In concluding these reports, the results obtained will be summarized as follows:- (1) Some 95 foci of Imaichi aftershocks. were located by the data obtained at the single station, Nishi-oashi, which had three components of seismographs at its desposal. (2) Epicenters of these aftershocks were mainly concentrated in the area bounded by a triangle whose apices were Nishi-oashi, Imaichi and Nikko, and this fact was in good accord with the result obtained by the difference time method. (3) Occurrence of shocks, though small in number, in the northwest and southwest quadrants was confirmed. (4) The incident angle of the first P wave was calculated. In this, as the vertical motion seismometer had the free oscillation period of 0.1sec., Tsuboi's method was made use of for the purpose of determining the true initial amplitude of the vertical component. (5) The majority of shocks had incident angles of about 10°～30° and the assumption of the existence of a discontinuity surface with a slightly bigger velocity within the earth was found to provide favourable explanation to it. The depth of this discontinuity will not be greater than 2.5km. (6) With respect to these shocks with small incident angles the depth of foci and the epicentral distance were calculated. (7) Some observations were made regarding the distribution of the first impulsions of aftershocks. In conclusion the author wishes to express his hearty thanks to Professor T. Hagiwara for his valuable advices. A part of the expense of this study was defrayed from the Scientific Research Fund of the Department of Education.
今市地震の余震觀測は今市,日光,船生,鹿沼及西大芦で行はれそれらの結果より求められた余震の震央穿布については別に報告されてゐる.筆者は西大芦に於て水平動の微動計2台,上下動の加速度計1台を用ひ三成分觀測を行つた・微動計はしばしに針が外れ觀測は困難であつたが初動は極めて明瞭な記録が得られたので之等の記録よリー點觀測による震央を求めた結果,4點觀測にょる震央の位置とazimuthにして10°位の差違を生ずるが,その結果は充分信{ｮするに足るものであることが解つた.求められた震央は大部分西大芦の北東,日光今市西大芦を結ぶ三角形の中で發生しその他西大芦鹿沼間にも可なりの地震が發生したことが知られた.入射角を計算するにあたつては坪非教授の求められた初動の型を用ひてもとめた結果10°～30°の入射角をもつものが最も多いことが知られた.又極めて淺い地震についても震央距離の大きいものの方が入射角が反つて小となることが知られ之等より地下数粁の所に一つの層を考へうれば之等の現象を説明するのに極めて好都合であることをのべた.
雑誌名
東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo
巻
28
号
3-4
ページ
401 - 413
発行年
1951-03-25
ISSN
00408972
書誌レコードID
AN00162258
フォーマット
application/pdf
日本十進分類法
453
出版者
東京大学地震研究所
出版者別名
Earthquake Research Institute, University of Tokyo
アイテムへのリンクの際はタイトル右肩にある「Permalink」をご利用ください！
Please use "Permalink" at the top right of the title when linking to items!
30. Aftershocks of Imaichi Earthquake Observed at Nishi-oashi Station
ji0283014.pdf
(1.29MB)
