Quality factor of seismic coda waves from earthquakes in northern Morocco

Abstract

The main objective of this work is to analyze seismic attenuation (1/Q_c) using a single backscattering model hypothesis of Aki and Chouet (1975). For this purpose, the recordings of 66 local earthquakes (epicentre distance < 100 km) during 2008 in Northern Morocco have been used with a magnitude (Ml ) less than 4. The Q_c quality factor values have been computed at four central frequencies 0.75, 1.5, 3, 6 and 12 Hz and analyzed for two horizontal and vertical components for performing the average values. Four lapse time windows seconds from 30 to 60 duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of Qc. We obtained a strong average frequency dependence follow a power law Q_n=Q₀fⁿ where Q₀ is Q_c at 1Hz and n is the power of frequency dependent .The frequency dependent relationships obtained are Q_c=(143.75±1.09)f^{(0.864±0.006)} for the vertical component , Q_c=(149.12±1.08)f^(0.85±0.05) and  for the N component , and Q_c=(140.42±1.81)f^{(0.902±0.04)} for the E component. The values estimated of coda Q shows independent on the component of wave motion consistent with (Jen-Kuang Chung 2009 and Priyamvada Singh 2012 ), thus only one component sufficient to treat the attenuation in this region. The mean values of the estimated Q_c of the vertical component vary from 76 (at 0.75) to 1147.6 (at 12 Hz) for 30  seconds coda window length, for 40 seconds coda window length Qc vary from 122.48 (at 0.75) to 1255 (at 12 Hz ) while for 50 seconds coda window length Q_c vary from 141.4 (0.75) to 1420.8 (at 12 Hz ) . Similarly for 60 seconds coda window length Q_c vary from 173.89 to 1495. The increase in Q_c values with lapse time shows the depth dependence which agree with many studies. The results obtained with this model are presented and then compared to results from the literature.