Constraints on the Aleutian Subduction Zone from the Shumagin Gap to Kodiak Asperity from New MCS and OBS Data of the ALEUT Project
Abstract
In July-August 2011, the Alaska Langseth Experiment to Understand the megaThrust (ALEUT) program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data along a part of the Aleutian subduction zone that exhibits the full spectrum of coupling, from locked to freely slipping. The goal of this program is to characterize variations in the geometry and properties of the megathrust, over-riding and downgoing plates and other structures and relate them to downdip and along-strike changes in slip behavior and seismogenesis. Our study encompassed 1) the freely slipping Shumagin Gap; 2) the locked Semidi segment, which last ruptured in 1938, and 3) the locked western Kodiak asperity, the western extent of the 1964 M9.2 rupture. We acquired 3700 km of MCS data with the R/V Langseth along a series of strike and dip profiles that span the entire locked zone on the megathrust (as indicated by GPS data and estimated rupture zones of past earthquakes), its updip and downdip transitions to stable sliding, bending of the downgoing plate, and preexisting structures in the oceanic crust. Data were acquired with a 6600 cu. in. air gun array and two 8-km-long streamers. The source and one of the streamers were towed at a depth of 12 m to maximize low frequencies (and deep imaging) while the second streamer was towed at 9 m for better imaging of the sediments and upper crust. Refraction data were acquired using the same source and short period OBS spaced at ~15 km along two ~400-km profiles coincident with MCS data across the Shumagin Gap and Semidi segment. Eight seismometers were also deployed onshore in the summer of 2011 that recorded the entire offshore experiment plus local and regional seismicity. Here we present initial results from MCS and OBS data regarding the megathrust and the hydration of the downgoing plate. Initial images reveal deep reflections from the megathrust from both the locked region and the estimated downdip transition to stable sliding. Other reflections in the overriding plate appear to delineate one or more large splay faults and associated basins. Farther seaward, MCS data reveal remarkable lateral variations in the structure of the downgoing plate. Abundant normal faulting, thin sediments and clear Moho reflections are observed in oceanic lithosphere subducting in the weakly coupled Shumagin Gap, while less bending-related deformation and Moho reflectivity is associated with the subducting plate in the Semidi segment. These structural variations are accompanied by changes in the velocity structure of the downgoing plate based on initial inspection of OBS data; lower velocities are associated with the oceanic crust and mantle that are subducting in the Shumagin Gap, possibly due to hydration. These preliminary observations may imply an important role for the properties of the downgoing plate in coupling at subduction zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T33A2387S
- Keywords:
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- 3025 MARINE GEOLOGY AND GEOPHYSICS / Marine seismics;
- 3060 MARINE GEOLOGY AND GEOPHYSICS / Subduction zone processes