著者

筒井 智樹 井口 正人 為栗 健 上田 義浩 大島 弘光 植木 貞人 大湊 隆雄 及川 純 市原 美恵 野上 健治 中道 治久 大倉 敬宏 清水 洋 宮町 宏樹 八木原 寛 前川 徳光 堀川 信一郎 吉川 慎 園田 忠臣 平野 舟一郎 末峯 宏一 林 幹太 加藤 幸司 長尾 潤 池亀 孝光 松末 伸一 五藤 大仁 河野 太亮 簗田 高広 田中 窓香 渡辺 竜一 長岡 優 前原 祐樹 吉田 沙由美 小林 由実 栢橋 志郎

出版者

京都大学防災研究所

雑誌

京都大学防災研究所年報. B = Disaster Prevention Research Institute Annuals. B (ISSN:0386412X)

巻号頁・発行日

vol.53, no.B, pp.241-260, 2010-06-01

著者

為 栗健 八木原 寛 筒井 智樹 井口 正人

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.67, no.1, pp.69-76, 2022-03-31 (Released:2022-04-26)

参考文献数

22

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We obtain a three-dimensional seismic velocity structure below the Aira caldera at a depth shallower than 15 km, southwest Japan, applying seismic tomography inversion method to analyze 14,652 P-wave onsets and 10,935 S-wave onsets of natural earthquakes observed by 45 seismic stations, and 3,121 P-wave onsets generated by artificial explosions. An anomalous zone of low S-wave velocity is discriminated at depths deeper than 12 km below the center of the Aira caldera. The S wave velocity is 18-55 % lower than the surrounding area. The volume of the anomalous zone is 139~255 km3 at shallower depths than 15 km, and the anomalous zone includes about 7 % melt (10~18 km3). Accumulation of magma in the anomalous zone activates a pressure source at the top of the zone, where velocity contrast of the S-wave is intense, and the pressure source induces inflationary ground deformation around the Aira caldera.

著者

中尾 茂 八木原 寛 平野 舟一郎 後藤 和彦 内田 和也 清水 洋

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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The earthquake (JMA Magnitude 7.1) occurred on November 14, 2015 in the area of west off Satsuma peninsula. The epicenter is located in Okinawa Trough where is in about 160 km west from Makurazaki City in Kagoshima Prefecture. This earthquake is one of the largest earthquakes in this area. Seismicity in this area is low in last twenty years. Two continuous GNSS sites are operated by Kagoshima University, one is UJIS site in Uji island which is 84 km to east from the epicenter and the other is MESM site in Meshima island which is 121 km north from the epicenter. At UJIS seismic observation is also operated by Kagoshima University and it is operated by Kyushu University at MESM. We went to those sites in order to get GNSS and seismic data because GNSS and seismic data are not telemetered at those sites. In this research, co-seismic crustal deformation and activity of aftershocks are reported.We relocated the main shock and aftershock until 10:00 on November 16. Length of aftershock area is about 60 km. Its Strike is the same of Okinawa Trough. The epicenter of the main shock is located at the south-west end of the aftershock area and maximum aftershock, which is occurred on November 15, is at north-east end. Activity of aftershock in northern part of aftershock area is high. However, in southern part it is low except aftermath of occurrence of the main shock.GNSS data analysis is by Bernese GNSS software Ver. 5.2 with CODE precise ephemeris. Daily site coordinates of UJIS and MESM are calculated with GEONET sites. Coseismic deformation is estimated by the difference between two days averages before and after the main shock. Displacement at UJIS and MESM is 0.82 cm and 0.65 cm, respectively. The theoretical coseismic deformation is estimated by a strike slip fault model (Okada, 1992). Fault length, strike, dip angle and fault position are estimated by the length of aftershock area. Fault width is assumed a half of the fault length. Amount of fault slip is estimated by the relationship between earthquake magnitude and moment (Sato, 1979). JMA moment magnitude 6.7 is used (JMA, 2015). Theoretical displacement at UJIS and MESM is 1.3 cm and 1.1 cm. Direction of observed displacement is coincident with that of theoretical displacement. However, amount of observed displacement is smaller than theoretical one.

