[S-SS08] 地震活動とその物理

[SSS08-09] Space and time changes in stress state and b-value off the coast of Tohoku and Hokkaido districts

*楠城 一嘉^1,2,3,4、堀 高峰^4,1 (1.静岡県立大学、2.静岡大学、3.統計数理研究所、4.海洋研究開発機構)

キーワード：地震活動、応力状態、プレートカップリング、地震ハザード

Monitoring the Earth's stress state plays a role in our understanding of an earthquake's mechanism and in the distribution of hazards (Nanjo, 2020). Crustal deformation due to the occurrence of the 2011 Tohoku earthquake of magnitude (M) 9 and the 2003 M8 Tokachi earthquake caused stress perturbation in nearby regions. However, implications of future seismicity remain uncertain. Here, the changes in the Gutenberg-Richter frequency-magnitude b-value, which characterizes the relative number of small compared to large earthquakes, was evaluated as a function of space and time in northern Japan. Assuming that the b-value is negatively correlated with differential stress (Scholz, 1965, 2015), an analysis based on b-values of seismicity off the coast of Tohoku district (Nanjo et al., 2012; Nanjo, 2024 in press; Iwata & Nanjo, 2024 in press) was conducted to infer that the stress relieved by the 2011 Tohoku earthquake has not yet recovered to levels found before the earthquake. Contrary to the previous study on b-values (Tormann et al., 2015), it was argued that the hazard of a great tsunami-producing earthquake off the coast of Tohoku district is unlikely to already be almost as high as it was just before the 2011 event. We agreed with another previous study (Buergmann et al., 2016) stating that many centuries of stress accumulation are required to rebuild stress and produce another great event. Moreover, near the high-slip zone of the 2003 Tokachi rupture (Yagi, 2004), a pronounced decade-scale decrease in b was shown. This is interpreted as an indication that the stress is accumulating and the Tokachi event did not contribute to relaxing stress. We compared our results with inter-plate coupling obtained from using GNSS (Iinuma, 2018), seismic quiescence (Matsu'ura, 2019), and slow earthquakes (Nishikawa et al., 2019), to show that the zone of a decrease in b fell into an area of stronger coupling. Considering regional seismicity and numerical simulation (Satake & Atwater, 2007; Hori et al., 2009; Hori & Miyazaki, 2011; Awata, 2020), we proposed that this zone is in the process of stress buildup for a mega-quake that is larger than events belonging to M8-class. Furthermore, we noted that another zone of low b-values was mostly overlapped with the high-slip areas of the 1968 M7.9 Sanriku earthquake (Yamanaka & Kikuchi, 2004), implying that the possibility of the recurrence of a Sanriku-type event is non-negligible. Future research requires careful monitoring of b-values as well as plate coupling off the coast of Hokkaido district.

We thank the Japan Meteorological Agency for the earrthquake catalog, and N. Hirata and K. Shiomi for valuable comments. This study was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research) (K.N., T.H.) and under STAR-E (Seismology TowArd Research innovation with data of Earthquake) Program Grant Number JPJ010217 (K.N.), and the Chubu Electric Power's research based on selected proposals (K.N.).