[S-CG50] 変動帯ダイナミクス

[SCG50-14] Effects of ETS on seismicity and stress in the subducting oceanic plate

*北 佐枝子¹、Houston Heidi²、矢部 優³、田中 佐千子⁴、浅野 陽一⁴、澁谷 拓郎⁵、須田 直樹⁶ (1.建築研究所、2.南カリフォルニア大学、3.産業技術総合研究所、4.防災科学技術研究所、5.京都大学防災研究所、6.広島大学)

キーワード：スロースリップ、スラブ内地震、応力場、地震活動、西南日本、紀伊半島

Slow slip phenomena appear related to geofluids, but the detailed relationships among slow slip, geofluids, and intraslab earthquakes remain unknown. Here we report changes in seismicity rates, b-values, and stresses in the oceanic slab before and after slow slip episodes beneath Kii Peninsula, Japan. One month before slow slip occurrence, the seismic rate and b-value become high, consistent with the effects of fluid-induced seismicity. Intraslab stress orientations below the plate interface in the ETS zone change only slightly, whereas below the plate interface updip of ETS, the intraslab stress orientations rotate by a larger amount, suggesting that transient aseismic slip occurs on the interface updip. The existence of repeating earthquakes and seismicity on the plate boundary also implies that aseismic slip occurs on the updip zone after the ETS, which could reset the stress state of the slab and the boundary to its prior condition. The proposed methodology, which is based primarily on intraslab earthquakes, may help monitor plate boundary conditions and slow slip phenomena, which can signal the beginning stages of megathrust earthquakes in subducting plates.