2021年10月7日に発生した千葉県北西部の地震について (first report: 2021/10/08, last update: 2021/10/08)
Earthquake summary on 7 October, 2021, Northwestern of Chiba prefecture, Japan

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1. 地震の概要 Summary of earthquake

 2021年10月7日 22時41分頃に千葉県北西部を震源とするMj5.9の地震が発生しました.震源深さは約75kmと推定されています.防災科学技術研究所のHi-netによる自動震源メカニズム解によると,東西方向に圧縮軸を持つ逆断層型地震であったことを示唆しています.本地震は関東地方に沈み込むフィリピン海プレートと太平洋プレートの境界付近で発生したと考えられています[1]が,詳細な分析結果はこれから発表されるものと考えられます.本震源域付近で想定されているいわゆる首都直下地震よりは地震の規模が小さかったことから,影響は限定的なものであったと推察されます.

 本地震では,埼玉県川口市,宮代町,東京都足立区において震度5強が観測されました[1].地震による被害状況に関しては,土木学会地震工学委員会 地震被害調査小委員会のサイト[2],ならびに各機関から公開されている情報等[3]を参考にしてください.

 An earthquake of Mj5.9 occurred at 22:41 on October 7, 2021 at locat time (UTC: 13:41, Oct. 7), with the epicenter in the northwestern part of Chiba Prefecture. The depth of the epicenter is estimated to be about 75 km. According to the automatic focal mechanism by Hi-net of the NIED, it was a reverse fault type earthquake with an east-west compression axis. This earthquake is considered to occur near the plate boundary between the Philippine Sea Plate and the Pacific Plate, which subducts beneath the Kanto region [1], but detailed analysis results will be opened in the future. The magnitude scale of the earthquake was smaller than that of the so-called Tokyo inland earthquake, which is expected to occur in the vicinity of the epicenter of the earthquake. The impact of the earthquake was probably limited.

 The largest seismic intensity scale by JMA was 5+ recorded in Kawaguchi City, Miyashiro City, and Adachi Ku. For detail information on damage caused by earthquakes, please refer to the website of the committee on Earthquake Engineering, JSCE [2], as well as information published by relevant organizations.

2. 地震動の概要 Summary of ground motion



 The velocity waveforms and acceleration response spectra (5% damping constant) of representative records from the JMA and the NIED strong-motion observation networks (K-NET and KiK-net) are compared. At K-NET Kawaguchi (SIT011), which had a seismic intensity about 5-, ground motion with PGA of 0.13g and PGV of 0.21 m/s was observed. At K-NET Gyotoku (CHB029) and K-NET Urayasu (CHB008), recording the seismic intensity was 5-, ground motions with PGV of about 0.19 m/s and 0.16 m/s were observed, respectively. After a 5 second horizontal motions, which is considered to be S-wave, the motions continued for several tens of seconds, although the amplitude was relatively small.

 The acceleration response spectra of K-NET Kawaguchi (SIT011) and Urayasu (CHB008) show a shape with a peak around 1.0 second, and those of K-NET Gyotoku (CHB029) show an additional shape with a peak around 0.5 seconds. The responses are significantly small compared to the JMA Kobe record (1995 Kobe earthquake), which is the record of past damage earthquakes. The impact on the main structures might be limited.



謝辞 Acknowledgment

 In this report, I used strong ground motion records of JMA and NIED. I appreciate for the efforts.

参考文献 References

[1] 気象庁:令和3年10月7日22時41分頃の千葉県北西部の地震について.Link

[2] 土木学会地震工学委員会 地震被害調査小委員会.Link

[3] 国土交通省:千葉県北西部を震源とする地震について.Link

文責 後藤浩之 (Hiroyuki Goto)京都大学防災研究所 DPRI Kyoto University

Mail to : gotocatfish.dpri.kyoto-u.ac.jp (@は半角の@に置き換えて下さい)