Strain Variations of the Yamasaki Fault Zone, Southwest Japan, Derived from Extensometer Observations. Part 2 --On the Short-term Strain Variations Derived from Strain Steps--

Abstract

Coseismic strain steps during the Yamasaki fault earthquake sequence and some other distant large earthquakes have been observed with the extensometers at the Yasutomi Observation Vault of the Yamasaki Fault Observation Station. Usually, the amplitudes of those strain steps are larger than those theoretically expected and the polarities of strain steps also disagree with theoretically expected ones. The directions of the principal strain axis of each strain released by strain steps were frequently normal or parallel to the fault strike. Therefore, the pattern of the strain release strongly depends on the neighboring structure, namely, existence of the fault. In order to confirm this mechanism, a mechanical model for numerical simulations is presented. The results of these simulations could explain reasonably well the characteristics of the observed strain steps. The rather large amount of strain released by strain steps is considered to be the strain which has been previously accumulated in and around the observation site and is released by seismic waves. In the case of the Yasutomi Observation Vault, this kind of strain accumulation is originated by thermal deformation of the basement rock caused by surface temperature variations.