HR: 1340h
AN: S23B-0318    [Abstracts]
TI: Envelope Broadening of High-Frequency P- and S-Waves in Relation to Quaternary Volcanoes in Northeastern Honshu, Japan
AU: * Takahashi, T
EM: taka@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Graduate School of Science, Tohoku University, Aramaki-Aza Aoba 6-3, Aoba-ku, Sendai, 980-8578 Japan
AU: Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Graduate School of Science, Tohoku University, Aramaki-Aza Aoba 6-3, Aoba-ku, Sendai, 980-8578 Japan
AU: Nishimura, T
EM: nishi@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Graduate School of Science, Tohoku University, Aramaki-Aza Aoba 6-3, Aoba-ku, Sendai, 980-8578 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: NIED, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AB: Envelope broadening of high-frequency ($\ge$ 2Hz) seismograms of microearthquakes reflects the scattering effect due to small scale medium heterogeneities. The peak delay times from onsets of P- and S-waves ($t_p$ in later) are known as good measure of accumulated forward scattering effect since they are not susceptible to intrinsic attenuation. Recently, we showed that S-waves propagating under Quaternary volcanoes have quite large values of $t_p$ as compared with those propagating in non-volcanic areas in northeastern Honshu, Japan (Takahashi et al., 2003). In this study, extending the envelope analysis to P-wave seismograms in the same region, we examined the scattering effect for P-waves. P-wave's $t_p$ was measured from Root Mean Square envelope of three-component velocity seismograms recorded by Hi-net (NIED). Microearthquakes occurred around the subducting Pacific plate with focal depths of 32$\sim$150km are used for the analysis. We restricted data to those that a clear peak of P-wave appears in the first half of the time window from P to S arrivals. Peak delay $t_p$ is found to increase generally with hypocentral distance $R$, for example the regression line is $\log t_p = 1.5\log R-3.2$ for 8$\sim$16Hz. The deviation of the logarithm of $t_p$ from its regression line $\Delta \log t_p$ represents relative strength of scattering effect along the ray path. Dividing the whole region into small blocks, we evaluate the distribution of minimum values of $\Delta \log t_p$ in small blocks. The result shows relatively large $\Delta \log t_p$s beneath Quaternary volcanoes at a shallow depth of less than 20km in the northern part of study area. Beneath other volcanoes in the southern part, large $\Delta \log t_p$ regions extend to the depth of about 40$\sim$50km. On the other hand, the result of S-wave analysis showed that the large $\Delta \log t_p$s are observed at regions extending to a depth of about 40$\sim$50km beneath most of the Quaternary volcanoes in the study area. This difference found in P- and S-wave scattering implies a regional variation of the ratio of P- to S-wave velocity inhomogeneities and/or a variation of fluid contents which may help us to understand deep structure of volcanoes.
DE: 7280 Volcano seismology (8419)
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting