著者

西川 友章 松澤 孝紀 太田 和晃 内田 直希 西村 卓也 井出 哲

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2019年大会

巻号頁・発行日

2019-03-14

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Subduction zone megathrust earthquakes result from the interplay between fast dynamic rupture and slow deformation processes, which are directly observed as various slow earthquakes, including tectonic tremors, very low-frequency earthquake (VLFs) and slow slip events (SSEs), and indirectly suggested by a temporal change in the frequency of repeating earthquakes and the occurrence of episodic earthquake swarms. Some megathrust earthquakes have been preceded by slow earthquakes and terminated near the areas where slow earthquakes were frequently observed. While capturing the entire spectrum of slow earthquake activity is crucial for estimating the occurrence time and rupture extent of future megathrust earthquakes in a given subduction zone, such an observation is generally difficult, and slow earthquake activity is poorly understood in most subduction zones, including the Japan Trench, which hosted the 2011 Mw9.0 Tohoku-Oki earthquake. Here we reveal the slow earthquake activity in the Japan Trench in detail using tectonic tremors, which we detected in the seismograms of a new ocean-bottom seismograph network, VLFs, SSEs, repeating earthquakes, and earthquake swarms. We show that the slow earthquake distribution is complementary to the rupture area of the Tohoku-Oki earthquake and correlates with the structural heterogeneity along the Japan Trench. Concentrated slow earthquake activities were observed in the afterslip area of the Tohoku-Oki earthquake, which is located to the south of the fore-arc geological segment boundary. Our results suggest that the megathrust in the Japan Trench is divided into three segments that are characterised by different frictional properties, and that the rupture of the Tohoku-Oki earthquake, which nucleated in the central segment, was terminated by the two adjacent segments.

著者

増田 滉己 井出 哲

雑誌

日本地震学会2023年度秋季大会

巻号頁・発行日

2023-09-29

著者

西川 友章 井出 哲 西村 卓也

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

松澤 孝紀 武尾 実 井出 哲 飯尾 能久 伊藤 久男 今西 和俊 堀内 茂木

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.56, no.1, pp.75-88, 2003-06-02 (Released:2010-03-11)

参考文献数

21

被引用文献数

3

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We estimated S-wave attenuation (QS-1) in a wide frequency range between 4Hz and 60Hz using the twofold spectral ratio [Matsuzawa et al. (1989)] in the western Nagano region, Japan, where the 1984 Naganoken-Seibu earthquake (M6.8) occurred and the seismicity is still active. In the region, there are 49 seismic stations in a range of around 10km in diameter and station separation is several kilometers. In this analysis, 156 shallow (depth <10km) events (0.9≤MW≤2.6) are used. We can effectively reduce the errors of the estimation by using a number of ray paths. We also determined the focal mechanisms of these events and corrected the waveform amplitudes using them. The direct S-wave portions of the seismograms are relatively small in a high frequency range (above 60Hz) at surface stations compared to the lower frequency waves, and contaminated by P-coda waves. Thus, to estimate QS-1 value, we used only the waves whose S/N ratios are greater than 2, where the noise levels are calculated for the time windows just before S-wave arrivals. Obtained QS-1 values show strong frequency dependence below 10Hz, but weak above 10Hz. These values are slightly larger than the ones estimated by Yoshimoto et al. (1998) from the coda-normalization method. This difference is probably owing to the fracture area of the 1984 Naganoken-Seibu earthquake that has strong attenuation.

著者

井出 哲

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.77, no.4, pp.337-342, 2003-03-31

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Magnitude of Earthquake Research Institute (ERI), University of Tokyo, ME, is determined using an empirical equation of Watanabe (1971). Based on the omega-square source model, constant stress drop scaling, constant Q, and instrumental response, we calculated a typical waveform for each event with a given seismic moment and MW to estimate its ME. This leads to theoretical relationships, which show 1)saturation for MW&gt;6, 2)overestimation of ME for 3&lt;MW&lt;5, and 3) underestimation of ME for MW&lt;2. Under MW 2, ME scales with 1.5MW, which may affect the estimation of b-value of the Gutenberg and Richter equation. Catalogue values of ME and MW for small and medium events show the predicted trend.

著者

酒井 慎一 山田 知朗 井出 哲 望月 将志 塩原 肇 卜部 卓 平田 直 篠原 雅尚 金沢 敏彦 西澤 あずさ 藤江 剛 三ヶ田 均

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.110, no.2, pp.145-155, 2001-04-25 (Released:2009-11-12)

参考文献数

6

被引用文献数

38 54

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From June 26, 2000, an intensive earthquake swarm started under Miyake-jima Island, 180 km south off Japan. This swarm was closely related to the eruption of Miyake-jima Island, probably dominated by underground magmatic activity. The swarm spread toward the northwestern ocean region from Miyake-jima Island, in which a huge number of earthquakes (over 100, 000) including five large events of M>6.0 were detected over about two months. This earthquake swarm was the most active since we started seismic observations in the 1970's.Although there are some telemetered observation stations on the Izu volcanic islands, no offshore instruments were operated in the area of this earthquake swarm. To understand both the spatial and temporal changes of this activity, we conducted a series of ocean bottom seismometer observations. According to the variation in the seismic activity with time, we changed the array configuration of OBSs six times. Furthermore, real-time seismic observations were undertaken using a buoy-telemetering OBS system.Combining the OBS data with those of the island stations, very precise earthquake locations were determined. The epicenter distribution obtained strongly indicates a northwest-southeastern lineament. The vertical cross-section of the events shows two characteristic trends. Deeper (7- 13km) events are forming a very thin (2-km thick) plane, while shallower ones (< 7 km) show a much thicker distribution. These distribution patterns will provide important constraints on the physical mechanism for understaning magma migration.