ARTICLE - GEOPHYSICIST QUESTIONS

5 QUESTIONS FOR GEOPHYSICIST ERIC CALAIS ON THE NEWLY DISCOVERED FAULT IN HAITI

Eric Calais, professor of geophysics at Purdue University, has studied the faults of Haiti and the Hispaniola region for more than two decades. He was one of the first scientists on the scene in the aftermath of the Haiti earthquake of 2010, which struck unexpectedly in the late afternoon nine months ago today, on January 12. Calais was also recently selected by the United Nations Development Programme to lead a project aimed at reducing the risks and damage associated with earthquakes in Haiti.

Calais and his team of researchers reported earlier this year that movement at a previously unknown fault had been responsible for the devastating Haiti earthquake. Britannica science editor Kara Rogers went in search of more information about this new fault and how it contributed to the 2010 earthquake. Calais kindly agreed to answer her questions. His responses reveal a tectonic world in turmoil beneath Haiti and offer insight into the future of hazard assessment in the region.


* * *
Britannica: Movement at the Enriquillo fault, which runs east-west a short distance south of Port-au-Prince, was believed to have been the cause of the Haiti earthquake of 2010. What led you to suspect that the earthquake may have originated with movement at a different fault?

Calais: The first give-away was the fact that the earthquake did not rupture the surface, which is quite unusual for large earthquakes on vertical faults with horizontal slip such as the Enriquillo. Then geologists discovered dead coral reefs along the coastline from Leogane to Petit-Goâve, indicating that the region to the north of the Enriquillo fault moved upward during the earthquake (hence bringing corals above sea level). This again was not quite consistent with the motion pattern expected from slip on the Enriquillo fault. The final solution came from space geodetic data, namely global positioning system (GPS) measurements on the ground and radar interferometry from satellites. These techniques allow for the very precise measurement of the ground displacement caused by the earthquake. From these measurements of ground displacement, one can infer the location and dip angle of the earthquake fault.

Britannica: Why hadn’t the fault been detected earlier, and how did you discover it?

Calais: Shortly after the geological observations were made, geodetic information became available from satellite radar interferometry (InSAR) and GPS data. The data was used to calculate the permanent ground displacement due to the earthquake by comparing data acquired before the event with data acquired right after it. The picture that emerged was clearly not compatible with horizontal slip on a vertical fault. Further calculations indicated that, in order to explain the data (both geological and geodetic), it was required that most of the earthquake slip occurred on a fault different from the Enriquillo.

This is not too surprising since major faults are usually accompanied by smaller ones on either sides. This newly discovered fault is probably one of several smaller faults that run along the Enriquillo main fault. Research done in the 1980s by Haitian geologists actually identified and mapped some of these ancillary faults offshore of the southern peninsula of Haiti. But very little geologic research has been done in Haiti since then, and many faults are probably going unnoticed. It took an earthquake to reveal this one, it will take a significant (but indispensable) research effort to find the other ones.

Britannica: What are some of the notable characteristics of the new fault?

Calais: This unmapped fault that ruptured on January 12 is slightly oblique to the Enriquillo. Its surface projection is close to the Enriquillo fault, but the fault dips about 60 degrees to the north, underneath the area of Léogâne—no wonder it was so severely shaken by the earthquake. We also find that fault slippage during the earthquake combined horizontal motion and vertical (contractional) motion.

Britannica: How does the fault fit with the known tectonic features of the region of Hispaniola?

Calais: The combination of horizontal and contractional slippage during the earthquake is consistent with what we know happens between earthquakes. GPS measurements show that during these periods, Haiti is both sheared in an east-west direction and compressed in a north-south direction. This slow, but constant and inexorable “squeezing” of Haiti (actually the entire island of Hispaniola) builds up stresses that are released episodically during large earthquakes—very much like a rubber band that snaps if pulled too much.

The question remains whether the “new” fault is part of the Enriquillo fault system (such as a small splay from it) or is part of a different fault system. There are reasons to suspect that the new fault is part of the “Haitian fold and thrust belt,” a series of mountain ranges that cut obliquely across Haiti in a NW-SE direction. Most of these ranges are not seismically active anymore, but some research has shown that its southernmost part is still active. Only more research will tell.

Britannica: How will knowledge of the new fault be used in the future of earthquake prediction in Haiti and surrounding regions?

Calais: First, let me say that science cannot predict earthquakes (yet). We are not trying to predict earthquakes in Haiti (because it cannot yet be done) but rather trying to assess the best we can the level of threat posed by earthquakes in the country and the surrounding region. One of the key ingredients to assessing seismic threat is to identify all potential sources of earthquakes—i.e., all the seismically active faults. The January 12 earthquake is a reminder that we still have a very partial catalog of faults at our disposal. Future hazard assessment will include that fault and hopefully other similar, yet unknown, features. We must look for them if we ever want to find them.