地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
68 巻, 4 号
選択された号の論文の4件中1~4を表示しています
解説
論説
  • ——オンサイト早期地震警報への利用を目的として——
    宮腰 寛之, 津野 靖士
    2015 年 68 巻 4 号 p. 91-105
    発行日: 2015/11/10
    公開日: 2016/01/16
    ジャーナル フリー
    For the Japanese train system, two kinds of the earthquake early warning systems (EEWS) have been installed to protect high-speed running vehicles against earthquakes. One is the on-site EEWS, which issues a warning to the areas where earthquake ground motions larger than a certain threshold are observed. The other is the regional EEWS, which issues a warning to the areas where railway damages will be expected by the information of the magnitude and the epicentral distance. These source parameters are estimated using the initial P-wave data at the closest seismic station to the epicenter. However, a method to directly predict S-wave amplitudes on the ground surface from the P-wave amplitudes at the seismic bedrock could be able to issue earthquake early warnings more simply and surely than the present method. Therefore, we quantitatively investigated the influence of source, path and site effects on the empirical relationship between P-waves at the seismic bedrock and S-waves on the ground surface, using seismic records of KiK-net in the Kanto Basin.
    At first, spectral ratios and peak amplitude ratios of S-waves to P-waves observed at the seismic bedrock (α1), and those of S-waves observed on the ground surface to S-waves observed at the seismic bedrock (α2), were evaluated theoretically and empirically. Here, α1 and α2 correspond to the influence of source effects (VP/VS around the source region and radiation pattern) and path effects (Attenuation of P-wave and S-wave) and the influence of site effects (Green’s function of S-wave in the sedimentary basin), respectively. Our evaluation shows that α2 are larger α1; therefore, site amplification factors more largely affect the prediction of S-waves on the ground surface using P-waves at the seismic bedrock than the source and path effects. On the other hand, logarithmic standard deviations of α1 (σα1) were larger than those of α2 (σα2); therefore, the variability of source and path effects by individual earthquakes more largely affected the variability of the prediction of S-waves on the ground surface using P-waves at the seismic bedrock.
    Secondly, we computed α1 from the theoretical equation as a function of VP/VS around the source region, radiation coefficients, and the wave velocity and the quality factor beneath the Kanto Basin. As a result, α1 values were mainly affected by VP/VS around the source region and the path effects in the high-frequency range. On the other hand, σα1 values were mainly affected by radiation patterns in the low-frequency range and path effects in the high-frequency range.
  • ——庄屋史料と藩史料の比較から分かる庄屋史料の有用性と地殻変動推定の可能性——
    弘瀬 冬樹, 中西 一郎
    2015 年 68 巻 4 号 p. 107-124
    発行日: 2015/11/10
    公開日: 2016/01/16
    ジャーナル フリー
    The 1854 Ansei Nankai Earthquake caused widespread damage from Kii peninsula to Kyushu region with Japanese seismic intensities of V to VI. A large amount of descriptions related to this earthquake are found in the archives “New Collection of Material for the History of Japanese Earthquakes”. The no. 5 of supplements to vol. 5 has collected documents related to the Ansei Tokai Earthquake (Nov. 4th, Ansei 1 in the old (lunisolar) calendar, or Dec. 23rd, 1854 in the new (Gregorian) calendar), Ansei Nankai Earthquake (Nov. 5th), and the largest aftershock in the Bungo channel (Nov. 7th), whose total number of pages amounts to 2,528. These collection books have quoted from many historical documents published by local governments, and reprinted from original historical material related to the earthquakes. However, when the published books were used, no reprint of original material was made. Purpose of this study is to reprint photocopy of an original private record handed down to Warabioka family who was a head of Masaki village of Ainan-town in the southernmost of Ehime Prefecture in Southwestern Japan. Furthermore, we compare our reprinted text with published ones, and make detailed investigation on natural phenomena and damage by the Ansei Nankai earthquake. It is identified that a lack of description about natural phenomena and damage and an error of the number of the dead in the texts adopted in the “New Collection of Material for the History of Japanese Earthquakes”. In particular, it is clear that the number of the dead in Fukaura village is not 101 but only one. This is consistent with the record of the necrology and gravestone. Preliminary estimation for the crustal deformation indicates a lowering of groundwater level in Masaki region by volume dilatation associated with the earthquake. This result is consistent with the record that the groundwater level was lowered. Furthermore, we compared our reprinted text with seven historical documents of Uwajima-Date family who had governed this area at that time in order to know the difference of information in the historical documents of the two families. As a result of comparison, there is no contradiction about descriptions of tsunami damage. However, the comparison shows no description about inland natural phenomena and damage such as well water dried up and rock fall in the historical documents of Uwajima-Date family, whereas it is written in the record of Warabioka family. These results show that the private record written by the village head has also valuable information on the earthquake.
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