Dai-Shimizu Tunnel is the world-longest tunnel recently constructed for a bullet-train railway. Underground survey was conducted by two LaCoste and Romberg gravimeters G 210 and G 581 on June 4 and 5, 1982, This paper describes the outline of the gravity survey and the reduction procedures including terrain correction. The apparent density is determined as 2.04 g/cm³ by using the gravity-elevation relationship. The Bouguer anomaly is closely related to the geologic structure of the region. A negative anomaly is located on a massive quartz-diorite which forms extensively the mountain range, and a positive one on basic rocks in the northern half of the tunnel.

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Formulas in [1] and their related theorems are strictly proved with the application of the generalized inverses.

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Formulas for a station adjustment are constructed through the generalized inverses of a matrix; in particular, a formula A⁺=(1/u)A^T is strictly proved, in which A indicates a design matrix in the adjustment, u the number of columns in the design matrix.

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Nihonkai-chubu Earthquake (M7.7) of May 26, 1983, generated tsunami on the whole area of Japan Sea. Crustal tilt and strain by the load of the tsunami were successfully observed at the Yahiko crustal movement observatory by using two water-tube tiltmeters and three fused-silica extensometers. Based on the result of a Fourier analysis, we conclude that the free oscillation of sea-surface has periods of 45 and 18 min.

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Taiwan is located along the north-west margin of the Philippine Sea plate and has a complicate tectonics. To clarify the tectonics in this region, secular horizontal strain in eastern Taiwan was obtained from the data of recent retriangulation. The strain along the Longitudinal Valley, a probable collision boundary between the Philippine Sea and the Eurasian plates, has a large strain rate of 2 to 3 micro-strain/yr and the axis of maximum compression intersects the valley with an angle of 60°, suggesting that the boundary has a component of left lateral movement. On the other hand, the strain pattern of north-eastern Taiwan shows a marked contrast to that along the valley. The predominant strain is extensional one there, and its axis has a direction of nearly E-W, This suggests a right lateral movement on the plate boundary extending from the northeast coast of Taiwan to the southern end of the Ryukyu Trough.

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We made for the first time absolute gravity surveys at the Aobayama Earthquake Observatory, Tohoku University, for the period from January 30 to February 7, 1984, using a transportable absolute gravimeter completed at the International Latitude Observatory of Mizusawa. A cat's eye falls about 30 cm in vacuum and a Michelson interferometer, which is illuminated by an iodine stabilized He-Ne laser, measures its falling distance every 1 ms. Timing signals are synchronized with a rubidium frequency standard. The inside of the vacuum chamber, in which a dropping mechanism is installed, is kept at a high vacuum better than 2.6×10^-5 Pa. Piezo electric ceramics, which are attached to the feet of the absolute gravimeter, cancelled ground vibrations in order to stabilize the Michelson interferometer. The rotation of the cat's eye around its horizontal axis during free fall was monitored by an autocollimator and a two dimensional position sensor, and was proved to be within 10 seconds of arc. The arithmetic mean of results obtained by the present absolute gravity measurements was approximately 8.5 × 10-7m/s² (85 μgal) smaller than that obtained by two LaCoste and Romberg gravimeters referring to the IGSN 71 system.

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