[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P39] Estimation of variations in the shallow crustal velocity structure in the Nagano region, Japan, from dense seismic observations, using seismic interferometry

*Louisa Bagot¹、Bogdan Enescu¹、Shiro Ohmi²、Masatoshi Miyazawa²、Tetsuya Takeda³ (1.Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan、2.Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan、3.National Institute for Earth Science and Disaster Resilience, Tsukuba, Japan)

On the 11th of March 2011, the magnitude 9.0 Tohoku-oki earthquake occurred, leading to a devastating tsunami, causing extensive damage and many casualties. Thanks to Japan's dense seismic networks, this event was one of the most well recorded large earthquakes. 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 (on 2011-03-12, at 3:59 JST), followed, one month later, by a Mw 5.4 earthquake (e.g., Shimojo et al., 2014). In this study, we focus on the Northern Nagano region, and assess the seismic wave velocity temporal evolution using seismic interferometry. The method uses ambient seismic noise and essentially consists in computing continuous noise signal correlations to estimate the temporal velocity variations of the medium (Shapiro and Campillo, 2004; Shapiro et al., 2005; Sens-Schönfelder and Wegler, 2006). We use the permanent NIED Hi-net network, as well as a dense seismic array which was deployed from 2008 to 2012, during the "Intensive Observations and Research in the High-Strain-Rate Zone of Japan" (or on short, HIZUMI project) (Enescu et al., 2008; Takeda, 2010; Sekiguchi, 2013), which is used for the first time to retrieve velocity changes in this area. We compute both the auto-correlation and cross-correlation functions for some stations, for different frequency bands. For the auto-correlation results, we can observe a time shift due to the Tohoku-oki earthquake, which shows that it had an impact on the region. For the cross-correlation results, we retrieve the surface wave component of the Green's function of the medium between pairs of stations. Using the stretching method (Sens-Schönfelder and Wegler, 2006), we estimate the wave velocity variations for the time period between October 2010 and September 2011, to assess the impact of the 2011 events on the Northern Nagano region. Our ongoing work is to improve these results to obtain more stable velocity variations. The goal of this study is also to compare the velocity changes due to the Mw 6.2 Nagano earthquake to previous results (Hobiger et al., 2016), on a more local scale thanks to the HIZUMI project stations, as well as showing a potential link with the Coulomb stress change caused by the event.