Yearly mean sea levels in many Japanese tidal stations show slowly upward change in 1900-1950, and considerably large downward change in 1950-1970. On the other hand, mean sea levels in the Pacific Ocean side of Tohoku and Hokkaido districts show considerably large upward change in 1950-1970. This is clearly seen in Fig. 1 which shows yearly speed of mean secular change in recent 20 years at each tidal station. These phenomena should be understood that they are due to vertical change in height of land in area of large scale, or that they are due to change in sea level itself in the circumference of Japan Islands. This paper adopts the latter judgment.The change of yearly mean sea level in Japan in 1894-1950 is expressed as follows:
ΔL₁ = +0.88(t-1900) mm.This relation can be applied for all Japanese tidal stations except the eastern part of the Hokkaido. The changes of yearly mean sea levels in almost all tidal stations in Japan except the next stated area in 1950- 1970 are expressed as follows:
Δ₂ = -3.92(t-1950) mm.Those in the Pacific Ocean side of Tohoku and Hokkaido districts are expressed as fol-lows:
ΔL₃ = +4.12(t-1950) mm. The movements of the earth's crust in long range in preseismic stages for many earthquakes can be obtained by subtracting change in mean sea level ΔL₁, ΔL₂ or ΔL₃ from mean sea level at each tidal station. These results are shown in Figs. 2 and 3, and also in Table III. Crustal movement in 1950-1970 at each tidal station is obtained by the similar method as the above stated one. The result is shown in Fig. 4. The tendencies of vertical movements in the Pacific Ocean side of the Tohoku district, southern part of the Kanto district, Tokaido, Nankaido, and the Seto inland sea are interesting from a standpoint of the plate tectonics.

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Building of Department of Geology and Mineralogy, where the National Fundamental Station of Gravity in Japan is situated, has been rebuilt in the summer of 1968. The station has been reconstructed in the same position and height as for the former one. According to the precise gravity measurements, the new value of gravity is about 0.06 mgal smaller than the old value. The old building was made of bricks (two-storey building with a basement floor), and the new one has been made of ferroconcretes (five-storey building with a basement floor). The difference between the new and old gravity values may be explained by the difference of attractions due to the new and old buildings.

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Precise gravity survey by means of six LaCoste & Romberg gravimeters was carried out in Miura and Boso Peninsulas during the period of November 11 to December 10, 1969 according to the recommendation of the Working Group for Gravity Survey and Precise Levelling in Japan. It was carried out under the predetermined routine, along the levelling route, to keep the quality of obtained data as uniform as possible, and a great care was taken in read-ing and handling for the gravimeters. Results of the survey are summarized in the present report. Mean value of closure error along the loops of measuring routes was about 0.07 mgal and the measuring ac-curacy of each gravimeter was estimated to be 0.02 mgal.

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After the accomplishment of the World Magnetic Survey (W. M. S.), the aeromagnetic survey of the total force has been carried out by the Geographical Survey Institute. The surveying regions are the south-western part of Japan: Kanto district in 1966, Kinki district in 1967, Chubu district in 1968, Shikoku district in 1969 and Kyushu district in 1970 (see Fig. 1). The height of measurement is generally about 3, 000 m and the instrument used is a proton precession magnetometer, whose accuracy is ±10 gammas. The transient magnetic variations and the perturbing magnetic field due to the craft are reduced. The continuous records of the total force at the Kanozan Geodetic Observatory (G. S. I.) are used for cancelling the transient magnetic variations. After two corrections the final accuracy of the total force is estimated as about ±20 gammas. The lines of equal total force and the isanomalic charts are shown in Figs. 3, 4 and 8. The isanomalic charts are derived by taking difference between the observed and the computed values (I. G. R. F.) in the height of 3, 000 m. Especially, the isanomalic charts are obtained on the plane 10 km high in order to clarify the regional pattern of the underground structure. The magnetic anomalies over Chubu, Kinki and Shikoku districts are negative. The magnetic low along the Median Tectonic Line seems to be related with the low sus-ceptibility of metamorphic rocks covered the earth's surface. The negative anomalies with large scale are found over the central Chubu district, the western Shikoku district, the south-western part of Iki island and the north-western part of Sakura-jima volcano. The positive anomalies with large scale exist over the northern, central and southern parts of Kyushu district. It seems unlikely that these anomalies are caused by volcanoes directly. The negative and positive anomalies with large scale may be related with the crust and mantle structures.

