JpGU-AGU Joint Meeting 2020

講演情報

[E] 口頭発表

セッション記号 S (固体地球科学) » S-SS 地震学

[S-SS03] Seismological advances in the ocean

コンビーナ:利根川 貴志(海洋研究開発機構 地震津波海域観測研究開発センター)、悪原 岳(東京大学地震研究所)、Pascal Audet(University of Ottawa)、平 貴昭(カリフォルニア大学バークレー校地震研究所)

[SSS03-03] Seismic ambient noise interferometry applied to dense OBS array of off Ibaraki region

*山谷 里奈1望月 公廣1悪原 岳1西田 究1市村 強1藤田 航平1山口 拓真1堀 高峰2 (1.東京大学地震研究所、2.海洋研究開発機構)

キーワード:地震波干渉法、堆積層、S波速度構造

Off Ibaraki region is located at the southern end of the focal area of the 2011 off the Pacific coast of Tohoku Earthquake (Tohoku Earthquake). A dense network of 32 ocean bottom seismometers (OBSs) was deployed at this region with a station interval of about 6 km from October 2010 (11 OBSs started from February 2010) to October 2011. A large number (> 10,000) of aftershocks following the 2011 Tohoku earthquake were detected by this network. However, precise determination of these hypocenters and focal mechanisms is challenging due to uncertainties of seismic properties of thick sediment layers beneath the seafloor. The P-wave velocity structure has been reasonably constrained by active-source seismic surveys (Mochizuki et al., 2008), but the S-wave velocity structure is still unrevealed despite its importance.
To constrain the S-wave velocity of the shallower portion, we apply the ambient noise interferometry to the short-period OBS data in this study. After dividing the data into ten-minute segments, we deconvolve the data with instrumental response function, remove trends, and discard data dominated by seismic events. Then, we apply a one-bit normalization and spectrum whitening. Finally, we calculate cross-correlations for vertical-vertical, radial-radial, and transverse-transverse components to retrieve Green's functions.
We measure average phase velocity in the array using spatial auto-correlation method (Aki, 1957; Nishida et al., 2008). The phase velocities of the fundamental Rayleigh, the first-higher Rayleigh, and the fundamental Love modes are 0.5 to 2.5 km/s (in the frequency range of 0.1 to 0.3 Hz), 0.8 to 1.5 km/s (0.17 to 0.3 Hz), and 0.5 to 2.0 km/s (0.25 to 0.1 Hz), respectively. Next, we infer the 1-D average S-velocity isotropic structure by non-linear inversion, whose sensitivity is mainly ~5 km. The results show ~1000 m thick sediment with S-wave velocity of 300–1000 m/s immediately beneath the seafloor. At last, we apply band-pass filter with frequency range of 0.125 Hz and measure travel-time anomaly of the phase velocity in each frequency range, following Nagaoka et al. (2012). We apply non-linear inversion (Rawlinson & Sambridge, 2003) and find low-velocity anomalies in the deeper of the northern part and in the shallower of the center part.