<\/p>\n

In this part of the Mexican Pacific coast there is also what\u2019s called a \u201ctriple-junction zone<\/a>,\u201d meaning that the boundaries of three tectonic plates intersect here \u2013 in this case, the Cocos, North America and Caribbean plates. Each of them moves at a different speed and in its own direction<\/a>, which makes predicting seismic activity in such areas extraordinarily difficult.<\/p>\n

Learning and rebuilding<\/strong><\/p>\n

Here\u2019s what we do know. The Tehuantepec earthquake happened just offshore, within the Cocos plate, at a depth of approximately 37 miles.<\/p>\n

The plate tectonics here get complicated, but essentially the oceanic Cocos plate slides underneath the North American continental plate, dipping some 50 degrees. The rupture that caused the quake started there, inside the subducted Cocos plate, along a highly unstable, almost vertical fault.<\/p>\n

The stresses released during this rupture were transferred mostly upwards and northwest, in the Earth\u2019s upper crust. The magnitude of the main event was so great that it reactivated numerous shallow faults, triggering a seemingly endless series aftershocks: the release of energy accumulated over decades.<\/p>\n

For hours after the initial quake, residents of Chiapas and Oaxaca felt aftershocks<\/a>, some of them with magnitudes greater than 5.5 \u2013 significant earthquakes on their own merits.<\/p>\n

These are the facts as we know them now, but many important scientific questions remain unanswered. For example, most of the aftershocks occurred in a narrow southeast-to-northwest band. Does this mean that the energy from the rupture was shunted more strongly in one direction \u2013 what\u2019s called directivity effect?<\/p>\n

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Aftershocks from the Tehuantepec quake. Blue dots show the deepest shocks, followed by green and red (shallowest). The large dot is the main shock.
Image adapted from the Special Report of Servicio Sismol\u00f3gico Nacional, UNAM.<\/p><\/div>\n

Seismologists around the world are also curious about the characteristics of the hardest-hit areas. Is there something in the earth below the city of Juchit\u00e1n, which was almost completely destroyed in the quake<\/a>, that caused the ground motion there to be especially intense?<\/p>\n

This is called a site effect, and it is important to understand because it helps governments craft building codes based on expected quake impact.<\/p>\n

Finally, has this earthquake now \u201cfilled\u201d the Tehuantepec gap? In other words, did the massive September 7 quake release all the seismic stresses accumulated in that region, or do some areas of the plate remain unbroken? Since we don\u2019t know, the future risk of quakes at the Tehuantepec gap<\/a> remains unclear.<\/p>\n

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Watch a building in #Juchit\u00e1n<\/a>, #Oaxaca<\/a>, weakened by 8.2 mag #earthquake<\/a>, collapse today. #Mexico<\/a> #Video<\/a> pic.twitter.com\/DELhwJeMTv<\/a><\/p>\n

— Mexico Times' 1st Breaking News (@1stBreakingNews) September 9, 2017<\/a><\/p><\/blockquote>\n