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
monitoring programs. Strain is a secondary physical quantity whose variation varies depending on the elastic
coefficient of the medium, and it can yield potentially valuable information as well. This article describes the
development of a borehole stress meter that is capable of recording both continuous stress and strain at a depth
of about 1 km. Specifically, this paper introduces the design principles of the stress meter as well as its actual
structure. It also describes a newly developed calibration procedure and the results obtained to date for stress and
strain studies of deep boreholes at three locations in Japan. To show examples of the observations, records of stress
seismic waveforms generated by the 2011 Tohoku earthquake (M 9.0) are presented. The results demonstrate that
the stress meter data have sufficient precision and reliability.