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Geophysical Institute
University of Alaska Fairbanks
Fairbanks, Alaska 99775-7320
(U.D., N.N.B.)
Science Applications International Corporation
10260 Campus Point Drive
San Diego, California 92121-1522
(D.A.A.)
Department of Civil Engineering
State University of New York at Buffalo
Buffalo, New York 14320
(A.P.)
The S-wave attenuation of the crust and upper mantle in
south-central Alaska is determined from shear-wave spectral data of 60
earthquakes with magnitudes 3.06.3 recorded by a network of 22 strong
motion stations in Anchorage. The direct S-wave attenuation
(
) and coda attenuation
(
) were estimated by the generalized
inversion and coda decay methods, respectively. The scattering and intrinsic
attenuation coefficients,
s and
i, were
determined by applying a multiple scattering model to the variation of
integrated spectral energy with hypocentral distance for three consecutive
lapse time windows (015, 1530, 3045 sec), starting from
the onset of the S-wave arrival. In order to obtain the dependence of
attenuation coefficients on frequency, the spectral energies over an octave
bandwidth for three central frequencies, 1.0, 3.0, and 6.0 Hz, were
calculated. The results show the following: (1)
is higher than
for frequencies (f)
between 0.6 and 3 Hz and less for f > 3.0 Hz; (2) for 1.0 Hz
f
6.0 Hz,
i increases and
s
decreases gradually with f; (3) the seismic albedo,
B0 =
s/(
s +
i), of the medium decreases from 0.48 to 0.15 with increasing
frequency for 1.0 Hz
f
6.0 Hz; (4) the total attenuation
coefficient,
,
is nearly frequency independent; (5) at low frequencies (
1.0 Hz) the
scattering attenuation
(
,
k is the wavenumber) is comparable to the intrinsic attenuation
(
),
while for high frequencies (f
3.0 Hz)
dominates the attenuation process
compared to
; and (6) at 1.0 Hz,
lies in between
and total attenuation,
, but is closer to
for frequencies higher than 1.0
Hz.
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