著者

八木原 寛 角田 寿喜 後藤 和彦 清水 洋

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.47, no.1, pp.53-61, 1994-06-14 (Released:2010-03-11)

参考文献数

30

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On January 30, 1992, a shallow earthquake of magnitude 4.9 followed by about 300 aftershocks occurred in a northern area of the Koshiki channel, north-western Kagoshima Prefecture. We located seismic events observed at two stations of NOEV (Nansei-toko Observatory for Earthquakes and Volcanoes) and four stations of SEVO (Shimabara Earthquake and Volcano Observatory), using Joint hypocenter determination (JHD). Hypocenters of the mainshock and its aftershocks were nearly vertically distributed at depths from 5km to 13km in a small area.Initial motions at the seismic stations of NOEV, SEVO and FMO (Fukuoka Meteorological Observatory) suggest a focal mechanism of strike slip fault type with a T-axis of NNW-SSE direction: the mechanism is very similar to those reported for the earthquakes in and around the area. The nodal plane striking in NE-SW agrees with trends of the fault system in the channel and the other WNW-ESE plane is parallel to the earthquake alignment along Amakusanada-Izumi-Kakuto areas. Hypocenters of the event and aftershocks nearly vertically distributing are, however, not consistent with any of the planes.In March of 1991, about 10 months before the M 4.9 event, an earthquake swarm (Mmax 2.9) occurred at depths around 5km almost within the same epicentral area. Namely, two different types of earthquake sequence occurred at different depths in the same area: the swarm occupied a shallower zone than the focal zone of the M 4.9 event. Although some volcanic process may be inferred from hypocenters vertically aligning, it is probably difficult to explain the fact that the earthquake swarm at shallow depths occurred about 10 months before the M 4.9 event at a deeper depth without accompanying any notable foreshocks.

著者

田城 陽菜 八木原 寛

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.68, no.2, pp.75-82, 2023-06-30 (Released:2023-07-27)

参考文献数

14

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We investigated temporal changes in the peak frequency of harmonic tremors accompanied by eruptions at the Showa crater of the Sakurajima volcano from January 1 to September 30, 2015. The peak frequency characteristics of harmonic tremors at the summit crater of Minamidake from 1982 to 2002 were also compared. We calculated the running spectrum of the harmonic tremors of eruptions at the Showa crater and detected their peak frequency based on the findings of Maryanto et al. (2008) by modifying the parameters. Based on the value of peak frequency and associated temporal changes, we classified harmonic tremors at the Showa crater into the following three types: nearly constant (NC), positive gliding (PG), negative gliding (NG), and others. A sharp transition, with abrupt peak frequency of the harmonic tremor changes over a brief period, was also observed at the Showa crater. Assuming that the fundamental frequency corresponds to changes in the length of the gas pocket, the length of the gas pocket was estimated to change from 110 m to 220 m at the Showa crater. Compared with the features of peak frequencies of harmonic tremors at the summit crater of Minamidake (Maryanto et al., 2008), those at the Showa crater present three differences: 1) Compared with HTB of the summit crater of Minamidake (Maryanto et al., 2008), Depending on the event, NC values had different frequency bands at the Showa crater. HTBs occur several hours after swarms of B-type earthquakes and their peak frequencies remained within a certain range (Maryanto et al., 2008). 2) The increase in f0 of PG at the Showa crater was smaller than that of HTE at the summit crater of Minamidake. HTEs occur several minutes after an eruption, as well as accompanying remarkably strong eruptions and their peak frequencies showed a gradual increase (Maryanto et al., 2008). 3) NG and sharp transitions were only recognized in harmonic tremors during volcanic activity at the Showa crater.

著者

八木原 寛 角田 寿喜 宮町 宏樹 後藤 和彦 平野 舟一郎 日野 亮太 金澤 敏彦 海宝 由佳 塩原 肇 渡邊 智毅 望月 将志 根本 泰雄 島村 英紀

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.49, no.2, pp.227-239, 1996-08-23 (Released:2010-03-11)

