ABSTRACT
Though seismic precursors surely exist, it is considered that they are very small in comparison with co-seismic signal caused by the earthquake. However, observations are mostly performed on the surface of the earth where precursory signals are disturbed by the artificial noise and so on, and may not be able to be detected. One possibility to solve these problems is to make observations in deep underground of bedrock where earthquakes occur.
Being based on this idea we have developed multi-component borehole instrument including multi-component strainmeters with high sensitivity, tiltmeters, seismometers, accelerometers, a thermometer and a gyro for earthquake prediction study. The instrument has a diameter of 10 cm, and is 150 - 500 cm long, depending on purposes and combinations of sensors.
For earthquake prediction study the multi-component borehole instruments were installed into boreholes at depths from 150 m to 800 m in seismically active areas. They are recording good data relating to earthquake activities and crustal deformations.
One of the instruments was installed in a borehole 150 m deep in the eastern part of Izu peninsula where sometimes earthquake swarms occur and the ground is uplifting at a rate of about 2 cm/year. The instruments recorded precursory tilt and strain variations before earthquake swarms. Precursory phenomena indicate tectonic influence and it was clarified that multi-component observation in deep borehole is useful.
We have next developed the strainmeters for rock mechanics and civil engineering purposes. For this purposes we have developed a wireless intelligent type strainmeter for the measurement of in situ rock stress. The instrument includes SRAM, RTC, A/D converter, MPX, a microprocessor, and batteries in addition to strainmeter sensors. We can install it in deep boreholes and easily take it out by overcoring as the instrument has no electric cables outside. We can determine initial stress of deep position by doing so.
