عنوان مقاله [English]
As seismic energy propagates through the earth medium, its energy (amplitude) decays due to geometrical spreading, intrinsic attenuation and scattering. Owing to anelastic absorption, intrinsic attenuation converts the seismic energy to heat while scattering redistributes the energy at random heterogeneities. Knowledge of the relative contributions of scattering and intrinsic attenuation is important for appropriate subsurface material identification, tectonic interpretations and quantification of the ground motion. Besides, investigating seismic wave attenuation inside lithosphere allows for a more thorough knowledge as to Earth’s deep structures. The attenuation of short-period S waves, expressed as the inverse of the quality factor (Q−1), helps fathom the physical laws related to the propagation of the elastic energy of an earthquake through the lithosphere. Coda wave attenuation is considered as the combination of scattering and anelastic attenuation. In this study, the quality factor of coda wave was estimated in NW Iran making use of single back scattering method of Aki and Chouet (1975). For this purpose, we analyzed 3720 waveforms recorded by 8 short-period stations of Tabriz network from 1996 to 2013. So as to calculate the frequency relationships for Qc, nine frequency bands with central frequencies of 1.5, 2, 3, 4, 6, 8, 12, 16 and 20 Hz were considered and the lateral and depth variations of Q0 (Qc in 1 Hz) were investigated in the research area. In order to study the lateral variations, we chose coda waves recorded in epicentral distances less than 80 km, in a lapse time window of 30 s. The reason for the selection of such short distance (< 80 km) and narrow lapse time (30 s) was to avoid coda waves reflected from deep scatterers, which ultimately helps compare and contrast the attenuation of shallower structures in the study area. Investigation of lateral attenuation variation demonstrated that in the northwest of Sahand volcano (in station AZR), in the northwest of Sabalan volcano (in station SRB) and around Marand (station MRD), the attenuation underwent a faint increase relative to other areas. Because of the shortage of significant lateral variations in the study area, we presented an average frequency relationship for coda quality factor in a lapse time window of 30 s as Qc = 68±1 f 0.84±0.01. The low amount of the quality factor (= 68) in the mentioned lapse time window reveal the thermal effects of the study area on the estimation of the quality factor.
In order to investigate the depth variation of Qc , seventeen lapse time windows from 30 s to 100 s (time interval of 10 s) were extracted for two different datasets, one including an epicentral distance <= 80 km, the other comprised of a distance range of 80-150 km. The Qc factor was calculated for each lapse time in both datasets. The obtained quality factor indicated that Q0 increased with the augment in the lapse time due to the effects of wave propagation inside the deeper parts. Frequency relationship parameter presented unexpected variations; it increased with the increase in the lapse time which is the opposite of typically-observed trends. Anomalous variations in frequency relation parameter versus the lapse time show heterogeneous uppermost mantel beneath the study area. The average frequency parameter obtained in this research was ~ 1.0, a value indicating that the frequency dependency of lithospheric attenuation is negligible in NW Iran.