Open Access

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

  • Yasuhiro Asai19Email author,
  • Makoto Okubo19,
  • Hiroshi Ishii19,
  • Harumi Aoki19,
  • Tsuneo Yamauchi29,
  • Yuuichi Kitagawa39 and
  • Naoji Koizumi39
Earth, Planets and Space201457:BF03352568

Received: 30 November 2004

Accepted: 28 March 2005

Published: 24 June 2014


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.

Key words

The 2004 off the Kii peninsula earthquakesstrain-stepprincipal strainIshii-type borehole strainmeterquartz-tube extensometermodification of strain field