Although precursory signs of an earthquake can occur before the event, it is difficult to observe such signs with
precision, especially on earth’s surface where artificial noise and other factors complicate signal detection. One
possible solution to this problem is to install monitoring instruments into the deep bedrock where earthquakes
are likely to begin. When evaluating earthquake occurrence, it is necessary to elucidate the processes of stress
accumulation in a medium and then release as a fault (crack) is generated, and to do so, the stress must be
observed continuously. However, continuous observations of stress have not been implemented yet for earthquake
Observing precursory phenomena of earthquakes is difficult;
however, a case in which precursory fluctuations in strain
and inclination were actually observed does exist, although
the observations were discovered after the earthquake. This
case was related to the swarm earthquakes that often occur
in the Izu Peninsula, which is located approximately
100 km southwest of Tokyo (Ishii et al. 1998; Okada et al.
2000; Ishii et al. 2002). The eastern part of the Izu Peninsula
is being uplifted at an annual rate of about 2 cm/year. One
of the borehole meters that we developed is installed in a
deep borehole in the eastern Izu Peninsula. This meter
recorded precursory inclination and strain fluctuations
before the occurrence of the swarm earthquakes. It
was discovered that the precursory phenomena were
indicative of tectonic effects and that multi-component
observations in deep boreholes is an effective technique
for evaluating earthquake occurrences.
Several types of strain observation meters are used in
boreholes. To date, volumetric strain meters (Sacks et al.
1971) have been installed at more than 30 locations in the
Kanto and Tokai regions by the Japan Meteorological
Agency. This type of meter can be used to observe
changes in volumetric strain only. The principle of the
meter is to convert the movement of silicone oil sealed
inside a cylindrical vessel into changes in strain. Sakata
(1981) developed a three-component strain meter by
applying a Sacks-like principle. These meters are sensitive