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    Solid earth sciences

    Temporal change in seismic velocity associated with an offshore MW 5.9 Off-Mie earthquake in the Nankai subduction zone from ambient noise cross-correlation

    Ikeda T, Tsuji T

    Nankai trough, 2016 Off-Mie earthquake, Monitoring, Ambient noise, Seismic interferometry

    (a) Temporal variation of the average seismic velocity in each node of DONET. The vertical dashed lines show the 30-day time window influenced by the mainshock. (b) Spatial and temporal seismic velocity variations estimated using cross-correlations from 1 to 30 April 2016. Squares represent the locations of nodes, and their colors represent averaged velocity changes at each node. Circles represent the locations of DONET stations, and their colors represent velocity changes at each station. Colored lines represent velocity changes between each station pair. The star marks the epicenter of the 2016 Off-Mie earthquake, and red dots are the epicenters of earthquakes in the period from 3 h before to 48 h after the mainshock (Wallace et al. 2016). The velocity changes relative to averaged pre-earthquake values are displayed.

    The Nankai subduction zone off the Kii Peninsula, Japan, has a large potential to generate megathrust earthquakes in the near future. To investigate the temporal variation of stress or strain in the Nankai subduction zone, we estimated the temporal variation of seismic velocity by using cross-correlations of ambient noise in the frequency range 0.7–2.0 Hz, which was dominated by ACR waves, recorded by the DONET offshore seismic network from 1 October 2014 to 30 November 2017. The 1 April 2016 Off-Mie earthquake (MW 5.9) and its aftershocks occurred beneath the seismic network. Our results document a clear decrease in seismic velocity at the time of the earthquake. These coseismic velocity drops were correlated with peak ground velocities at each station, suggesting that dynamic stress changes due to strong ground motions are a primary factor in coseismic velocity variations. Differences in the sensitivity of seismic velocity changes to peak ground velocity may reflect subsurface conditions at each station, such as geological structures and effective pressure conditions. We also observed a long-term increase in seismic velocities, independent of the 2016 earthquake, that may reflect tectonic strain accumulation around the Nankai subduction zone. After removing the long-term trend, we found that the coseismic velocity drops had not completely recovered by the end of the observation period, possibly indicating nonlinear effects of the 2016 earthquake. Our results suggest that ambient noise cross-correlation might be used to monitor the stress state in the Nankai accretionary prism in offshore environments, which would contribute to a better understanding of earthquake processes.