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UT Arlington faculty conclude that previously unknown fault caused Haiti earthquake

A new study co-authored by UT Arlington College of Science Dean Pamela Jansma, earth and environmental sciences professor Glen Mattioli and researchers at several other universities presents strong evidence that the Jan. 12 Haitian earthquake was caused by a previously unmapped fault and not one experts first suspected.

The researchers say "a significant seismic threat for Haiti and for Port-au-Prince in particular" remains because the earthquake didn't release significant accumulated strain from the Enriquillo-Plantain Garden fault, as first believed. Their paper, "Transpressional rupture of an unmapped fault during the 2010 Haiti earthquake," will be published in the November issue of the journal Nature Geoscience and is already available online.

The researchers concluded that a previously unmapped fault called the Léogâne fault caused the quake.

Eric Calais, a Purdue University professor of earth and atmospheric sciences, led the research team, which also included experts from University of Miami and institutions in Saudi Arabia and Haiti. Calais, Jansma and Mattioli have worked together since the mid-1990s and data for the current study was obtained, in part, from their 2004 National Science Foundation funded study of surface deformation and seismic activity on the island of Hispaniola, which includes Haiti and Dominican Republic.

Mattioli, who traveled to Haiti on Jan. 29 to gather data with Calais, said the latest results add new dimension to scientists' understanding of the area where the Caribbean and North American tectonic plates meet.

"Just because there was a magnitude seven quake that happened on this Léogâne fault doesn't necessarily mean the risk is diminished on the Enriquillo-Plantain Garden fault, especially to the east and the west of the rupture zone," Mattioli said. "Ultimately, there's going to be more earthquakes in this region of the world, and the fact is we don't have a lot of good information about how frequently these faults rupture."

More than 200,000 people died in the Jan. 12 quake. Damage has been estimated at more than $8 billion. Originally, researchers thought the quake was due to a rupture on the Enriquillo-Plantain Garden fault, which extends from western Dominican Republic to beyond the western tip of the Tiburon-peninsula of Haiti, passing near major population centers of Port-au-Prince and Léogâne. To explore that idea, Calais' team used global positioning system observations and radar interferometry measurements of ground motion.

The scientists found that instead of moving the ground east to west, as they believed a quake from the Enriquillo-Plantain Garden fault would, the quake had actually moved the ground toward the fault and upward, causing shortening of the ground surface in the region near the earthquake, Mattioli said. Those measurements, along with data about the location of the quake, led them to believe a rupture of the Enriquillo-Plantain Garden fault had not occurred.

Instead, Calais and his team inferred that the Jan. 12 quake actually resulted from a rupture on an unmapped north-dipping fault they called the Léogâne fault. That fault is subparallel to the Enriquillo-Plantain Garden fault, researchers believe.

The Léogâne fault is what is known as a "blind thrust" because the fault does not reach to the surface and can't therefore be seen, said Jansma, who is an expert in tectonics of the northern and eastern Caribbean.

"A blind thrust is the type of fault on which the Northridge earthquake in southern California occurred in 1994. These faults are known to exist, but difficult to detect," Jansma said. "This is particularly the case in places like Haiti where the detailed mapping and subsurface investigation haven't been done to the same extent as other places, such as southern California."

In a story from the Purdue University news service earlier this week, Calais said the new paper highlights the need for more exploration of the fault system in Hispaniola.

"The fault system is more complex than we originally thought, and we don't yet know how the January earthquake impacted the other faults," Calais said. "Preliminary measurements indicate that the Enriquillo fault did not release any accumulated seismic energy and, therefore, remains a significant threat for Haiti, and Port-au-Prince in particular."

He added that the "inexorable" earthquake risk in Haiti should be motivation for a focus on proactive measures to adapt to earthquake hazards and save lives. The latest research was supported by grants from the National Science Foundation and the National Disaster Risk Management System Development Program - UNDP Haiti.