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Crustal Attenuation Characteristics in Northwestern Turkey in the Range from 1 to 10 Hz

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 (M_L <4.5) following the 1999 Izmit earthquake (M_w 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 Q_S(f) = 17f^0.80 for 1 f 10 Hz and Q_P(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 Q_T 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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