Haiti sits between two massive seismic plates, the Gonave Plate, part of the larger North American Plate, and the Caribbean Plate to the south. The capital, Port Au Prince lies less than 20 km from the Enriquillio-Plaintain Garden Fault (EPGFZ), the a convergence point between the two plates.
This fault is a ‘strike slip fault
,’ where there is a deep vertical fracture between the two paltes, and friction is accrued horizontally. The Haiti quake, was caused by the pressures from the eastward motion of the Caribbean plate, which moves as shown in the model below from Purdue University Seismologist Eric Calais
This model, from Caltech’s Anthony Sladen’s source model, dramatically represents the what happened when these plates unstuck, by showing how the surface of the earth was displaced by the event.
The area has built up seismic tension, since the EPGFZ’s last series of quakes in the mid 18th century. There were three major seismic events in a twenty year span, as the seismic tensions was relieved through a series of quakes. Here is the recent seismic history of the region, via the University of Texas Jackson School of Geosciences
Strangely enough, there quakes were predicted a year ago, yet there was little action taken. In 2008 Paper, presentation to the Caribbean Conference, Professor Calais predicted that Haiti was due for a magnitude 7 or earthquake, owing to the seismic pressure the Enriquillo Plantain Garden Fault had accrued a from ‘slip debt’ of over two meters. Take a look at this prediction from Professor Calais, who is earning a reputation as a seismological prophet:
We confirm that the oblique convergence between Caribbean and North America in Hispaniola is partitioned between plate boundary parallel motion on the Septentrional and Enriquillo faults in the overriding plate and plate- boundary normal motion at the plate interface on the Northern Hispaniola Fault. To the east, the Caribbean/North America plate motion is accommodated by oblique slip on the faults bounding the Puerto Rico block to the north (Puerto Rico subduction) and to the south (Muertos thrust), with no evidence for partitioning. The spatial correlation between interplate coupling, strain partitioning and the subduction of buoyant oceanic asperities suggests that the latter enhance the transfer of interplate shear stresses to the overriding plate, facilitating strike-slip faulting in the overriding plate. The model slip rate deficit, together with the dates of large historical earthquakes, indicates the potential for a large (Mw7.5 or greater) earthquake on the Septentrional fault in the Dominican Republic. Similarly, 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. (Source)
It is important that the world takes Calais’ warning about the Septentrional Fault, with a great deal of Urgency. The fault, which runs through the Northern Dominican Republic is due for a quake even larger than that which occurred in Haiti.
The Dominican Republic should learn all that it can from Haiti’s experience, as they are proverbially walking down a geological hallway with a large kick me sign affixed to their back.
Today’s 6.0 aftershocks, combined with the historical patterns of quakes appearing waves have raised suspicions that this only the beginning of a larger regional alleviation of seismic pressure. The aftershocks are shown below from the USGS’s nifty google earth quake tracker