The 2004-2005 Les Saintes (French West Indies) seismic aftershock sequence observed with ocean bottom seismometers

On November 21, 2004 an M(w)6.3 intraplate earthquake occurred at sea in the French Caribbean. The aftershock sequence continues to this day and is the most extensive sequence in a French territory in more than a century. We recorded aftershocks from day 25 to day 66 of this sequence, using a rapidly-deployed temporary array of ocean bottom seismometers (OBS). We invert P- and S-wave arrivals for a tomographic velocity model and improve aftershock locations. The velocity model shows anomalies related to tectonic and geologic structures beneath the Les Saintes graben. 3D relocated aftershocks outline faults whose scarps were identified as active in recent high-resolution marine data. The aftershocks distribution suggests that both the main November 21 event and its principal aftershock, on February 14, 2005, ruptured Roseau fault, which is the largest of the graben, extending from Dominica Island to the Les Saintes archipelago. Aftershocks cluster in the lower part of the Roseau fault plane (between 8 and 12.6 km depth) that did not rupture during the main event. Shallower aftershocks occur in the Roseau fault footwall, probably along smaller antithetic faults. We calculate a strong negative Vp anomaly, between 4 and 8 km depth, within the graben, along the Roseau fault plane. This low Vp anomaly is associated with a high Vp/Vs ratio and may reflect a strongly fracturated body filled with fluids. We infer from several types of observation that fault lubrication is the driving mechanism for this long-lasting aftershock sequence. (C) 2010 Elsevier B.V. All rights reserved.