六甲鶴甲断層運動観測室(以下,「六甲鶴甲」と略す)において実施されてきた地殼変動の連続観測結果を用いて,大月断層の破砕帯付近の岩盤の力学的特性が調べられた.1977～1984年の観測から求められた永年変化の平均ひずみ速度は,10^-6～10^-5/年のオーダーであった.このような大きなひずみ速度は,弾性ひずみではなく,粘性流動あるいは塑性流動によるものと考えられる.また,1984年5月30日に山崎断層で発生したM5.6の地震(Δ≒60km)に伴って観測された余効変動は,震源の変位が弾性的に伝播してきたものと考えるには大きすぎ,地震発生に関連して生じた地殼応力の再配分による観測点付近の粘弾性変形であると考えられる.これらの観測結果を説明するための破砕帯付近の岩盤の力学モデルとしては,Maxwell要素とKelvin-Voigt要素の直列結合であるBurgersモデルが適当である.　六甲鶴甲での永年変化の最大せん断ひずみ速度の観測値と六甲諏訪山観測点で測定された水平最大せん断応力の値とを用いて,Maxwell要素の粘性係数の値を推定すると,破砕帯の外部では7×10¹⁸Pa・s,内部では1×10¹⁸Pa・sが得られた.また,地震の余効変動の時定数と六甲鶴甲の岩盤の弾性定数とを用いて,Kelvi-Voigt要素の粘性係数の値を推定すると,破砕帯の外部では7×10¹⁴Pa・s,破砕帯内部の最も強く破砕された場所では2×10¹⁴Pa・s,その他の破砕帯内部では4×10¹⁴Pa・sが得られた.　これらの粘性係数の値をもつBurgersモデルの力学的特性によって,気圧変化に起因する地殼ひずみ変化の観測結果をある程度説明できることが確かめられた.また.今回求められたMaxwell要素の粘性係数の値は,西南日本の第四紀地殻変動から推定されている地殼の平均的な粘性係数の値1.5×10²¹Pa・sと比べて,約2～3桁も小さい.さらに,その値は,根室半島沖の地震発生サイクルとそれに関連する地殻の上下変動から推定されているプレート境界の粘性係数の値10¹⁸Pa・sとオーダーで一致する.これらのことから,求められた粘性係数の値は,活断層付近の地殼の粘性係数としてほぼ妥当なものであると考えられる.

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　中国ではM7以上の大地震が多数起きており,そのうち52の大地震が地質学的に研究されている.本論文では構造地質学,ネオテクトニクスおよび関連する活断層の地震テクトニクスにもとついて,　hの構造区においてM8以上およびM7級の大地震の発生の地質学的背景を分析した.それぞれの構造区における対応する地質学的背景に基づいて,将来のMS以上およびM7級の大地震の地震発生危険地域を認定した.これらの地震危険地域は古地震学的データ,とくに固有地震の線形モデル(同一再来間隔)に基づく古地震の平均的再来期間によって定義されている.

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The Geographical Survey Institute has carried out astronomical observations at 454 stations between 1947 and 1992. The density of the stations is one station per approximately 30 km×30 km. Interpolation is one of the best ways to obtain a dense estimation. The observed deflections of the vertical are affected very strongly by topography, so that the interpolation accuracy depends on the topographic irregularity. To eliminate the interpolation error due to the topographic effects, the following steps have been performed: (1) The topographic effects are removed computationally by subtracting them from the observed deflections of the vertical. (2) After removing the topographic effects, interpolation is made at an unknown station by the method of least squares interpolation. (3) As the final step of this method, the topographic effects are restored by adding them to the interpolated values at unknown stations. Residual, the difference between the interpolated and observed values, is computed at each astronomical observation station to estimate the interpolation error. The rms value of residuals is 2.6 seconds in this precise method, and is 4.4 seconds in the direct interpolation. These results show that the direct interpolation leads to poor results because of topographic irregularity, and that it is extremely advisable to factor the topographic effects into the interpolation. Mean interpolation error is estimated to be around 2.0 seconds at unknown stations among the observation stations.

