14:45 〜 15:00
[SCG49-05] The upper crustal structure across the central part of the NE Japan Arc revealed from the 2019 onshore seismic refraction/wide-angle reflection profiling
キーワード：東北日本弧、上部地殻構造、屈折/広角反射法探査、地殻変形、日本海、背弧拡大
In 2019, an extensive onshore-offshore seismic expedition was undertaken from the Japan Trench to the Yamato bank across the central part of the NE Japan arc (Sato et al., 2020a). The onshore seismic line in the NE Japan Arc was laid out from the Shonai Plain on the coast of the Sea of Japan to the Kitakami Mountains on the Pacific coast, crossing the Dewa Hills, Shinjo Basin, Ou Backbone Range and Kitakami River Valley from the west to the east. In the western part of the profile (from the Shonai Plain to the eastern boundary of the Shinjo Basin), seismic reflection and refraction/wide-angle reflection surveys were simultaneously carried out under the “Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan” funded by MEXT (Sato et al., 2020b). A refraction/wide-angle reflection experiment in the eastern half of the profile, on the other hand, was supported by funds from MEXT under its 2nd Earthquake and Volcano Hazards Observation and Research Program and ERI, the University of Tokyo. The total length of the profile is about 155 km, on which 1,667 receivers were set to record 24 large energy shots. As seismic sources, we used explosives at 3 shot points and 4 vibroseis trucks with 150 stationary sweeps at the other 21 points.
In this paper, we present the first velocity structure model based on the ray-tracing technique. The travel-times from the above 24 shots were highly undulated except beneath the Kitakami Mts. The uppermost part of the crust from these data is composed of 4 layers of Vp=1.6-2.0, 1.8-3.5, 3.5-4.5 and 4.5-5.5km/s, respectively, representing sedimentary and volcaniclastic rocks. Their geometry shows significant change along the profile line. At the boundary of the Shinjo Basin and Dewa Hills, the upper 3 layers abruptly reduce in thickness from 5 to 1 km to the east. This position is almost consistent with the location of the Aosawa fault. In the Ou Backbone Range, the thickness of the above 3 layers is less than 1.5 km, but it again increases to 2-2.5 km beneath the Kitakami River Valley. As in the case of the Aosawa fault, the uppermost parts of the major faults developed around the profile are well correlated to the locations at which the sedimentary part shows abrupt change in thickness. Such complicated structure represents the crustal deformation associated with the Miocene backarc spreading of the Sea of Japan. In Kitakami Mts., on the other hand, the structure is very simple, where the 5.0-5.5 km/s layer is almost outcropped, showing marked contrast with highly deformed crust from the Shinjo Basin to the Kitakami River Valley. Such structural variation of the uppermost crust is well consistent with those from the tomographic analyses by Sato et al. (2020a) and Kurashimo et al. (2021).
The velocity of the crystalline part of the crust is about 5.8 km/s. Although the deeper part of the crust is not modelled as yet, weak arrivals beyond offsets of 40-50 km indicate that the crustal velocity increases to 6.3 km/s around at a depth of 8 km. In some shot records, later phases 0.2-0.5 s appearing behind the first arrivals in an offset range of 40-70 km suggest the existence of some velocity discontinuities within the crust.
References
Kurashimo et al., 2021. Seismic structure from the forearc region off Miyagi to the central part of Northern Honshu arc, Japan, revealed by onshore-offshore seismic experiment. submitted to 2021 JpGU Meeting.
Sato et al., 2020a. Seismic transect across the central part of Northern Honshu, Japan, 2020 JpGU-AGU Joint Meeting, MIS03-P05.
Sato et al., 2020b. Deep seismic reflection profiling across the Shonai and Shinjo basins, northern Honshu, Japan, 2020 Spring Meeting of JAPT, 016.
In this paper, we present the first velocity structure model based on the ray-tracing technique. The travel-times from the above 24 shots were highly undulated except beneath the Kitakami Mts. The uppermost part of the crust from these data is composed of 4 layers of Vp=1.6-2.0, 1.8-3.5, 3.5-4.5 and 4.5-5.5km/s, respectively, representing sedimentary and volcaniclastic rocks. Their geometry shows significant change along the profile line. At the boundary of the Shinjo Basin and Dewa Hills, the upper 3 layers abruptly reduce in thickness from 5 to 1 km to the east. This position is almost consistent with the location of the Aosawa fault. In the Ou Backbone Range, the thickness of the above 3 layers is less than 1.5 km, but it again increases to 2-2.5 km beneath the Kitakami River Valley. As in the case of the Aosawa fault, the uppermost parts of the major faults developed around the profile are well correlated to the locations at which the sedimentary part shows abrupt change in thickness. Such complicated structure represents the crustal deformation associated with the Miocene backarc spreading of the Sea of Japan. In Kitakami Mts., on the other hand, the structure is very simple, where the 5.0-5.5 km/s layer is almost outcropped, showing marked contrast with highly deformed crust from the Shinjo Basin to the Kitakami River Valley. Such structural variation of the uppermost crust is well consistent with those from the tomographic analyses by Sato et al. (2020a) and Kurashimo et al. (2021).
The velocity of the crystalline part of the crust is about 5.8 km/s. Although the deeper part of the crust is not modelled as yet, weak arrivals beyond offsets of 40-50 km indicate that the crustal velocity increases to 6.3 km/s around at a depth of 8 km. In some shot records, later phases 0.2-0.5 s appearing behind the first arrivals in an offset range of 40-70 km suggest the existence of some velocity discontinuities within the crust.
References
Kurashimo et al., 2021. Seismic structure from the forearc region off Miyagi to the central part of Northern Honshu arc, Japan, revealed by onshore-offshore seismic experiment. submitted to 2021 JpGU Meeting.
Sato et al., 2020a. Seismic transect across the central part of Northern Honshu, Japan, 2020 JpGU-AGU Joint Meeting, MIS03-P05.
Sato et al., 2020b. Deep seismic reflection profiling across the Shonai and Shinjo basins, northern Honshu, Japan, 2020 Spring Meeting of JAPT, 016.