12:00 〜 12:15
[SCG46-12] Uncovering mechanism of massive shallow coseismic slip without significant stress drop in 2011
キーワード：2011東北地方太平洋沖地震、 日本海溝、微小地震活動、余効変動
The reanalysis of the source model of the 2011 Tohoku-Oki earthquake has revealed that the large-scale fault slip in the shallowest part of the oceanic-continental plate boundary near the Japan Trench, which was the cause of the massive tsunami, did not accompany significant coseismic stress drop. This suggests that the shear stress level on the fault plane in the shallow slip area was already close to zero just before the earthquake occurred. Nevertheless, the coseismic slip became large due to the release of slip deficit accumulated by kinematic coupling caused by strong locking in the deeper region. This hypothesis assumes a significant difference in mechanical coupling strength between the shallow and deep parts of the plate boundary.
To verify this variation in coupling strength, we initiated a study based on seafloor observations conducted in the source region after the Tohoku-Oki earthquake. Right after the mainshock, normal-faulting earthquakes became widely active in the overriding plate above the source region, while reverse-faulting earthquakes occurred only at the landward end of the area where the coseismic stress drop was nearly zero. This suggests that the seismic activity was influenced by spatial variations in the post-earthquake stress state in the overriding plate. Furthermore, observations of seafloor crustal deformation indicate that kinematic coupling has recovered in this region immediately after the earthquake. If there were significant differences in coupling strength between the shallow and deep parts of the plate boundary, corresponding changes in the stress state within the overriding plate would be expected. Therefore, we aim to observe the present intraplate seismic activity more than ten years after the Tohoku-Oki earthquake and compare it with the seismicity in 2011. Through this comparison, we seek to elucidate the spatiotemporal variations in the stress state within the overriding plate during the post-seismic stress reaccumulation process and clarify the differences in mechanical coupling strength between the deep and shallow parts of the plate boundary.
To verify this variation in coupling strength, we initiated a study based on seafloor observations conducted in the source region after the Tohoku-Oki earthquake. Right after the mainshock, normal-faulting earthquakes became widely active in the overriding plate above the source region, while reverse-faulting earthquakes occurred only at the landward end of the area where the coseismic stress drop was nearly zero. This suggests that the seismic activity was influenced by spatial variations in the post-earthquake stress state in the overriding plate. Furthermore, observations of seafloor crustal deformation indicate that kinematic coupling has recovered in this region immediately after the earthquake. If there were significant differences in coupling strength between the shallow and deep parts of the plate boundary, corresponding changes in the stress state within the overriding plate would be expected. Therefore, we aim to observe the present intraplate seismic activity more than ten years after the Tohoku-Oki earthquake and compare it with the seismicity in 2011. Through this comparison, we seek to elucidate the spatiotemporal variations in the stress state within the overriding plate during the post-seismic stress reaccumulation process and clarify the differences in mechanical coupling strength between the deep and shallow parts of the plate boundary.