HR: 09:45h
AN: S41A-08    [PDF]
TI: High-resolution Imaging of Subduction Zone Seismicity: Evidence for a Fluid-controlled Double-seismic Zone in the Nazca Plate
AU: * Waldhauser, F
EM: felixw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia Univ., Route 9W, Palisades, NY 10964 United States
AU: Rietbrock, A
AF: Department of Earth Sciences, Univ. of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AB: We present high-precision relocations of large numbers of earthquakes associated with the subducting Nazca Plate below the Central Andes. The new locations are computed by applying waveform cross correlation and double-difference relative location to locally and globally recorded phase pick and/or waveform data, reducing both measurement and velocity model errors, respectively. Relative location uncertainties are typically smaller than the scale-length of the tectonic units in which the events occur, ranging from a few 100 m to a few km (depending on the distribution and quality of available data). Relocations of intermediate-depth earthquakes (80-130 km), using local seismic data and a 3-D model to predict travel-time differences, reveal a fine-scale double-layered Wadati-Benioff zone (WBZ). Correlation with steep-angle reflection data locates the upper and lower band of seismicity at the top of the 8-9 km thick oceanic crust and in the uppermost oceanic mantle, respectively. Analysis of focal mechanisms and waveform similarities suggest that earthquakes in the oceanic crust occur on pre-existing normal faults which slip due to hydraulic embrittlement caused by increasing pore pressure from metamorphic dehydration. Accumulation of small earthquakes at the top of the oceanic crust may demonstrate repeated rupturing of a low permeability seal that acts as a barrier against the upwelling fluids. In both layers extensional faulting predominates, suggesting a weak mechanical coupling between the oceanic crust and mantle. Relocations at around 200 km depth, using globally recorded phase pick and waveform data, reveal a sharp, approximately 15 km thick WBZ, right below what seems to be an (aseismic) bend in the subducting plate. The thickness of the WBZ at these depths is similar to what is observed at shallower depths, and about 50% narrower than what is seen in existing earthquake catalogs. There is some indication of fine-scale layering of seismicity within the plate, but a more detailed analysis of all available data, in particular smaller events, is needed to confirm this observation. Earthquakes at around 530 km depth show strong spatial and temporal clustering that persists over the 25 year observational period. The two clusters investigated occur in a narrow depth range, and form linear structures that parallel the general strike of the subducting plate.
DE: 7215 Earthquake parameters
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8102 Continental contractional orogenic belts
DE: 9360 South America
SC: Seismology [S]
MN: 2003 Fall Meeting