Interpretation of Periodic Changes in Rock Resistivity Observed at Aburatsubo, Central Japan

*Olivier Gensane[1] ,Hisashi Utada [1]
Earthquake Research Institute, University of Tokyo[1]

In the underground cavity of Aburatsubo, central Japan close to the sea,the ground apparent resistivity is recorded since 30 years. It wasobserved resistivity variations at tidal frequencies and coseismic jumpsof the resistivity (Yamsaki, 1965, Rikitake and Yamasaki, 1969). Theseobservations are understood as a result of crustal deformations, and arereported like this in several publications (e.g Park, 1993). Thesepresentation want to pay attention to this interpretation. From a detaildata analysis of resistivity and related parameters (crustal deformations,tide, temperature and atmospheric pressure) and a review of the morerecent publications (Yoshino et al, 1998, Utada et al, 1998, Holschneideret al, 2000, 2001), we can object some doubts to the role playing by thecrustal deformation on the observations described above. We show theimportance of the watertable level variations induced by oceanic tide onthe tidal variations of the resistivity, the influence of atmsophericpressure and ground temperature on the resistivity changes over periods of1 day. A numerical experiment which takes into account heat and waterdiffusion in rock allows us to clarify quantitatively the contributionof each parameters. As a result of this study, we can translate annualresistivity variations in another physical parameters (like watersaturation). Then, following Utuda et al, 1998 and holschneider et al2001, we propose to take into account the influence of the perturbationinduced by seismic waves on diffusive processes (heat, water) to explaincoseismic jumps of the resistvity. From data analysis and this model, weshow the relation resistivity-strains linear at small strain (<10^{-5}), unlike it is generally reported.