The 7-magnitude shallow-depth earthquake of January 12, which had its epicentre about 15 kilometres southwest of Port-au-Prince, Haiti’s capital, ruptured the long Enriquillo-Plantain Garden Fault for a length of about 75 km and width of 13 km to 15 km.
The extent of rupture along the fault will become clearer after detailed studies are carried out. While the quake relieved a certain amount of accumulated stress, the Fault, according to the United States Geologic Survey (USGS), has not been ruptured “appreciably” and still stores accumulated stress. An earthquake results when the rocks fail and the accumulated stress is suddenly released.
According to the USGS, aftershocks of magnitude 7 will continue for months; there is also a “small chance” of subsequent quakes being larger than the calamitous one of January 12. It is well known that a sudden release of strain at one point loads another area along the same fault or adjacent faults, and may hasten the occurrence of another quake. The loading-unloading of stress becomes all the more pronounced after a major earthquake.
Haiti lies in a seismically active zone between the North American and Caribbean tectonic plates. Another major strike slip fault, the Septentrional Fault, runs across the country. The country is also sandwiched by two thrust faults, one in the north and the other in the south.
Yet Haiti and the rest of the Caribbean region, which resembles a small-scale ring of fire like the one encircling the Pacific Ocean, are largely ignored by the scientific community. Quakes produced by the strike slip faults, Enriquillo and Septentrional, occur at relatively shallow depths. Even smaller magnitude quakes can be felt on the ground, and poorly constructed buildings can get weakened or damaged. Unfortunately, smaller magnitude earthquakes are generally ignored by the global network of seismic stations, which report only quakes of magnitude 4.5 and above. This underlines the need for studying regional seismic activity.
Indonesia, which was sparsely instrumented prior to the 2004 tsunami, is better studied today. It is important to study even the smaller magnitude earthquakes in seismically active zones because, over the long term, they may anticipate a remotely possible large earthquake.
Establishing or improving building codes will become possible only when a thorough seismic hazard assessment is made. The good news is that it is possible to fast-track the assessment to get a better understanding of the likelihood and nature of quakes over different time frames.