日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG48] Science of slow earthquakes: Toward unified understandings of whole earthquake process

2019年5月29日(水) 10:45 〜 12:15 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:井出 哲(東京大学大学院理学系研究科地球惑星科学専攻)、廣瀬 仁(神戸大学都市安全研究センター)、氏家 恒太郎(筑波大学生命環境系)、波多野 恭弘(東京大学地震研究所)

[SCG48-P08] Constraining S Wave Velocity of the Source Region of Shallow Very Low-Frequency Earthquakes

*悪原 岳1利根川 貴志2 (1.東京大学地震研究所、2.海洋開発機構 地震津波海域観測研究開発センター)

キーワード:浅部超低周波地震、地震波速度構造、海底地震計、ベイズ統計

Shallow very low-frequency earthquakes (VLFEs), which are believed to occur in the vicinity of megathrusts, have been often associated with elevated pore fluid pressure. Lines of evidence for this idea include a small amount of stress drop, high tidal sensitivity, and reduced P wave velocity inferred from active source seismic surveys. Revealing the physical properties of the source region of shallow VLFEs will lead to a better understanding of their generation mechanisms. However, the challenge lies in the difficulty of acquiring S wave velocity in fine resolution.

Recent achievements by the authors may help overcome this issue. Akuhara et al. (2019) developed a robust technique to estimate Green's functions of receiver side structures from offshore seismograms, which are typically ill-conditioned due to water reverberations. Tonegawa et al. (2017) inverted Rayleigh wave admittance (i.e., a ratio of ground displacement to hydraulic pressure) for shallow S wave velocity structures beneath DONET seafloor observatories at the Nankai Trough subduction zone. Their velocity models show low-velocity zones that characterize the source regions of shallow VLFEs. However, a smoothing constraint imposed on the models renders their interpretation somewhat difficult.

This study presents preliminary results from transdimensional inversion of receiver-side Green's function estimated by the method of Akuhara et al. (2019), where we adopt a prior constraint by the velocity models of Tonegawa et al. (2017). Complementary to the Rayleigh admittance, the Green's function approach is sensitive to velocity contrasts and thus will offer an improved resolution.