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An international gravimetric connection between Japan and China as well as domestic ones in both countries were carried out during the period from October 1985 to February 1986 by means of four LaCoste & Romberg gravimeters (G-196, G-305, G-484 and G-605) for the purposes of (1) calibrating scale values of each gravimeter employed, (2) precisely determining a gravity value at each measured station occupied, (3) establishing a precise gravity net composed of 24 gravity stations in the standard baseline field of gravity values in Lushan Mountains recently arranged by the Chinese authority, (4) experimentally reconfirming specific characteristics of mechanical sensitivity of La Coste & Romberg gravimeters revealed on some gravimeters through the international gravimetric connections along the Circum-Pacific zone performed in 19791982, and (5) clarifying the dependency of gravimeters, from data obtained under various measurement conditions, on disturbing environmental effects. The present article deals with the purpose (4) among them. During the whole course of the present investigations, a direct calibration of all the gravimeters employed was carried out as frequently as possible together with a sensitivity calibration at the time of every gravity measurement. Results obtained through examinations on the static characteristics of the gravimeter are summarized as follows: (1) The dependency of an optimal offset angle for the optical method of gravity measurements on a gravity value at the measured station has fully been ascertained through the investigations. On the contrary to this fact, an optimal offset angle for the readout method of gravity measurements has been ascertained to have quite different characteristics; namely, it depends on a gravity value and has time-dependent character. This character was revealed to be caused by instability for the zero-offset control of a readout amplifier. Without taking frequent and careful adjustments for the zero-offset of the readout amplifier, measurement accuracy for the readout method of gravity measurements becomes significantly worse according to a setting error along the longlevel of the gravimeter. (2) The readout system equipped in the gravimeter is not guaranteed for both longterm and short-term stabilities. Unlike its instantaneous high resolution, this problem in the short-term instability guarantees measurement accuracies in dial readings with the readout method for gravity measurements of only about the same order or less compared to those with the optical method for gravity measurements. (3) The resolution of bubble-type levels for the gravimeter's setting is limited to about ±10 seconds of arc in ordinary measurement conditions and it may reach up to ±15 seconds of arc in extremely severe measurement conditions. Therefore, it strongly requires to adjust bubble positions close to the optimal ones for attaining a high measurement accuracy concerning the setting positions of the gravimeter. (4) The gravity measurements had been accomplished with an accuracy of a few pgals usually and 5 pgals at maximum concerning the setting accuracies of the gravimeter throughout the whole period of the investigations.

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Spatial and temporal variations of room temperature in the gravity observation room of Kyoto University were investigated with 13 quartz thermometers for clarifying the effect of room temperature changes on precise gravity observation with superconducting gravity meters. Observational data of room temperature and gravity in July 10, 1993-July 9, 1994 were analyzed. Although the air conditioner has not been operating in the observation room, annual and diurnal changes of room temperature are 10°C and 0.2°C at the most, respectively. Ratios of these changes to atmospheric temperature changes were about 0.5 and 0.03, respectively. The effect of room temperature changes on gravity observation is considered to be caused by thermal expansion of piers of gravity meters, movements of the tilt control system depending upon temperature changes, as well as thermal noise of electric circuits. Gravity changes due to thermal expansion of piers are in the order of a few ngals in a year and 0.01 ngal in a day at the most. Those due to movements of the tilt control system depending upon temperature changes are lower than 1 ngal. On the other hand, thermal noise of electric circuits might cause gravity changes in the order of 1 Fugal in proportion to annual change of room temperature. In a period band shorter than a few days, however, it causes gravity changes by 0.01 μgal at the most. Therefore, the effect of room temperature changes on gravity observation can be ignored in that shorter period band, even if the air conditioner is not used.

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