Shallow crustal seismic velocity variations in the Nagano region, Japan, imaged by ambient noise seismic interferometry

The magnitude 9.0 Tohoku-oki earthquake occurred on March 11, 2011, leading to a devastating tsunami, causing extensive damage and many casualties. It also triggered seismicity all over Japan, such as in the Nagano region, where the Mw 6.2 Northern Nagano earthquake occurred 13 hours later, or in the Mount Fuji area, where the Mw 6.0 East Shizuoka earthquake occurred on March 15. In this study, we focus on the Northern Nagano region, assessing the temporal evolution of the seismic wave velocity, using ambient noise seismic interferometry. By applying the same approach, we also estimate the seismic wave changes around Mount Fuji. Velocity changes can be caused by coseismic damage, so we compare these results with the distribution of Peak Ground Acceleration (PGA).

   The method used in this study consists in correlating continuous noise recordings to estimate the temporal seismic velocity variations in the medium. We use the NIED Hi-net three components waveform data from 110 stations to compute daily cross-correlation functions, which are then averaged with a moving window of 30 days. We retrieve the components of the Green’s function of the medium between pairs of stations, for all the component combinations. Using the wavelet cross-spectrum analysis, we estimate the seismic wave velocity variations of all stations, for the period between January-August 2011, for 0.1-0.9Hz. The same approach is also applied around Mount Fuji area. We thus obtain the spatio-temporal distribution of seismic velocities for both Northen Nagano and Mount Fuji regions. We compare these velocity variations to the distribution of the PGA observed for both regions. Overall, areas with larger velocity decreases experienced larger PGA during the mainshocks. Nevertheless, some other patterns can also be observed; for example, we observe anomalously large velocity drops for some stations located near volcanoes, in line with previous observations.

  Our ongoing work aims to estimate the velocity variations from two events in the Kyūshū region: the 2016 Kumamoto earthquake and the 2024 Hyūga-nada earthquake. We could then compare these non-triggered events, of smaller magnitudes, to events triggered by the Tohoku-oki earthquake, and this analysis may help identify similar patterns comparable with the Nagano area, especially near volcanoes.