Rupture within tectonic plate is probable cause of Mexico earthquakes

Mexico’s most recent earthquakes did not directly involve two tectonic plates clashing, as is commonly the case. Seismologist Dr Stephen Hicks explains

We are often reminded about the force and devastation from earthquakes that occur around the Pacific Ring of Fire. The titanic collision of two tectonic plates, which firmly lock together and accrue strain over tens to hundreds of years, eventually releases this pent-up energy as a large earthquake. We have seen such quakes striking Indonesia, Chile and Japan over the past 15 years. Mexico, too, lies on the Ring of Fire and is no stranger to such quakes: the 1985 8.0 magnitude earthquake that devastated Mexico City was a fairly typical “thrust” earthquake that ruptured the shallow portion of the tectonic plate boundary.

The two earthquakes that struck Mexico this month were different.

The first tremor, a magnitude 8.1 earthquake, had a fairly typical epicentre location – just offshore of the coastline. Yet its depth, pinpointed to 70km down, raised eyebrows. In this case the rupture took place inside a plate itself, not at the locked zone between plates. Following its head-on collision with the North American plate (to which Mexico belongs), the Cocos plate (a piece of crust beneath the Pacific Ocean) gradually sinks deeper into the earth under its own weight. As the Cocos plate plunges deeper into the bowels of our planet, fractures inside the slab bend and pull apart. This process probably caused the magnitude 8.1 earthquake.

The location of the 8.1 magnitude earthquake, 7 September.
The location of the 8.1 magnitude earthquake, 7 September.

If the 8.1 tremor raised eyebrows, then Tuesday’s magnitude 7.1 earthquake left everyone stunned. The earthquake’s epicentre was unusually located, in central Mexico. Its depth was puzzling: only 50km down, but the faulting mechanism implied rupture inside a plate.The reasonably shallow depth suggested that this particular part of the Pacific Ring of Fire does not behave as we expect. Based on previous scientific studies, the slab begins to behave as normal at shallow depths, but then it suddenly flattens for a distance of around 200 km, before again plunging into the deep Earth. This final kink in the Cocos plate is located right where the 7.1 earthquake happened, and directly beneath the Valley of Mexico, in which Mexico City is located. This warping of the Cocos plate caused a large earthquake to occur close to Mexico city at a relatively shallow depth. The combination of the earthquake’s location, radiated seismic energy, and the very foundations of Mexico City – thick, loose soils that behave like a bowl of jelly during earthquakes – created the conditions for devastation.

The 7.1 magnitude earthquake, 19 September.
The 7.1 magnitude earthquake, 19 September.

The two earthquakes occurred within just 11 days of each other and both were within the sinking Cocos plate. Preliminary calculations show that because the earthquakes occurred more than 600km from each other, the first rupture would have only increased stress on Tuesday’s ruptured fault by a tiny amount. So there is no immediate indication that these two quakes are directly linked.

Although these types of earthquake are rare, the amount of damage caused by them is not globally unprecedented. The world’s largest recorded earthquake, which struck Chile in 1960, was not the country’s most damaging earthquake. A magnitude 8 earthquake that struck the city of Chillán in 1939 contributed to the deaths of nearly 30,000 people. This quake in Chile occurred at a similar depth, with a similar faulting mechanism, and beneath a highly populated area in a sedimentary basin, similar to the two recent Mexico earthquakes

We know very little about the geometry of these faults within plates, how they behave from one earthquake to another, and the frictional properties of the rocks that fracture. Our detailed earthquake records from instruments only go back a hundred years or so, and these deep quakes do not leave any noticeable imprint in the geological record from surface ruptures or large tsunamis. So, it is difficult to tell where and when these types of ruptures have occurred in the past.

Whilst geophysicists continue to try and unravel the factors that drive these deep earthquakes inside tectonic plates, the key for people living in earthquake-prone areas is to prepare. Mexico has made great strides in improving building codes since the 1985 earthquake. However, as this week’s events have shown, more work is still to be done in retro-fitting older buildings to bring them up to modern standards.

Mexico, was one of the first countries, along with Japan, to develop an earthquake early-warning system. These systems analyse recordings of the weak and faster-travelling seismic wave arrivals (P-waves) to try to estimate when the slower travelling, but more damaging waves (S-waves) will arrive and how strong they will be. Mexico City was given 10-15 seconds of warning during the magnitude 7.1 earthquake.

If building structures are improved, early warning systems continue to be implemented and the public is educated about what action to take to protect themselves when the warning sirens sound, then cities located in earthquake-prone areas will be making good progress to better protect their infrastructure and people during future quakes.


Stephen Hicks

The GuardianTramp

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