参考文献数

23

被引用文献数

1 1

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We investigated seismic activity around Tokara Channel north off Amami Oshima, Nansei Syoto (Ryukyu) Islands of western Japan, using 12 ocean-bottom seismographs (OBS), as well as two temporal stations at Yaku Shima and Amami Oshima islands, operated from April 16 to May 10, 1992. One-dimensional velocity structure and station corrections were inverted from P and S times of 51 events provisionally well-located in the OBS network. We then relocated precisely 239 events in the studied region, using the inverted velocity structure and station corrections.Seismicity was highest in an area of about 10km×10km near the trench axis northeast off the OBS network: the largest event of MJMA 5.6 and other 40 events (probably aftershocks) were located at shallow depths. A mechanism solution of normal fault type with a T-axis of NW direction for the largest event was concordant with bending process of the Philippine sea plate. On the other hand, 18 events at depths of about 30km in a small area north of the OBS network were presumably due to interplate thrusting, because a composite mechanism solution for three events was of reverse fault type with a P-axis of ESE direction. A cluster of 17 events at depths from 10km to 25km was found in a southwest area of the network. These shallow events were probably crustal earthquakes within the Eurasian plate.We found an area of very low seismicity in the southeast of the network during the period studied. It is also identified at the nearly same location in the epicenter distribution from 1984 through 1991 obtained by Japanese Meteorological Agency (JMA) and possibly corresponds to the aftershock area of the 1911 Kikaijima Earthquake (M 8.0).Although we could not confirm any discernible alignments of shallow earthquakes along the Tokara Channel which is a notable tectonic line, the dipping angle of the intermediate-deep seismic zone changes discontinuously from about 65° on the north of the channel to about 40° on the south.

著者

宮町 宏樹 小林 励司 八木原 寛

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.67, no.4, pp.471-478, 2022-12-31 (Released:2023-01-30)

参考文献数

9

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We carried out a temporary seismic observation using 4 ocean bottom seismographs in and around the Aira caldera to obtain the precise hypocenter distribution, especially in the Wakamiko caldera. During the observation period from July to September 2020, we observed two short-time seismic swarms that occurred inside the Wakamiko caldera and revealed the precise distribution of these hypocenters and the focal mechanism solution. In contrast to our results, the JMA unified hypocenters of the two swarms are widely distributed in the western region outside the Wakamiko caldera. This difference in the hypocenter distributions insists that the seismic observation in the sea area around the Wakamiko caldera is required to reveal the precise seismic activity in the caldera.

著者

及川 輝樹 八木原 寛 高 美喜男 平野 舟一郎 中野 俊 中尾 茂 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.4, pp.563-567, 2013-12-27 (Released:2017-03-20)

著者

木股 文昭 石原 和弘 植木 貞人 内田 和也 小山 悦郎 佐藤 峰司 鈴木 敦生 高山 鐵朗 竹田 豊太郎 辻 浩 寺田 暁彦 中坊 真 浜ロ 博之 平野 舟一郎 松島 健 宮島 力雄 森 済 八木原 寛 山本 圭吾 渡辺 秀文

出版者

京都大学防災研究所

雑誌

京都大学防災研究所年報 (ISSN:0386412X)

巻号頁・発行日

vol.42, no.1, pp.35-43, 1999-04

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1998年以降, 火山活動が活発化している岩手山火山において, 火山活動に伴う地殻上下変動とその圧力源を議論する目的で, 水準路線を設置し, 1998年7, 9, 11月に精密水準測量を実施した。1998年9月3日, 水準測量実施中に, 直下でM6.1の地震が発生し, 20cmに達する断層運動を水準測量で検出した。岩手山南麓ではこの4ヶ, 月間に4cmに達する山側隆起の上下変動が観測され, その圧力源は岩手山西方に深さ3km前後と推定される。Earthquake swarm is observed around the Iwate-san Volcano, Northeast Japan since 1998. The leveling route with distance of 36 km was set up around the volcano and the precise levelings have been repeated to discuss the crustal deformation four times in July, September, September and November in 1998. When the precise levelingis doing in September 3, 1998, earthquake of M6. 1 was occurred close to the volcano. One leveling team was making leveling in the epicenter area, Re-levelings were repeated since the next day of the earthquake, and coseisimic deformations of 20 cm are detected along the leveling route. However the precursor of the vertical movements is not recognized in the leveling data made just before the earthquake. Uplift of the Iwate-san Volcano is observed and which amounts to 4 cm in the period of July to November in 1998. The pressure sources of the vertical deformations are estimated to be under the west side of the volcano with depth of 3 kim, which is the almost the same location of the pressure estimated by GPS measurements and the DInSAR (Differential Interferometric SAR).

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.

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

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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

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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.