HR: 0800h
AN: S51B-0164    [Abstracts]
TI: Flat-Slab to Steep Subduction Transition Zone in Central Chile-Western Argentina: Body Waves Tomography and State of Stress
AU: * Pardo, M
EM: mpardo@dgf.uchile.cl
AF: Dpto. Geofisica, U. de Chile, Blanco Encalada 2002, Santiago, RM 6511227 Chile
AU: Monfret, T
EM: monfret@geoazur.unice.fr
AF: UMR Geosciences Azur-IRD, 250 Albert Einstein, Valbonne, 06560 France
AU: Vera, E
EM: evera@dgf.uchile.cl
AF: Dpto. Geofisica, U. de Chile, Blanco Encalada 2002, Santiago, RM 6511227 Chile
AU: Yanez, G
EM: GYane003@codelco.cl
AF: Dpto. Geofisica, U. de Chile, Blanco Encalada 2002, Santiago, RM 6511227 Chile
AU: Yanez, G
EM: GYane003@codelco.cl
AF: Codelco-Chile, Teatinos 258, Santiago, RM 6511227 Chile
AU: Eisenberg, A
EM: aeisenbe@dgf.uchile.cl
AF: Dpto. Geofisica, U. de Chile, Blanco Encalada 2002, Santiago, RM 6511227 Chile
AB: Two temporary networks were deployed for about 100 days in the flat-slab subduction zone of Central Chile from Nov. 1999 to Feb. 2000 (30-32S, 70-71.5W; 38 SP stations), and in the transition zone from flat-slab to steep subduction zone of Central Chile-Western Argentina from Nov. 2002 to March 2003 (31.5-34S, 67-71.5W; 29 BB stations). Both networks recorded in continuous mode and were complemented with the permanent stations of the Univ. of Chile within the zone. Several thousands earthquakes were recorded and hypocenter locations were obtained for about 50 events/day. Accurate focal mechanisms were determined for a subset of the best recorded events, using first arrival polarities of P-waves, which permitted to estimate the stress tensor in different regions along the subducted Nazca plate. Local tomography for P- and S-waves velocities from data of each network shows an image of the subduction zone from the flat-slab to the steep subduction. A sharp transition zone at around 32.2S is well correlated with the subduction of the Juan Fernandez Ridge within the slab. Seismicity was relocated using the 3D velocity model obtained from tomography. It is distributed around the trench, along the coast with mainly compressive interplate events at depths down to 50-60 km, along the subducted slab with mainly extensional intraslab events at maximum depths of 150 km at the flat-slab zone and 200 km at the steeper dip zone, and crustal shallow events (depths < 30 km) along the overriding continental plate. This seismicity clusters around the subducted Juan Fernandez Ridge along its entire track within the downgoing slab. More events of high magnitude are observed on that area, hence more seismic moment and energy is released as compared with the surrounding regions. Crustal shallow seismicity occurs mostly at the Chilean side of the Andes Cordillera and foothills, and at the back-arc in the Argentinean Precordillera. This observation indicates a different stress regime and deformation styles at the very steep frontal Andes, relative to the back-arc with smoother topography. Besides a NS differentiation associated with the subduction style is also observed: the seismicity is higher at the front-arc in the steep subduction zone segment, and higher at the back-arc in the flat-slab subduction segment. Finally, a new model of the isodepth contours of the subducted Nazca plate (27.5-34.5S) was obtained from the best hypocenter locations of the temporary networks, including the catalog of the permanent stations of the Univ. of Chile and a previous temporary network at the northern end of the flat-slab segment. This model shows that the subducted Juan Fernandez Ridge controls the transition from flat to steep subduction, and that its buoyancy is associated with the flattening of the slab in the northern part of the studied area.
DE: 7230 Seismicity and seismotectonics
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting