日本地球惑星科学連合2019年大会

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[J] ポスター発表

セッション記号 S (固体地球科学) » S-SS 地震学

[S-SS11] 地震波伝播:理論と応用

2019年5月28日(火) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:西田 究(東京大学地震研究所)、白石 和也(海洋研究開発機構)、新部 貴夫(石油資源開発株式会社)、澤崎 郁(防災科学技術研究所)

[SSS11-P12] Regional shear waves from an unusually deep earthquake beneath Ogasawara Islands: 660 km discontinuity and anisotropy

*久家 慶子1 (1.京都大学大学院理学研究科地球惑星科学専攻地球物理学教室)

Following a large deep earthquake beneath Ogasawara Islands on 30 May, 2015, an aftershock (mb 4.9) took place at a depth of 678 km (USGS). P and S waves from the aftershock were recorded by F-net broadband stations in Japan at distances of 1 to 18 degrees. The aftershock is small enough to neglect source effect. Here I explore structure effect on the shear wave propagation. SH waves recorded by the transverse directions in western Japan were generally characterized by a single pulse. The peaks of the SH waves tend to arrive earlier than those of the SV waves in the radial directions. The dominance of a single pulse in the SH waves can be explained by the iasp91 model with the 660 km discontinuity shallower than the aftershock. When the 660 km discontinuity is assumed to be deeper than the aftershock, additional signals predicted in radial components can apparently cause different peak time arrivals between SH and SV waves. However, in these cases, the SV peaks arrive earlier than the SH peaks, which cannot fit the observations. It is thus more likely that the observed peak time difference is attributed to anisotropy. Because signals due to crustal reflection are predicted in the radial components ~6 sec after the direct S waves, shear wave splitting parameters were carefully estimated by using initial portions of S waves. The fast directions obtained in western Japan are nearly in the east-west, which looks different from the fast direction observed for a nearby deep earthquake in the Wadati-Benioff zone. We cannot see systematically early arrivals of SH peaks from Fiji earthquakes in a similar back azimuth. The anisotropic regions may be located deep.