An earthquake cannot be observed near the hypocenter as it occurs at a significant depth underground. Accordingly, an earthquake is observed on the surface of the earth, where signals of the earthquake will be disturbed by artificial noises and other factors. Therefore, it is difficult to detect weak signals, including precursory movements. A possible solution to such problems lies in installing a monitoring instrument in the deep bedrock where S/N ratio is improved much better. Based on such an idea, we have been developing a multicomponent borehole instrument (a comprehensive crustal activity observation instrument) that is capable of observing crustal activity at significant depths for predicting earthquakes. This instrument can be equipped with highly sensitive stress meters, strain meters, tilt meters, seismographs, accelerometers, thermometers, and declinometers, and it is capable of observing multiple components of various types of signals. The stress meters were recently developed and can observe both stress and strain as reported in a previous paper about development of stress meters.
Stress and strain meters recorded seismic waveforms generated by the 2011 Tohoku earthquake (M9.0), which occurred on March 11, 2011, and provided useful data for research. In addition, favorable stress waveforms and strain waveforms of an earthquake that occurred in Chile (M8.7) on April 2, 2014 were also recorded．The epicentral distance is approximately 15000 km from the observation point. The tidal movements of the earth were also well recorded by the stress and strain meters.
This paper examined the reliability of the data observed by these instruments. We obtained invariants in elastic dynamics from earthquake waveforms and the earth tide recorded by the stress and strain meters and checked whether the invariants satisfy elastic invariant theory. The analysis revealed that the invariants obtained by the stress and strain meters were in good agreement with the theory. We concluded from these results that the developed stress and strain meters used for observation have been properly installed in the bedrock and have adequate accuracy and reliability.
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