According to recent surveys, it has been found that considerable earth's strain has been accumulated in the South Kanto District after the Great Kanto Earthquake of 1923. The strain has been deduced from the comparison of the present electro-optical distance measurement with results of revision triangulation after the earthquake. Considering the accuracy of both measurements, errors of strain are almost dependent on accuracy of triangulation. Hence, the authors have examined the accuracy of that revision triangulation. In this study, we have calculated the corresponding covariance matrix with regard to the revision triangulation in the ordinary and free adjusted network, and drived the size and direction of the major and minor axes of the error ellipses, and the standard deviation of adjusted side length joining two stations. In conclusion, the accuracy of the earth's strain is ±0.5×10^-5, in average, except the triangles containing Ooshima Island. The strength of triangulation network being insufficient around the Sagami Bay, it might be considered that the accuracy of the strain is comparatively large in this area; ±1.0-1.5×10^-5.

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This paper is the serial report concerning the continuous observation of crustal deformation at Inuyama since 1967. Digital records of crustal strain have been analyzed to separate the annual change of strain from the secular one. It has been found from the present analyses that the secular change in strain seems to be in accordance with both the horizontal displacement which was developed during the Mino-Owari earthquake in 1891 and the movement of geologic fault in the Quarternary epoch. The maximum shear strain rate developed by the tectonic stress in the NW-SE or W-E direction was found to be the order of 1×10_-6/year with the direction of about N10°W or N80°E . Geodetic data also show the same tendency. These facts suggest that the tectonic movement of the Quarternary epoch is still in the progressive stage. Some characteristics of the annual change in strain become clear. The clear and simple sinusoidal annual change is considered to be generated partly by the periodic thermal strain of rocks in the observational vault. However, most of the annual change which is amounting to 10_-6 in strain is thought to be generated by change of pore-water pressure in the fractured zone existing in the inner part of the vault. The mechanism of the annual variation in strain is explained by penetration of rain water into the fractured zone and discharge of pore-water out of it .

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Change in elevation of the fixed point at Aburatsubo tidal station is re-examined in detail. Before 1950, the leveling line in the Miura Peninsula was a single line (west line). In order to give condition to this line, a main circuit of Ko-F25-116-Ko (Fig. 1) is selected. Adjustment of this circuit gives the conditional elevation at bench mark F25. After 1952, the east line was constructed, and a circuit in the Miura Peninsula was made. In ordinary method, a net of two circuits (main and Miura Peninsula circuits) should be adjusted simultaneously. But separate adjustment is made by the following reasons. (1) Survey year of each circuit was not always the same. (2) Extraordinary large closure of circuit sometimes appeared in one circuit. In this case, simultaneous net adjustment gave wrong correction to other circuit. So first the main circuit is adjusted, and next, the circuit in the Miura Peninsula is adjusted by using the conditional elevation of F25. As the main circuit in 1950, Ko-35-70.1-92.1-116-Ko (Fig . 2) is adopted after examination of extraordinary large closure of circuit. In 1960, the main circuit was changed to Ko-35-10349-11054-Ko (Fig. 1) by the reason of route change of survey Further, after 1963, a circuit of including kilo-post bench marks in part was adopted, but the shape of the main circuit was nearly the same . Observed height differences of partial lines and closures of the main circuit are shown in Tables I, II-2, IV, V, VI, VII, VIII, IX, X and XI. In examining all surveys, orrection of ground subsidence was made for the survey in 1960.5, the survey in 1970.1 was abondoned by the reason of doubtful change (Fig. 3), and erroneous observation (Fig. 4) was corrected for the survey in 1970.9. The calculated elevations of F25 in all observed years, in which the datum station is fixed, are shown in Table X13III . Next subsidence of the datum station in the period from 1923 to 1972 is investigated. Using the apparent upheaval at several bench marks near the datum station (Table XII), the following values of datum subsidence are determined: This secular change is shown in Fig. 5. Now the change in elevation of F25 corrected by datum subsidence can be calculated, which is shown in Table XIII and Fig. 5. The secular change in elevation, that is, vertical movement of the crust at the bench mark of Aburatsubo tidal station is obtained after adjustment of the circuit in the Miura Peninsula in which the corrected elevations of F25 give condition. The final result is drawn in Fig. 6. This curve shows nearly linear subsidence of -3.0 mm/year after the Great Kanto earthquake of 1923. The present author believes this vertical movement is the best one among those obtained in the past.

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　海岸昇降検知センターの資料から平均海水面変化を調べてみると,八丈島東方沖地震前には,三宅島における平均海水面変化は,その附近の布良,小名浜,油壷などと比較して,甚だしく大きく変化しているのが見られる.そして,この変化の模様は1941年のリスボン沖大西洋の海底地震の際のリスボンにおける平均海水面の著しい変化とよく似ているのは面白いと思う.　つぎに,油壷の年平均海水面変化をみると,関東大地震,東南海地震および南海道地震前には急激に下っているのが,三陸方面や房総沖方面の地震のときには急激に上っているのが認められるので,油壷を中心として西方海域の大地震と東方海域の大地震とを区別して,また,年代によって地震の資料に対する信頼度が異なると考えて,西方のT₁,T₀,東方のT3と3つの周期を計算してみた・地震の規模については7.4以上のもののみをとって,T₁=(91±15)年,T2=(65±13)年,T3=(29±6)年を得た.なお,目下のところ,油壷の年平均海水面は上昇をたどっているから,関東大地震程度の大地震は起こらないものと推測される.また,三陸方面では,1933年以来,磐城沖地震,房総沖地震と規模7.5以上のものが3つも起こっているから,関東地方に被害を与える規模8程度の地震のエネルギーは少しずつ解放されているものと想像される.

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Precise gravity measurement had been carried out on the 1st order bench marks, where gravity difference from the Gravity Station of Geophysical Institute of Kyoto University (gravity value 979.721583 gal) is smaller than 1 mgal, along the levelling route of the Kinki District. Besides them, the 1st order bench marks with the gravity value 979.700 gal±1 mgal were newly added in due consideration of distribution of the stations . Two sub-stations were subjointly established near each bench mark for the precise gravity measurement in order to keep the stations for a long time and also to make clear the accuracy of gravity change measured. Gravity values were determined, as a first step, on those stations by using a single LaCoste & Romberg gravimeter G-196 in March and April 1973. The standard deviation of the present measurement was obtained to be about 0 .01 mgal. Precise gravity measurement is now in progress by means of 3 LaCoste & Romberg gravimeters .

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We estimate changes in the geoidal height associated with a land-deformation, assuming that the corresponding surface density disturbances are restricted to a cap of radius ψ₀ on the earth's surface.

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