Estimation of relative site effect, coda Q and scattering strength from the records obtained by a large airgun experiment

By using a large-capacity marine airgun, we carried out a seismic refraction experiment around the Bandai volcanic area, in the southern part of Tohoku, NE Japan. The analysis of the observed waveform data from the airgun shots shows that the coda wave amplitude is independent of hypocentral distance after a lapse time of about 9 s at any observation site with epicentral distance < 30 km. This fact indicates that most of the coda wave energy, after this lapse time, is distributed homogeneously in the medium beneath the observation sites. It is found that the coda wave amplitude correlates well with the amplitude of the direct P wave of a distant earthquake. The relative site amplification of coda wave amplitude is nearly the same as that of the direct P wave amplitude. Based on this observation, relative site amplification factors can be evaluated by using the coda wave amplitude at each observation site. The inverse coda Q values (Q_c^-1) estimated from the time decays of coda wave amplitudes are about 4.6 × 10^-3, 2.1 × 10^-3 and 1.5 × 10^-3 at 5, 10 and 20 Hz, respectively. The frequency dependence of Q_c^-1 is expressed by a power of frequency in Hz (Q_c^-1 ∝ f{hook}ⁿ), where the power n is found to be about -0.8. In the earlier part of the lapse time, the coda wave energy concentrates around the seismic source, which indicates that the scattering effect is very strong in the study area. The shapes of waveform envelopes at 5, 10 and 20 Hz show that the forward scattering factor is stronger at higher frequency than that at lower frequency.