HR: 0830h
AN: S11E-0345    [PDF]
TI: Anomalous P-wave Attenuation Around an Active Volcano: An Interpretation Based on a Localized Random Inhomogeneity
AU: * Saito, T
EM: saito@zisin.geophys.tohoku.ac.jp
AF: Geophysics, Tohoku University, Sendai, 980-8578 Japan
AB: \ \ Around active volcanoes, strong attenuation bodies were often detected by seismic experiments and intrinsic absorption is considered as a physical mechanism of attenuation. Paying an attention to that seismograms near the attenuation bodies often contain incoherent later-phases, random inhomogeneity model would be a candidate to lead direct-wave attenuation and coda excitation due to scattering. In this study, I numerically simulate wave propagation through a medium having a localized inhomogeneity to explain the observed characteristics. \ \ In a 2D medium of background velocity Vp = 5km/s and Vs = 2.9km/s, a disk of localized random inhomogeneity with diameter of 6km is placed. The random inhomogeneity is given by fluctuation of velocities and density (30% in RMS fractional fluctuation and 500m in correlation distance). A P-wave source is 10km apart from the center of localized inhomogeneity and receivers are 15km apart from the source concentrically. Finite difference method is used for the simulation of wave propagation for a source radiation of Ricker wavelet with 10Hz dominant frequency. Receivers are divided into shaded receivers to which the direct ray travels across the inhomogeneity and unshaded receivers to which the direct ray avoids it. Significant differences of wave traces are seen. (1) Clear P-wave is seen at each unshaded receiver, while P-wave is attenuated at shaded receivers. Peak amplitudes at shaded receivers are about 1/5 times as large as ones at unshaded receivers. (2) At shaded receivers, scattered waves having amplitudes as large as attenuated direct P-wave continue for long duration as P coda in radial and transverse components. They are leaked from the localized inhomogeneity where waves are localized by multiple scattering. (3) At unshaded receivers, amplitudes of scattered waves in transverse component are much smaller than that of P-wave in radial component. On the other hand, at shaded receivers, amplitudes of P coda in transverse component increasing with lapse time are as the same order as attenuated direct P-wave in radial component. \ \ Scattering by a localized random inhomogeneity explains the features of seismograms around active volcanoes, anomalous peak-amplitude attenuation and excitation of coda. In addition, it predicts localization of scattered waves and P-wave leakage into transverse component.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2003 Fall Meeting