Secular ground tilts and strains observed with tiltmeters and extensometers show sometimes characteristic changes which can fully be represented by exponential functions with time constants of several years or shorter. We estimate the time constants of such exponential secular changes on the results obtained from various stations in Japan, excluding changes with time constants shorter than several months. It is found that these exponential secular changes are classified into two groups; one has rather long time constants of about 5 years or more and the other has shorter time constants of about 2 years. The origin of the former is considered to be instrumental drift due to stress relaxation within instrumental materials and that of the latter being real ground deformations due to stress relaxation in rocks around observation instruments. The latter might have some connection with post-seismic crustal movements usually observed in epicentral regions.

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A three-component ocean bottom magnetometer has been constructed . Using this magnetometer a preliminary measurement of geomagnetic field was made at the sea floor of the Pacific Ocean during a cruise of R/V Hakuho-maru of the Ocean Research nstitute in July through August 1980. In this paper the design of the magnetometer and results from the measurement are reported.

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A statistical procedure to extract the drift component (d) from the record of Crustal movements is proposed. This procedure decomposes the data (y) into the form:yi=di+ri+ei, (i=1, 2, ..., n) (1)where ri represents the response to the luni-solar forces and other phenomena such as atmospheric pressure;; e; denotes the irregular component. The assumptions on each component are as follows: a) The time difference of appropriate order of the drift component is a zeromean Gaussian white noise with variance sd². b) The response to the associated phenomena is linear. c) Irregular component is a zero-mean white Gaussian noise with variance se², which is independent of the drift. Parameters, such as sd², se² and/or differencing order of the drift term are adjustable and chosen to minimize a Bayesian Information Criterion (ABIC). The present approach allows the existence of occasional steps of the original record and/or missing observations. The positions and amounts of the steps can also be estimated minimizing ABIC.

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Since the laser tracking system of the satellites was put into practice, the possibility of the detection of crustal motion by satellite observations greatly increased. It is, however, difficult to monitor a large area from ground-based stations because the number of the stations to be set up are generally too few compare with the extent of the ob jective area. Spaceborne laser ranging system opened a new horizon in the observation of crustal motion. In this system, a ranging equipment is mounted on a satellite and range measurements are done from the satellite to ground markers distributed in objective large area on the earth's surface. This technique enable to survey the position of markers in a short period of time within the accuracy of a few centimeters. The project using this system is in progress at NASA and the first test operation is scheduled by using Space Shuttle in near future. In Japan, there is no practical plan concerning the spaceborne laser ranging system. However, one of the authors proposed a system of this kind including the detection method of precise satellite orientation.

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This report surveys present status of VLBI projects on crustal deformation and tectonic plate motion, conducted and planned mainly in U.S.A. and Japan, together with some theoretical background of VLBI technique and various VLBI systems.

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Since August 1971, a continuous observation of crustal movement based on the short base-line length in the vault has been conducted at the Erimo Geophysical Observatory, Hokkaido University (this observation will be called COCM hereafter). The purpose of the observation is to obtain such a continuous and precise crustal movement near the observatory as we cannot observe by geodetic surveys. The COCM has been adopted as one of the most important subjects to be studied in the earthquake prediction research, because this observation could probably reveal premonitory phenomena of earthquakes [1]. In fact, the COCM is excellent as a practical method possible to watch continuous crustal movement. However, the COCM has still problems not only whether the results based on the COCM always agree with those to be obtained by the geodetic surveys or not, but also whether the data by the COCM are contaminated with any other disturbances due to the known or unknown factors or not. For the problems, the geodetic surveys around the observatory should be also carried out along with the COCM so as to observe the same quantities of crustal movement, and so as to reveal factors which would affect the data as noise. After careful comparison of the data obtained by both methods was made, discussion of the premonitory or abnormal crustal movement should be given. Under these conditions, the COCM had started at the Erimo Geophysical Observatory in August 1971, where the vault with a total length of about 160 m was constructed for three sets of the quartz-tube extensometer with a length of 30 m and two sets of the water-tube tiltmeter with a length of 30 m. At the time, the occurrence of an earthquake with a magnitude of about 8 had been forecasted off the Nemuro Peninsula in the near future on the basis of the seismicity gap conception by Utsu [2]. On June 17, 1973, the earthquake (Ms=7.7, NOAA) occurred just in the predicted area about 2 years after the beginning of the COCM at Emimo. Just after the event, there was the discussion about the possibility whether a large earthquake would occur again around there or not because the magnitude of the event was smaller than that estimated [3], [4], [5]. Until now, another earthquake in question has not occurred in the area. The 1973 earthquake was the largest one at Erimo during the observation period from 1971 to 1981. According to the data observed by the extensometers during this period an abnormal variation of Crustal strain was recognized only before and after this event. The summary of this phenomenon will be presented.

