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Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Department of Civil Engineering, University of Alaska, Anchorage, 3211 Providence Drive, Anchorage, Alaska 99508

M. K. Sen

Jackson School of Geosciences, University of Texas at Austin, 10100 Burnet Road, Building 196, Austin, Texas 78758

N. Biswas

Geophysical Institute, University of Alaska, Fairbanks, P.O. Box 757320, Fairbanks, Alaska 99775-7320

Z. Yang

Department of Civil Engineering, University of Alaska, Anchorage, 3211 Providence Drive, Anchorage, Alaska 99508

This study deals with shallow sedimentary structure of the Anchorage basin in Alaska. For this purpose, inversion of site response [SR(f)] data in the frequency range 0.5–11.0 Hz from various sites of the basin has been performed using the simulated annealing method to compute subsurface layer thickness, shear-wave velocity (β), density, and shear-wave quality factor. The one-dimensional (1D) models for the aforementioned parameters were obtained with preset bounds on the basis of available geological information such that the L-2 norm error between the observed and computed site response attained a global minimum. Next, the spatial distribution of the important parameter β was obtained by interpolating values yielded by the 1D models. The results indicate the presence of three distinct velocity zones as the source of spatial variation of SR(f) in the Anchorage basin. In the uppermost part of the basin, the β values of fine-grain Quaternary sediments mainly lie in the range of 180–500 m/sec with thickness varying from 15 to 50 m. This formation overlies relatively thick (80–200 m) coarse-grain Quaternary sediments with β values in the range of 600–900 m/sec. These two Quaternary units are, in turn, overlain on Tertiary sediments with β>1000 m/sec located at depths of 100 and 250 m, respectively, in the central and western side along the Knik Arm parts of the basin. The important implication of the result is that the sources of spatial variation of SR(f) in the Anchorage basin for the frequency band 0.5–11 Hz, besides in the uppermost 30 m, are found to be deeper than this depth. Thus, use of commonly considered geological formations in the depth intervals from 0 to 30 m for the ground-motion interpretation will likely yield erroneous results in the Anchorage basin.