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.
4 0 0 0 地震波干渉法による茨城沖領域の構造推定
- 著者
- 水野 真理子 佐藤 利典 篠原 雅尚 望月 公廣 山田 知朗 金澤 敏彦
- 出版者
- SEISMOLOGICAL SOCIETY OF JAPAN
- 雑誌
- 地震 第1輯 (ISSN:00371114)
- 巻号頁・発行日
- vol.62, no.1, pp.19-23, 2009
- 被引用文献数
- 2
2 0 0 0 OA 巨大地震アスペリティの実体解明のための海域構造調査および解析手法の開発
- 著者
- 望月 公廣
- 出版者
- 公益社団法人 東京地学協会
- 雑誌
- 地学雑誌 (ISSN:0022135X)
- 巻号頁・発行日
- vol.126, no.2, pp.207-221, 2017-04-25 (Released:2017-06-12)
- 参考文献数
- 39
- 被引用文献数
- 2 3
Seismicity accompanying subduction of one plate under the other is active in subduction zones. This seismicity is not uniformly distributed along strike; aseismic regions may be identified in a band of active seismicity. Such an uneven distribution may be attributed to the heterogeneity of frictional properties along the plate interface. Variations in the physical properties of the top-most layer over the subducting plate and rough topography along the plate interface are considered to be factors determining frictional properties. Recent active-source seismic surveys have revealed characteristics of the plate interface in detail and provide an understanding of how regional seismicity and slip distributions of major earthquakes are controlled. Seismicity around the northern limit of the 2011 Tohoku-oki earthquake shows a good correlation with seismic reflectivity along the plate interface, and suggests that the water content and/or the amount of clay minerals in a thin layer over the top of the subducting plate is a major factor. A subducting seamount was identified from a seismic survey around the southern limit of the 2011 Tohoku-oki earthquake off Ibaraki Prefecture. Repeating M 7 earthquakes share the same source at the subduction front of the seamount, while the source area of the largest aftershock of the 2011 Tohoku-oki earthquake lies next to it. The fault slip of these earthquakes, once initiated at the base of the seamount, did not propagate over the seamount. The same relationship between fault slip and subducted seamount is identified for a large slow slip at the Hikurangi margin. Physical or structural properties controlling seismicity or fault slip along the plate interface appear to be characteristics of the subducting plate. The importance of geophysical surveys of the incoming plate can now be further emphasized to provide a better understanding of seismicity along the plate interface.
- 著者
- 望月 公廣 山下 裕亮 Savage Martha Warren-Smith Emily Jacobs Katie Wallace Laura
- 雑誌
- JpGU-AGU Joint Meeting 2020
- 巻号頁・発行日
- 2020-03-13
The Pacific Plate subducts beneath the Australian Plate at a rate of ~5 cm/year along the northern Hikurangi trough off the North Island, New Zealand, and the seismic activity is very high. In addition to regular earthquakes, slow earthquakes including slow slip events (SSEs) and tremor have been observed offshore on the shallow plate interface. SSEs are relatively frequent in the northern part of the Hikurangi subduction margin, occurring every 1-2 years. Therefore, this frequent, repeating occurrence offers an excellent chance to capture accompanying seismicity as well as the SSE itself using temporary deployments of ocean bottom instruments directly overlying the shallow (<10 km) SSE source.We conducted an international collaborative observation from May, 2014, through June, 2015, using 15 ocean bottom seismometers (OBSs) and 24 ocean bottom pressure gauges (OBPs). We were successful in capturing an SSE directly beneath the network, and obtained a precise slip distribution of the event. The slip reached near the trough axis, and that the slip was reduced in the area of subducted seamounts. The tremor activity initiated near the end of the SSE and it lasted for more than two weeks, within a limited region over one of the subducted seamounts. Stress inversions from focal mechanisms of earthquakes during the SSE cycle revealed temporal variations in stress orientations suggestive of an increase in pore fluid pressure within the slab and along the plate interface during the period prior to SSEs in the area, and a subsequent reduction of pore fluid pressure following SSEs. Furthermore, variations of shear wave splitting delay times and Vp/Vs suggested that filling and emptying of cracks and pore spaces accompanied the fluid pressure changes. These observations of long-lasting tremor activity and temporal variation of focal mechanisms and cracks in tandem with the occurrence of SSEs suggest a potential role of fault-valving in the generation of slow earthquakes.We conducted another temporary OBS deployment from Oct. 2018 through Oct. 2019 using 5 OBSs in the same region of the 2014-2015 observation spanning the subducted seamount where we recorded offshore tremor activity previously. A large SSE occurred during the observation period in April-May 2019 around our OBS network, and we were successful in capturing seismic activity accompanying the SSE. The seismicity increased around the start of the SSE and continued throughout the event. Some of the activity shows a large low frequency component, and it is likely that tremors were activated. We are conducting further investigations.
