HR: 0830h
AN: S11E-0350 [PDF]
TI: Transparent Paths between Quaternary Volcanoes in Northeastern Japan Revealed from S-wave Envelope
Analysis
AU: * Takahashi, T
EM: taka@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Graduate School of Science, Tohoku University, Aramaki-aza Aoba, 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, 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, Aoba-ku,
Sendai, 980-8578
Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: NIED, 3-1, Tennodai, Tsukuba, 305-0006
Japan
AB:
S-wave envelopes of deep and intermediate depth microearthquakes observed at stations located in the back arc side of
northeastern Japan are significantly different from that observed in the fore arc side. Apparent durations and peak delays of
S-wave envelopes are relatively long at stations in the back arc side, while are not prominent in the fore arc side (Obara
and Sato [1995]). In this study, we examine the path dependence of S-wave envelope of shallow(35km$\sim$100km) earthquakes
which occurred along the subducting Pacific Plate to clarify the regional difference of medium heterogeneity. To characterize
an S-wave envelope, we measure the peak delay time from the S onset $t_p$. By looking at the $t_p$ obtained from the data
having almost the same hypocentral distance (150km$\sim$200km) and the same earthquake magnitude, we find significant
difference in their $t_p$. For example, at a station located near the Iwate volcano, $t_p$ in 16Hz$\sim$32Hz band is 3.6s
for a ray propagating beneath the volcano and that is as short as 1.7s for a ray passing far from the volcano. From the
systematical examination of $t_p$ for many different pairs of stations and events, we further find that $t_p$s are large for
rays traversing beneath Quaternary volcanoes in northeastern Japan, while $t_p$s are small for the rays propagating in narrow
paths between Quaternary volcanoes in depth down to about 40km. Our peak delay analysis revealed the spatial variation of
medium heterogeneity and the existence of transparent and less scattering paths between Quaternary volcanoes in the back arc
side.
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting