Some triangulation stations located in the northern part of the Izu Peninsula, Japan, were moved to SSE direction by more than two meters during the 1923 Kanto earthquake. These movements were generated by faulting along the Sagami-Bay fault and the Nishi-Sagami-Bay fault. Succeeding to this event, the 1930 Kita-Izu earthquake occurred. Around the occurrence of the earthquake, anomalous crustal movements were associated during pre- and post-seismic period. Parameters of a creep dislocation model for the observed anomalous crustal movements during the period 1931-1933/1934 are given as follows;strike of fault φ=N7°E, length of fault L=20 km, width of fault W=30 km, dip angle δ=45°, dept of upper side of fault plane H1=4.2km, depth of lower side of fault plane H2=25.5km, dip slip Ud=43cm, left-lateral strike slip Us =43cm. A similar creep dislocation might precede the occurrnce of the 1930 Kita-Izu earth-quake, and additional strain given by a creep dislocation triggered the 1930 Ito earthquake swarm. Migration of the seismic activities from 1923 to 1930's may be mediated by creep dislocation at the depth of the thrust along the East-off-Izu tectonic line.

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In the previous report, we deduced the horizontal earth's strain preceding the 1923 Kanto earthquake in the west part of the Tokyo Bay, Japan, by analysing the old survey data of the second and the third order triangulations and found the anomalous maximum shear strain in the area. In this paper, we give the strain before the 1923 Kanto earthquake in the other Kanto district. The supplemental stations of the first order trigangulation (average distance between the two stations is 25 km) were surveyed in the west part of the Kanto district during 1883-1885 and in the east part of the district during 1897-1899, while the first order triangulations (average distance between the two stations is 40 km) were carried out in the district during 1890-1892. We calculate horizontal strain for some first order triangles using the results of these two sequences of observations. We can not find any anomalous horizontal strain during the seven years before the 1923 Kanto earthquake in the Kanto district except in the west part of the Tokyo Bay. The survey errors are larger than the reasonable strain in the other region. The authors give an explanation for the detected precursory strain.

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In Geographical Survey Institute (GSI) in Japan a few kinds of solutions [1] [2] [3] have been used in adjustment of any geodetic free networks by means of the method of least squares. The defect of rank in normal equation can be easily determined by con-sidering how many geodetic quantities we need at least in order to make it fix on the surface of the reference ellipsoid. When observations are only horizontal angles and directions in a geodetic network, that is pure triangulation, we can know its shape alone. Innumerable geodetic figures with the similar shape are able to exist for capable solutions in such a case, because we have no information of its size and azimuth. When longitudes and latitudes of any two stations in the network are given, it is fixed on the reference ellipsoid. Accordingly the defect in the normal equation in such a case is obviously four. If kind of observation is only length of side, that is pure trilateration, the defect will be three, because we can fix the network by giving a station its position (longitude, latitude) and the direction of a side its azimuth. The defect is reduced to three from four when some sides or azimuths are additionally observed in pure triangu-lation, because either size or rotation of the network is restrained from being free. As a matter of course the defect is two in such a network in which its shape, size and azimuth are known through observations, but its location is still unknown in spite of the most perfect observations. In other words, location of the adjusted network is essentially free, and there are infinite possibilities in the ways of putting it upon the original one which is made up of approximate coordinates given for every station at the beginning of the computation. If those approximate coordinates indicate old positions of the stations decided in the previous surveying, the difference between the approximate position and the corresponding adjusted one for each station will give its displacement-vector Vi. Out of infinite locations of adjusted network a desirable solution can be settled according to an appropriate condition that should exist among those displacement-vectors. Now in GSI in the Universal Program using ellipsoidal coordinates (longitude, lati-tude) for any geodetic network the both conditions ΣV=0 and ΣV2=min. are actually used. It is very important that we pay attention to the fact that the free network solution of ΣV2=min. is universally available for any geodetic networks, but on the contrary the utilization of V=0 is restricted in such a free network alone in which the defect in normal equation is two. The reasons are as follows. When the network can freely rotate on account of no observation of azimuth, if we assume an azimuth in it we will be able to find one location of the network satisfying ΣV=0 while we are making it move around keeping the azimuth. When the network can freely dilatate or contract on account of no observation of side-length, even though azimuths are observed, we can still find one location of the network satisfying ΣV=0 for every assumed size. As a matter of course infinite solutions satisfy ΣV=0 in both cases mentioned above. Therefore we cannot use the method of ΣV=0 except the cases in which the defect is two. The Universal Program has utterly taken the above into consideration.

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This report outlines the Japanese activities in the field of gravimetry for the period from 1 July 1974 to 30 June 1978. It has been prepared for submission to the International Gravity Commission of the International Association of Geodesy at its Eighth Meeting to be held in Paris (September 1978). The report was compiled by the editor from accounts submitted by various Research Institutes and University research groups. The editor is grateful to Drs . I. YOKOYAMA, K. HOSOYAMA, Y. HAGIWARA, T. HAYASHI, A. SINZI, J. SEGAWA, J. CHUJO, J. NAKAI, Y. KONG, E. ABE, K. ISHIHARA and M. SATOMURA for their assistance.

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