A new gravimetric network, named the Japan Gravity Standardization Net 1975 (JGSN 75), was established and new gravity values were determined on the basis of the International Gravity Standardization Net 1971 (IGSN 71). A reliable gravimetric net has been organized in Japan by combining results of the GSI pendulum and gravimeter measurements by the Geographical Survey Institute and gravity values were determined in the Potsdam system referring to the value at the old pendulum station in Tokyo. The IGSN 71 published in 1974 contains 16 common stations with the national net. If the old reference value in Tokyo has no error, a discrepancy in gravity values at every common station must be equal to -14.0 milligals; a correction to the Potsdam absolute value. The present comparison showed that the discrepancy took mostly a constant value within a range of ±0.06 milligals. The mean value and the standard deviation were -13.80±0.03 milligals for the most districts and -14.00±0.03 milligals for the Kyushu District. No systematic inclination against the gravity value was found. This means that the scale of the Japanese gravimetric net agrees well with that of the IGSN 71. Therefore, the gravity value hitherto adopted in Japan can be converted into new one based on the IGSN 71 only by adding a constant value. New gravity values were obtained for 122 stations throughout the country and are given in this report. These are the results of repeated measurements including many new ones with LaCoste & Romberg gravimeters of which calibration constant values were corrected with our pendulum or the IGSN 71 results. A relative accuracy in this net is considered to be ±0.035 milligals and an absolute accuracy is the same as that ofthe IGSN 71. The JGSN 75 will serve as the framework of the gravimetric net in the country and should be used for all gravity works. The old Potsdam system values in Japan should be changed into new ones based on the JGSN 75.

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Results of continuous observation made at the Fujigawa Crustal Movement Observa tory for the period from July 1970 to December 1975 are described in the present paper. The Observatory is equipped with three components of extensometers (EXT), two com ponents of water-tube tiltmeters (WTT) and a set of borehole tiltmeters (BHT). Main features of the results are as follows : (1) Two modes of transient movements were recorded in the initial period of the observation . One is characterized with rapid movements of duration about half a year, as noticed on ground contraction (EXT-2 and EXT-3) and on inclination (BHT-2) . This mode may be attributed to artificial causes due to tunnel excavation and others . The other is more gradual in time duration of about two years, as indicated by ground contraction (EXT-2 and EXT-3) and by monotoni WSW-down tilting (WTTs) . (2) The EXT-1 started to record a large amplitude variation both in secular change and in short-period movements suddenly in November 1973. It may be attributed to a change in underground water pressure conditions, resulting in amplification of the strain changes. (3) Secular movements of the BHTs are inconsistent with those of the WTTs both in amplitude and in direction . From this, it is considered that a BHT, which records the tidal events nicely, is less reliable than a WTT so far as the long-term movements are concerned. However, the change in tilting modes of the BHTs may have some meanings . (4) The ground strain has ac cumulated at an average strain rate of about 0.4×10^-6/year in a general sense of extension in NS-direction and contraction in EW-direction . These trends are parallel to strike and dip directions of the surrounding geologic structures, respectively. (5) The ground tilt has accumulated in a general sense of WSW-down, which was especially active in the first two years of the observation, and less active thereafter. Although the tilting rate is not unif ornn, its average rate throughout the period is approximately 0.5×10^-6/year. -(6) The results obtained here with the EXTs, WTTs and BHTs do not seem to harmonize precisely with those from triangulations and levellings . But, it is too early to draw any definite conclusion now, as the present events on the instruments are still transient. (7) A remarkable change in strain and tilt accumulations was observed in 1973. Anomalous changes of similar modes in the same period have been reported, independently by several other observatories in Japan . This might support the idea that the modes of crustal deformations may vary almost simultaneously in the whole of Japan, as previously hypothesized by SHICHI.

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Geomagnetic fields around Japan are usually approximated by the 2nd degree polynomials of latitude and longitude until the present. However aeromagnetic surveys around Japan are going to cover a wide area and to be more precise in recent years, and surveys of the geomagnetic total f orce by means of proton magnetometer are being carried out simultaneously with those of three components by using fluxgate type magnetometer in the same region. Under these circumstances, a method to represent geomagnetic fields by the 3rd degree polynomials, being satisfied the relations of both (rot F)_r=0 and F²=X²Y²+Z², was required. Considering the above two conditions, this method seems not only to compensate low accuracy of the three components X, Y and Z but to fairly free from their systematic survey errors. The method was applied, for example, to aeromagnetic data of surveys conducted by the Hydrographic Department of Japan in 1969 and 1970, and thefollowing facts were ascertained through the present investigation. (1) Standard errors of the 3rd order polynomials obtained were 215γ, 263γ, 126γ and 343γ for total force, north, east and downward components, respectively. These values will be able to make smaller by smoothing the data. (2) The area where the mutual consistency among each component was satisfied, became larger by considering the above two conditions. (3) The negative regional magnetic anomaly of the total force was recognized in the area around the Japan Sea and the Okhotsk Sea. (4) The horizontal vector anomaly in the adjacent sea of the Tohoku District mostly directed southward.

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