Abstract
We have carried out a seismic refraction experiment around Bandai volcanic area, the southern part of Tohoku, by using a large capacity marine airgun. Analysis of observed seismograms from the airgun shots shows that the coda wave amplitude at each observation site is independent of hypocentral distance. This indicates that the coda wave energy is homogeneously distributed in the medium beneath the refraction profile. The coda wave amplitude correlates well to the amplitude of the direct P wave from distant earthquakes and inversely correlates to the Bouguer anomaly. The relative site amplification of the coda wave is nearly the same as that of the direct P wave, and is considered to be related to the average density or the thickness of sedimentary layer just beneath the observation site. By using the coda wave amplitude at each observation site, we can evaluate the relative site amplification. Correction of the site amplification factor gives a smooth and less scattered distribution of the P wave amplitude from the airgun shots along the refraction profile.
