Estimation of Quality Factor of Coda Waves (QC) in Meghalaya Region

Abstract

The earthquake recorded in the form of a seismogram is a combination of source, path, and site. To know the seismic hazard of a region it is necessary to know each of these factors. In the present thesis, the path/medium characteristic of the Meghalaya region has been studied by estimating the quality factor of coda waves (Qc) using local earthquakes. This report also includes the literature reviews on the estimation of the quality factor of coda waves which have been carried out in the region by various researchers along with the importance and objectives of coda Q as medium characteristics as well as earthquake precursory parameter over other parameters. Attenuation of seismic waves is described by the dimensionless quantity called quality factor ‘Q’ (Knopoff, 1964), which expresses the decay of wave amplitude during its propagation in the medium. It is a combination of both intrinsic attenuations, the loss of elastic energy to heat or another form of energy, and scattering, which is deflection and/or mode conversion of elastic energy due to heterogeneities in the transmitting medium. In the present study, Qc estimates have been obtained by analyzing coda waves of 126 seismograms from 21 local earthquakes recorded in the Meghalaya Himalaya recorded digitally in the region during January-August 2018. For this purpose, the coda waves in a seismogram has been analyzed at seven central frequencies using the single backscattering model. The earthquakes have their epicentral distances within 100 km, focal depths up to 42.7 km with 3.0 <ML < 4.2. For the 30-sec coda window length, Qc values have been computed at seven central frequencies of 1.5, 3.0, 6.0, 9.0, 12.0, 18.0, and 24 Hz for different earthquake stations pairs falling in the lapse time window of 30-60 s. The mean value of Qc shows a dependence on frequency, varying from 207 at 1.5 Hz to 3585 at 24 Hz. The frequency dependence average Qc relationship (Qc = Q0 fn) has been obtained for the region as Qc = 136f1.01 which indicates that attenuation at higher frequencies is less prominent. The seismic wave attenuation in the Meghalaya region at lower frequency range, i.e. 1.5, 3.0, 6.0 and 9.0 Hz is low while at higher frequencies (12 ,18 and 24 Hz) attenuation is low (high Q).