2:00 PM - 2:15 PM
[S06-03] Lateral structure variation across the central part of the NE Japan Arc deduced from the 2019 onshore seismic profiling -II
Since 2021, we have conducted intensive analyses for seismic data which were collected in 2019 on the onshore seismic reflection/refraction line across the central part of the NE Japan arc (Sato et al., 2020a, b). This line,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. A velocity structure model so far obtained shows interesting structural features including the highly deformed sedimentary layers, the complicated Moho with a 2-km thick transition zone and the upper mantle reflectors at 38 and 46 km depths (Iwasaki et al., 2021a,b, 2022a,b, 2023).
Geologically, the surveyed area has been seriously affected by several tectonic processes since the Miocene backarc spreading (Sato et al., 2020a, b). Although our model shows the highly complicated sedimentary structure in the uppermost crust, the lateral structural variation, probably existing also within the crystalline upper/middle crust, has been left unclarified. Since 2022, we have reanalyzed our data to elucidate such structural heterogeneity by combining travel-time analysis and amplitude calculation based on the asymptotic ray-theory (Iwasaki et al., 2022b, 2023).
As the first step of the present study, we reevaluated the seismic velocity at the top of the crystalline crust. After removing trave-time fluctuations on the individual seismic traces arising from the shallow and local structure by using time-terms obtained along the profile (Iwasaki et al., 2022b), we made stacked section to enhance the first arrivals travelling the uppermost crust. This enabled us to trace the first arrivals 5-10 km further as compared with the original (unstacked) section and confirm our travel-time picking. The upper crustal velocities obtained through this process were almost consistent with the previous results (Iwasaki et al., 2022b, 2023).
Next, we calculated synthetic seismograms for 22 additional shots in the seismic reflection line (the western part of the profile) as well as 24 large energy shots used in the previous analyses. This calculation was aimed to identify shallow structural variation and local reflectors. The seismic data from the added shots revealed more precise structure of the sedimentary part in the western part of the profile. Particularly in the westernmost part, the uppermost low velocity layer with 2.2~3.1 km/s in the previous model is separated to the upper high velocity gradient part and lower part of low velocity gradient. A high velocity contrast (~0.8 km/s) existing in the sedimentary package at a depth of 3.5-4 km produces large amplitude reflection within an offset of 5-10 km. Such structural features are clearly different from those further east. The upper crystalline crust is divided into two parts. The upper part, 1.5-2.5 km thick, has a velocity of 5.65~5.8-5.8-5.9 km/s This velocity is relatively high (5.8 km/s) in the western part (the backarc basin basalt area) but slightly decreases to 5.65~5.7 km/s in the middle and eastern parts beneath the felsic caldera complex (Ou backbone range) and the Kitakami river valley. In the felsic caldera complex, we recognized local reflectors with a velocity contrast of ~0.1 km/s at a depth of 4-6 km. The velocity contrast between the upper and lower parts is 0.1-0.2 km/s probably with a slight lateral change along the profile.
The uppermost crust in the easternmost part of the profile (beneath the Kitakami Mts.) is characterized by a very thin sediment and higher velocity crystalline part (5.8-5.9 km/s). This high velocity block descends to the west beneath a thick sedimentary package beneath the Kitakami river valley. Such structural geometry is constrained down to about 5-km depth both from the travel times of the first arrivals and later phases.
Now our analysis is being focused on the middle crustal level. The boundary between the uppermost and middle crust is estimated to be at a depth of 10 km, while the boundary between the middle and lower crust at around 12-14 km depth. Wide-angle reflections from these interfaces were recognized in some shot records in the central and eastern parts of the profile, indicating that their depths and velocity contrasts show lateral change of 1-2 km and 0.1-0.2 km/s, respectively.
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. Iwasaki et al., 2023. 2023 JpGU Meeting, SCG62-P02.Sato et al., 2020a. 2020 JpGU-AGU Joint Meeting, MIS03-P05. Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.
Geologically, the surveyed area has been seriously affected by several tectonic processes since the Miocene backarc spreading (Sato et al., 2020a, b). Although our model shows the highly complicated sedimentary structure in the uppermost crust, the lateral structural variation, probably existing also within the crystalline upper/middle crust, has been left unclarified. Since 2022, we have reanalyzed our data to elucidate such structural heterogeneity by combining travel-time analysis and amplitude calculation based on the asymptotic ray-theory (Iwasaki et al., 2022b, 2023).
As the first step of the present study, we reevaluated the seismic velocity at the top of the crystalline crust. After removing trave-time fluctuations on the individual seismic traces arising from the shallow and local structure by using time-terms obtained along the profile (Iwasaki et al., 2022b), we made stacked section to enhance the first arrivals travelling the uppermost crust. This enabled us to trace the first arrivals 5-10 km further as compared with the original (unstacked) section and confirm our travel-time picking. The upper crustal velocities obtained through this process were almost consistent with the previous results (Iwasaki et al., 2022b, 2023).
Next, we calculated synthetic seismograms for 22 additional shots in the seismic reflection line (the western part of the profile) as well as 24 large energy shots used in the previous analyses. This calculation was aimed to identify shallow structural variation and local reflectors. The seismic data from the added shots revealed more precise structure of the sedimentary part in the western part of the profile. Particularly in the westernmost part, the uppermost low velocity layer with 2.2~3.1 km/s in the previous model is separated to the upper high velocity gradient part and lower part of low velocity gradient. A high velocity contrast (~0.8 km/s) existing in the sedimentary package at a depth of 3.5-4 km produces large amplitude reflection within an offset of 5-10 km. Such structural features are clearly different from those further east. The upper crystalline crust is divided into two parts. The upper part, 1.5-2.5 km thick, has a velocity of 5.65~5.8-5.8-5.9 km/s This velocity is relatively high (5.8 km/s) in the western part (the backarc basin basalt area) but slightly decreases to 5.65~5.7 km/s in the middle and eastern parts beneath the felsic caldera complex (Ou backbone range) and the Kitakami river valley. In the felsic caldera complex, we recognized local reflectors with a velocity contrast of ~0.1 km/s at a depth of 4-6 km. The velocity contrast between the upper and lower parts is 0.1-0.2 km/s probably with a slight lateral change along the profile.
The uppermost crust in the easternmost part of the profile (beneath the Kitakami Mts.) is characterized by a very thin sediment and higher velocity crystalline part (5.8-5.9 km/s). This high velocity block descends to the west beneath a thick sedimentary package beneath the Kitakami river valley. Such structural geometry is constrained down to about 5-km depth both from the travel times of the first arrivals and later phases.
Now our analysis is being focused on the middle crustal level. The boundary between the uppermost and middle crust is estimated to be at a depth of 10 km, while the boundary between the middle and lower crust at around 12-14 km depth. Wide-angle reflections from these interfaces were recognized in some shot records in the central and eastern parts of the profile, indicating that their depths and velocity contrasts show lateral change of 1-2 km and 0.1-0.2 km/s, respectively.
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. Iwasaki et al., 2023. 2023 JpGU Meeting, SCG62-P02.Sato et al., 2020a. 2020 JpGU-AGU Joint Meeting, MIS03-P05. Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.