HR: 0800h
AN: S11B-0175    [Abstracts]
TI: Localization of scattered waves revealed from active sources around the Iwate Volcano, northeastern Japan
AU: * Kosuga, M
EM: mkos@cc.hirosaki-u.ac.jp
AF: Hirosaki University, Bunkyo-cho 3, Hirosaki, 036-8561 Japan
AB: I have investigated the characteristics of seismic wavefield from active sources around the Iwate Volcano, northeastern Japan, by using the data from a dense seismographic array. The active seismic survey was carried out in October 2000 under the National Project for the Prediction of Volcanic Eruptions to clarify a three-dimensional velocity structure beneath the volcano. The survey was extensive with nine chemical explosions observed by 330 temporary seismic stations around the volcano. I examined the time evolution of wavefield by measuring band-pass-filtered rms amplitudes of seismic waves, and then spatially interpolated them to display an amplitude map for each sliding time window with a length of 1 s. The general feature of wavefield is a circular spread of wavefront from the explosion and gradual decay of coda amplitudes with time. However, there remains significant seismic energy at areas including the explosions and the summit. This localization of seismic energy is particularly evident for the explosion located at the eastern flank of volcano and in the frequency ranges lower than 8 Hz. The plot of amplitude versus epicentral distance takes spindle shape, suggesting a contribution of strong scattering. By applying a diffusion model to the amplitude data, I estimated a mean-free-path of about 800 m. This value is longer than that of 200 m estimated at the Vesuvius volcano, Italy, probably reflecting lower volcanic activity of Iwate volcano. The area of energy localization roughly coincides with a low-velocity (LV) zone beneath an area covering the summit and the eastern flank of volcano. LV materials are interpreted as relatively younger volcanic edifices that reflect the evolutionary history of volcano. From the spatial distribution of P-wave amplitudes corrected for geometrical spreading, the LV zone is characterized by moderate attenuation. The above observations suggest an effective excitation and entrapment of seismic waves by small-scale heterogeneities with large velocity and/or high density embedded in a LV matrix.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7218 Lithosphere (1236)
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: Fall Meeting 2005