Propagation Time of Seismic Waves at KiK-net Hino Observation Site during, before and after the 2000 Tottori-ken Seibu Earthquake

The 2000 Tottori-ken Seibu earthquake caused severe damages in Tottori and Shimane Prefectures. The KiK-net Hino observation site, operated by the National Research Institute for Earth Science and Disaster Prevention, is situated near the seismic fault. The vertical array records observed at the site provide valuable information about wave propagation at large strain level. In this study, temporal changes of propagation times of seismic waves were examined based on Normalized Input-Output Minimization (NIOM) analysis. These examinations revealed that (1) the S-wave propagation time increased from 0.197 s to about 0.35 s due to nonlinear behavior of soils; (2) gradual decrease of the propagation time of S wave was observed a thousand days after the mainshock, however, the propagation time was still larger than the value before the mainshock; and (3) the P-wave propagation time also increased due to the principal motions during the mainshock. Subsurface soil properties were also examined based on the above results and pointed out that (4) an abrupt decrease of shear rigidity to 20 MPa at strain of 3×10-3 during the principal motion of the mainshock, and gradual increase up to 100 MPa after the mainshock were observed, whereas (5) the Poisson ratio was about 0.46 to 0.47 and almost independent of strain level.