Abstract

The 120  km2 Léogâne fan delta in southwestern Haiti borders the epicentral region of the devastating magnitude 7.0 Haiti earthquake of 12 January 2010. The flat plain of the Léogâne area experienced some of the worst shaking, destruction of buildings, and loss of life caused by the Haiti earthquake. This intense shaking was attributed by previous workers to either activation of a blind (no surface expression) thrust fault some 4 km beneath the Léogâne fan delta or to strike-slip motion along a shallow, ground-breaking fault that ruptured the uppermost part of the fan delta. Our research team from the University of Houston and the Haiti Bureau of Mines and Energy collected shallow seismic and gravity data in the fan delta where previous studies of earthquake aftershocks, coastal uplift of coral reefs, and radar interferometry all indicated a maximum amount of coseismic uplift. Our objective was to acquire geophysical information on the subsurface stratigraphy, structure, and material properties of the fan. S-wave seismic studies revealed an average velocity of 215  m/s for the first 30 m. These velocity values suggest that the near-surface sediments at Léogâne are a seismically hazardous class D sediment type (National Earthquake Hazard Reduction Program). Interpretation of our various seismic data sets has indicated prolonged sedimentary environments of fluvial channeling and channel migration to a depth of approximately 350 m as expected in this fan delta setting. There was no clear evidence on our seismic reflection lines for substantial faulting in the seismically slow, shallow fan delta sediments. Integrated geophysical data analyses indicated south-dipping seismically slow layers on the southern end of the Léogâne fan with a less well defined northward dip with the broad, anticlinal axis aligned with the area of maximum coseismic uplift at the coast. The sudden, coseismic upheaval of the ground surface above the proposed blind thrust combined with extreme shaking of the seismically weak sediments contributed to the destructiveness of the earthquake on the Léogâne fan delta.

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