Aron Mirwald, M.Sc., Temblor, Inc.
Citation: Mirwald, A., The riddle of the 19 September 2017 Mexican earthquake, Temblor, http://doi.org/10.32858/temblor.017
Only 16% of the energy that was released by the M=7 earthquake was converted into seismic waves, which are responsible for the damage. But the quake nevertheless produced the strongest bedrock shaking ever recorded. Had more of its energy been transformed into waves, its toll would have been vastly greater.
The magnitude 7.1 earthquake, which struck Mexico City on 19 September 2017, caused devastation in the city and its surroundings. 369 people died, 57 buildings collapsed, and a much larger number was seriously damaged.
A recent study, carried out by a team of seismologists at the National University of Mexico (UNAM) including myself, found that the 2017 Mexican earthquake was very ‘inefficient’. What does this mean and how does it relate to other earthquakes in Mexico?
Here, you can see the 19 September 2017 earthquake (star) and other similar events (green points), which occurred within the slab that is being shoved beneath Mexico. The red patches are where major coastal earthquakes have occurred. One example is the 1985 Mexican earthquake that caused more than 10,000 deaths in Mexico City. The dotted purple line marks the position of the cross-section below.
Graphic by Carlos Villafuerte.
What makes the 19 September 2017 Mexican earthquake special?
The 2017 earthquake caused the strongest shaking (technically, the largest ground acceleration) ever to be measured at Ciudad Universitaria (CU), a bedrock site in the south of Mexico City (57 ).
Another peculiarity is that the quake struck very close to the city, only 127 km (79 mi) away, while most of the great earthquakes originate in the Pacific coast, about 300 km (186 mi) away. Moreover, the rupture was 57 km (35 mi) deep; much lower than most major earthquakes in Mexico, which take place at depths less than 30 km (19 mi) below the ground.
This cross-section shows Mexican earthquakes and their relation to the tectonic plates. The light blue and light green circles are coastal earthquakes, which occur where the two plates are in direct contact. The dark green and red earthquakes, on the other hand, occur within the slab that is being pushed beneath Mexico. These ‘in-slab’ earthquakes can occur very close to large cities in central Mexico. The 2017 earthquake is to the very right (1).
Singh et. al (2018)
As we show in the map above, earthquakes occur within the descending Cocos slab that is being shoved beneath Mexico. These ‘in-slab’ earthquakes occur at depths between 40-80 km (25-50 mi). Although less frequent and generally smaller, they represent a major seismic hazard in Mexico. In fact, researchers have estimated that in-slab earthquakes and coastal earthquakes have similar destructive potential (Singh et. al., 2015).
But here is a problem: Seismologists do not even know how earthquakes this deep can happen. Let me explain.
Rock flows below 30 km depth
At the earth’s surface, rocks are hard and brittle. That means that if enough force is applied, they will break. This is how most earthquakes are triggered. The stresses in the rock build up until it breaks, and the violent sliding of the two sides of the fault generates the seismic waves.
At depths below 30 km rock behaves differently, because of the high pressure and great temperature: Stresses are accommodated by flow-like deformation, similar to Taffy when it is chewed. Under these conditions, it is hard to imagine how breaking and sliding, sufficiently violent to generate seismic waves, can occur. But, it does, and seismologists are still searching for an explanation.
On the left, there is an example of brittle deformation. The shift of the layers can be explained by the rock breaking and sliding along the vertical fracture. On the right, we see that the rock has undergone ductile, or ‘flowing’ deformation.
Simulating the earthquake
In our study, we simulated the breaking and sliding process of the 2017 earthquake at the hypocenter.
We only knew the point of origin of the earthquake and the movement it caused on the surface, and we wanted to know the whole process of rupture. In order to model the phenomenon respecting its physical laws such as the fault friction, we had to use novel procedures that solve the equations governing the rupture and sliding process and the propagation of waves till the Earth’s surface.
In the video below, the result of one simulation is shown. You can see how the rupture propagates across the fault.
This animation shows the sliding velocity along the plane. At first, only a small part of the rock has broken (ruptured). Then, the rupture radiates from the initial spot, and the following sliding causes seismic waves. In order to obtain this, we solved the equations that govern the breaking and the slip.
Using models like the one shown in the video above, we evaluated the energy released by the earthquake. The result of this analysis was a surprise: only 16% of the energy that was released by the earthquake was converted into seismic waves. That means, that the shaking that was felt was only a small part of the overall earthquake energy. And even then, it was enough to produce the largest acceleration measured so far at CU!
But what happened to the other 84%?
Most likely, the rest of the energy was converted into heat in the fault zone. Considering the type of rocks that are found where the earthquake occurred, it is probable that this caused the rock to melt. This opens the possibility of formulating an explanation about the physics of deep earthquakes (Prieto et. al., 2012). Recall that we don’t know how earthquakes can happen below 30 km.
One possible explanation is that the rock, at those depths, does not break in the same manner as it would at the surface. Instead, a thin layer of rock suddenly melts and allows for the fast sliding, producing seismic waves. The reason this could happens is the local variability of the rocks conditions. For example, if a layer of rock is a little bit weaker than its surroundings, it will deform more, causing it to heat up and eventually melt. Of course, this is only a hypothesis that has to be tested further. For the moment, I can tell you that it is consistent with our results.
Unfortunately, more to come
Knowing the exact mechanism of in-slab earthquakes will help us to better evaluate their risk. In the meantime, we will have to rely on statistical information. Dr. Singh and his collaborators calculated that the ‘return period’ of earthquakes like this is around 150 years (Singh et. al., 2018). That is, an in-slab event causing similar damage in Mexico City can be expected to happen on average every 150 years.
But you must not lower your guard! Remember that, until today, there is no technology that allows us to predict when or where an earthquake will occur.
If you want to know more about the 2017 Mexican earthquake, check out the articles that have focused on the effects and implication in Mexico city, the damage caused by the earthquake, the stress configuration within the tectonic plate, the relation with other earthquakes, and the best strategy to survive.
Mirwald, A., Cruz‐Atienza, V. M., Díaz‐Mojica, J., Iglesias, A., Singh, S. K., Villafuerte, C., & Tago, J. (2019), The September 19, 2017 (MW 7.1), intermediate‐depth Mexican earthquake: a slow and energetically inefficient deadly shock. DOI: 10.1029/2018GL080904, Geophysical Research Letters.
Prieto, G. A., Beroza, G. C., Barrett, S. A., López, G. A., & Florez, M. (2012). Earthquake nests as natural laboratories for the study of intermediate-depth earthquake mechanics. Tectonophysics, 570, 42-56.
Singh, S. K., Ordaz, M., Pérez-Campos, X., & Iglesias, A. (2015). Intraslab versus interplate earthquakes as recorded in Mexico City: Implications for seismic hazard. Earthquake Spectra, 31(2), 795-812.
Singh, S. K., Reinoso, E., Arroyo, D., Ordaz, M., Cruz‐Atienza, V., Pérez‐Campos, X., Iglesias, A., Hjörleifsdóttir, V. (2018). Deadly intraslab Mexico earthquake of 19 September 2017 (M w 7.1): Ground motion and damage pattern in Mexico City. Seismological Research Letters, 89(6), 2193-2203.
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