The Haitian earthquake of January 12, 2010, was caused by rupture of a fault that forms part of the complex boundary between the Caribbean and North American plates. Together these faults accommodate about 20 mm/year of east-northeastward motion of the Caribbean plate, relative to the North American plate. Over the past few centuries, this plate boundary has generated numerous destructive earthquakes in the lands astride it – Guatemala, Jamaica, Haiti, the Dominican Republic and Puerto Rico, among others.
Earthquake scientists will be working out the details of the rupture of the Enriquillo-Plantain Garden Fault Zone (EPGFZ) for months after the January 12th earthquake. Nonetheless, the location of the mainshock epicenter, just west-southwest of the Haitian capital city of Port au Prince, seismological analysis of the mainshock, and the initial pattern of aftershocks imply that the source of the earthquake is approximately delineated by the red line. Unbroken sections of the fault (between the arrows) may produce large earthquakes in coming decades.
The EPGFZ is clearly visible (between the arrows) in satellite imagery on Google Earth. The large white arrows indicate the sense of motion of the fault. The block north of the fault moves west relative to the block on the south. We call this type of fault a left-lateral (or sinistral) fault, because an observer looking across the fault from one side would see the other side of the fault move left during a big earthquake.
A basic analysis of the landforms in Figure 3 show that left-lateral (sinistral) motion of the EPGFZ fault, probably in thousands of large earthquakes over a couple million years, has offset two large river courses by many kilometers.