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Article |
1 Istituto Nazionale di Geofisica e
Vulcanologia
via Bassini 15
20133 Milano, Italy
(D.B.)
2 GeoForschungsZentrum
Potsdam
Telegrafenberg
14473 Potsdam, Germany
(S.P., H.G.,
C.M.)
3 Ministry of Public Works and
Settlement
General Directorate of Disaster Affairs
Earthquake Research
Department
P.O. Box 763
Ankara, Turkey
(S.K.)
We have analyzed the aftershocks (ML <4.5) following
the 1999 Izmit earthquake (Mw 7.4) to infer the
frequency-dependent attenuation characteristics of both P and
S waves, in the frequency range from 1 to 10 Hz and in the
distance range from 10 to 140 km. A linear-predictive model is assumed to
describe the spectral amplitudes in terms of attenuation and source
contributions. The results show that both P and S waves
undergo a strong attenuation along ray paths shorter than 40 km, while the
secondary arrivals significantly contribute to the spectral amplitudes over the
distance range from 40 to 60 km, as also confirmed by the computation of
synthetic seismograms. For longer ray paths, the decrease in attenuation
suggests an increase in the propagation efficiency with depth. Finally, the
spectral attenuation curves are flattened, or sloped upward at low frequencies
in the range from 100 to 140 km, due to the contemporary arrivals of direct
waves and postcritical reflections from the Moho. In terms of geometrical
spreading and anelastic attenuation, the attenuation in the range from 10 to 40
km is well described by a spreading coefficient n = 1 for both
P and S waves, and the quality factors can be
approximated by QS(f) =
17f0.80 for 1 
f 10 Hz and
QP(f) = 56f0.25 for 2.5
f 10 Hz. For ray paths in the range from 60 to 80 km, the
attenuation weakens but the interaction between seismic waves and propagation
medium is more complex. The multilapse time window analysis (MLTWA)
is applied to quantify the amount of scattering loss and intrinsic absorption
for S waves. The seismic albedo B0 decreases
from 0.5 at 1 Hz to 0.3 at 10 Hz, while the total quality factor
QT increases from about 56 to 408. The multiple lapse
time-window analysis (MLTWA) results provide only an average estimate
of the attenuation properties in the range from 10 to 80 km. In fact, by
neglecting the variation of attenuation with depth, the MLTWA results
underestimate attenuation for distances less than 40 km, and do not capture the
significant features caused by the integrated energy of the secondary arrivals
observed in the range from 40 to 60 km.
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