3 Questions: Understanding Haiti’s Earthquakes | MIT news

On August 14, a 7.2-magnitude earthquake struck Haiti. The largest earthquake in the region since 2010, the disaster left at least 2,000 people dead, 12,000 injured and nearly 53,000 homes destroyed. Two assistant professors in MIT’s Department of Earth, Atmospheric and Planetary Sciences discuss why the region experiences earthquakes and what has changed—in Haiti and in seismology—since the devastating 2010 event, when the country had only one seismometer.

Camila Catania is a seismologist with expertise in numerical modeling, earthquake physics and statistical seismology. William Frank is a geophysicist who focuses on the physical mechanisms that control deformation within the Earth’s crust.

Q: Why is Haiti prone to earthquakes?

Catania: I’ll start with the broad tectonic setting. The island of Hispaniola, which includes Haiti and the Dominican Republic, lies between the North American plate in the north and the Caribbean plate in the south. Haiti is primarily on a small plate sandwiched between the two. At each plate boundary there are faults and fractures within the Earth’s crust that extend approximately from east to west. The earthquake occurred in the southernmost fault system, called the Enriqué Plantin Garden fault system, where there are faults with slightly different directions, creating complex fault geometries. The northern plate is moving to the west while the southern plate is moving towards the east, causing earthquakes along this fault zone.

Explicit: Not only do you have east-west sliding motion, but you also have flexural or flexural motion at the board boundary that is absorbed by other nearby imperfections. For example, one of the big questions of the 2010 earthquake is: What really went wrong? It looked like it was right after the Enriquillo-Plantain Garden fault system, but was it a transit fault, slippage, or compressive fault? There are a lot of questions hanging about the complexity of what seems simple from afar.

Catania: The area transitions between horizontal motion, where the plates slide past each other, to the compressive motion described by William, which has some vertical motion. Even in this earthquake, prototypes show that there was very little of both.

Another question would be: Why now? Why have there been two earthquakes recently? The Enriquillo-Plantain Garden fault system has been associated with the earthquakes of 1751, 1770, and 1860, without much in between. An extended period of time without earthquakes can increase your likelihood of experiencing an earthquake because you have more time to build up stress. Furthermore, the 2010 earthquake, which occurred on a sub-fault, increased stress at the site of the 2021 earthquake.

Q: What is the same and what is the difference in this earthquake versus the 2010 earthquake?

Frank: The 2010 earthquake occurred on a previously unidentified fault, one of the faults that accommodates the compressive motion of the plate boundary. The question we now ask is whether this latest quake is on the main translation error, or whether it is also on another fault accommodating the compressive motion. If so, it will be the same board boundary, but a different error scheme.

Catania: The reason there are so many unknowns is that this area had very little until 2010, when Haiti did not have a permanent seismic network. Now the area has more seismographs, and people also have portable, low-quality seismographs in their homes that provide a large amount of measurements. The quality of the data we have from this earthquake is superior to anything we had in 2010 or before. I think we’ll have more answers in the future to some of these questions than we did before because the hardware has improved between these two events.

Frank: The increased hardware allows us to get a better picture of what happens in the fault zone during the main earthquake and the aftershocks that followed. The parallel story about why this is possible is that during the 2010 earthquake, there was no seismology at Haiti State University. Now, there is a Department of Earth Sciences that recruits and trains seismologists.

An informative site is the result of an exciting collaboration between Earth science researchers in Haiti and the University of Nice in France, where they publish real-time locations and aftershock discoveries. It provides huge amounts of publicly available data. In general, there’s a lot more activity within Haiti, from hardware, and general interest in earthquake risk, and people to study the data, than there was during the 2010 earthquake.

Catania: Another difference between these events is their scale. the first was 7; This latest was 7.2. But the location was also different – the former was closer to Port-au-Prince and generally more densely populated areas. The fact that it is stronger does not necessarily mean that it is more harmful.

Q: What does your research tell us about future earthquakes in this region? What do we know as a scientific community?

Catania: We cannot predict with certainty the location or magnitude of mega earthquakes in this region or anywhere else. However, we do know the typical characteristics of aftershocks. Basically, you’ll feel hundreds of earthquakes in the first few weeks, and then that number gradually decreases unless one of those earthquakes is big enough to start a new sequence.

How does an earthquake affect the fault system? We had an earthquake in 2010 that happened east of the current earthquake, and it increased the amount of stress where the 2021 earthquake happened. If you look at a map of this area, obviously there are other parts of the same fault system that haven’t had major earthquakes for a long time. There is a possibility of another destructive earthquake on the same fault system.

Frank: For me, my research is most concerned with developing effective methods for detecting, identifying, and characterizing aftershocks. We have developed signal processing techniques that we can use in seismic data to identify earthquakes, and once we can identify them, we can get good locations. We are able to study the rate at which these aftershocks occur.

Aftershocks catalogs are very important for understanding the extent of the rupture and identifying the actual faults and planes on which they occur. There are two simple ways to define the structure. You can look at the main earthquake itself, or the rupture zone of the main earthquake, where aftershocks often determine where the main earthquake occurred. Once you can identify, locate and characterize those aftershocks, you can better model the earthquake.

Catania: My work was about including engineering complexity in aftershock forecasts. When you are trying to figure out where aftershocks occur, you need to know as much as possible about the direction of the faults present, and sometimes you have to make simplistic assumptions about it. I’ve developed methods that help to better include everything we know, using the data and the kind of information William was describing, to try to infer how the aftershock would develop given what the fault geometry would look like and how varied it is in this region. My methods allow you to get accurate information about fault engineering to produce better predictions for aftershocks.

Frank: That’s why I’m excited to be here with Camila – because we can make that direct connection.

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