Japan Geoscience Union Meeting 2016

Presentation information


Symbol S (Solid Earth Sciences) » S-SS Seismology

[S-SS28] Seismic wave propagation: Theory and Application

Mon. May 23, 2016 1:45 PM - 3:15 PM A07 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Kiwamu Nishida(Earthquake Research Institute, University of Tokyo), Hisashi Nakahara(Solid Earth Physics Laboratory, Department of Geophysics, Graduate School of Science, Tohoku University), Jun Matsushima(School of Engineering, The University of Tokyo), Tatsuhiko Saito(National Research Institute for Earth Science and Disaster Prevention), Chair:Takuto Maeda(Earthquake Research Institute, the University of Tokyo), Yohei Yukutake(Hot Springs Research Institute of Kanagawa Prefecture)

2:30 PM - 2:45 PM

[SSS28-09] Isotropic radiation of S waves at volcanoes revealed by numerical simulations of high-frequency scattered wavefields

*Hanae Morioka1, Hiroyuki Kumagai1, Takuto Maeda2 (1.Graduate School of Environmental Studies, Nagoya University, 2.Earthquake Research Institute, the University of Tokyo)

Keywords:Scattering, Volcano, Isotropic radiation of S waves, Amplitude source location method, Finite-difference method

Volcanoes have highly heterogeneous structures, which produce scattered seismic wavefields in seismic wave propagation. At many volcanoes, the amplitude source location (ASL) method has been used to locate volcano-seismic events and tremor. The ASL method uses high-frequency seismic amplitudes under the assumption of isotropic radiation of S waves. This assumption has been interpreted to be valid in a high-frequency band because of the path effect caused by the scattering of seismic waves. Kumagai et al. [JGR, 2011] simulated seismic waveforms in heterogeneous medium with topography to investigate the validity of this assumption. However, they could not reproduce isotropic radiation and suggested that strong short-scale structural heterogeneity is required to achieve isotropic radiation. To validate the isotropic radiation assumption and to investigate the characteristics of scattered seismic wavefields at volcanoes, we performed numerical simulations of high-frequency seismic waveforms with stronger heterogeneous media. We synthesized seismic waveforms with a 3-D finite-difference method at Taal volcano, Philippines. We used topography of this volcano and heterogeneous structural models characterized by von Karman-type power spectral density function (PSDF). We used the correlation distance (a) in a range between 50 and 1000 m and the root-mean-square amplitude of velocity fluctuation (ε) between 0.05 and 0.2, in which a constant value of 0.5 for κ was used. To test isotropic S wave radiation, we used synthetic seismograms at actual station locations to determine the source location using the ASL method in various frequency bands (0.2-2, 1-6, 3-8 and 5-10 Hz). We found that the source location was determined near the input location when using a = 50 m and ε = 0.2 in a frequency band of 5-10 Hz. In this condition, ka is around 1 and the mean free path is about 1500 m, where k is wavenumber. This mean free path is similar to those estimated at volcanoes, which are around 1000 m [e.g., Yamamoto and Sato, JGR, 2010]. Our study strongly supports that isotropic S wave radiation is achieved by the path effect caused by seismic scattering in heterogeneous media similar to actual volcanoes.