Abstract
The attenuation of coda wave in Alborz and NW Zagros has been estimated using the single backscattering (SBS) and the single isotropic scattering (SIS) methods at 9 frequency bands with central frequencies of 1, 1.5, 2, 3, 4, 6, 8, 12 and 16 Hz. The database consisted of 677 local earthquakes (418 events in the Alborz region and 259 events in the NW Zagros region) with the M N magnitude range from 3.0 to 5.7. A total of 8,717 seismograms with signal-to-noise ratios of greater than 3 and epicentral distances less than 200 km were used. To investigate the depth variation of attenuation in this study, the coda quality factor Q c was estimated in each area at epicentral distance range of R<100 km and 100<R<200 km, through 11 coda window lengths between 10 and 60 s. The estimated average frequency-dependent relation of Q c = Q 0 f n at coda window length of 10 to 60 s varies from 66 f 1.04 to 164 f 0.72 in Alborz and 66 f 0.99 to 157 f 0.76 in NW Zagros. In this study, the value of the frequency-dependent parameter decreases with increasing the coda window lengths, and suggests that the lithosphere becomes more homogenous with increasing depth. Furthermore, the coda wave quality factor was determined from all three components.We found that there is no significant difference in obtained results by using the vertical and horizontal components. This indicates that the seismic waves encounter similar heterogeneities and attenuation in the vertical and horizontal directions in the study area.The average frequency- dependent relations of coda waves have been derived Q c =109 f 0.87 and Q c = 108 f 0.86 , for the Alborz and NW Zagros regions, respectively, usingthe SBS model at the coda window length of 25s. Comparison of estimated coda- Q values show that, the obtained Q c values using the SIS method are slightly higher than that those obtained using the SBS method.
Highlights
IntroductionThe attenuation of seismic waves in the lithosphere is an important property for studying the regional earth structure and seismotectonic activity [Kumar et al, 2005]
The seismic attenuation, as an important characteristic in the modern seismology, has an inevitable effect on the determination of the source mechanism, the waveform modeling, the simulation of strong ground motion and the seismic hazard analysis [Jackson and Anderson, 1974; Farrokhi et al, 2016; Soham and Abhishek, 2016].The attenuation of seismic waves in the lithosphere is an important property for studying the regional earth structure and seismotectonic activity [Kumar et al, 2005].Seismic attenuation as amplitude decay through wave propagation is caused by two distinct physical processes: elastic and anelastic properties of the medium
We concentrated on the study of coda waves at shorter coda window lengths and for smaller epicentral distances, because they are less affected by deeper parts of the lithosphere, and the heterogeneity of the upper lithosphere is better manifested in them
Summary
The attenuation of seismic waves in the lithosphere is an important property for studying the regional earth structure and seismotectonic activity [Kumar et al, 2005]. Seismic attenuation as amplitude decay through wave propagation is caused by two distinct physical processes: elastic and anelastic properties of the medium. Attenuation is usually expressed as the inverse of the quality factor (Q) and is generated by two main sources: scattering due to heterogeneities and intrinsic absorption. The existing random heterogeneities throughout the earth’s lithosphere are responsible for the generation of late-arriving wave trains in the tail portion of seismograms -after the arrival of major wave types such as P, S, and surface wavesrecords of the local and regional events, which are called ‘coda waves’ [Aki, 1969; Aki and Chouet, 1975; Sato, 1977; Parvez et al, 2008; Sato et al, 2012; Obermann et al, 2012; Havskov et al, 2016]. Coda waves are backscattered body waves [Aki, 1969; 1980], which their amplitude decreases due to attenuation (including scattering) and geometrical spreading [Havskov et al, 2016]
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