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タイトル: 14. 地震動の尾部について(其の2)
その他のタイトル: 14. On the Coda Wave o f Earthquake Motions(Part 2)
著者: 表, 俊一郎
著者(別言語): Omote, Shun'itiro
発行日: 1946年7月30日
出版者: 東京帝国大学地震研究所
掲載誌情報: 東京帝国大学地震研究所彙報. 第22冊第2/4号, 1946.7.30, pp.140-169
抄録: In Part 1 the author investigated the coda perids of the earthquakes whose epicentral distances are less than 1,000 kilometres, and showed that, the nearer the epicentres are, the shorter the predominant perieods; and the greater the distances, the longer the periods. In this Part 2, we have investigated the coda periods of the earthquake motions, with reference to twelve distant earthquakes whose epicentral distances are greater than 1,000km but less than 10,000km, on the bases of the seismograms recorded by the same seismograph that was used in investigating the coda periods of near earthquakes, its free oscillation period having been throughout keptup at about sixty seconds. The frequency distribution curves of different periods of coda waves are given in Figs.2~13. In preparing these figures the relative freqnency Np has been used, the definition of which is given in the previons paper. The peaks of the frequency diagrams are not sharp enough to enable one to determine the predominant periods of the coda waves. Therefore, the mean period To of every one minute of coda waves has been determined with each of the 12 earthquakes in the same way as in the previous report, and the result obtained is given in Figs.14~25. These mean peiod curves of distant earthruakes differ from those of the near ones in that the S waves are immediately followed up by surface waves with extremely long periods and large amplitudes. In the earthquakes now under consideration which happened in the South Seas regions these large waves appeared about ten minutes after the comr encement of the P waves and lasted some ten minutes or so. After the large waves died away the vibration periods of the earthquake motions usually became almost uniform and remained so during the interval of 30 to 60 minutes or longer. These uniform waves, however, are followed up again by large waves of long period, which are presumably the Love and Rayleigh waves propagated along major arc of the earth's surface. It will be clearly seen from the mean period curves in Figs.14~25 that, so far as a single earthquake is concerned, the mean period of one minutte has an almost uniform value at any stage in the entire duration of the coda waves, while on the other hand it will also be clear from the frequency curves in Figs.2~13 that waves with periods of approximately the same value as this uniform value actually exist in a large number in the coda waves so that we can safely take this uniform value of period as the predominant period of the coda waves of the respective earthquake motions. In the case of the Amami-Oosima earthquake of April 23, 1936, suitable seismograms were obtained concerning both the east-west and north-south components. The mean period curves Obtained from the respective seismograms of the two components fairly alike, and the predominant periods deduced from them have almost the same value. In the case of the Celebes earthquake of May 20, 1936 also, as in the case of the Amami-Oosima earthquake, the predominant period calculated from the E-W component wag almost equal to that calculated from the N-S component. From this we can reasonably conclude that the value of the coda period of the earthquake motions will never be affected by the directions,from which the waves reach the observing station. In other words the azimuth of the epicentre has nothing to do with the length of the coda period. The predominant periods of the coda wave determined with the twelve distant earthquaks are tabulated in Table III. The relation between the predominant periods and epicentral distances are shown in Fig.26. From this it will be clear that, the greater the epicentral distances are, the longer are periods predominant, and the smaller the distances, the shorter the peridds, a fact which perfectly agrees with the result of the investigations of near earthqua'es. It will be seen froth this curve, however, that as the epicentral distance increase, the predominant periods does not increase at the same ratio, and it seems probable that the predominant period of an extremely distant earthquake approaches a certain defin to value, which marks the limit of the increase.
URI: http://hdl.handle.net/2261/10619
ISSN: 00408972


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