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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):
- 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
93 mm/yr (Prentice et al., 2003), in agreement with GPS
estimates (Calais et al., 2002).
- 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.
- 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.
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.
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