Tsuneo Yamauchi

Co-seismic strain-steps associated with the 2004 off the Kii peninsula earthquakes-Observed with Ishii-type borehole strainmeters and quartz-tube extensometers

The 2004 off the Kii peninsula earthquakes (Mj7.1 and Mj7.4) occurred at the Nankai trough, on 5 September 2004. Clear strain-steps associated with these earthquakes were observed with Ishii-type borehole strainmeters and quartz-tube extensometers in the Tokai and Kinki districts. We investigated the spatial and depth distribution of the observed principal strain changes and compared the observed strain-steps and theoretical calculations at all observatories. The following results were obtained: the observed strain-steps at all observatories are generally consistent with the polarities of the theoretical values, and the observed strain-step increases with depth at the same place. As for the Togari site, the following relationships are obtained: the strain-step and the tidal strains increase with depth and increasing of the modulus of elasticity, namely, hardness of rock. We consider that the geological structure around the observatory may cause a modification of the strain field.

In situ stress measurement by the stress relief technique using a multi-component borehole instrument

We have developed a two-dimensional method for calculating principal stresses on a plane by measuring strain changes obtained by the stress relief technique using a multi-component borehole instrument. If the ratio of the radius of the core to that of the borehole is 1.4–2.0, a uniform strain change occurs in any stress field due to the overcoring. It is therefore desirable that the radius of the core obtained by overcoring is larger than that of the borehole by a few orders of magnitude. We applied the proposed calculation method to a stress measurement performed at the Hishikari mine, southwest Japan. The maximum and minimum principal stresses were determined to be 5.0 and −0.2 MPa, respectively. The nearly vertical stress was estimated to be 1.5 MPa, which was merely 30% of the predicted vertical stress (5.6 MPa). One of causes of the small vertical stress obtained in this study is considered to be the limitation of the two-dimensional method for calculating principal stresses.