The 1891 great Nobi earthquake is considered to have been the greatest (M-8) intraplate earthquake in Japan in the past 100 years. It provides much information on the long-term prognosis of earthquakes in this district which is affected by postseismic movements. The postseismic movements on and near the ruptured fault zone (the Neodani-Umehara fault) in the period of almost 100 years since the 1891 quake have been inves-tigated by analyzing the data from repetitive leveling and triangulation surveys ever since 1889 in the south Chubu district of central Japan. The postseismic vertical movement due to aseismic slip on and near the edge of the south-eastern section of the ruptured fault zone continued to subside for about 80 years since the occurrence of the quake, but in this last 20 years, the movement has changed to an uplifting movement. That is, following the final stage of o-phase due to aseismic slip in the phase cycle of seismic crustal movement in this district, the first stage of a-phase started around 1970. Two zones showing relatively large maximum shear strains, were detected along the main fault and along the direction of a curved line from WSW to E nearly perpen-dicular to the main fault. The latter zone seems to agree with the leading edge of the Philippine sea plate. These facts show that the strain energies were accumulated elastically by postseismic slow deformation in the period of about 80 years after the 1891 quake and mostly released by generating earthquakes in the periphery of the fault. A non-local anomalous upheaval around Ookuwa-mura near the Atera fault was detected from the leveling data during the period from 1966-7 to 1984 just before the Western Nagano prefecture earthquake (1984 M 6.8). It is necessary to conduct further investigation especially around the Atera fault or Mt. Ontake to comfirm whether the upheaval was released by the earthquake or not.

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The Survey of Kenya and the author carried out the first order levelling near the Rift Valley in Kenya after twenty five years' absence. As a result of the levelling we deduce that the bottom of the Rift Valley have been sinking at the rate of 12 mm/year.

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An international gravimetric connection between Japan and China as well as domestic ones in both countries were carried out during the period from October 1985 to February 1986 by means of four LaCoste & Romberg gravimeters (G-196, G-305, G-484 and G-605) for the purposes of (1) calibrating scale values of each gravimeter employed, (2) precisely determining a gravity value at each measured station occupied, (3) establishing a precise gravity net composed of 24 gravity stations in the standard baseline field of gravity surveys in Lushan Mountains recently arranged by the Chinese authority, (4) experimentally reconfirming specific characteristics of mechanical sensitivity of LaCoste & Romberg gravimeters revealed on some gravimeters through the international gravimetric connections along the Circum-Pacific zone performed in 19791982, and (5) clarifying dependencies of gravimeters, from data obtained under various measurement conditions, on disturbing environmental effects. For the domestic gravimetric connections in China, three other LaCoste & Romberg gravimeters (G-793, G-808 and G-818) belonging to the Seismic Observation Brigade, State Seismological Bureau, took part in the present investigations. These purposes have successfully been attained, although the present article only deals with the purposes (1), (2) and (3) among them. The methods of data analyses followed to those developed through the data analyses for the international gravimetric connections along the Circum-Pacific zone, except for newly taking periodic errors (inequality of a gear system) into account. The scale values of three gravimeters (G-196, G-305 and G-484) participated in the Circum-Pacific gravimetric connections have been rerevised by the present inves-tigations, although the amounts of rerevision were very small. For the others, their scale values were revised by the present investigations for the first time. Taking advantage of these revised scale values and also taking the periodic errors into consideration, gravity values at all the measured stations, including those in the standard baseline field of gravity surveys in Lushan Mountains, have precisely been determined so that these stations will contribute to calibrate scale values of the other gravimeters and/or to examine a possible gravity change in the future. It is emphasized that the information on both the scale values and the periodic errors for each gravimeter is essentially important to a high precision gravimetry, because errors arisen from these sources are neither negligibly small nor random.

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A formula to obtain an atmospheric correction that is required in the electro-optical distance measurement (EDM) under any atmospheric conditions is deduced, and tested in the field observations. The present method is a modification and extension of the one developed by ANGUS-LEPPAN and BRUNNER considering the three atmospheric regions proposed by ANGUS-LEPPAN. These three regions are classified according to the Obukov length that is calculated from the mean height along the light pass and other suitable parameters. The usual correction by using mean temperature and pressure obtained from the observations at the both end points is called normal correction and the new one A-B correction. Test observations are carried out in two networks with short range (3-4 km) and moderate range (several-10 km) side-length. The results of this study are:(1) When we define efficiency ratio as η=[{(s.d. of normal correction)-(s.d. of A-B correction)}/(s. d. of normal correction)]×100, η is several percentage under short range side-length, and(2) The method cannot be applied to the side-length more than 10 km.

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If the earthquake phenomena are caused by the external force as the result of the large scale internal activities of the Earth which are steady and continuous in the geophysical time scale, such phenomena should indicate the progress of time in considerably certain accuracy in the crust and the uppermost mantle of similar physical properties. In this point of view, the writer reported several papers. In the present paper, the writer arranged the points of arguments of the former reports and established the following five a priori hypotheses: 1) Gutenberg-Richter's formula of N. 2) The precursor duration time T is inversely proportional to N. 3) The general idea of the earthquake province-domain is proposed, and in one province NT=1. 4) Giant earthquakes (>8.05) occurr sequentially. 5) The maximum scale of giant earthquakes Mmax=8.40.4) and 5) are partly established by evidencies.These hypotheses must be a posteriori verified. In the Part 1 of this report, the writer examined the erathquakes occurred from 1600 A. D. to the recent times near the main land of Japan, and the earthquakes along the Circum-Pacific zone from Kamchatka to Taiwan in the recent period. In the results, he attained the following conclusions: a) The activity of large earthquakes seems to begin after a quiet period of 30 or more years and ends in an average cycle of about 117 years±several years. b) At the end of the cycle, the relation NT =1 is concluded quite accurately. c) Giant earthquakes larger than 8.05 play the lead in large earthquakes, and the earthquakes of M8.05-7.45 display the supplementary activity. The total energy of giant earthquakes seems to be more than 60% of the total energy of earthquakes in the active seismic zone.

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The earth strain has been observed simultaneously by strain gages and extenso meters in the tunnel of the Nokogiriyama Crustal Movement Observatory during the period from January 1986 to March 1987. The data processing is made by the least square method and the BAYTAP-G program. The strain response to atmospheric pressure changes are much affected by the topography over the observational tunnel. Tidal factors obtained by extensometers are 210 times greater than those obtained by strain gages.

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This paper demonstrates magnitude of the systematic distortions of horizontal control net when the geoid is neglected in projection computation to a reference ellipsoid. To see the geoid negligence effects, two adjustments are carried out with/without geoid information for the north-east portion of Japanese trilatelation network. The geoid reported by R. H. RAPP is used for the projection computation. Results of a comparison of those adjustments as well as the NNSS and conventional positions are described. It is concluded that the geoid undulation should not be neglected in trilatelation adjustments. A large local rotation of HOKKAIDO district in the conventional net is found.

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