23. Division of the South-western Pacific Area into Several Regions in each of which Rayleigh Waves Have the Same Dispersion Characters
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In the previous papers, the writer classified the dispersion curves of Rayleigh waves into seven kinds, and gave them the characteristic numbers from "1" to "7". These curves were revealed to be not independent of each other. For example, when Rayleigh waves pass across three differing regions with dispersion characteristic numbers "1", "3" and "7" with a pass length ratio of 3:3:4, the resultant dispersion character along the total path shows the character "5". Similar relations could be found also with the other curves (Fig. 3). Using these relations, a trial was made to divide the complicated oceanic area in the western side of the "Andesite line" into four regions "1" (purely oceanic type), "3" (sub-oceanic type), "5" (subcontinental type) and "7" (purely continental type). At first, the present oceanic area was divided into four regions by using the dispersion data obtained along thirty-two paths for Tsukuba Station (Fig. 4). Secondly, the same work was done by adding other dispersion data along thirteen paths for three stations, Hongkong, Honolulu and Suva (Fig. 11). In order to check the results, the group velocities of Rayleigh waves across these divided regions were calculated by least square method (Figs. 6, 12). The result showed that our division map was quite satisfactory. Judging from the division map obtained, it was recognized that the crustal structure in the western side of the "Andesite line" was certainly more continental than in the eastern side of it. In the sea region deeper than 4km, for instance, Rayleigh waves show the characteristic number "1" in the eastern side, while they show the sub-oceanic one "3" in the western side. The region in which Rayleigh waves show the dispersion characterized by type "1" is, in the western side of the "Andesite line", limitted to regions with a much greater ocean depth, the depth being greater than 6km. Rayleigh waves through the East China Sea or the Borneo Sea show the purely continental dispersion character "7". Only in the central part of the South China Sea or in small parts of the Celebes Sea, does the dispersion of Rayleigh waves show the sub-oceanic character "3". Four kinds of dispersion curves in the divided regions were compared with the theoretical ones, and it was found that the shift of the dispersion character from "1" to "7" could be explained by decreasing the compressional waves velocity in the crust from 6.9km/sec to 5.0km/sec and by increasing the thickness of the crust from 5km to 20km. (Fig. 13).|前論文で，海洋を伝わつて日本にやつてくるレーリー波の分散曲線を7種に分類したが,これらの分散曲線は,実は夫々独立なものではなくて,例えばレーリー波が,夫々の内部では"1","4","7"という分散特性を示す様な領域を夫々3:3:4の割合で含む径路を通つてきた場合には,全体としての分散特性は"5"になる,といつた具合に,お互いに関連性をもつていることも前論文で示されている(第3図).この事実を逆に利用することによつて,西南太平洋,特にいわゆる安山岩線の西側の海域を,レーリー波が全く陸的な分散を示す海域"7",かなり陸的な分散を示す海域"5",かなり海的な分散を示す海域"3",完全に海的な分散を示す海域"1"の4つの領域に分割することを試みた.先ず最初は,東大震研の筑波支所の記録から得た合計32本の径路に沿つての分散資料を用い(第4図),次に,これらの径路と交叉する径路をとるものとして,香港,ハワイ,フィジー島の3ケ所における記録から調べた計13ケのレーリー波の分散資料を加えて,それによつて前の結果に若干の補正を加えた(第11図).
- 1962-03-15
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