Three methods of analysis of earth tides-Lecolazet's, least square's and Fourier integral's-were compared with each others using one month's data obtained with an Askania gravimeter in Kyoto, Japan. Both a modified Lecolazet's method to exclude systematic errors of analysis and the least square's method assumed to be present seven components of tides, give excellent results in gravimetric factor and phase lag, using hourly values covering the duration of one month. Observational data obtained in Kyoto (December 1967-March 1968) and those in Mizusawa (November 1967-February 1968) were analyzed continuously by means of the modified Lecolazet's method. These analyses show a similar variation in gravimetric factor and phase lag, and it is considered to be a part of seasonal variation in comparison with the previous observation made in Kyoto (August 1959-August 1960). Further, the present results for gravimetric factor give greater values, in general, than the previous ones. This suggests secular change or long period variation in gravimetric factor.

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Since the International Geophysical Year, gravimetric tidal observations were made at many stations in the world. But the obtained values of tidal factor and phase lag of gravity show fairly large fluctuations with time even at the same station, and they are too large to be attributed only to observational errors. In order to investigate the causes of such fluctuations, simultaneous observations of earth tides with four Askania gravimeters and two LaCoste & Romberg gravimeters were made from August 30 to November 17, 1968 at the International Latitude Observatory of Mizusawa. In succession, simultaneous field survey including two other LaCoste & Romberg gravimeters was carried out for the purpose of calibrating the scale constant of the gravimeters along the route from Mizusawa to Hachinohe (gravity difference is approximately 210 mgal). The results obtained through the present investigations are as follows:(1) There appears to be the discrepancy among the gravimeter type.(2) Large fluctuations of the tidal factor recognized by the Askania Gs-11 gravimeters amount to about ±35%, and it may be considered that they are mainly due to non-linearity of the recording system.(3) It is to be noted that trustworthy results cannot to be expected so far as continuous observations of earth tides are made by a single gravimeter.

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Strain steps associated with earthquakes have been observed with super-invar-bar extensometers at Iwakura, Amagase and Donzurubo Observatories for earthquakes of magnitude 3.2-7.9. The stability of these instruments for vibration was confirmed experimentally by two methods. Strain step amplitude dependance upon distance seems to be R^-2.4, and based on a few assumptions, fault length is related to earthquake magnitude by the following equation, M = -8.2+2.2log10 Lwhere L is fault length in cm.

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If the earthquake is the result of the accumulation of strain energy in the crust and the upper mantle, it might be expected that the necessary period of energy storage will be related to the scale of the resulting earthquake. For shallow earthquakes, it is expected that the crustal deformation accompanied by the strain accumulation may be observed by frequent surveys of geodetic nets in the area concerned. Although it is quite difficult at the present stage to find geodetic data which clarified the period of duration of the preseismic crustal deformation, the writer looked for following three examples which might have the possibility of suggesting the preseismic duration.Name Year and Mag. Assumed duration T Nankaido Earthquake 1946 M 8.1 92 years Kitamino Earthquake 1961 M 7.0 12 years Omi Earthquake 1967 M 5.0 0.3 years Considering that these three data show a linear relation between M and log T, the writer calculated the coefficient of log T = bM+a by using method of least squares as follows;log T = bM+a = 0.79 M-4.44. (1)Further he checked this equation by applying it to some examples which suggest the durations of preseismic deformations in somewhat insufficient features comparing the above mentioned data, and found considerable consistency (Table I and Fig. 5). If the earthquake may occur when the linear strain per unit length of the rock of the earth's upper layer attained to some limiting value and Eq. (1) is concluded, then we may have the following equation;eT = const. (2)where e is the velocity of strain accumulation per unit length per year. This shows that the small earthquake will be caused by quicker accumulation of strain than large one, which is favourable condition for the purpose of earthquake prediction. From Eq. (1), the duration of crustal deformation of the earthquake of M8.6 (which may be the maximum to be expected) is about 220 years. If we assume that, in the volume surrounding the epicentral region which stores most part of seismic energy, the average strain per unit length will be about 5 x 10-5 immediately before the earthquake, then Eq. (2) becomeseT≈5 x 10-5, (3)and for the earthquake of M8.6e≈2.2 x 10-7 per year.Let us assume furthermore that the horizontal average diameter of this seismic volume be about 200 km, then the crustal movement of the circumference of this area will be about 4.5 cm/year, which nearly corresponds to the value suggested by the theory of continental drift and ocean bottom spreading. The writer assumed the time interval To and the block (described as a seismic province in the following) which covers so-called a seismic cycle and contains one complete sequence of earthquakes including one earthquake of maximum scale. Let the number of earth-quakes of almost same magnitude per year be N, and T be the period of preparation which might be considered as duration of progressive strain accumulation, then in one seismic province the following equation might be obtained, assuming statistically the sequential collation of duration periods for earthquakes of nearly the same magnitude, ToNT = To, as ToN is the number of earthquakes in To years. Then NT =1. (4)After Gutenberg and Richter log N = β M+α. (5)From Eqs. (1), (4) and (5), b = - β and α = - a. (6)According to the estimation by Gutenberg and Richter, the coefficients 3 and a for Japan area β=-0.80 and α=+5.5. (7)Comparing (1) with (7), the first condition of (7) is almost satisfied and the second may also hold, if we assume about ten seismic provinces for Japan. To confirm the reality of these relations, more data and investigations are expected.

