Haiti subsurface imaging project: Seismic and gravity results

Abstract

The Haiti earthquake of 12 January, 2010 was initially reported to have occurred along the main strike slip fault (Enriquillo–Plantain Garden fault zone, EPGFZ), but more recent studies proposed slips on previously unrecognized, neighboring faults (Léogâne). As a participant in SEG's Geoscientists Without Border program, we have investigated the shallow subsurface at Léogâne Fan Delta, Haiti using seismic and gravity techniques. We found compressional (P)-wave velocities of 400-2350 m/s for an approximately 800 m deep section from seismic data analysis. Shear (S)-wave velocities (estimated from surface wave inversion) vary from 140-240 m/s for the top 20 m. The prominent change in both P-wave and S-wave velocities (around 200 m and 400 m offset) are interpreted as the effects of faulting. Extracted reflection coefficients also support a reflective structure at these offsets. Near-surface reflection seismic analysis provided deeper information indicating multiple layers with varying velocities, intersected by a number of faults. Detailed ultrasonic measurements on hand samples provide an average of 6250 m/s for the sample A, 2700 m/s for sample B and 3450 m/s for sample C. The estimated Free air gravity profile suggests that the variation of the gravitational field may be related with the proposed faults.