Test Observation of Crustal Strains by Two Extensometers with Super-Invar Rods of Different Coefficients of Expansion

Abstract

We carried out test observation of crustal strain by two extensometers on common bases in an observation vault of Takajo observatory at Miyazaki prefecture in Southwest Kyushu, Japan. These extensometers are composed of two different type invarrods, one of which is conventional super-invar (S-I) and the other is new super-invar whose coefficint of expansion is extremely lower than that of conventional one. The new S-I extensometer is less influenced by temperature changes in the vault. Especially in the periods showing large drift after installation of instruments and for the duration of rainfall disturbances, when the air temperature in the vault changes rapidly, the availability of the low coefficient of new S-I reveals evidently. Coefficient of linear expansion of both materials were numerically estimated from the observation data in the period of initial drift and from the heating experiments data. The coefficient of expansion of conventional S-I is about 1×10^-6/°C and that of new S-I is less than 1×10^-7/°C, sign of which changes according to the temperature.