Abstract
AbstractThe 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.
Highlights
Strain-steps associated with large earthquakes have been reported during the last few decades
During the last 10 years, in the Tokai and Kinki districts of central Japan, several crustal movement observatories were built by the Tono Research Institute of Earthquake Science (TRIES) and Geological Survey of Japan (GSJ), and stable continuous observations have been performed with a 20 bit A/D resolution, far superior to that of the previous resolution
The theoretical principal strain associated with the foreshock and the main shock at each observatory were calculated by the four fault models of the Geographical Survey Institute (GSI) (M-1 to M-4 in Table 3; Geographical Survey Institute, 2004) which are inferred from the co-seismic horizontal movements by GEONET (GPS Earth Observation Network; Miyazaki et al, 1997)
Summary
Strain-steps associated with large earthquakes have been reported during the last few decades. Kasahara (1974) and Okada (1975) investigated the space distribution of the co-seismic strain-steps observed with extensometers which are widely distributed throughout Japan, and compared the co-seismic strain-steps with the theoretical values from a dislocation model of the earthquakes, showing many strain steps which agree with the polarity of the theoretical value and some strain steps which do not agree. At the Toyohashi borehole observatory (TYH1) and the Ohara borehole observatory (OHR), belonging to GSJ, the instruments were installed in the pre-Tertiary sedimentary rocks and pre-Tertiary volcanic rocks, respectively At these borehole and vault-type observatories, crustal strains were measured by Ishii-type borehole strainmeters ASAI et al.: CO-SEISMIC STRAIN-STEPS ASSOCIATED WITH THE 2004 OFF THE KII PENINSULA EARTHQUAKES
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