13:45 〜 15:15
[SCG62-P02] 2019年陸域屈折/広角反射法探査解析に基づく中部東北日本弧の構造変化
キーワード：地殻・上部マントル、屈折/広角反射法探査、東北日本弧、島弧、プレート沈み込み、背弧拡大
In 2019, an extensive onshore-offshore seismic expedition was undertaken from the Yamato bank to the Japan Trench across the central part of the NE Japan arc (Sato et al., 2020a, b). The onshore profile line of this expedition, 150 km in length, was laid out from the Shonai Plain on the coast of the Sea of Japan to the eastern margin of the Kitakami Mountains on the Pacific coast. So far, we have constructed the whole crustal structure model from the onshore data set (Iwasaki et al., 2021a,b, 2022a,b), from which we found interesting structural features including the highly deformed sedimentary layers, the complicated Moho boundary and the upper mantle reflectors. On the other hand, the structural variations within the crystalline part of the upper crust and middle crust have been left unclarified because their information is hidden by the undulated sedimentary layers. However, the simple travel-time inversion by the extended time-term method (Iwasaki, 2022b) indicates that the uppermost crustal velocity shows a slight lateral change of 0.1-0.15 km/s along the profile.
Incorporating the above inversion result, we have been carrying out intensive re-analyses for all the shot records based on the asymptotic ray theory. As in the previous studies, we applied the distance correction to suppress systematic travel-time shift arising from the 2-D ray-tracing. To correct local/systematic travel-time misfits at near offsets appearing in the former analyses, the velocities and layer geometry within the uppermost sedimentary part are slightly modified (within 0.1-0.2 km/s and 500 m) from the previous models, but their general structural features including the correspondence to the active faults and caldera systems remains unchanged. The velocity of the uppermost crystalline crust is 5.65-5.70 km/s from the Shinjo Basin to the Kitakami River Valley, 0.1-0.15 km/s lower than those in the surrounding areas. The upper crystalline crust consists of two parts with velocities of 5.65-5.9 and 6.0-6.25 km/s. Their boundary is not so clear, but has a contrast of 0.1-0.2 km/s beneath the eastern half of the profile (east of the Shinjo Basin). The prominent structural change occurs between the regions of the Kitakami Mountains and Kitakami River Valley, where the higher velocity uppermost crust (5.8-6.25 km/s) of the Kitakami block is descending westward beneath the lower velocity (5.65-6.1 km/s) crust under the Kitakami River Valley. It is interesting that the crustal earthquakes are distributed below this westward descending structural boundary. The travel-time analysis and computation of synthetic seismograms indicate the boundary between the upper and middle crust is located at about 10-km depth, below which the middle crustal velocity is estimated to be 6.4-6.5 km/s. The thickness of the middle crust is 5-7 km. At the present stage of our analysis, however, the structural variation in the middle crustal part is not clear.
The revise of the upper and middle crustal model mentioned above does not require significant structural changes at the lower crustal and/or the uppermost mantle levels. The main part of lower crust, which has a velocity 6.6 to 7.1 km/s, is generally reflective with less seismic activity. The Moho is situated at 30.5-32 km depth, below which Pn velocity is about 7.7 km/s. At the bottom of the lowermost crust, there exists a 2-km thick transition zone (7.1-7.4~7.5 km/s), which reduces the velocity contrast at the Moho to be only 0.2~0.3 km/s. The uppermost part of the mantle is also reflective, containing two velocity discontinuities with a contrast of 0.1~0.2 km/s at depths of 38 and 46 km.
References
Iwasaki et al., 2021a. 2021 JpGU Meeting, SCG49-05. Iwasaki et al., 2021b. 2021 Fall Meeting of SSJ, S06-03. Iwasaki et al., 2022a. 2022 JpGU Meeting, SCG50-04. Iwasaki et al., 2022b. 2022 Fall Meeting of SSJ, S06-06. Sato et al., 2020a. 2020 JpGU-AGU Joint Meeting, MIS03-P05. Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.
Incorporating the above inversion result, we have been carrying out intensive re-analyses for all the shot records based on the asymptotic ray theory. As in the previous studies, we applied the distance correction to suppress systematic travel-time shift arising from the 2-D ray-tracing. To correct local/systematic travel-time misfits at near offsets appearing in the former analyses, the velocities and layer geometry within the uppermost sedimentary part are slightly modified (within 0.1-0.2 km/s and 500 m) from the previous models, but their general structural features including the correspondence to the active faults and caldera systems remains unchanged. The velocity of the uppermost crystalline crust is 5.65-5.70 km/s from the Shinjo Basin to the Kitakami River Valley, 0.1-0.15 km/s lower than those in the surrounding areas. The upper crystalline crust consists of two parts with velocities of 5.65-5.9 and 6.0-6.25 km/s. Their boundary is not so clear, but has a contrast of 0.1-0.2 km/s beneath the eastern half of the profile (east of the Shinjo Basin). The prominent structural change occurs between the regions of the Kitakami Mountains and Kitakami River Valley, where the higher velocity uppermost crust (5.8-6.25 km/s) of the Kitakami block is descending westward beneath the lower velocity (5.65-6.1 km/s) crust under the Kitakami River Valley. It is interesting that the crustal earthquakes are distributed below this westward descending structural boundary. The travel-time analysis and computation of synthetic seismograms indicate the boundary between the upper and middle crust is located at about 10-km depth, below which the middle crustal velocity is estimated to be 6.4-6.5 km/s. The thickness of the middle crust is 5-7 km. At the present stage of our analysis, however, the structural variation in the middle crustal part is not clear.
The revise of the upper and middle crustal model mentioned above does not require significant structural changes at the lower crustal and/or the uppermost mantle levels. The main part of lower crust, which has a velocity 6.6 to 7.1 km/s, is generally reflective with less seismic activity. The Moho is situated at 30.5-32 km depth, below which Pn velocity is about 7.7 km/s. At the bottom of the lowermost crust, there exists a 2-km thick transition zone (7.1-7.4~7.5 km/s), which reduces the velocity contrast at the Moho to be only 0.2~0.3 km/s. The uppermost part of the mantle is also reflective, containing two velocity discontinuities with a contrast of 0.1~0.2 km/s at depths of 38 and 46 km.
References
Iwasaki et al., 2021a. 2021 JpGU Meeting, SCG49-05. Iwasaki et al., 2021b. 2021 Fall Meeting of SSJ, S06-03. Iwasaki et al., 2022a. 2022 JpGU Meeting, SCG50-04. Iwasaki et al., 2022b. 2022 Fall Meeting of SSJ, S06-06. Sato et al., 2020a. 2020 JpGU-AGU Joint Meeting, MIS03-P05. Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.