EVIDENCE OF LATE HOLOCENE SURFACE RUPTURING ON THE ENRIQUILLO-PLANTAIN GARDEN FAULT ZONE AND THE EARTHQUAKE HAZARD IN HAITI

The disastrous 12 January 2010, Haiti earthquake was modeled to be caused by slip on a 55º N-dipping blind thrust fault associated with the Enriquillo-Plantain Garden fault zone (EPGFZ). The EPGFZ is the boundary between the Caribbean plate to the south and the Gonave microplate to the north, extends at least 350 km across the southwestern part of Hispaniola, and poses a major seismic hazard to Haiti. It had been recognized previously in Haiti but was largely unstudied even though geodetic models suggested that it may accommodate 7±2 mm/yr of left-lateral slip and may be the source of at least three major historical earthquakes. Following the 2010 earthquake, we conducted a detailed ground-based assessment of the fault in the epicentral area and the first aerial reconnaissance of the entire fault in Haiti.

The geomorphic expression of the EPGFZ is especially prominent east of the 2010 epicenter where the Rivière Momance and Rivière Froide flow along 24 km of strike valleys about 10 km south of Port-au-Prince (PAP). Here, the EPGFZ has an average strike of 085º and a near-surface dip of 60-80ºS, which contrasts with the N-dipping modeled fault for the 12 January earthquake. Along a 12-km-long part of the Rivière Momance, we measured geomorphic features that have been left-laterally offset between 1.3 m and 160 m. At nine locations we measured offsets of 1.3–3.3 m in small drainage channels that were not visible on high-resolution imagery including LiDAR. We attribute these small offsets to the youngest surface rupture because they are sharp and well preserved in very young deposits and likely formed during the 1751 or 1770 earthquakes. We confirm that no significant surface rupture occurred on the main EPGFZ during the 2010 earthquake.

East of PAP near the town of Dumay, we measured 6.6 and 4.8 m of sinistral offset on two nested terrace risers. Near the Dominican Republic border we found active folds that warp gently sloping alluvial surfaces of probable late Quaternary age into broad, growth anticlines. This folding could be related to deep shearing on the EPGFZ.

Our observations of offset landforms confirm a long history of repeated surface rupture on the EPGFZ. Because the main EPGFZ did not rupture in 2010, considerable strain remains to be released in an earthquake that poses a major hazard to densely populated parts of Haiti, including PAP.