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　上記の会合がさる6月23～27日にコロラド州ボールダーのコロラド大学で開催された.主催はIAGであるが,アメリカで開くについてはAGUの招待があり,またESSAが共催役をつとめた.主題の示す通り,その目的はIAGの第1・19特別研究グループ課題の「電子距離測定」と同じく第1・23グループの「大気屈折」との両分野における最近の研究進展を展望しようとするものである.　従来もこの種の問題,特に電子距離測定は何度か国際シンポジウムの対象となつて来たが,どちらかといえば測定の原理や装置に重点が置かれていた感じがする.その点,今回は下記のプログラムに見られる通り,測定精度や実用結果に関する論文が多く読まれたようである.それだけこの技術が測地学の分野に定着した実績を示すものではなかろうか.　あいにく日本からの出席者はなかつたが,国土地理院提出の論文(レーザージオジメーターに関する)はDr.Bostrom(ワシントン大学)によつて代読され,地震研究への適用の方向を示すものとして出席者の関心を呼んだと伝えられる.このたび提出論文の別刷一式が公式の会合記録とともに筆者宛送られて来たので,ここにプログラムを転載し御参考に供する次第である.

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　国際地球潮汐会議が,5年ぶりに,フランス国のストラスブール市で,9月15日から20日までの6日間にわたり,20力国から約70名の関係者が参加して開催された.参加者の大部分はヨーロッパの人たちであり,彼等にまじつてアメリカやカナダなどからの代表の姿も見られたが,ソ連からは1人の参加者もなかつたことは残念であつた.わが国からは,緯度観測所の奥田所長と筆者がこの会議のために派遣され,折から滞欧中の緯度観測所の細山謙之輔・若生康二郎両氏を加え,4名が会議に出席した.遠路をはるばる4名の日本人が会議に参加したことは,日本の地球潮汐研究に対する熱意の表われであると,参加者一同から極めて好感をもつて迎えられた.　会議は,改築後まだ日の浅いストラスブール大学数学および地球物理学教室の講義室の一室で,別掲のプログラムにしたがつて行なわれた.会議に報告された論文は40篇であつたが,別に,8篇の論文がソ連から送付されてきており,Melchior教授によつて,それら8篇の論文の概要が紹介された.会議での講演は,数年以上にわたる長期間の地球潮汐観測資料の解析結果,Ms分潮の検出,海洋潮汐の影響の除去,極地方における観測の開始,あらたに考案された計器による地球潮汐の観測,地球潮汐と極運動との関係などが中心課題であつたが,地球潮汐の理論的研究に関する成果が乏しかつたことは,なにか物さびしい感じをうけた.　ストラスブールはドイツとの国境に近い町であり,9月中旬というのに,早朝はすつぼりと霧につつまれて,数10メートル先が見えない日が少なくなかつた.それでも,日中は明るい陽光を浴びるのがつねであつた.筆者は第5回国際地球潮汐会議の直後(1964年6月)にストラスブール大学を訪れたことがあつたが,その当時の古い建物はごく一部を残すだけでほとんど跡形もなく,近代建築のすばらしい大学に変つているのには,まつたく驚いた.　9月18日には,Alsace地方へのexcursionが催された.あいにくの悪天候であつたが,参加者約50名が1台のバスに乗り,まず,Strasbourg市から南へ数10キロメートルの距離にあるColmar市では博物館を訪れ,Ste-Marie-aux-Minesの町では町長主催の歓迎会でワインを味わい,Alsaceの山谷をドライブののち,KaysersbergとRiquewihrでは中世の街並みを見物した.Alsace地方はブドー酒の本場として名高いところであるだけに,excursionでは,あちらこちらで,土地柄のワイソに舌鼓をうつた.なかでも,Riquewihrでは,ワイン工場でワインの製造過程を見学ののち,そこの地下室にある中世のままの姿の酒蔵で味わつたワインの味は,また,格別であった.　今回の国際地球潮汐会議に出席して強く感じたことは,日本のおかれている自然環境ということであつた.ヨーロッパは,一般に,地球潮汐という点においては,極めて安定しているので,その観測結果にも局所性はあまり認められず,極めて良好な結果が得られているのが普通である.ところが,これとは対照的に,日本は周囲を海に包まれているので,海洋潮汐の影響が極めて大きく,不安定な場所にあるので,観測結果も何らかの原因でつねに擾乱をうけており,地球潮汐の観測という点では,明らかに不利な立場にあるということができる.したがつて,日本での観測結果は,そのままヨーロッパなどの観測結果と一致しなのは当然であり,そのちがいを徹底的に調べることが,一つの課題でもある.このような事情を考慮にいれて,地球潮汐研究の現状をふりかえるとき,理論的研究と基礎的実験が欠けているように思われる.日本の地球潮汐研究の発展のためには,関係機関の協力が切に望まれる.

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