HR: 1340h
AN: T53A-1124    [Abstracts]
TI: Seismic Structure Reconnaissance of the Lesser Antilles Subduction Zone
AU: * Roux, E
EM: eroux@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: Laigle, M
EM: laigle@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: Sapin, M
EM: sapin@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: Hirn, A
EM: hirn@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AB: The Lesser Antilles is an active subduction zone which has experienced in 1843 a M > 7.5 probably mega- thrust earthquake that destroyed Pointe-a-Pitre city on Guadeloupe island. But the location in depth of the mega- thrust subduction fault and its geometry along the arc were unknown until recently.
A first reconnaissance of the seismic structure and activity has been undertaken by Sismantilles project which has been carried out at a regional scale from Northern Guadeloupe to Martinique islands. The project focused more particularly on the detection, mapping and characterization of the potentially seismogenic part of the interplate subduction fault.
For this purpose, wide angle reflection and refraction (WARR) data have been collected by a broad but sparse array of 37 3-components Ocean Bottom Seismometers (OBS) that were deployed offshore Guadeloupe and Martinique islands over severals weeks and recorded continuously the MCS shots from the French N/O Nadir vessel as well as regional and local seismicity.
Joint modeling of MCS / wide-angle data on three transects across the arc enables us to identify the contact zone between the forearc crust and the subducting oceanic crust, which is considered as a proxy for the seismogenic part of the interplate. The most striking feature is that its location with respect to the deformation front and the volcanic arc and its downdip size appear significantly variable along the arc.
Relocation of recorded earthquakes has been improved by a better network geometry including 3- components OBS just above the sources and thanks to the application of stations corrections for OBS's observations. The recorded earthquakes are mostly located downdip of the contact between the two crusts. Depending on the real geometry of the interplate, which is not yet constrained deeper than 20 km depth, seismicity occured either in the forearc mantle, either at the interplate or in the subducting plate. In the upper plate, a part of microseismicity has been clearly located in the arc crust and mantle.
This first study reveals an unexpected strong variation along the arc of the forearc structure and allows to target for finer measurements in the contact zone between the two crusts.
DE: 3025 Marine seismics (0935, 7294)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting