The disasters keep on coming. This summer has seen massive mudslides, catastrophic monsoon flooding, and hurricane rain intense enough to depress an entire landmass, to note just a few. Now it's Mexico's turn, with a one-two punch from two of the largest earthquakes of the 21st century, plus a wallop from yet another hurricane and a volcanic eruption.
Geology plays a part in where and how all of these disasters happen. Mexico is particularly seismically active: it sits on the intersections of five jostling plates (The North American, Pacific, Cocos, Caribbean, and Rivera). The tectonic stresses caused by so much subduction and transform faulting lead to some world-class earthquakes and violent volcanoes. This has been spectacularly demonstrated this month, with the restless earth shaking central and southern Mexico hard. Let's zoom in and take a look at the back-to-back disasters.
Chiapas: A Shaky Past
The Mexican states of Chiapas and Oxaca are no strangers to massive earthquakes. Just since the beginning of the 20th century, the subduction zone between the subducting Cocos and the overriding North American and Caribbean plates have generated six earthquakes greater than magnitude 7 in Chiapas and neighboring Guatemala, and one magnitude 8 event in Oaxaca.
The fault zone shared between Chiapas and Guatemala sketches the outlines of a rather defuse triple junction where the Cocos meets the North American and Caribbean plates. The oceanic Cocos dives under the two larger plates, generating the hugely destructive earthquakes that have regularly rattled both areas.
The 9/7/2017 Chiapas Earthquake
Two weeks ago, a bit of the subducting Cocos plate ruptured near Pijijiapan in Chiapas, severely shaking southern Mexico and rattling Mexico City 650 kilometers to the north. The intraplate earthquake, which occurred at a depth of around 58 km, killed at least 96 people and left tens of thousands homeless. The most severe shaking occurred in the Gulf of Tehuantepec, devastating cities along the coast and causing a small tsunami. Fortunately, the rupture stopped when it reached about 40 km below the surface; if it had reached nearer the ocean floor, the seismic sea wave would have been far more devastating.
In a bit of good news amidst the devastation, the quake and its numerous aftershocks may have eased some of the pressure on a worrisome seismic gap in the vicinity. If so, southern Mexico has avoided a far worse seismic event in its future. But with all the stresses and strains the region is under, future huge quakes are a certainty.
As if the blow from a massive earthquake wasn't enough, the same region was overrun by Hurricane Max just one week later. Because of its location, Mexico is regularly subjected to hurricanes, and an earthquake in hurricane season means a lot of people with damaged or destroyed homes getting lashed by intense wind and rain. Fortunately, Max was only a Category 1 by the time it made landfall, and it didn't tarry like Harvey. Still, that's a heck of a beating for an already wounded area.
Disastrous Encore: The 9/19/2017 Central Mexico Earthquake
Less than a week after Max made landfall, with the shaking from the Chiapas Earthquake still a sharp and scary memory, central Mexico was walloped by a M7.1 earthquake that struck just after 1pm Eastern time, killing at least 248 people, destroying structures and trapping residents in the rubble.
Just two hours earlier, people in Mexico City had finished commemorating the horrific 1985 Michoacan Earthquake, which had killed nearly ten thousand people in and around the city. Now, 32 years to the day later, residents once again found themselves riding out the earthquake waves while dozens of buildings collapsed around them. Fortunately, the earthquake drills performed earlier in the morning, and the 20-second warning from the city's SASMEX earthquake early warning system, saved lives in the city.
Why did a smaller quake kill so many? Besides being densely populated, Mexico City is built on the soft, deep sediments of vanished Lake Texcoco. The damp sands and clays of the former lake bed amplify seismic waves, and are appallingly prone to liquefaction, both of which cause the damage from earthquakes to drastically increase. This is why an earthquake 650 km away rattled the city, and why this quake, with an epicenter 122 km to the southeast, caused dozens of buildings to collapse.
Fire and Earth
Mexico City's Popocatépetl volcano erupted shortly after the earthquake, reportedly causing a church to collapse, killing the worshipers inside. It's probable that the earthquake triggered this eruption, but Popocatépetl has been menacing the city for a long time. The volcano has been quite restless over the last few years. Steam and ash eruptions occur frequently, and the city lives with the ever-present threat of a truly catastrophic eruption.
As if triggering a volcano wasn't enough, the quake could also cause catastrophic landslides. For survivors already reeling from disaster, these would be a terrible further blow.
Though closely related in time, the Central Mexico Earthquake is not an aftershock of the Chiapas Earthquake. It is a sibling: both quakes were intraplate ruptures of the descending Cocos slab. Like it's older sibling, the Central Mexico quakes was a result of normal faulting at a depth of around 50 km.
How You Can Help
PBS has a comprehensive list of relief organizations you can donate to. The people of Mexico have been there for us in our times of need. We can be there for them now.
Geologic forces create much of the natural beauty in our world, and also a vast amount of our wealth. But these processes are quite often violent, and frequently leave us vulnerable to dangerous meteorological events. No place on Earth is immune. All we can do is learn to live with disaster, and be there for each other when geology hurts.