An ocean bottom tiltmeter is under development for the purpose of detecting the precursors to predict earthquakes. The precursors having the periods between several minutes and a month are the object of the observation by the tiltmeter. The real-time signals of the tiltmeter will be transmitted by FM-FDM system through the submarine co-axial cable. This cable will also serve as the power line. The tiltmeter will be laid on the ocean bottom. Two force-balanced pendulum type and a bubble type tiltmeters are used as sensors. The result of the long term stability test of the sensors shows the long term drifts are less than 2×10^-6 radian per month and that the reliabilities are within the order of 10^-7 radian. The attitude of the sensors is controlled by a forced gimbal with the clamp mechanisms using the worm gears. A quartz thermometer is installed on the wall of the gimbal to monitor the temperature changes that affect the signals of sensors. The apparatus is composed of the pressure vessel which contains the gimbal and the transmitter and the platform which supports the pressure vessel. The ocean bottom tiltmeter is expected to be settled at about 20 m depth point near the marine observation tower about 1 km off Hiratsuka in Sagami Bay. The observation on the ocean bottom will give us fundamental informations about the quality of the coupling between the ocean crust and the tiltmeter on the sedimentary layer of sand and silt, and about the effects on the tiltmeter of the oceanographical factors such as the changes of the water temprature and the bottom current.

View full abstract

Absolute measurements of gravity were carried out at Shizuoka University for the period from August 15 to August 20, 1980. As the observation site is located only 150 m apart from the Tomei Express Highway, ground vibration is predominant in a high frequency range (about 5 Hz). Because of the high-level ground vibration, the results obtained for each drop were scattered as much as ± 2.5 mGal. The mean gravity value obtained from thirty-three drops isg=979730.71±0.17 mGal. Some considerations show that the high-frequency ground vibration effects on gravity could cause scattering of ± 1 mGal even if ground vibration corrections were properly made. It becomes evident that such effects may be one of the most serious difficulties to be overcome for absolute gravimetries.

View full abstract

Two typhoons (T7916 and T7920) passed through the southern part of Kinki district in 1979. On these occasions, remarkable changes of ground-strains were observed with two components of laser extensometers which were installed in the underground tunnel of the Amagase Observatory. Assuming that the atmospheric perturbation caused by the passage of typhoon is a negative load acting on an elastic homogeneous and isotropic half-space, we calculated the change of strain-field around a cavity and compared it with observational results . As a result, observed values were one order larger than calculated ones. This suggests that observed ground-strains of the order of 10-7 at the time of the passage of typhoon were caused by the another source, for instance, the change of the water level of Amagase reservoir which exists near the observatory.

View full abstract

A surface ship gravity meter has been newly designed and manufactured by using a servo accelerometer as the gravity sensor, aiming at mechanical toughness and immunity from the ship's environments. This meter is characterized by its sensor, in which a proof mass is supported electromagnetically in order to balance the gravitational force. The platform where the gravity sensor is placed is kept horizontal by being remotely controlled by a vertical gyroscope installed beside. A servo accelerometer is usually so rugged that it can withstand the acceleration as large as ±10G. This device, however, is of the type analogue device, and so it has been thought difficult to attain a high accuracy level . The gravity sensor used for this gravity meter has been manufactured by solving most of the difficulties in virtue of selection of materials, high stabilization of electric voltage and efficient temuerature control. Tests both on land and sea showed that the gravity meter is accurate to 0.1-0.5 mgal order. This gravity meter has been manufactured originally for the purpose of installing on board the Ice Breaker “FUJI” that is used for the Japanese Expedition of the Antarctica. The name “NIPRORI” is taken after a combination of the abbreviations of the National Institute of Polar Research (NIPR) and the Ocean Research Institute (ORI) of the University of Tokyo.

View full abstract

For the purpose of investigation of conductivity anomaly of the ocean floor, an ocean bottom vector magnetometer consisting of fluxgate sensors was designed, and trial device was built. The philosophy of the design of the device is as follows; (1) geomagnetic main field is cancelled by rotating permanent magnet, (2) temperature effect is compensated in off-line data analysis by use of temperature data observed simultaneously, and (3) electric power is supplied intermittently to save power. Trial device of single component was tested at Kakioka Geophysical Research Laboratory. The result obtained was in good agreement in detail with that obtained by Kakioka Standard Magnetometer.

View full abstract

In deducing crustal displacement vectors from geodetic data with two or more surveying epochs, selection of some fixed stations and accumulated survey errors determined by net-adjustment are the most important problems. In this paper, methods by which fixed stations should be selected are proposed as follows; (1) The fixed stations should be within the area where tectonic movements are as small as possible. The tectonic movements may be able to checked by tentativelly calculated shear strains. (2) If the fixed stations are selected far from the discussed area, survey errors will be much accumulated. On the other hand, location of the fixed stations should be out-side of the above-mentiond area. For instance, the 1923 KANTO earthquake was as large as M7.9 and supposed TOKAI earthquake will be as large as M8.0 respectively, then the areas influenced by these two earthquakes are as large as about 104 km2 res-pectively. The fixed stations shoud be outside of this areas. (3) Location of the fixed stations should be selected considering the tectonic move-ments. As drifting of the Philippine Sea plate is supposed in the direction NNW, the displacement vectors may be neglected when the fixed stations line is selected parallel to the direction of the plate motion, but when the fixed stations line is selected perpendi-cular to the drifting of the Philippine Sea plate the movement may be ref rected in the calculated displacement vectors. In discussing crustal movement of KANTO-TOKAI district, we select the fixed statians TSUKUBASAN, TERUISHIZAN, DODAIRAYAMA, KOKUSHIDAKE II, KAIKO-MAGADAKE and ENASAN on the basis of above mentioned criteria. The fixed stations line is selected perpendicular to the direction of the Philippine Sea plate motion and located about 150 km far from the convergent plate boundary of the Philippine Sea plate.

View full abstract