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Observations of magnetic variations were made by means of portable fluxgate type magnetometers at Akagane (cp = 39°10:6 N, λ =141°20:6 E), Ishibuchi (ψ = 39°06:7 N, A = 140°54:6 E), Yuzawa (ψ = 39°10:2 N, A = 140°29:8 E) and the Mizusawa Geodetic Observa-tory (ψ = 39°06:5 N, A = 141°12:4 E) for the purpose of studying the conductivity anomaly along the east-west line including the Observatory in the Tohoku district of Japan. Using the observed events having the duration time longer than 30 minutes, the ele-ments of Parkinson Vector at four stations are obtained as shown in Table, whose direc-tions are eastward and opposite from those obtained by using the data of geomagnetic micropulsations having the periods of 10-60 seconds. A brief discussion was made about the relation between the conductivity anomaly and the crust and upper mantle structure in the Tohoku district which has typical geo-physical characteristics among circum-pacific island arcs.

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In the present report, some themes are examined from statistical standpoint by using the reading data obtained through precise gravity survey in Miura Peninsula, and the following results are noticeable.Those are:1. Five successive readings at each station are unnecessary under the circumstances of other influential disturbing factors.2. Irregularities in instrumental drift do not seem to affect the result. The fact that similar drift patterns are sometimes appeared in different instruments, suggests common causes of drift. 3. It is rather difficult to keep the measuring accuracy within 1/100 mgal always by increasing the numbers of instruments used.4. Systematic deviations of results of respective instrument from mean values are the most serious problem. Without finding the way to minimize these deviations, the improvement of measuring accuracy is difficult. Similar patterns of deviations which often appear in different gravimeters like the case of drift would be a key of solving this problem.

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Possible gravity changes accompanying crustal movements may be classified into two; one is a secular change or growing of Bouguer anomalies through out geologically long periods especially accompanied with foldings and the other is a short period gravity change due to density change within the crust or upper mantle. (a) One of the typical features of some foldings in Japan formed through the Quater-nary period and still progressive is that the axes of the anticlines having short wave length Bouguer positive anomalies are relatively rising, while the axes of the synclines have the negative anomalies and are relatively subsiding. These short wave length Bouguer anomalies discussed by Y. Hagiwara seem to have been growing with an approximate rate of change of 0.1 pgal/year or less by the progressive geometrical deformation of sub-crustal higher density layers. (b) The order estimation of short period gravity change before and after earthquake occurrence is discussed by using a model of crustal deformation due to uni-axial horizontal compression in which the change of crustal density just before great earthquake occurrence might originate the change of Bouguer anomaly of about 0.2 - 0.3 mgal compar-ed with the values observed past 10 years or more near the epicentral area.

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In point of the vertical displacement of crust, the levelling, tide and gravity are discussed. A new method by which the tidal data are connected with the levelling ones is proposed. Let the levelling surveys be carried out at to and ti. In this method, the tidal variations during the period of toti are considered in the net adjustment of the levelling survey. Comparing with the ordinary method in which the relative positions of mean sea-levels are given as boundary conditions in the net adjustment of the levelling survey, the new method is free from the levelling error and the Datum displacement, as shown in Table I. The variation of mean sea-level during the period of to-ti can be obtained by various methods. The mean sea-level is primarily affected by pressure, water temperature and current. The pressure and water temperature show annual changes approximately. But the oceanographic conditions sometimes generate the noises with the period of several years in tidal records. Then, the variation of mean sea-level at a tidal station in to-ti, may be derived from the linear variation of annual mean sea-levels at the tidal station in more than 10 years including the period of t0-t1. For the earthquake prediction, the abnormal vertical displacement, the discrepancy between the observational and normal vertical displacement peculiar to the area, must be considered. And the short-term characteristics of vertical displacement have to be checked in order to improve the accuracy of the earthquake prediction. The tidal data are also valuable, because they supply the continuous information of the vertical displacement near the coast of Japan where most of great earthquakes have occurred. The gravity value might change in accompany with the variations of height and/or underground density. According to the order estimate, the gravity change of 1/100 mgal corresponds to the height change of about 3 cm and 5 cm in cases of normal free-air and Bouguer gradients respectively. And the density change might cause the gravity change of about 0.1 mgal in occurrence of a great earthquake. Although the gravity survey using several LaCoste & Romberg gravimeters has the long-term aim mentioned above, the present purpose is to determine the instrumental errors individually. According to our indoor experiment on the pressure effect, the gravity change is about 30 minutes later than pressure change and the coefficient is about -0.01 mgal/100 mmHg shown in Fig. 4. From experiences of the field survey, it seems that there are some problems in the vibration-induced drift of LaCoste & Romberg gravimeters.

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