Estimation of Coda Wave Attenuation Quality Factor from Digital Seismogram Using Statistical Approach

Jwngsar Brahma

doi:10.5923/j.scit.20120201.01

Abstract

The main objective of the paper is to estimate seismic wave attenuation from the decay rate of coda wave amplitudes of digital seismogram of local earthquakes (epicentral distance < 100 km) following statistical approach. Three earthquake events have been selected for estimation of coda wave attenuation quality factor (Qc). The coda wave of 9 seismograms from 3 local earthquakes recorded digitally by three seismic stations in the region have been analyzed for this purpose at ten central frequencies (1, 1.5, 2, 3, 4, 6, 8, 12, 16 and 18Hz) and three lapse time windows of 20, 30 and 40 seconds duration. Single backscattering model proposed by Aki (1969) and extended by Aki and Chouet (1975) and Sato (1977) is employed to determine Qc values. The mean values of the estimated Qc vary from 31+12 (at 1Hz) to 1974+211 (at 18Hz) for 20 seconds coda window length. For 30 seconds coda window length Qc vary from 46+16 (at 1Hz) to 1977+ 256(at 18Hz). Similarly for 40 seconds coda window length Qc vary from 97+ 22 to 2552 +312. It is observed that Qc value for the study area is frequency dependent and increase with increase in frequency. Moreover the observed Qc values increase with increasing lapse time at all frequency bands. The estimated Qc values show a frequency dependent relationship of the form Qc=Qof n, where Q0 is Qc at 1Hz and n represents degree of frequency dependence. The frequency dependent empirical attenuation relationship for 20, 30 and 40 seconds window lengths are obtained as Qc = 21.49+1.17 f 1.48+0.08, Qc = 48.6+1.11 f 1.29+0.06 and Qc = 88.86+1.12 f 1.19+0.06 respectively.

Highlights

Attenuation is one of the fundamental properties of seismic waves from which the material and physical conditions in the Earth’s interior can be inferred (Aki, 1980)
Attenuation of seismic wave is described by a dimensionless parameter called the quality factor Q (Knopoff, 1964) that expresses the wave amplitude decay that occurs when a wave propagates through a medium the
The quality factor (Qc) values for all lapse time and frequency ranges are listed in Table 3a, Table 3b and Table 3c

Summary

Introduction

Attenuation is one of the fundamental properties of seismic waves from which the material and physical conditions in the Earth’s interior can be inferred (Aki, 1980). The decay of seismic wave amplitude with distance defines the attenuation of the medium. Seismic attenuation is usually considered to be a combination of two mechanisms – intrinsic absorption and scattering loss. As seismic wave propagate through the Earth’s interior and arrives at seismic stations on the surface, its energy (amplitude) decays due to conversion of elastic energy to heat or other forms of energy (intrinsic attenuation) as well as energy redistribution in the heterogeneities present in the lithosphere (scattering). The measurement and interpretation of elastic wave attenuation is important for studying the medium through which seismic wave propagates. Attenuation of seismic wave is described by a dimensionless parameter called the quality factor Q (Knopoff, 1964) that expresses the wave amplitude decay that occurs when a wave propagates through a medium the

Objectives

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Conclusion