[S-SS06] 地震波伝播：理論と応用

[SSS06-P05] 近地地震を用いた関東地方深部地殻構造の反射波イメージング

*白石 和也¹、渡辺 俊樹² (1.海洋研究開発機構、2.名古屋大学)

キーワード：反射波イメージング、地殻構造、近地地震、関東地方、スラブ

We developed a novel method for seismic imaging of subsurface structures from passively observed earthquake records. The proposed method is based on reverse time migration (RTM) using reflection waves of earthquakes without source information (Shiraishi and Watanabe, 2021). By correlating two wavefields extrapolated forward and backward in time from all receiver locations using the same observation records, multiple reflections between the earth surface and subsurface boundaries illuminate crustal structures. In this study, we applied this method to local earthquake records obtained by the Metropolitan Seismic Observation Network (MeSO-net) to image deep crustal structures in Kanto region where the Philippine Sea (PHS) plate and the Pacific (PAC) plate are subducting beneath the Okhotsk plate. Various models of the subducting slabs were documented in previous studies based on seismic tomography and receiver function analyses (e.g., Hirose et al., 2008; Nakajima et al., 2009; Uchida et al., 2010; Ito et al., 2019; Ishise et al., 2021) and controlled source seismic surveys (e.g., Sato et al., 2005). Our objective of this application is to obtain reflection profiles by using earthquakes in the deep subsurface where reflection imaging is difficult by the controlled source seismic surveys. Two crossing pseudo-survey lines, a 191-km-long WSW-ENE line and a 131-km-long SW-NE lines, are arranged using the MeSO-net stations. After collecting local earthquake records of magnitudes greater than 2.5 based on the Japan Meteorological Agency (JMA) unified earthquake catalog in an approximately 180 km × 110 km area surrounding the MeSO-net stations, we selected high-quality 200 earthquake records from the initially collected 722 records according to root-mean-square amplitude ratios between the earthquake signals and background noises. To obtain P-wave reflection profiles, we extracted P-wave velocity sections along the pseudo-survey lines from an existing three-dimensional velocity model obtained by seismic tomography using earthquakes (Matsubara et al., 2019). We applied the two-dimensional acoustic RTM with the extracted velocity models to 60-s-long vertical component records after bandpass filtering with 0.5-3 Hz and automatic gain control with a gate length of 3 s. Stacking RTM sections from the selected earthquakes and applying a two-dimensional wavenumber filter to the stacked sections, we obtained the final reflection profiles. Several continuous reflectors are imaged approximately 20-70 km in depth in the reflection profiles of both two lines. The major reflectors correspond to each other on the intersection of two profiles, and the reflection patterns are consistent with the velocity variations. The dipping reflectors at the depth of 20-50 km are likely the top and bottom boundaries of the PHS slab, which are in the similar depth range of the subducting slab proposed in the previous studies. Other dipping reflectors at the depth of 40-70 km are possibly relevant to the PAC slab. The en-echelon reflection patterns around 20 km depth imply reflective boundaries within the inland crust above the subducting slabs. Our result suggests high potential to explore crustal structures, including the depth of subducting slabs, using passive seismic data of earthquakes recorded by recent high-dense seismic observations. The passive seismic reflection imaging technique is also useful in the area where controlled source seismic surveys are difficult such as the metropolitan area.

Acknowledgements: This research is supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Scientific Research C (JP19K04028). We used the unified earthquake catalog of the Japan Meteorological Agency (JMA) and seismic records of the Metropolitan Seismic Observation Network (MeSO-net) managed by the National Research Institute for Earth Science and Disaster Resilience (NIED).