Gravity surveys have been frequently repeated in the South Kanto District since 1955. Secular gravity changes can be determined by comparing gravity data obtained by two surveys which were made successively. As a result, the significant gravity differences could not be detected except for bench marks on Miura Peninsula. The South Kanto District has a general tendency to increase gravity. This may be caused by the gravity decrease at the Fundamental Gravity Station, University of Tokyo, where the gravity was originally assumed to be constant. The gravity decrease rate, about 0.011 mgal/year during 1963 to 1969, based on the Kakioka Fundamental Gravity Station, can be explained by changes in underground water level.

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Data observed by the extensometers at the Akita Geophysical Observatory for the period of July 14, 1970-December 31, 1971 are analyzed in order to investigate the effect of rainfall on strain change. It is found that abrupt strain change due to a rainfall does not depend on the load of rain, but penetrating water into the ground. It is clarified that areal strain change due to a rainfall is always minus and strain change does not depend upon total precipitation but types of a rainfall. It is also shown that strain change due to a rainfall is explained by the theory of a porous medium, comparing the observed strain curves with the theoretical ones.

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The results of geodetic surveys (triangulation, leveling, gravity, magnetism) before and after the in-land type earthquake concerning the earthquake area of central part of Gifu prefecture, Sept. 9, 1969 (φ=35°47', λ=137°04', depth 0-5 km, M=6.6, Japan Meteorological Agency) is reported, referring to seismic data in this area. By comparing the crustal movement of pre-, co-, and post earthquake deduced from these data with the Niigata earthquake (1964) 's pattern, it is clarified that the priorearthquake phenomena exist in this earthquake as 6-stage or dilatancy stage proposed by Scholz et al. [15] and others. Migratory crustal movement is suggested to cause to occur the new earthquake of M=6, 0, Aug. 31, 1972 in the middle point between the Kitamino earthquake (1961) and this earthquake, transforming a part of stress energy into density variation energy.

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This paper is the fourth report concerning the continuous observation of Crustal deformation at the Inuyama Crustal Movement Observatory since 1967. Records of tilt observed with water tube tiltmeters have been analyzed. Observational errors are eli-minated by a suitable high cut digital filter. Short period variation in tilting that amounts to 10^-8 or 10^-7 radian is mainly related to the change of discharged water from the observational vault. Vertical movement at Inuyama has been revealed by analyzing the secular change that is separated from annual one by a simple method reported in the former report. Tilting rate of maximum dip changed greatly, at a nearly constant rate, from 0.89×10^-6 rad/year to 0.16×10^-6 rad/year early in 1970, but the direction of maximum dip stayed about N40°W which coincides with that of minor principal strain. This tilting rate and direction of maximum dip is almost concordant with geodetic data around the observatory. We can find the same tendency in the Quarternary vertical movement, so it is considered that not only horizontal strain but also present vertical movement is in the progressive stage, developing the Quarternary tectonic movement in and around the Nobi Plain. Some problems relating to earthquake occurrence were examined. A phenomenon which might be related to earthquake occurrence was detected for the frequency com-ponents of tilt and strain having periods longer than one year. From about one year before the occurrence of the earthquake (M=6.6) in the central part of Gif u Prefecture on 9th September 1969, all the secular variations of tilt rates and strain rates began to change simultaneously. Again, they changed rapidly in the opposite sense about one or two months before the occurrence of the earthquake. This pattern of change in tilt rates and strain rates may be compared with the three stages of Crustal movement (α·β·γ) before and after the occurrence of major earthquake. Relating to this pheno-menon, problems concerning digital filter technique and meteorological effects have been examined, but there was no negative feature to obscure the possibility mentioned above.

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The probability of earthquake occurrence is discussed on the assumption that strain of the earth's crust increases linearly with time and the earthquake occurrence time is governed by a Weibull distribution. The associated hazard rate, density and reliability functions are determined from the geodetic data available for the South Kanto District.

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