In the aftermath of the catastrophic M7 earthquake in Haiti there is an urgent need to record aftershocks and capture ephemeral data that will help to assess both short term (immediate) and long term earthquake risk in the region. Reliable models of strain accumulation and seismogenic release along the fault zone are urgently needed. A U.S. team is already on the ground, installing additional seismometers and GPS stations and mapping features associated with the earthquake, and a French team is preparing to deploy a suite of ocean bottom seismometers offshore. The January 12 mainshock ruptured a relatively small segment of the Enriquillo-Plantain Garden fault zone (EPGF), the southern of two parallel E-W sinistral transforms accommodating most of the motion between the North America and Caribbean plates. The EPGF follows the core of the southern peninsula and is associated with several large historic earthquakes, but its pre-1700?s paleoseismicity is unknown. A number of NW-SE faults and folds intersect the EPGF and may also be active. Many of these structures are unmapped, especially offshore, including a possible one in the Baie de Port au Prince. This survey of the offshore portion of the main rupture and of some of the secondary structures associated with it along the southern peninsula and the Baie de Port au Prince will complement those efforts by documenting the near surface effects of the earthquake offshore and characterizing the seismogenic structures. The survey will include shallow water (<300 m) multibeam mapping, with coincident and targeted sidescan sonar, and chirp (0.5-12 kHz) sonar profiles, and spot coring. The principal broader impact of this study is its very high and immediate societal relevance.
The January 12, 2010 M7.0 earthquake was catastrophic for Haiti, devastating the capital and surroundings. It was also one of the deadliest in history. The earthquake occurred along or near the Enriquillo-Plantain-Garden fault zone (EPGF), a fault that is part of the tectonic boundary between the North America and Caribbean plates (Fig. 1). The earthquake was unusual in that the main shock indicated horizontal motion along the fault, in agreement with the long-term motion of the plates. But the aftershocks provided evidence of thrust faults that indicated shortening. Teams of geologists who were surveying the fault on land did not find evidence of surface rupture but documented variable uplift and subsidence along the coastline. Because the fault continues offshore, a marine survey was rapidly organized. Researchers and students from the US and Haiti sailed from the R/V Endeavour to map the submarine portions of the EPGF (Fig. 2). The team using sound-based instruments mapped the sea-floor trace of the EPGF around a restraining bend where aftershocks were concentrated and uplift was reported. The core samples recovered a sedimentation event that was linked to landslides triggered by the 2010 earthquake (Fig. 3). This event covered the basin for over 50 km2 and is ~ 1 m thick. Additionally, a 600-m-thick sediment plume was still present in the lowermost water column at this location nearly two months after the earthquake. The 2010 turbidite, and older ones observed beneath it, display characteristic stratigraphy indicative of long waves and bathtub like oscillations that are consistent with locally reported tsunamis. A second and much older event dated at 2000 years BP was also recovered in the cores. The ~2000 year gap in between earthquakes is much rare than typical earthquakes along the EPGF that are generally 250 years apart. This raises a testable hypothesis that shortening and uplift associated to the unusual 2010 earthquake may also have occurred 2000 years ago. Earthquakes on strike-slip faults can produce devastating natural hazards. However, because they consist predominantly of lateral motion, these faults are rarely associated with significant uplift or tsunami generation (Figs 4, 5). Although submarine slides can generate tsunami, only a few percent of all tsunami are believed to be triggered in this way. The 12 January Mw 7.0 Haiti earthquake exhibited primarily strike-slip motion but nevertheless generated a tsunami. Our comprehensive field survey around the epicenter documented that modest uplift together with slope failure caused the tsunami and likely was the source of the turbidite and sediment plume observed in the deep water Canal de Sud. In terms of hazards, submarine landslides caused the most severe tsunami locally. Our analysis suggests that slide-generated tsunami occur an order-of-magnitude more frequently along the Gonave microplate than global estimates predict. The earthquake also caused liquefaction at several river deltas that prograde rapidly and are prone to failure. We conclude that coastal strike-slip fault systems such as the Enriquillo–Plantain Garden fault produce relief conducive to rapid sedimentation, erosion and slope failure, so that even modest predominantly strike-slip earthquakes can cause potentially catastrophic slide-generated tsunami—a risk that is underestimated at present.
