著者

武村 俊介 馬場 慧 矢部 優 山下 裕亮 汐見 勝彦 松澤 孝紀

雑誌

日本地震学会2022年度秋季大会

巻号頁・発行日

2022-09-15

著者

伊尾木 圭衣 山下 裕亮 加瀬 善洋

雑誌

JpGU-AGU Joint Meeting 2020

巻号頁・発行日

2020-03-13

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Hyuga-nada region is located at the south-western part of Nankai Trough, in the Pacific Ocean. M7-class interplate earthquakes are repeatedly occurred by the subducting Philippine Sea plate beneath the Eurasian plate. The largest earthquake in this area was the 1662 Hyuga-nada earthquake (M=7.6) which occurred off Miyazaki Prefecture, south-eastern area of Kyushu region, Japan, and generated tsunami (after called the 1662 tsunami). The tsunami heights were estimated at least 4-5 m along the coast of Miyazaki city by historical records. The 1662 tsunami was much larger than tsunamis generated by usual M7-class interplate earthquakes. This region is also active area of the shallow slow earthquakes. It is known by the 2011 Tohoku earthquake that focal area of shallow slow earthquakes also become a tsunami source area. So, we hypothesized that the 1662 unusual large tsunami was caused by the coseismically slipping of focal area of shallow slow earthquakes. We firstly constructed the fault model of the 1662 earthquake based on the recent result of geophysical observation. To examine the tsunami source of the 1662 earthquake, we surveyed the 1662 tsunami deposits in the lowland along the coast of south-eastern Kyushu region. As a result, sandy event deposits interbedded with clay (organic clay) were recognized at several surveyed points. Based on facies features, these event deposits were possibly formed by the 1662 tsunami. Numerical simulation of the tsunami was carried out using the constructed fault model. Calculated tsunami inundation area can explain distribution of the likely tsunami event deposits at Komei, Miyazaki Prefecture. Furthermore, this study compares calculated tsunami inundation areas, distribution of other surveyed tsunami deposits and tsunami heights of historical records. Tsunami source of the 1662 earthquake proposed by our study could better explain geophysical, geological and historical records.

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

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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.

著者

宮町 宏樹 泊 知里 八木原 寛 井口 正人 為栗 健 山本 圭吾 大倉 敬宏 安藤 隆志 尾西 恭亮 清水 洋 山下 裕亮 中道 治久 山脇 輝夫 及川 純 植木 貞人 筒井 智樹 森 済 西田 誠 平松 秀行 小枝 智幸 増田 与志郎 加藤 幸司 畠山 謙吾 小林 哲夫

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.58, no.1, pp.227-237, 2013-03-29

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2008年に実施された屈折法地震探査によって得られたP波初動走時により,姶良カルデラおよび桜島火山の深さ3kmまでの速度構造を推定した.本研究地域の基盤層である四万十層群は4.6-5.0km/sのP波速度を持ち,姶良カルデラの中央部に向け傾斜している.姶良カルデラの中央部には,4.2-4.4km/sの低速度域が深さ1.5-3kmに存在している.そして,この低速度域はカルデラ下に存在する深部マグマ溜まりからのマグマ供給系が活発であることを示唆している.また,基盤層は鹿児島地溝帯の北西域の境界に沿って深さ1kmから2.5kmに急激に落ち込んでいることがわかった.桜島火山の速度構造は3.6-3.7km/sの領域が存在することで特徴づけられる.桜島火山の山頂直下で発生している火山性地震の震源域と速度構造の比較から,地下構造が種々の火山性地震の震源域の広がりに強い影響を与えていることを示した.