New insights into subaqueous paleoseismology from the preserved imprints of paleo-earthquake markers on a normal fault scarp (Roseau Fault Lesser Antilles, France)

Assessment of seismic and tsunami hazards along coastlines requires knowledge of past earthquakes and their recurrence times along active submarine faults. To this end, subaqueous paleoseismological studies are performed and are based on sediment cores and seismic reflection images of faults. However, local site conditions sometimes preclude coring or seismic surveys and, even when possible, the resulting data may be limited. In addition to traditional geophysical and sedimentological data, seafloor geophysical data from submersibles can help elucidate the paleoseismic history of submarine faults. Here, we conducted a near-bottom geological survey using a Remotely Operated Vehicle (ROV) along the Roseau normal fault (Lesser Antilles, France) to study the fine morphology and paleoseismic history of an active submarine fault scarp. This fault hosted the Mw 6.3 2004 Les Saintes earthquake and shows a coseismic ribbon at its base. We used the submersible data to map and characterize several scarp morphologies including abrasion bands, notches, roughness changes, dark bands, and uplifted sediments  along the fault scarp. We propose that these markers, which all formed at the seafloor, can ultimately be used to reconstruct the exhumation history of the fault scarp, because they are linked to base level changes (i.e. sedimentation and tectonic exhumation). At one site along the Roseau fault, the scarp’s detailed morphology can be explained by the occurrence of three earthquakes coupled to several episodes of rapid sedimentation. The penultimate earthquake may have generated a vertical offset of 3 m, where at the same location the 2004 event slipped by ~1.4 m. The penultimate earthquake was at least as energetic as the 2004 event, the Roseau fault being able to host a M7 event if broken entirely. Sediment rates from cores sampled near the fault show that the penultimate earthquake probably occurred within the past ~2.8 kyr. These observations highlight the potential of studying offshore faults with ROV optical imagery to better understand the seismic history of crustal faults.