Oaxaca earthquake: A chain reaction?

Six dead, many injured and hundreds of buildings damaged or destroyed. This is the preliminary assessment of the severe earthquake in southern Mexico on June 23rd. It is just the last of many major earthquakes that have hit the state of Oaxaca in the past few decades. An extremely extremely active region with slow slip events, earthquake swarms and severe earthquakes which forms a complex system of  triggering events. Yesterday's quake could be the latest part of a chain reaction that has been going on for three years.

The effects of the earthquake are moderate: with six confirmed deaths, multiple injuries and around 500 damaged buildings it is a comparatively low amount of damage for an earthquake of this magnitude. However, in the coming days the civil protection is going to continue its damage assessment, which might lead to a much higher number of damaged structures in the end.

ShakeMap basing on the USGS finite fault model and earthquake activity since June 22nd as recorded by SSN. Main- and aftershocks were located offshore while the USGS epicenter, marked with a star, is located inland.

So far, damage has mainly been reported from the epicentral region in Oaxaca where numerous buildings collapsed, fires broke out and landslides occurred. There was also building damage in the neighboring state of Veracruz and also in the capital Mexico City, 500 km away (there were also two injuries). Most of the Mexico-City urban area is located on the deposits of an ancient lake. The geological setting within a basin causes an amplification of earthquake waves from severe earthquakes, even from those hundreds of kilometers away. Instead of barely noticeable vibrations as on the outskirts of the city, which are built on solid volcanic rocks, the ground shaking in the center reached intensity V or more. An effect that was even stronger in this earthquake since above average energy was emitted in form of long-period seismic waves.

While the Mexican earthquake service SSN determined magnitude 7.5 for the quake on Tuesday, the USGS corrected the quake down from magnitude 7.7 to magnitude 7.4 on Tuesday evening - a reaction basing on precise determination of the rupture mechanism and thus also the released energy. Differences, also in the location of the epicenter, which will be revised by further evaluations in the coming days. At least the approximate location of the rupture area with a size of around 5000 km² is known: According to the USGS evaluation, it is located on the southeastern coastal section of the state of Oaxaca. The model of the USGS also includes an area off the coast, which was the cause of the tsunami.

Immediately after the quake, the Pacific Tsunami Warning Center issued tsunami warnings for the coasts of Mexico, Guatemala, Nicaragua and El Salvador, later expanded to Ecuador. After several hours, these warnings were lifted: wave heights of up to around 70 centimeters were recorded on several coasts of Mexico, including Acapulco area: harmless if you are not directly on the beach.

The area on the coast of Oaxaca has had numerous major earthquakes in the past, many of which had a similarly moderate impact as the recent one. Quakes of ~ magnitude 7.5 from 1999, 1978 and 1965 are known (with 2020 it forms some certain regularity). A quake in 1931 was stronger (M7.8). Quakes above magnitude 8 are also known from historical times. This region belongs to a subduction zone. Off the coast, the small Cocos Plate dips under the North American plate. The process takes place continuously at a speed of several centimeters per year. Stresses are built up at depths of 10 to 30 kilometers due to the friction between the plates, which are released in form of earthquakes.
Another kind of activity takes place above and below this so-called “seismogenic zone”. So-called "Slow Slip Events" (SSE), also known as "slow earthquakes", are more or less periodic, long-lasting movements similar to an earthquake, but happening significantly slower without a sudden movement, so that there are no noticeable vibrations. These SSEs release roughly the same amount of energy as severe earthquakes do. They can only be detected by GPS stations.

SSEs are very common in some subduction zones, especially off the coast of Oaxaca. They sometimes cover a very large area and are also known to trigger severe earthquakes.

In 2012, a magnitude 7.6 quake shook the neighboring state of Guerrero. Studies showed that an SSE that started in Oaxaca triggered this earthquake. SSEs can also be triggered by severe earthquakes, just as an SSE in Guerrero immediately followed a severe earthquake (M8.8) in Chile. Later studies showed that the passing of the seismic waves from Chile initiated the SSE beneath Guerrero.
In the past few weeks, the first traces of a new SSE in Oaxaca have been measured at GPS stations. A SSE which may have triggered yesterday's earthquake. A statement by the Mexican earthquake service is planned for Friday (June 26).

However, the Chiapas earthquake (M8.2) in September 2017 could also have contributed to the nucleation of yesterday's quake: Calculations from 2017 show that the epicentral region belongs to an of ​​increased tectonic due to the Chiapas earthquake. Every major earthquake releases stress, but at the same time additional stress is built on faults in certain areas. As a result, further earthquakes become more likely in these areas.
This was demonstrated last year when an unusually large swarm of earthquakes shook the coastal areas of Oaxaca, likely some distant triggered "aftershocks" of the Chiapas quake. The rupture area of this swarm overlaps with the rupture area of yesterday's quake.

A glance at the past shows that the subduction zone off the coast of Oaxaca has a very complex seismic behaviour. Frequent, almost regularly occurring severe earthquakes are present as well as slow slip events and earthquake swarms, partly connected, partly triggered by other (even distant) earthquakes. However, since not every SSE leads to a major earthquake, and it cannot be predicted when a large earthquake may trigger SSEs, it is not possible to make any predictions of their occurrence. Further & more in-depth observations of this activity may greatly contribute to a better understanding of the dynamic processes involved, allowing possible warning signs to be seen in the future.