Coseismic velocity change in and around the focal region of the 2008 Iwate-Miyagi Nairiku earthquake

The 2008 M7.2 Iwate-Miyagi Nairiku earthquake occurred in NE Japan where dense seismic networks exist. Using the data from these networks, we determined the coseismic seismic velocity change associated with this earthquake by two different methods: ambient noise interferometry and vertical array interferometry of coda wave. The purpose of this article is to reveal the spatial distribution and the cause of the velocity change by integrating these two approaches. Ambient noise interferometry revealed a coseismic velocity decrease of Rayleigh wave by 0.1-0.5% at 0.25-0.5 Hz. We also estimated the spatial distribution of the velocity change by a tomographic inversion. The velocity decrease was distributed in and around the focal area. In the second method, we applied cross-correlation analysis to coda waves observed by the KiK-net vertical array. We detected a shear velocity decrease of approximately 5% in shallow layers up to a few hundred meters depth. Quantitative comparison of the two results reveals that the 5% shear velocity decrease in shallow layers can explain the 0.1-0.5% decrease of Rayleigh wave velocity. The distribution of the velocity decrease is similar to that of the strong ground motion and the static stress change. However, a significant velocity increase is not observed in the compression area, where a velocity increase is expected. Based on the observations, we consider that the primary factor affecting the velocity change is damage in shallow layers due to strong motion. The effect of the static stress change might be masked by the larger effect of the strong motion.