The Mizusawa Astrogeodynamics Observatory (JPG 882K) performs tidal observations and absolute gravity measurements.
The former is made at the Esashi Earth Tides Station.(GIF 134K) Three quartz
tube extensometers (JPG 279K) and two water tube tiltmeters (JPG 279K)have regularly been used for observing the tidal and secular variations of the
ground movements since June, 1979. Strain observation at the middle
point of each extensometer began at the beginning of 1980 for
the purpose of checking reliability of the extensometer against
the strain steps caused by earthquakes. A borehole strainmeter
started observations in January 1985. In addition, atmospheric
pressure, rain fall and air temperature are also measured. Auxiliary
information on entrances into the observational tunnel, offset
of the signals, and occurrences of earthquakes, etc., is available
upon request in machine-readable forms. Development of the
transportable
absolute gravimeter (JPG 404K) started in 1979 and many improvements have
been made. The present accuracy of gravity values obtained with
this gravimeter is 2 Gals. A cryogenic super
conducting gravimeter (JPG 207K) was installed in 198x for
detecting tiny signals from the core of the Earth.
The Esashi Earth Tides Station (GIF 143K) is located on the north side of Mt. Abara.(JPG 743K) Details of the station were discribed by Tsubokawa et al. (1979). The coordinates of the entrance of the observation tunnel referred to the Bessell ellipsoid with the Tokyo datum point are as follows:
| Latitude | 39 08 53 N |
| Longitude | 141 20 07 E |
| Altitude | 393m above mean sea level |
All the observations are made with analogue sensors. The signals are recorded on dot point recorders at the station and simultaneously transmitted to the Mizusawa Astrogeodynamics Observatory via two NTT telecommunication data lines in digital form (Tsubokawa, 1989).
The digital data received at Mizusawa are recorded in two ways: (1) digital records sampled every one minute and every 10 minutes on magnetic tapes, (2) analog records converted from the sampled digital data with 1 sec sampling intervals. The converted analog signals are monitored on dot point recorders.
Bayesian Tidal analysis Program-Grouping
model (Ishiguro et al., 1983, 1984 and 1985; Tamura et al., 1989)
is used in the tidal analyses for the strain and tilt. Figures
3 to 6 show the results after removal of the effects of disturbances,
such as entrances into the tunnel and offset of the signals.
Three extensometers (JPG 279K)
are placed along north-south (NS), east-west (EW) and north-east
(NE) directions, respectively. The extensometer consists of a
quarts-glass tube, a supporting frame and a transducer. The tubes
are suspended in a similar way as that of a Benioff strainmeter.
One end of each tube is fixed on a bedrock and the other is kept
free. The instrumentation is summarized in the following table
and the details were given by Sato et al.(1980).
| Azimuth | N 00 .00 E | N 90. 00 E | N 45 .00 E | |
| Effective length(m) | Free end | 35.77 | 35.69 | 50.69 |
| Mid-point | 17.99 | 19.97 | 27.97 | |
| resolution(10-11) | Free end | 3 | 3 | 2 |
| Mid-point | 3 | 3 | 2 |
The displacements of the pedestals at the
free end (indexed by F) and the middle point (indexed by M) relative
to the fixed-end are measured with differential transformers with
primary signal of 5 kHz and 2.5 Vp-p. They are mounted on sliding
stages fixed to the pedestals. Drift adjustments and calibrations
of the transducer are performed by using a differential micrometer
installed on this sliding stage.
A Sacks-Evertson (Sacks et al., 1971) strainmeter was installed in October 1984, under the ground adjoining the east side of the recording room of the Esashi Earth Tides Station. Its depth is 102.15 m. A differential transformer attached to a bellows converts the pressure change of the oil inside a sensing tube of the strainmeter into an electric signal. The signal from the transformer is led to an amplifier. The output signal from the amplifier is recorded as a long period signal (DC component). Then the DC component is led to a high pass filter whose cut-off frequency is about 7 10-4 Hz. This filtered signal is recorded as a short period signal (AC component). Specifications of the instruments are summarized in the following:
| Instrument | Sacks-Evertson strainmeter |
| Transducer | differential transformer |
| Primary signal | 4kHz, 10Vp-p |
| Effective length of the sensing tube | 3.680m |
| DC resolution | 3.4x10-10 |
| AC resolution | 3.4x10-11 |
| Depth | 102.15m |
Details of the instrumentation were given by Sato (1986).
The observation with the strainmeter on a regular basis started in January 1987.
The calibration of the borehole strainmeter is carried out by heating the oil in the sensing tube with an electric heater. The oil expands in proportion to the intensity of the current fed to the heater. Usually the current intensity is set to be 500 mA, which causes a strain change of 9 10-8 in the sensing oil.
The data of the DC component is processed similarly as in the cases of the extensometers. The difference of elastic constants among the tube of the strain meter, the surrounding rock and the mortar layer between the tube and the rock yield output signals greater than true volumetric strain changes (Sato, 1986). The boring itself also has the similar effect. These, however, are not corrected for in the present report.
The specifications of the water tube tiltmeters (JPG 279K) given by I.Tsubokawa et al. (1981) and T.Tsubokawa (1989) are summarized as follows:
1) instrument: water tube tiltmeter with floats
2) transducer: differential transformer
primary signal 5 kHz, 2.5 Vp-p
3) water tubes:
| Instrument | water tube tiltmeter | |||||
| Transducer | differential transformer | |||||
| primary signal | 5kHz | 2.5Vp-p | ||||
| Water tubes | ||||||
| azimuth | N 00 .00 E | N 90 .00 E | ||||
| effective length | 36.32m | 36.24m | ||||
| inner diameter | 16.0mm | 16.0mm | ||||
| resolution | 0.02mas | 0.02mas | ||||
| diameters of the vessels and floats | ||||||
| vssel | 119.5mm | 119.6mm | 119.2mm | 117.7mm | ||
| float | 80.1mm | 80.2mm | 80.3mm | 80.4mm | ||
A calibration vessel is attached to each tube approximately at the middle position between the two vessels. Calibrations are performed by adding or subtracting water of known volume into or out of the calibration vessel (Sato et al., 1983).
Observed data obtained by the tiltmeter are processed in the same way as that for the extensometers.