Two earthquakes just 12 days apart in Mexico have killed hundreds of people, injured thousands, flattened buildings, and highlighted the enormous seismic risk the region continually faces.
Just before midnight on Sept. 7, a 8.1-magnitude earthquake–the most powerful to hit Mexico in a century–rattled the country, doing the brunt of its damage to its southern part, which was closest to the quake’s epicenter off the Pacific Coast. Then on Tuesday, as cleanup and recovery efforts continued, an earthquake with a preliminary magnitude of 7.1 struck about 100 miles southeast of Mexico City, causing severe, sustained shaking in the capital. It happened 32 years to the day that a massive earthquake in 1985 struck Mexico, killing as many as 5,000 people.
Gavin Hayes, a research geophysicist with the United States Geological Survey, spoke with the New York Times and NPR about these earthquakes and others. Based on that, here are the basics of what we know:
223: the number of confirmed dead from the most recent quake, which is thought to be the deadliest since the 1985 quake. (If you’re looking to donate or help locally, here are a few places to start.) Here is footage of the rescue of two young children earlier yesterday at the Enrique Rébsamen school, where 21 children and four adults have been confirmed dead as a search through the rubble continues:
— ABC News (@ABC) September 20, 2017
7.1: magnitude of the quake that struck Mexico City. Both this and the earlier earthquake this month occurred in a fault within the sinking oceanic Cocos plate. Earthquakes that happen inside a plate tend to be weaker than those that occur between plates—but they also tend to be closer to the surface. That can cause major damage to whatever is sitting directly above them.
The Sept. 7 earthquake was stronger than the one that struck less than two weeks later, but its epicenter was farther away from densely populated areas; the epicenter of the more recent quake was much closer to Mexico City. The city is built on a sedimentary basin, a geology that amplifies an earthquake’s effects more so than an area with more bedrock. During quakes, basins like that are “like a bowl of jelly shaking around, and it just keeps on reverberating.”
Surface waves: These are often created by shallow quakes in this region, which travel at a frequency that is especially damaging to 10- to 20-story buildings; these buildings tend to resonate with these waves, synching with them and shaking even more.
Why did such large quakes strike in such rapid succession? It’s not yet known if the initial earthquake altered stresses nearby, leading to subsequent quakes, including ones as far away as the second one was. But it’s not likely, says Hayes. Yet the area is home to lots of faults that lie along and around the junction of two plates, the Cocos and the North American, across an area that runs along the western coast of Central America, from Central Mexico to Panama—one of the Earth’s so-called subduction zones.
Subduction zone: The parts of the earth where one slab of the crust is slowly sliding under another. In Mexico’s case, the oceanic Cocos plate is gradually sinking beneath the continental North American plate. Over time, stress builds because of friction between the slabs, and at some point, the strain becomes so immense that the pent-up energy is released in the form of an earthquake. These quakes are the world’s most powerful earthquakes. Had one of the Mexico earthquakes occurred between the Cocos and the North American, it would likely have produced a megathrust.
Megathrust quakes: Relatively recent examples of “megathrust” quakes include the magnitude-9.1 quake off Japan in 2011, the magnitude-9.1 quake in Indonesia in 2004, a magnitude-9.2 quake that struck Alaska on Good Friday in 1964, and a magnitude-9.5 quake that struck Chile in 1960–the strongest quake ever recorded. Quakes at plate boundaries, in so-called subduction zones, usually involve larger faults and thus release more energy, and can produce magnitudes of 9.0 or higher. But they also usually occur farther from the surface than quakes within plates.
— USGS (@USGS) September 19, 2017
Once a year: how often, typically, a quake of magnitude 8 or higher occurs somewhere in the world; there are about a dozen quakes of magnitude 7 or higher annually.
A “quiet year”: how Hayes described global seismic activity this year to the Times. According to USGS data, about 4,200 earthquakes of magnitude 4.5 or higher have occurred around the world so far this year. Over the same period in 2016 and 2015, about 5,100 quakes of the same strength occurred. In 2014 there were closer to 6,000.
“Any day now”: when the next “big one” could happen in the U.S., according to seismologists and others: In the Cascadia subduction zone, which runs along the Pacific Coast on the western borders of Oregon and Washington, the Juan de Fuca Plate is edging east and slipping slowly beneath the North American Plate. The zone last generated a magnitude-9.0 earthquake in the Pacific Northwest in 1700, meaning that another is due. The quake and the tsunami it would create would be “devastating” to both Oregon and Washington, especially their coasts, Dr. Hayes told the Times. (The New Yorker wrote about the “really big one” in 2015.)
The other subduction zone in the United States, which includes Alaska, generated the 9.2 quake in 1964, meaning that another quake of that magnitude isn’t likely for hundreds of years. The San Andreas fault, which runs north to south along most of western California, is capable of producing an earthquake with a magnitude as high as 8.2; the last major temblor there occurred 160 years ago.
“Human-induced” earthquakes: the so-called result of wastewater being pumped into the ground by oil and gas drilling operations. Tremors have been recorded in Oklahoma with magnitudes as high as about 5.8, but it’s not clear how much stronger they can get.