On Viscoelastic Changes Appearing on Tiltmetric and Extensometric Records of the Ground

Abstract

Secular ground tilts and strains observed with tiltmeters and extensometers show sometimes characteristic changes which can fully be represented by exponential functions with time constants of several years or shorter. We estimate the time constants of such exponential secular changes on the results obtained from various stations in Japan, excluding changes with time constants shorter than several months. It is found that these exponential secular changes are classified into two groups; one has rather long time constants of about 5 years or more and the other has shorter time constants of about 2 years. The origin of the former is considered to be instrumental drift due to stress relaxation within instrumental materials and that of the latter being real ground deformations due to stress relaxation in rocks around observation instruments. The latter might have some connection with post-seismic crustal movements usually observed in epicentral regions.