Tectonic Setting

Earthquakes result from the sudden rupture of faults put under stress by the motion of tectonic plates. The island of Hispaniola is located on the boundary between the Caribbean and North American plates, sliding past each other at a speed of 2 cm/yr. This motion between the Caribbean and North American plates creates high levels of stresses in the Earth's crust. These stresses build up with time, causing the Earth's crust in the vicinity of seismic faults to deform like a rubber band, without breaking. When these stresses are too high (exceeding the strength of faults), they are released by a sudden slip on a fault: the ``rubber band'' breaks, an earthquake happens.

Geologic and geophysical studies have shown that the relative motion between the Caribbean and North American plates in the Hispaniola area is accommodated by three major active tectonic structures (Figure 2.1):

1. The Septentrional Fault, a major left-lateral strike-slip fault in northern Hispaniola that follows the northern cost of Haiti (offshore) and continues eastward on land in the Dominican Republic, where it is responsible for the uplift of the Cordillera Septentrional and for active folding and faulting at its contact with late Neogene to Holocene units of the Cibao valley (Calais et al., 1992; Mann et al., 1998). Holocene slip rate on the Septentrional fault is 9$\pm$3 mm/yr (Prentice et al., 2003), in agreement with GPS estimates (Calais et al., 2002).
2. The Enriquillo Fault, the second major left-lateral strike-slip, plate boundary-parallel, fault in Hispaniola. It is particularly well-exposed in Haiti, where it is marked by a 200 km long narrow valley striking east-west through the southern peninsula. The Enriquillo fault ends abruptly in south-central Hispaniola and connects southeastward to low angle thrust motion at the western termination of the Muertos fault. A number of historical earthquakes affected towns of southern Hispaniola in the 17th, 18th, and 19th century suggesting that they have occurred on the Enriquillo fault. However, no geological estimate of slip rate is yet available for this fault.
3. The North Hispaniola Fault and Puerto Rico Trench: The North Hispaniola fault extends offshore northern Hispaniola in a roughly east-west direction. Sidescan sonar and seismic reflection data suggest a very low angle thrust, consistent with the occurrence of a series of M7.2-8.1 large thrust earthquakes in the period 1943-1953 (Dillon et al., 1992; Dolan et al., 1998; Dolan and Wald, 1998). The North Hispaniola fault is continuous with the Puerto Rico trench to the east which is the site of recent strike-slip and low-angle thrust faulting and a very strong (-400 mGals) negative gravity anomaly. Both the North Hispaniola and Puerto Rico trench faults mark the site of subduction for slabs of Atlantic lithosphere beneath Hispaniola and Puerto Rico, respectively.

Other seismic faults may be present in the central part of the island (in particular in the San Juan Valley, or in the Bonao area), but no adequate study has yet been done to clearly identify and map these faults.

Figure 2.1: Tectonic framework. Velocities are shown with respect to the North American plate. Note the excellent agreement between observed and rigid Caribbean velocities in the Lesser Antilles and Puerto Rico. Velocities deviate significantly from rigid Caribbean motion in eastern Hispaniola. Similarly, slip vectors of subduction earthquakes parallel the Caribbean-North America plate motion in the Lesser Antilles and Puerto Rico, but are rotated perpendicular to the plate boundary in Hispaniola. EF: Enriquillo Fault; SF: Septentrional Fault; MHF: North Hispaniola Fault; PRT: Puerto Rico Trench; MP: Mona Passage; MT: Muertos Trench; LAT: Lesser Antilles Trench.