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Since 1976, precise leveling has been made every year in the limited area of 700×800 m² near Sakashita, Gifu-prefecture, cutting across Atera Fault . From this leveling, it was clarifyed that the scalar value of tilt vector during five years from 1976 to 1981 was below 1.5 micro radian. This fact suggests that the fault has been locked at present.

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The trajectories of maximum horizontal tectonic stress in the Japanese Islands are inferred from Quaternary tectonic movements based on geological and geomorphological investigations, present-day crustal movements obtained by geodetic surveys, and earthquake focal mechanisms etc. Tectonically active regions can also be predicted from the rate of crustal strain variation. The earth's crust, however, is not a homogeneous, isotropic, elastic solid, but exhibits various types of non-elastic behaviour . The geological structure of the Islands is very complicated and there are many active faults and folds . Therefore, high rates of strain variation observed in geodetic surveys do not always indicate large accumulations of strain energy in the crust, particularly in the Island Arc. Absolute stress magnitudes in the crust are required to define these values . For the investigation of crustal deformations, it is very important to clarify the stress state in the uppermost crust, especially three dimensionally . In some regions, the directions of principal stress obtained by focal mechanism solution of shallow earthquakes do not always agree with those of principal strains by geodetic surveys . Moreover, sometimes one of the principal stresses in the top layer of the crust is not vertical . This situation occurs characteristically in the tectonically active, and topographically steep, Islands.

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Near the converging plate boundary, the overriding plate might be presumed to be compressed in the direction of relative plate motion up near to the leading edge. However, the geodetically observed azimuths of the maximum contractive axes in the Tokai and Kanto districts, Japan show a systematic spatial variation, from almost E-W in the west to N-S in the east, whereas the plates converge in NW-SE. This area lies very near, within 100 km, to the converging plate boundary. We interpret this observation as an effect of the change in the strike of the plate boundary. An approximate relationship, (1+σ) tanφ=-1/tan 2θis derived for a thin elastic plate with Poisson's ratio σ, where φ and θ denote the direction of plate convergence and the azimuth of the maximum contractive axis, respectively, each measured counterclockwise with respect to the strike of the plate boundary. On the basis of the above formula, we found that the present secular deformations are consistent with the position and strike of the mechanical plate boundary for the past hundreds of thousand years, proposed on the basis of the submarine tectonic relieves and geologic structures in this region.

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The accuracy of satellite laser ranging reaches 2-3 cm level [1] and some transportable laser ranging systems were completed [2]. Using such systems the Crustal Dynamics Project of NASA started in USA. A preliminary result [3] of this project shows that the repeatability of determination of 10 baselines whose lengthes are between 258 and 3930 km by satellite laser rangings is 5 cm, and the maximum difference between baselines determined by satellite laser ranging and those by VLBI is 6 cm. A laser ranging system with the precision of 3 cm is introduced at Simosato Hydrographic Observatory (33°34'N, 135°56'E) of the Hydrographic Department of Japan by the end of February, 1982. In oder to treat ranging data obtained by these systems, an orbital data processor for improvement of orbital elements, station coordinates and some geophysical parameters was developed by the anther. Some results of a trial determination of baselines between laser stations using range data are presented in this report.

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