The Iwate‐Miyagi Nairiku earthquake (14 June 2008, M 7.2, depth 8 km) caused severe damage in the southern part of Iwate and the northern part of Miyagi Prefectures in Japan. The KiK‐net Ichinoseki‐Nishi vertical array site (IWTH25), operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), is situated near the epicenter. The observed peak ground accelerations at the surface in the up–down, east–west, and north–south directions were, respectively, 3866 cm/s2, 1432 cm/s2, and 1143 cm/s2. The extreme ground motions recorded at this site provide valuable information about the wave propagation and soil degradation at a large strain level.
We examined the nonlinear soil behavior at the site due to the earthquake on the basis of normalized input‐output minimization (NIOM) analysis. First, we analyzed 279 records observed before the Iwate‐Miyagi Nairiku earthquake and estimated the vertical propagation time (initial value) between the bottom of the well and the ground surface by taking into account incidence angles. Then we analyzed the records of the Iwate‐Miyagi Nairiku earthquake and 179 events that occurred after it (from 14 June 2008 to 19 October 2009). The results revealed that (1) the S‐wave propagation time increased from 0.265 s to about 0.35 s because of the nonlinear behavior of the soil; (2) this propagation time change corresponded to a 60% reduction in the shear moduli of the upper soil layers (surface to 64‐m depth) and a strain level of 1×10−3; (3) a gradual decrease in the propagation time with decreasing seismic intensity was observed soon after the principal motions; however, (4) the propagation times in the coda part and for the events after the Iwate‐Miyagi Nairiku earthquake were still larger than the initial value, indicating that the shear rigidities of soils remained less than their values before the earthquake for more than a year.