The devastating earthquake that rocked Haiti in January was unleashed by a previously undetected fault line – not the well-known one scientists initially blamed, according to an analysis of new data.
It's unclear how dangerous the new, unmapped fault might be or how its discovery changes the overall earthquake hazard risk for Haiti, said Eric Calais, a professor of geophysics at Purdue University in West Lafayette, Ind.
He said the analysis shows that most, if not all, of the geologic movement that caused January's magnitude-7.0 earthquake occurred along the newly uncovered fault, not the well-documented Enriquillo fault.
Calais, who presented the findings this week at a scientific conference in Brazil, said they suggest Haiti's seismic zone is far more complex than scientists had anticipated. But the new fault's profile, including the possibility that it merges with the Enriquillo fault at some depth, won't be known until scientists intensively study the region.
"If there are other faults capable of producing earthquakes besides the Enriquillo and this new one we need to know about them. We need to go after them," he said from Brazil by telephone.
Calais said that at the time of the quake, Haiti had no seismic stations. Researchers who flocked to the Caribbean nation have since installed about 10 stations to monitor the earth's movement.
Ross Stein, a geophysicist with the U.S. Geological Survey in Menlo Park, Calif., said Calais' findings were fascinating and raise many questions about the complexity of Haiti's faults and what actually occurred during January's quake. But he said the discovery is not surprising, given the many unknowns about earthquakes.
Stein noted that even in California, whose many faults have been closely studied, about half of all moderate or stronger quakes occur on previously unknown faults.
"I work in a humbling field where we're constantly reminded of the depths of our ignorance," he said. "And if that's the case in California, then perhaps we shouldn't be surprised it also occurs to us in Haiti – a country that has barely been scoured at all."
The discovery is the sort of revelation that often comes after big earthquakes, when scientists descend on quake-ravaged sites to conduct intensive research, USGS geophysicist Bruce Presgrave said, adding "it's part of the learning process of science."
Earthquakes typically occur along fault lines, areas where two sections of the Earth's crust grind past each other. When decades or centuries of accumulated stress become too great at a fault boundary, the land gives way, causing an earthquake.
The first sign that the Enriquillo fault might not be to blame in the Haiti quake came when geologists didn't find any surface disturbance along the east-west fault. Instead, data pointed to new, unknown fault because an area north of the Enriquillo fault had been forced upward and to the south, Calais said.
The new findings are based on surface observations in the devastated region around Port-au-Prince, global positioning system measurements and other observations and data. Calais presented the research Tuesday at a meeting of the American Geophysical Union in Foz do Iguacu, Brazil.
In 2008, he warned that growing stresses in southern Haiti had left the Enriquillo fault ripe for up to a magnitude 7.2 quake. He said this week that the information then wasn't conclusive enough to say whether those stresses were building up along the Enriquillo fault, or some other fault.