![]() ![]() ![]() Much remains uncertain about this possible source of the region’s quakes, Ramírez-Herrera says. Other researchers have proposed similar mechanisms happening off New Zealand’s North Island, Gurney notes. This may be causing one plate to stick against the other, increasing friction and the possibility of quakes, says palaeoseismologist Ramírez-Herrera, who recently published a study suggesting just such a mechanism at work off the coast of Guerrero, Mexico. As the Cocos plate slides beneath North America, so too does this ridge. Oaxaca, however, sits near what’s known as the Tehuantepec fracture zone, where choppy crust forms a ridge that pokes up into the sediments along the seafloor. These sediments serve to lubricate the boundary region, helping one plate to slide against the other and limiting the buildup of stresses that causes earthquakes. In some subduction zones, sediments may be swept in along with the down-going plate. The situation near Oaxaca becomes even more complex. “It’s really, really rapid, which is why you get these quakes fairly regularly,” says Jamie Gurney, an independent geologist and founder of the UK Earthquake Bulletin. While that advance might not seem very quick, it’s a gallop in tectonic terms, and it facilitates the rocky friction that causes quakes. This occurs because of the Cocos plate’s steady march to the northeast between 50 and 70 millimeters a year. Near its southern region, the North American plate collides with the Cocos plate, which is forced underground in what’s known as a subduction zone. Mexico rides atop the North American tectonic plate. Restless EarthĪ series of tectonic plates make up Earth’s surface, and are constantly jostling for position. The many strong tremors are caused in part by the area’s complex geology-a furious collection of tectonic forces that primes Oaxaca for intense seismic activity. “The geologic scars of these events that happened in 1787 and then 1537 shows that this an area prone to large earthquakes and tsunamis,” says paleoseismologist María-Teresa Ramírez-Herrera of the National Autonomous University of Mexico. And hints of an even earlier strong quake, in 1537, also linger on land. In 1787, an even bigger quake-magnitude 8.6-rattled the same shores where yesterday's struck, creating a massive tsunami that flooded hundreds of miles of coastline. That magnitude 8.2 temblor struck to the southeast of yesterday's earthquake, collapsed buildings and killed hundreds of people, and sparked a torrent of aftershocks. Memories of a massive 2017 quake are still fresh in people’s minds. ![]()
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