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Imaging the Southern Alaska Subduction Zone
Josh A. Calkins (Lamont-Doherty Earth Observatory), Geoffrey A. Abers (Lamont-Doherty Earth Observatory), Douglas
Christensen (University of Alaska Fairbanks), Stéphane Rondenay (Massachusetts Institute of Technology), Jeffrey T.
Freymueller (University of Alaska Fairbanks)
The southern Alaska subduction zone, where the Pacific plate and the thick Yakutat block subduct beneath the accreted terranes
that make up the margin of the North American plate, produced one of the largest ever instrumentally recorded earthquakes:
the Mw 9.2 Good Friday earthquake of 1964. Between 1999 and 2009, two PASSCAL arrays were deployed between the
coast and 400 km inland, with the aim of imaging the source region of the 1964 earthquake. Using data from the southern array
(the “Multidisciplinary Observations of Onshore Subduction” or MOOS array), we produced 2-D scattered wavefield migration
images of the upper 80 km of the subduction zone (top center panel in the figure). The cross sections show results of inverting
forward and back-scattered S waves for perturbations in Vs, and highlight sharp changes or gradients in seismic velocities.
Our results reveal a shallowly north-dipping low velocity zone that is contiguous on its downdip end with previously obtained
images of the subducting plate further north [Rondenay et al., 2008], and suggest that both the dip angle and the thickness of the subducting
low velocity zone change along strike, across a roughly NNW-SSE striking line drawn through the eastern Kenai Peninsula.
Previous geodetic studies [Suito and Freymueller, 2009] indicate a distinct change in locking at the subduction interface along the
same NNW-SSE boundary. On the west end of the Kenai Peninsula, where seismically imaged downgoing crust appears oceanic, the
geodetic signal mainly reflects postseismic deformation from the 1964 earthquake as evinced by southeast trending displacement
vectors. While postseismic relaxation also continues on the eastern Kenai Peninsula, NNW-directed elastic deformation due to locking
at the plate boundary dominates the geodetic signal, and imaging reveals thickened Yakutat crust is subducting. The collocation of
sharp changes in both deep structure and surface deformation suggests that the nature of the plate interface changes drastically across
the western edge of the Yakutat block and that variations in downgoing plate structure control the strain field in the overriding plate.
References
Brocher, T. M., G. S. Fuis, M. A. Fisher, G. Plafker, M. J. Moses, J. J. Taber, and N. I. Christensen (1994), Mapping the Megathrust beneath the
Northern Gulf of Alaska Using Wide-Angle Seismic Data, J. Geophys. Res., 99(B6), 11663-11685.
Rondenay, S., G. A. Abers, and P. E. Van Keken (2008), Seismic imaging of subduction zone metamorphism, Geology, 36(4), 275-278.
Suito, H. and J.T. Freymueller (2009). A viscoelastic and afterslip postseismic deformation model for the 1964 Alaska earthquake, J. Geophys.
Res., 114 B11404.
Acknowledgements: This work was supported by NSF grant EAR-0814235.
Seismicity and migration imaging results from the BEAAR (Rondenay et al, 2008) and MOOS broadband arrays. Upper (center and right) panels show results of scattered
wavefield migration and clearly image the subducting oceanic crust as a prominent N- dipping low velocity (red) structure. Black circles mark the locations of
earthquakes located using the two arrays. Joint migration of the two broadband data sets is a work in progress, and when combined with results from the TACT experiment
(top left, Brocher et al., 1994)), these data will illuminate the structure along a 600 km cross-strike pro¬file of the southern Alaska subduction zone. The map
(bottom center) shows the location of the arrays, and the inset in lower left shows the locations of earthquakes as determined by the MOOS array.
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-179