Quantifying Coseismic Normal Fault Rupture at the Seafloor: The 2004 Les Saintes Earthquake (Mw 6.3) Along the Roseau Fault (French Antilles)

Thursday, 18 December 2014
Javier Escartin1, Frederique Leclerc1, Mathilde Cannat1, Sven Petersen2, Nico Augustin2, Antoine Bezos3, Diane Bonnemains1, Valérie Chavagnac4, Yujin Choi1, Marguerite Godard5, Kristian Haaga6, Cedric Hamelin6, Benoit Ildefonse5, John W Jamieson2, Barbara E John7, Thomas Leleu4, Miquel Massot-Campos8, Catherine Mevel1, Paraskevi Nomikou9, Jean-Arthur L Olive10, Marine Paquet1, Celine Rommevaux1, Marcel Rothenbeck2, Anja Steinführer2, Masako Tominaga11, Lars Triebe2, Rafael Garcia12, Nuno Gracias12, Nathalie Feuillet1 and Christine Deplus1, (1)Institut de Physique du Globe de Paris, Paris, France, (2)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (3)University of Nantes, Nantes, France, (4)University of Toulouse, Toulouse, France, (5)University of Montpellier II, Montpellier Cedex 05, France, (6)University of Bergen, Bergen, Norway, (7)University of Wyoming, Laramie, WY, United States, (8)Universitat de les Illes Balears, Palma de Majorca, Spain, (9)University of Athens, Faculty of Geology and Geoenvironment, Athens, Greece, (10)WHOI, Woods Hole, MA, United States, (11)Michigan State University, East Lansing, MI, United States, (12)University of Girona, Girona, Spain
Direct observations of coseismic fault displacement and rupture-related features are essential to understand seismic cycles, to quantify seismic hazard, and to constrain rupture dynamics. They are also needed to trace the paleoseismic history of active faults. Such observations in submarine environments are practically absent, but critical to assess associated tsunami hazard. The ODEMAR cruise studied a ~10 km section of the Roseau Fault (RF) off Les Saintes Islands (Guadeloupe, French Lesser Antilles), a normal fault that generated a Mw 6.3 earthquake in 2004 that triggered a tsunami (<3.5 m of run-up). Microbathymetric data and video observations conducted with the autonomous underwater vehicle ABYSS (GEOMAR) and the remotely operated vehicle VICTOR (IFREMER) allow us to document recent fault-related deformation features.

First, the RF hangingwall shows an indurated and ubiquitous rippled sediment layer, locally covered by recent, unconsolidated sediments reworked by currents. Seafloor photomosaics show the indurated layer disrupted by extensional cracks (up to few m long, several 10s of cm wide, ~30 cm deep) along >1 km and by the RF scarp base, that are certainly very young as they are not covered by unconsolidated sediments.

Second, video imagery reveals well-preserved, subvertical and polished fault planes exposed at the RF scarp base. Videomosacing and video derived 3D terrain models of a fault outcrop (~12 m long, ~5 m high) reveal on the fault plane a thin, continuous line of unconsolidated sediment sub-parallel to and 10s' of cm above the fault/sediment contact. The line represents the paleo-fault/sediment contact prior to a very recent displacement event, as strong currents in the area would otherwise have washed out the sediment. The fault surface above this line is rougher than that below it.

Based on the apparent young age of these features, and given the recurrence time of seismic events along the RF (hundreds to a few thousands of years for Mw 6 earthquakes), we attribute them to the 2004 Saintes event. High-resolution bathymetry, videomosaicing, and 3D terrain models thus provide critical constraints on coseismic deformation: We document the rupture of >1.5 km of the RF, and we quantify a coseismic vertical displacement of at least 0.8 m, constraining model-derived estimates of fault displacement.