Documentation for initial seismic hazard maps for Haiti

In response to the urgent need for earthquake-hazard information after the tragic disaster caused by the moment magnitude (M) 7.0 January 12, 2010, earthquake, we have constructed initial probabilistic seismic hazard maps for Haiti. These maps are based on the current information we have on fault slip rates and historical and instrumental seismicity. These initial maps will be revised and improved as more data become available. In the short term, more extensive logic trees will be developed to better capture the uncertainty in key parameters. In the longer term, we will incorporate new information on fault parameters and previous large earthquakes obtained from geologic fieldwork. These seismic hazard maps are important for the management of the current crisis and the development of building codes and standards for the rebuilding effort.

The boundary between the Caribbean and North American Plates in the Hispaniola region is a complex zone of deformation. The highly oblique ~20 mm/yr convergence between the two plates (DeMets and others, 2000) is partitioned between subduction zones off of the northern and southeastern coasts of Hispaniola and strike-slip faults that transect the northern and southern portions of the island. There are also thrust faults within the island that reflect the compressional component of motion caused by the geometry of the plate boundary.

We follow the general methodology developed for the 1996 U.S. national seismic hazard maps and also as implemented in the 2002 and 2008 updates. This procedure consists of adding the seismic hazard calculated from crustal faults, subduction zones, and spatially smoothed seismicity for shallow earthquakes and Wadati-Benioff-zone earthquakes. Each one of these source classes will be described below. The lack of information on faults in Haiti requires many assumptions to be made. These assumptions will need to be revisited and reevaluated as more fieldwork and research are accomplished.

We made two sets of maps using different assumptions about site conditions. One set of maps is for a firm-rock site condition (30-m averaged shear-wave velocity, Vs30, of 760 m/s). We also developed hazard maps that contain site amplification based on a grid of Vs30 values estimated from topographic slope. These maps take into account amplification from soils.

We stress that these new maps are designed to quantify the hazard for Haiti; they do not consider all the sources of earthquake hazard that affect the Dominican Republic and therefore should not be considered as complete hazard maps for eastern Hispaniola. For example, we have not included hazard from earthquakes in the Mona Passage nor from large earthquakes on the subduction zone interface north of Puerto Rico. Furthermore, they do not capture all the earthquake hazards for eastern Cuba.