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1 Environment and Natural Resources
Institute
University of Alaska Anchorage
707 A Street
Anchorage, Alaska
99501
(U.D.)
2 Institute of Technology
Shimizu
Corporation
4-17 Etchujima 3-chome. Koto-ku
Tokyo 135-8530,
Japan
(T.S., T.S.)
3 Department of Architecture and Urban
Design
Faculty of Human-Environment Studies
Kyushu University
6-10-1,
Hakozaki, Higashi-Ku
Fukuoka 812-8581, Japan
(H.K.)
4 Geophysical Institute
University
of Alaska Fairbanks
P.O. Box 757320
Fairbanks, Alaska
99775
(U.D., N.B., A.M.)
5 Department of Aerospace and
Mechanical Engineering
University of Southern California, 430
Los Angeles,
California 90089
(M.D.)
The array measurements of microtremors were carried out at nine sites
to estimate the subsurface S-wave velocity (ß) structures
of the sedimentary deposits
beneath the metropolitan area of Anchorage, Alaska. The data were recorded by
ten
three-component accelerometers arranged in a triangular manner for three
different
array sizes. The phase velocities (C(f)) were estimated at
each site from the vertical
component of the recorded microtremor data by using the frequency-wavenumber
technique. The C(f) data from different arrays were combined
after checking their
consistency for a series of overlapped frequency bands from different arrays and
were inverted using a stochastic least-squares inversion technique to estimate
the 1D
ß-structure underneath each site. The inversion results show that
the engineering
basement (ß >750 m/sec) lies at a relatively shallower depth
(
40 m) in the eastern
part of the basin along the foothills of the Chugach Mountains (CM)
and at deeper
depths toward the southcentral (100 m) and western (150 m) parts of
the basin
in accord with the general dip of the basin. Below the engineering basement, a
well-
developed low-velocity zone (LVZ) with ß-values in the
range of 900–1040 m/sec is
found to be present in the eastern as well as along the Knik Arm side in the
western
part of the basin at a depth of 200 m and 900 m, respectively. Moreover, the
central
part of the basin is associated with a weakly developed LVZ below the
engineering
basement depth. In the rest of the basin, the ß-value increases
gradually with depth.
The spatial variations of ß in the basin at different depths from
20 to 500 m are
represented by using 2D interpolation of the ß-structures obtained
from the inversion
of C(f) data. The depth to the crystalline basement of the
basin, however, could not
be ascertained and it seems to be much deeper than the maximum depth (2000 m)
resolved by the data gathered in this study. To validate the results of
inversion, the
spectral ratio between the horizontal and vertical components (H/V)
of the recorded
microtremor data at each site has been compared with the computed H/V
of Rayleigh
waves based on the respective ß-structure. The results showed good
agreement in
the frequency range of about 0.4–6.0 Hz. In this frequency range, the
H/V peaks are
due to the overall effect of the velocity contrasts between layers representing
the
subsurface ß-structure.
This article has been cited by other articles:
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  Z. Lawrence, P. Bodin, C. A. Langston, F. Pearce, J. Gomberg, P. A. Johnson, F.-Y. Menq, and T. Brackman Induced Dynamic Nonlinear Ground Response at Garner Valley, California Bulletin of the Seismological Society of America, June 1, 2008; 98(3): 1412 - 1428. [Abstract] [Full Text] [PDF]
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