First Assessment


Origin Time: 21:53:10 UTC; 16:53:10 Local time
Epicenter location: Latitude 18.457N, Longitude: -72.533W
Magnitude: 7.0 (Teleseismic Moment Magnitude)
Depth: 13 km (fixed by location program)
Data from: The Global Seismographic Network, the Advanced National Seismic System, and other contributing regional earthquake monitoring networks.
Source: U.S. Geological Survey, National Earthquake Information Center, Golden, Colorado (http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/us2010rja6/)

The Haiti earthquake had its epicenter about 25 km to the west of Port-au-Prince. The analysis of seismic and geodetic data (GPS and InSAR) indicate that the earthquake resulted from the rupture of a ~45 km long fault segment between about 2 and 20 km depth. This rupture lasted about 15 seconds and propagated from east to west, away from Port-au-Prince. The source mechanism of the earthquake (= description of the forces that caused the rupture), as determined from geological and geodetic data is a combination of left-lateral strike-slip and thrust motion. The geometry of the earthquake rupture remains to be accurately determined, but all data indicate that it involves at least one major segment dipping to the north at an angle of 45 to 65 degrees (depending on the data set used to ewstimate this parameter). Geodetic inversions indicate that a significant part of the seismic moment was released via thrust motion. This hybrid mechanism is consistent with the transpressional nature of the interseismic strain rate field, as determined from GPS measurements (Manaker et al., 2008).

Field geological investigations have determined that the earthquake rupture did not break the surface (work by P. Mann, R. Bilham, C. Prentice, R. Briggs). Geodetic inversions indicate that it stopped at a depth of about 2 km. Observations along the coast from Petit Goave to Leogane show that the earthquake was accompanied by regional-scale uploft (up to 0.5 m near Leogane, work by R. Briggs). A number of secondary effects were reported, such as cracks affecting the Leogane/Carrefour Dufort road, or lateral spreading in some coastal areas. They do not represent the trace of the earthquake rupture but are caused by ground/soil failure when shaken by the seismic waves lauched by the earthquake.

Past earthquakes have however left their mark along the Enriquillo fault, with clear surface offsets along some portions of that fault. This suggests that perhaps more than one fault has been involved in large earthquakes in southern Haiti, complicating a simple calculation of earthquake hazard potential. Interestingly, a portion of the 12 January rupture zone lies offshore in the Baie de Port au Prince. Marine geophysical surveys, such as the ones completed by the French research vessel Atalante (GeoAzur Nice, Ifremer, IPGP) and by the US research vessel Endeavor (Cecilia McHugh, Leonardo Seeber, Michael Steckler and others) will provide additional constraints on the 12 January as well as past ruptures.

The January 12 earthquake has been followed a a sequence of aftershock, that reflect stress redistribution caused by the main shock. This is a expected consequence of this event and happens after all major earthquakes. The aftershock sequence continues and will last for several months. Although the frequency of events will decrease with time, their magnitude can remain significant. The USGS computed a probability of 95% that an earthquake greater than magnitude 5 could occur in the next year. Aftershocks will therere remain a concern, even the smaller events can seriously damage buildings already weakened by the mainshock.