Crustal attenuation characteristics in northwestern Turkey in the range from 1 to 10 Hz

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 )
17f 0.80 for 1 f 10 Hz and QP( f ) 56f 0.25 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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