Abstract
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.
Highlights
The Nankai subduction zone, where the Philippine Sea plate is subducting beneath the Japanese Islands at approximately 4.1–6.5 cm/year (Fig. 1; Seno et al 1993; Miyazaki and Heki 2001), is a well-studied plate convergent margin (e.g., Tobin and Saffer 2009; Moore et al 2009; Bangs et al 2009; Park et al 2010)
If we assume that the codas of cross-correlations we analyzed were dominated by acoustic-coupled Rayleigh (ACR) waves, the estimated velocity changes would be sensitive to a depth of several kilometers below the seafloor as well as in the water layer, it is difficult to clarify the depth sensitivity because the predicted mode transition is complicated in the studied frequency range (Tonegawa et al 2015)
Our results showed that drops in seismic velocity were associated with the 2016 Off-Mie earthquake and were followed by postseismic recovery
Summary
The Nankai subduction zone, where the Philippine Sea plate is subducting beneath the Japanese Islands at approximately 4.1–6.5 cm/year (Fig. 1; Seno et al 1993; Miyazaki and Heki 2001), is a well-studied plate convergent margin (e.g., Tobin and Saffer 2009; Moore et al 2009; Bangs et al 2009; Park et al 2010). In light of the high seismic risk, geophysical and drilling data have been intensively acquired in the Nankai accretionary prism. As part of that effort, we need to monitor the temporal variation of stress or strain within the accretionary prism. To monitor seismic activity near the Nankai subduction zone, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) deployed a network of real-time seafloor observatories off the Kii Peninsula in southwestern Japan called the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) (Kaneda et al 2015; Kawaguchi et al 2015). DONET has been used effectively to identify and monitor seismic activity including slow earthquakes such as low-frequency tremors, impulsive low-frequency earthquakes, very low frequency events, and slow slip events, which are important for understanding slip behavior along plate boundary faults (e.g., Nakano et al 2013, 2014, 2018; Suzuki et al 2016; Araki et al 2017; Toh et al 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
