HR: 0800h
AN: T31A-0268    [Abstracts]
TI: The Southern Chilean Subduction Zone: Local Earthquake Tomography and State of Stress
AU: * Lange, D
EM: dietrich.lange@zmaw.de
AF: University of Hamburg, Bundesstr. 55, Hamburg, 20146, Germany
AU: Cembrano, J
EM: jcembrano@ucn.cl
AF: Universidad Católica del Norte, Avda Angamos 0610, Antofagasta, Ant, Chile
AU: Rietbrock, A
EM: a.rietbrock@liverpool.ac.uk
AF: University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AU: Haberland, C
EM: haber@geo.uni-potsdam.de
AF: University of Potsdam, Karl-Liebknecht-Strasse 24, Potsdam, 14476, Germany
AU: Dahm, T
EM: torsten.dahm@zmaw.de
AF: University of Hamburg, Bundesstr. 55, Hamburg, 20146, Germany
AU: Bataille, K
EM: bataille@udec.cl
AF: Universidad de Concepción, Casilla 160-C, Conception, Con, Chile
AB: While the northern and central part of the South American Subduction zone has been intensively studied the southern part has attracted less attention, which might be caused by its difficult accessibility and lower seismic activity. However, the southern part exhibits strong seismic and tsunamogenic potential with the prominent example of the Mw=9.5~May~22, 1960 Valdivia earthquake. Here we present data from an amphibious seismic array (Project TIPTEQ) located between 41.5--43.5°S reaching from the trench to the active magmatic arc incorporating the Island of Chiloé and the magmatic arc with the N-S trending Liquiñe-Ofqui fault zone~(LOFZ). 364 local events were observed in a 11-month period from November 2004 until October 2005 with magnitudes between 0.5 to 5.1~Ml. The observed seismicity allows us to constrain the current state of stress of the subducting plate and magmatic arc as well as the local seismic velocity structure. The downgoing Wadati-Benioff zone is readily identifiable as an eastward dipping plane with an inclination of about 29-33°. Besides events in in the Benioff Zone, 75~shallow crustal events with depths shallower than 25~km were observed mainly occurring in different clusters along the magmatic arc. These crustal clusters of seismicity are related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the LOFZ. Focal mechanisms were calculated using moment tensor inversion of amplitude spectra for body waves which mostly yield strike slip mechanisms with SE-NW oriented direction of~σ₁~for the LOFZ at this latitude. In contrast to the strike-slip mechanism of the events along the LOFZ, the focal mechanisms of the stronger events in the Benioff Zone yield mainly thrust mechanism down to 55~km depth. Focal Mechanism Stress Inversion (FMSI) was carried out for the crustal events and the events in the Benioff zone indicating a strike slip regime along the arc and thrust regime in the Benioff zone, respectively. We suggest that the observed deformation along the LOFZ combined with teleseismic observations is a confirmation for the proposed northward movement of the forearc sliver acting as a detached continental micro-plate as outlined by Forsythe~& Nelson (1985) and Beck et al.~(1993). A high quality subset of events was inverted for a 2-D velocity model using the SIMULPS inversion code. First results suggest a 10~km high mantle bulge (v_p>7.8~km/s) below the longitudinal valley and low velocity structure below the coastal cordillera.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: 2007 Fall Meeting