Tectonic context

A Mw 7.0 earthquake struck Haiti on January 12, 2010, causing intense damage in the capital Port-au-Prince. The event is thought to have occurred on the Enriquillo-Plantain Garden fault zone (EPGFZ), one of the main faults that accommodate the relative motion between the Caribbean and North American plates (Mann et al., 1995; Dixon et al., 1998; Figure 1). We have been studying this area with GPS geodesy since 2003: in collaboration with the Haitian Bureau of Mines and Energy and the Civil Protection Agency, we have installed and repeatedly measured a network of 35 survey benchmarks in Haiti. We have similar measurements in the Dominican Republic, with a network of 40 survey sites that have repeatedly measured, some of then since 1994. From this data set we have been able to determine a velocity field for that portion of the Caribbean-North America plate boundary (Figures 2 and 3), from which we estimated elastic strain accumulation on its major active faults (Manaker et al., 2008). Among the conclusions of this work, we wrote "the Enriquillo fault in Haiti is currently capable of a Mw7.2 earthquake if the entire elastic strain accumulated since the last major earthquake was released in a single event today" (Manaker et al., 2008). The results of this study was also integrated with the geology of the EPGFZ and presented during the 2008 Caribbean Geologic Conference (2008) in Santo Domingo, Dominican Republic, with Haitian geologists in attendance (Mann et al., 2008) along with private meetings between Calais and government officials in Haiti led by collaborators of the Haitian Bureau of Mines and Energy. Unfortunately, the earthquake scenario described in these presentations and publications unfolded on January 12 with some casualty estimates exceeding 200,000 people.

The Northeastern Caribbean region marks the tectonic transition between frontal subduction of the North American plate beneath the Caribbean plate in the Lesser Antilles and roughly east-west strike-slip motion along the Cayman Trough (Figure 1; Mann et al., 1984; Calais et al., 1991, 1992 a, 1992b, 1998; Dolan et al., 1998). GPS studies show that the interior of the Caribbean plate moves east-northeastwards (N70E) at a rate of 18-20 mm/yr relative to the North American plate (Dixon et al., 1998; DeMets et al. 2000), implying oblique convergence between the islands of Hispaniola and Puerto Rico on the Caribbean plate and the oceanic lithosphere of the North American plate (Mann et al., 2002).

Oblique convergence is strongly partitioned, in Hispaniola, between plate boundary-parallel motion on the Septentrional fault zone and EPGFZ in the overriding plate, and plate-boundary normal - or thrust - motion at the plate interface along the North Hispaniola fault zone (Figure 2; Dolan et al., 1998; Calais et al., 2002). The most recent GPS velocity field for the area (work in progress by Calais et al. is summarized in Figures 2 and 3) shows eastward, left-lateral motion of the Gonave and Caribbean plates in Hispaniola with respect to North America at a rate of 19 mm/yr. GPS velocities across Hispaniola show a north-south gradient, reflecting elastic strain accumulation on the major active faults (Figure 3). The observed GPS velocities for Haiti shown on Figures 2 and 3 are consistent with fault slip rates recently derived from a kinematic block model with strain accumulation on faults (Manaker et al., 2008). Model slip rates are 9+-2 mm/yr on the Septentrional fault, consistent with paleoseismic data (Prentice et al., 2003) and 7+-2 mm/yr on the EPGFZ.

We have inferred that the last major earthquake to strike the EPGFZ in Haiti occurred in 1751, followed in 1770 by another large event. Their magnitude is estimated to be around 7.5 (summary of historical earthquake catalog provided by Ali et al., 2008). No major earthquake has been reported in the southern part of Haiti since then, although smaller magnitude ones that are less well located occurred in 1701, 1784, 1860, 1864, 1953. Assuming that elastic strain has been accumulating at a constant rate of 7 mm/yr based on our GPS observations, the EPGFZ in Haiti was indeed capable of a Mw7.2 event (Manaker et al., 2008). The EPGFZ fault has not been investigated in detail with paleoseismological methods in Haiti and the penultimate event and a recurrence time for this fault is not known. The total accumulated slip on the EPGFZ fault is 1.90.4 m assuming that our GPS-derived rate of ~7 mm/yr is correct as well as our inference that the last major earthquake occurred in the Port-au-Prince area in 1751. This total slip, if distributed uniformly on an 80 x 10 km fault, the initial estimate of the January 12th 2010 rupture plane, would yield a seismic moment of 4.5e19 Nm (Mw7.0), which is in excellent agreement with the observed moment release for this event (USGS/NEIC).