High Sensitivity Tripartite Observation of Matsushiro Earthquakes : Part 3

The results of the tripartite observation carried out at Hoshina, Nagano Prefecture, within the periods from October 31 to December 18, 1965 and from February 20 to June 20, 1966 in order to investigate the Matsushiro earthquakes have been reported in our previous papers. The present paper is a continuation of these papers and are results obtained within the period from the latter part of June to the middle of September, 1966. The methods of observation and analysis are the same as were described in the first report. From an examination of 1050 felt earthquakes detected by the temporary seismographic network of the Earthquake Research Institute set up in the area around Matsushiro during the period from April to June, we determined the Omori coefficient k as 6.12km/sec with standard deviation of 0.61. After the last part of June, we used this value when determining hypocenters of the Matsushiro earthquakes. The high sensitivity tripartite observation of the same kind as at Hoshina was also made at Komatsubara, (36°37'N, 138°8'E), about 9 kilometers northwest of the town of Matsushiro, within the period July to August, 1966. The continuous tripartite observation using a magnetic tape recorder of 6-tracks capable of working for 29 hours continuously was started at Osa (36°26'N, 138°18'E), Sanada, Nagano Prefecture, about 6 kilometers northeast of Ueda city on September 5 and is continuing to the end of September, 1966. However, these observational results will be reported at the earliest opportunity.松代群発地震を調査するために長野県若穂町保科で行なつた高感度3点方式による極微小地震観測の結果は.すでに第1報および第2報で報告した.ここではその後の1966年6月下旬から同年9月中旬までの観測結果について報告する.観測方式および解析方法は第1報で述べた通りである. 1966年4, 5, 6月の1050個の有感地震を用いて松代臨時地震観測網のデータから大森定数kの値として新たに6.12km/sec,標準偏差0.61を得た.本報告では震源決定にこの値を採用することとした.1966年7, 8月には松代の北西約9kmの小松原(36°37'N, 138°8'E)においても保科と同じ方式の3点方式による極微小地震観測が行なわれた,さらに9月には上田市の北東6kmの真田町,長(36°26'N, 138°18'E)において同じく3点方式による極微小地震の連続観測が開始され9月末現在継続中である.しかし小松原および真田における観測結果については次の機会に報告をする