2012 IRIS Workshop

Imaging Mega Thrust Zone in Alaska Subduction Zone

YoungHee Kim: Lamont-Doherty Earth Observatory, Geoffrey A. Abers: Lamont-Doherty Earth Observatory, Jiyao Li: Lamont-Doherty Earth Observatory, Doug Christensen: University of Alaska, Fairbanks, Josh:

Tectonic framework with Pacific plate isodepth contours and 1964 earthquake rupture zone, and seismic observations. a. Regional map showing stations used in teleseismic migration and receiver function (RF), local seismicity recorded from the MOOS array, and tremors. b. Migrated scattered-wave dVs/Vs image for A-A’. c. Stacked RFs from the MOOS array. d. P-wave and S-wave velocity profiles for the low-velocity layer on top of the subducted crust, with the same color scheme as the RFs shown in c.

Full-resolution graphics file in original format: 0037.jpg

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) and teleseismic receiver functions using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these receiver function amplitudes shows a thin (~6.5 km) low-velocity layer (shear wave velocity of ~3 km/s), which is ~20 % slower than normal oceanic crustal velocities, between the subducted slab and the overriding North America plate. The observed low-velocity features (with Vp/Vs ratio exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-12 km. Both velocities and thickness of the low-velocity channel gradually increase as the slab bends in central Alaska, which agrees with previously published receiver function results [Ferris et al., 2003]. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane [Rondenay et al., 2008]. Subduction of this buoyant crust could explain the shallow dip of the thrust zone beneath southern Alaska.

Acknoweldgements: This work was supported by NSF-EAR Geophysics.

For further reading: Ferris, A., G. A. Abers, D. H. Christensen, and E. Veenstra (2003), High resolution image of the subducted Pacific (?) plate beneath central Alaska, 50-150 km depth, Earth Planet. Sci. Lett., 214, 575-588. Rondenay, S., G. A. Abers, and P. E. van Keken (2008), Seismic imaging of subduction zone metamorphism, Geology, 36, 4, 275-278, doi:10.1130/G24112A.1. doi: 10.1130/G24112A.1

Keywords: receiver_function, teleseismic_migration, subduction_zone

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