Sismotectonique du système de failles d’Enriquillo et du séisme du 12 janvier 2010 (Mw 7.0) en Haïti

In Haiti, the oblique convergence of 20 mm/yr between North American and Caribbean plates is partitioned into strike-slip and shortening. The strike-slip component is accommodated by two major ~E-W left-lateral strike-slip faults, the Septentrional Fault to the north and the Enriquillo-Plantain Garden Fault (EPGF) to the south. The shortening component is accommodated by numerous folds and thrusts, mainly located in the centre of the island of Hispaniola (Haiti and Dominican Republic). The overall faults imply a significant seismic hazard. They have broken several times producing large historical earthquakes (e.g., 18 October 1751, M~7.5; 21 November 1751, M~6.5; and 3 June 1770, M~7.5). The last one occurred on 12 January 2010, destroying the capital city of Port-au-Prince and killing more than 230 000 people. Combining satellite imagery, geological maps, bathymetric data, and field observations, we mapped at several scales the active faults in the southern peninsula of Haiti, around the 2010 earthquake epicentre. We identified several NW-SE-striking Quaternary folds and thrusts in the Enriquillo Valley and the Cul-de-Sac plain where lies the city of Port-au-Prince. These thrusts connect to the main strike-slip EPGF, which crosscuts the whole southern peninsula of Haiti. We showed that the EPGF was composed of more than four segments, each capable to generate Mw>7 earthquakes. We conducted field paleoseismological investigations in the Clonard pull-apart basin, 150 km to the west of Port-au-Prince, during which an exploratory trench was excavated. This trench showed stratigraphic evidence for one paleoearthquake. Radiocarbon dating in a paleochannel suggests that the paleoearthquake may be the 1770 historical event. We identified several geological and geomorphic markers of long-term deformation along the EPGF. The inferred Pliocene paleofan of Port-au-Prince was offset by ~8 km from its original drainage basin by the left-lateral motion of the fault. Older structures, such as Lower Miocene folds (Massif de la Selle and Massif de la Hotte) are offset by 40 ± 10 km by the fault. This implies minimum slip rate of 2-3 mm/yr. The main rivers, Froide and Momance, are offset by ~8 and ~15 km, respectively. Overall, our work allows us to better understand the seismotectonic context of the 12 January 2010 Haiti earthquake. A general consensus has emerged from the study of this earthquake: the coseismic rupture was complex, portraying both reverse and strike-slip motion, but lacking unambiguous surface break. Near the 2010 epicentre, we identified the WNW-ESE Quaternary Lamentin thrust, which cut across the coral reef platform of the bay of Port-au-Prince. The geometry and distribution of the aftershocks of the 2010 earthquake and the analysis of the regional geology allow us to place constraints on the connection at depth between the Lamentin thrust and the sinistral strike-slip EPGF. Inversion of geodetic data suggests that both faults may have broken in 2010, consistently with the regional geodynamical setting. The rupture initiated along the Lamentin thrust and further propagated along the EPGF due to instantaneous unclamping at depth. The corals uplifted around the Léogâne Delta Fan, contributing to the build-up of long-term topography between the Lamentin thrust and the EPGF. The 2010 earthquake increased the stress toward failure on unruptured EPGF segments as well as on the thrust fault sitting in the middle of the city of Carrefour, in the direct vicinity of Port-au-Prince, thereby increasing the seismic hazard in these areas.