First P- and S-wave tomographic results of a wide-angle seismic profile from the Haiti-TWiST campaign

The Septentrional-Oriente Fault Zone (SOFZ) and Enriquillo-Plantain Garden Fault Zone (EPGFZ) have a history of producing devastating earthquakes and tsunamis around Haiti. However, the deep structures and the role of fluids in their processes remain poorly understood, which limits our ability to assess their seismic potential. We present new insights into the crustal structure of the Haiti dual active transform fault system derived from an active-source Ocean Bottom Seismometer (OBS) profile (TWI1) collected during the Haiti-TWiST campaign (R/V “Pourquoi pas?”, June-July 2024), which uniquely spans across both offshore and onshore regions. This profile transects both the EPGFZ off the southern Haiti peninsula and the SOFZ offshore the northern Haiti peninsula. High-quality seismic data from 61 OBS stations yielded over 34,000 P-wave and 18,000 S-wave traveltimes, enabling forward modeling and tomographic inversion of the crustal structure. The resulting P-wave, S-wave, and Vp/Vs structures reveal oceanic plateau characteristics in the offshore crust in the south of the EPGFZ, with a thickness of 18–20 km, P-wave velocities of 4.0–7.5 km/s, S-wave velocities of 2.5–4.1 km/s, and Vp/Vs ratios of 1.75–2.0. The SOFZ exhibits pronounced structural variations with distinct high- and low-velocity boundaries. Additionally, a high Vp/Vs anomaly (2.0~2.1) in the upper crust on the southern flank of the EPGFZ suggests extensive fracturing, potentially linked to stress perturbation during strike-slip motion.