The quake's epicenter was roughly seven miles southeast of the coastal town of Leogane, some 20 miles west of Port-au-Prince on the country's southern peninsula.
The Enriquillo-Plantain Garden fault, in effect part of a boundary between the North American and Caribbean plates, runs through the area.
But when members of Dr. Hayes's team arrived and hunted for fresh surface evidence of a rupture along the fault in that area, they found none. Typically, a strike-slip fault such as this, in which one crustal plate slides along the edge of another, would show fresh jogs in once-straight stream beds or other signs of lateral shifts. But the team found none.
Instead, after reviewing seismic information, along with information from satellites and boots-on-the-ground visits to various locations in the area, the team found that the landscape between the fault and the coast has been raised by as much as two feet. Once-submerged corals along the coast were now high and dry.
That suggested a hidden thrust fault, a conclusion borne out by modeling experiments built on the data the team collected.
"Most or all of the slip was on the Leongane fault," says Carol Prentice, an earthquake scientist at the US Geological Survey's office in Menlo Park, Calif., and another member of the team. But, she cautions, it's a modeled fault and will require more study to determine if it truly has a real-world counterpart.