Mapping the Mw6.3 2004 Les Saintes earthquake seafloor rupture with deep-sea vehicles: Length, displacement, nature, and links between coseismic deformation and erosion/sedimentation

The Mw 6.3 2004 Les Saintes earthquake ruptured the Roseau normal fault off Les Saintes (French Antilles). A first survey in 2013 (ODEMAR cruise) documented a vertical coseismic displacement of 0.9 m at one site, and near-fault hangingwall cracking. In 2017 the SUBSAINTES cruise deployed an AUV and an ROV to acquire multibeam microbathymetry and obtain video for 3D reconstructions of outcrops. Operations showed that submarine field work can be conducted at scales and resolutions similar to subaerial ones. This is the first systematic, comprehensive mapping of a submarine coseismic rupture.

First, we mapped a clear coseismic scarp along >12 km of the Roseau Fault. The seafloor manifestation of the rupture varies laterally, with well-preserved sub-vertical fault planes in the central portion of the fault, ruptures through sediment/rubble toward the tips, and locally pervasive hangingwall cracking extending up to 100 m or more locally.

Second, videos and 3D models reveal coseismic displacement as freshly exposed surface lacking dark manganese crust deposited due to long-term seawater exposure. The maximum coseismic fault displacement documented exceeds 2.5m, tapering laterally. Displacement varies at spatial scales of 250 m or less as in subaerial ruptures, indicating local fault slip heterogeneities. Overall our mapping reveals significantly higher displacements than predicted by seismological models (>2.5 m vs. 0.9 m, Feuillet et al., 2011).

Third, the 3D models and geological observations reveal a clear correlation between seismicity and deformation/erosion. Fault planes that are adjacent to dejection cones at the mouth of gullies incising the cumulative fault scarp show a coseismic displacement tapers, while no displacement is observed along the gullies. Comparison of 2013 and 2017 seafloor photomosaics, primarily at the mouth of gullies, reveal no sedimentation nor material mobilization. These observations indicate coseismic reactivation of cones, with deposition locally covering ruptures. Ongoing dating of volcanic units will constrain recurrence and earthquake history of the Roseau Fault. Furthermore, gravity-driven erosion and associated sedimentation may be strongly modulated by the magnitude and temporal distribution of seismicity in submarine environments (e.g., mid-ocean ridges).