While Mexico is still counting the victims and assessing the damage from the last earthquake that struck the capital, estimated to be up to $10bn according to the USGS, we reflect on how the country is being hit left, right and centre by natural catastrophes.
To the East, hurricane season continued across the Atlantic Basin. The Mexican coastline was hit by Hurricane Katia, which made landfall just to the north of Tecolutla as a category 1 storm. Heavy rainfall was swept inland as the storm weakened, but this led to flooding and mudslides east of Mexico City. All this in quick succession with Hurricanes Harvey and Irma, which battered the Caribbean and U.S. coastline, has made a busy Atlantic Hurricane season. To the West, the Eastern Pacific may also develop a hurricane over the next few days, and we’re watching the National Hurricane Center’s forecasts closely.
While hurricane season rumbled on, a deadly offshore M=8.1 earthquake struck the Chiapas region, with many aftershocks causing panic across the population. In the centre, a M=7.1 earthquake in the region of Puebla collapsed buildings in Mexico City, 120 km away, killing hundreds of people. This happened on the 32nd anniversary of a deadly M=8.0 earthquake in Mexico City, which killed over 5,000 people and caused billions of dollars in damage.
Furthermore, the quake caused a small eruption of Popocatépetl, a volcano 70 km southwest of Mexico City, and 50 km north of the epicentre of the M=7.1 earthquake. Popocatépetl is one of the most active volcanoes in Mexico and a constant threat to the more than 9 million people living nearby. With more than 15 major eruptions documented in historical times, and a continuous period of activity with sporadic explosions and lava flows over the past decade, Mexico is proving to be an example of strength and resilience against adversity. But, is it over, or is there more to come?
Understanding the facts
The Willis Research Network (WRN) is working closely with one of its newest members, Dr. Ross Stein, an internationally renowned seismic expert, CEO and co-founder of Temblor.net, and his team, to understand the consequences and implications to the Re/Insurance Industry of these sets of events.
As explained in their recent blog: “Depending on the subduction model used, today’s earthquake could have either been within the subducting Cocos plate (Franco et al., 2005), or the overriding North American plate (Hayes et al., 2012). This difference is a matter of how much the dip of the subducting slab shallows.”
Professor Shinji Toda from IRIDeS, Tohoku University, Japan, and a WRN member, also highlighted that “even though intraslab earthquakes tend not to be as destructive as those occurring in subduction zones, they are much harder to predict as their sources are totally invisible”. Intraplate earthquakes are relatively rare, but can inflict heavy damage, particularly because such areas are not accustomed to earthquakes. Prof. Toda suggests that a flattened slab layer could be a third source of large earthquakes in Mexico.
Linking the evidence
Dr. Ross Stein and his team suggested on September 12 that the M=8.1 earthquake on September 7 could have been much worse. A week later, a new event, smaller in magnitude, struck in a totally unpredicted area and affected Mexico City. Further studies on September 20 showed that the Coulomb stress effect in the area yields little to no evidence to suggest the M=8.1 earthquake is related to the M=7.1. Their calculations also suggested that, if any, the first earthquake delayed the second one.
Dynamic triggering and Popocatépetel volcano?
Earlier we mentioned that the M=7.1 earthquake caused a small eruption of Popocatépetl volcano located 50 km north of the epicenter. This could potentially shift the forces that keep the volcanic system in equilibrium and cause a large eruption. But what if the other way around is also true? The study above suggests that the seismic waves from the M=8.1 earthquake in Chiapas might have caused dynamic triggering of the distant faults. The waves “pump pockets of fluids that slowly diffuse into nearby fault zones, lubricating them to the point of failure, as proposed by Parsons et al ”. Could that pocket of fluid have been the large magma reservoir underneath Popocatépetl?
At the moment there are many theories and ongoing studies trying to understand the consequences and implications of these events. Even though there’s not a clear connection between them, there’s also not enough evidence to rule out the possibility that these could facilitate, statically or dynamically, new earthquakes, or even volcanic eruptions.
Whilst we can’t predict the future, the immediate work after the event by Stein and Toda suggests the probability of a large earthquake in the Chiapas region is now potentially lower than before. However, we can’t be complacent and will of course continue to learn from what we’ve seen this year in order to better assess the future impact of events.