1 0 0 0 OA 海底地震観測からわかった2011 年東北地方太平洋沖地震の余震分布と応力変化
- 著者
- 村井 芳夫 東 龍介 篠原 雅直 町田 祐弥 山田 知朗 中東 和夫 真保 敬 望月 公廣 日野 亮太 伊藤 喜宏 佐藤 利典 塩原 肇 植平 賢司 八木原 寛 尾鼻 浩一郎 高橋 成実 小平 秀一 平田 賢治 対馬 弘晃 岩崎 貴哉
- 出版者
- 北海道大学大学院理学研究院
- 雑誌
- 北海道大学地球物理学研究報告 (ISSN:04393503)
- 巻号頁・発行日
- vol.76, pp.147-158, 2013-03-19
2011年3月11日に,太平洋プレートと日本列島を乗せた陸側のプレートとの境界で2011年東北地方太平洋沖地震が発生した.この地震は,日本周辺では観測史上最大のマグニチュード9という巨大地震だった.本震発生後には多数の余震が発生するが,大地震発生のメカニズムを解明するためには,正確な余震分布を調べることが重要である.全国の6つの大学と海洋研究開発機構,気象庁気象研究所は,本震発生直後から共同で100台以上の海底地震計を用いて余震観測を行った.2011年6月中旬までのデータから,震源域全体で約3か月間の精度の良い震源分布が得られた.余震の震源の深さは,全体的に陸に近づくにつれて深くなっていた.震源分布からは,本震時に大きくすべったプレート境界では余震活動が低いことがわかった.上盤の陸側プレート内では余震活動が活発で,正断層型と横ずれ型が卓越していた.太平洋プレート内の余震も多くが正断層型か横ずれ型だった.このことから,日本海溝付近の太平洋プレート内の深部と上盤の陸側プレート内では,本震の発生によって応力場が圧縮場から伸張場に変化したことが示唆される.
- 著者
- 村井 芳夫 東 龍介 篠原 雅尚 町田 祐弥 山田 知朗 中東 和夫 真保 敬 望月 公廣 日野 亮太 伊藤 喜宏 佐藤 利典 塩原 肇 植平 賢司 八木原 寛 尾鼻 浩一郎 高橋 成実 小平 秀一 平田 賢治 対馬 弘晃 岩崎 貴哉
- 出版者
- 北海道大学大学院理学研究院
- 雑誌
- 北海道大学地球物理学研究報告 (ISSN:04393503)
- 巻号頁・発行日
- no.76, pp.147-158, 2013-03
2011年3月11日に,太平洋プレートと日本列島を乗せた陸側のプレートとの境界で2011年東北地方太平洋沖地震が発生した.この地震は,日本周辺では観測史上最大のマグニチュード9という巨大地震だった.本震発生後には多数の余震が発生するが,大地震発生のメカニズムを解明するためには,正確な余震分布を調べることが重要である.全国の6つの大学と海洋研究開発機構,気象庁気象研究所は,本震発生直後から共同で100台以上の海底地震計を用いて余震観測を行った.2011年6月中旬までのデータから,震源域全体で約3か月間の精度の良い震源分布が得られた.余震の震源の深さは,全体的に陸に近づくにつれて深くなっていた.震源分布からは,本震時に大きくすべったプレート境界では余震活動が低いことがわかった.上盤の陸側プレート内では余震活動が活発で,正断層型と横ずれ型が卓越していた.太平洋プレート内の余震も多くが正断層型か横ずれ型だった.このことから,日本海溝付近の太平洋プレート内の深部と上盤の陸側プレート内では,本震の発生によって応力場が圧縮場から伸張場に変化したことが示唆される.