The aftershocks that have rocked the region near the 7.0 earthquake of January 12 in Haiti will continue for months, if not years, according to a report by the United States Geological Survey (USGS).  It is important to understand the nature of the seismic activity in this region as this knowledge will be significant in the near-term relief efforts, but perhaps more importantly, in the long term rebuilding of the infrastructure of the country near Port-Au-Prince, and the many surrounding communities.

M7.0 Haiti Earthquake and Aftershocks Map. Click on map to go to USGS active map site.

Here is a brief synopsis of the USGS assessment of the 7.0 earthquake and the aftershocks.

Aftershocks. According to the USGS, aftershocks will continue for months if not years in the area seen on the map here.  The map shows the initial earthquake (largest red circle), and the aftershocks as of January 21.  The USGS initial assessment includes the possibility of an earthquake greater than 7.0 (less than 3%), but earthquakes ranging from 5.0 to 6.0 have a higher probability.

Precautions.  As we watch the pictures and video from Haiti, we are all aware of the rescue activity that is in progress where the work is being done in damaged structures, which are extremely vulnerable to further destruction by these aftershocks.  Aftershocks (earthquakes) in the 5.0 range are quite powerful, especially since these earthquakes are fairly shallow with the energy being concentrated near the surface.

Concerns for the Near-term.  As I have discussed, the fault that caused the Port-Au-Prince earthquake is part of a very active seismic region existing between the North American and Caribbean tectonic plates.

The Caribbean Plate showing its boundaries with the North American, Cocos, & South American Plates

The Caribbean Plate is an oceanic tectonic plate that encompasses roughly 3.2×106 km² and underlies Central America and the Caribbean Sea off the north coast of South America.  As you can see on the map is located between not only the North American Plates, but the Cocos plate to the west, and the South American plate to the south.  The North American Plate is a tectonic plate covering most of North America, Greenland and parts of Siberia and Iceland.  The boundary between the North American and Caribbean plates is what geologists call a strike-slip fault or transform boundary.  It is along this boundary that the 7.0 Haiti earthquake occurred.  As seen on the map this boundary extends across quite a distance starting in east in the Virgin Islands, extending westward through Puerto Rico, Hispaniola (Haiti & The Dominican Republic), continuing to Cuba, and then into Honduras, Guatemala, and Belize.  This entire boundary is active seismically.

Topography along the Enriquillo-Plaintain Garden Fault, Haiti

In fact many geologists describe the area between these two plates as a micro-tectonic plate, that is being squeezed and ruptured by the three plates that surround it.  In the USGS report, geologists suggested that the earthquake did relieve some of the stress on the fault system, but they caution that the Enriquillo fault to the east of the January 12 earthquake is still under great stress, and could result in a damaging earthquake in the future. The red lines on the map indicate fault zones, which the black circles show the location and magnitude of earthquakes in the region.

Long-term Concerns.  As shown in the map above, Haiti has major fault zones cutting across the country, and the historical record shows that major earthquakes have happened in this region of intense seismic activity.   Rebuilding of structures in Haiti needs to take into account the effect of earthquake shaking on buildings.  Structures need to be designed that take this into account, and meet standards that will help new building withstand intense ground movements.  One area that the report emphasizes is that it is essential that structures such as hospitals and schools be rebuilt with greater ability to withstand earthquakes.

Caribbean Seismicity.  The Haiti earthquake was the result of earth movement along a fault that is part of the zone between the Caribbean and North American tectonic or crustal plates.  This region, is sometimes called a small-scale “ring of fire,” analogous to the Pacific rim “ring of fire.”  It includes Puerto Rico, Jamaica, Dominican Republic, Martinque, and Guadeloupe.  The map below shows the extent of the seismic activity in the Caribbean, as well as to the region further to the West along the coast of South America.  Rebuilding efforts in the region need to take into account the knowledge we have of seismic activity in the region.

Seismicity of the Caribbean Tectonic Plate region, USGS