17:15 〜 18:45
[SSS09-P06] 2019年陸域屈折/広角反射法探査データ解析に基づく中部東北日本弧地殻変形と地震活動
キーワード：島弧地殻・上部マントル構造、東北日本弧、制御震源地震探査、地殻変形、地震活動
Integrated analyses for onshore seismic refraction/wide-angle reflection data in the Central NE Japan arc revealed the crustal deformation since the Miocene backarc spreading along the eastern margin of Asia continent. Our seismic data were acquired on a profile line from the Shonai Plain to the Kitakami Mountains as a part of the 2019 extensive onshore-offshore seismic expedition from the Yamato bank in the Sea of Japan to the Japan trench (Sato et al., 2021a,b). The present analyses, which were composed of the travel time inversion (the extended time-term method (Iwasaki, 2002) and forward travel time and amplitude modelling by the asymptotic ray theory, provided more detailed structural variation particularly for the upper and middle crustal levels as compared with the previous results by Iwasaki et al. (2023a,b). Our present model has the following structural features.
(1) The uppermost part crust consists of 5 layers of Vp= 1.6-2.0, 1.8-3.5, 2.7-4.5, 4.0-4.8 and 4.5-5.5 km/s, representing sedimentary and volcaniclastic rocks. Their geometry shows significant lateral change along the profile line, well correlated with fault and caldera systems developed in the surveyed region under successive tectonic processes in the NE Japan arc.
(2) High velocity contrasts (~0.5-1 km/s) in the basal part of the 4-km thick sedimentary package beneath the Shonai basin produce strong wide-angle reflections on the record sections for the westernmost shots. Such structural features are different from those further east.
(3) The upper crystalline crust is divided to 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. The velocity at its top is relatively high (5.8 km/s) in the western part (the backarc basin basalt area) but decreases to 5.65-5.7 in the middle part from the felsic caldera complex (Ou backbone range) to the Kitakami river valley. The present amplitude analysis indicates local reflectors with a velocity contrast of ~0.1 km/s at a depth of 4-6 km beneath the felsic caldera complex. The velocity in the lower part is estimated as 6.0-6.2 km/s.
(4) The easternmost part (Kitakami Mts.) is characterized by a very thin (~0.5 km) and high velocity (~5 km/s) sediment overlying a high velocity (5.8~5.9-6.3 km/s) upper crust. This high velocity block descends to the west beneath a thick sedimentary package beneath the Kitakami river valley. This structure is consistent with the major geological structures including the Sue fault, Asahiyama flexure and Kashimadai fault.
(5) The middle crust with a velocity of 6.3-6.5 km/s is in a depth range of 8~9-14~16 km. The velocity contrast between the upper and middle crust fluctuates with an amplitude of 0.1-0.15 km/s and a wave length of 10-20 km.
(6) In the middle and eastern parts of the profile, most of crustal earthquakes are distributed within the upper/middle crust. From our model, the P-wave velocity at the brittle-ductile transition is 6.4-6.5 km/s. The seismic activity of the 2003 Northern Miyagi earthquake (Mj 6.4) occurred around the Sue fault, which corresponds to the western edge of the Kitakami block described in (4) (Kato et al., 2004).
(7) The lower crust, composed of three layers with velocities ranging from 6.6 to 7.1 km/s, is generally reflective with less seismic activity.
(8) The Moho is situated at 30.5-32 km depth. The 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). The resultant velocity contrast at the Moho becomes only 0.2~0.3 km/s. The uppermost part of the mantle contains two velocity discontinuities at depths of 38 and 46 km with a contrast of 0.1~0.2 km/s.
(9) Low frequency earthquakes are occurring at depths of 17-40 km just within the reflective zones in the lower crust and upper mantle in our model, suggesting upward fluid migration.
References
Iwasaki, 2002. EPS, 54, 663-677. Iwasaki et al., 2023a, JpGU Meeting, SCG62-P02. Iwasaki et al., 2023b. Fall Meeting of SSJ, S06-03. Kato et al., 2004, EPS, 56, 1369-1374. Sato et al., 2020a. 2JpGU-AGU Joint Meeting, MIS03-P05. Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.
(1) The uppermost part crust consists of 5 layers of Vp= 1.6-2.0, 1.8-3.5, 2.7-4.5, 4.0-4.8 and 4.5-5.5 km/s, representing sedimentary and volcaniclastic rocks. Their geometry shows significant lateral change along the profile line, well correlated with fault and caldera systems developed in the surveyed region under successive tectonic processes in the NE Japan arc.
(2) High velocity contrasts (~0.5-1 km/s) in the basal part of the 4-km thick sedimentary package beneath the Shonai basin produce strong wide-angle reflections on the record sections for the westernmost shots. Such structural features are different from those further east.
(3) The upper crystalline crust is divided to 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. The velocity at its top is relatively high (5.8 km/s) in the western part (the backarc basin basalt area) but decreases to 5.65-5.7 in the middle part from the felsic caldera complex (Ou backbone range) to the Kitakami river valley. The present amplitude analysis indicates local reflectors with a velocity contrast of ~0.1 km/s at a depth of 4-6 km beneath the felsic caldera complex. The velocity in the lower part is estimated as 6.0-6.2 km/s.
(4) The easternmost part (Kitakami Mts.) is characterized by a very thin (~0.5 km) and high velocity (~5 km/s) sediment overlying a high velocity (5.8~5.9-6.3 km/s) upper crust. This high velocity block descends to the west beneath a thick sedimentary package beneath the Kitakami river valley. This structure is consistent with the major geological structures including the Sue fault, Asahiyama flexure and Kashimadai fault.
(5) The middle crust with a velocity of 6.3-6.5 km/s is in a depth range of 8~9-14~16 km. The velocity contrast between the upper and middle crust fluctuates with an amplitude of 0.1-0.15 km/s and a wave length of 10-20 km.
(6) In the middle and eastern parts of the profile, most of crustal earthquakes are distributed within the upper/middle crust. From our model, the P-wave velocity at the brittle-ductile transition is 6.4-6.5 km/s. The seismic activity of the 2003 Northern Miyagi earthquake (Mj 6.4) occurred around the Sue fault, which corresponds to the western edge of the Kitakami block described in (4) (Kato et al., 2004).
(7) The lower crust, composed of three layers with velocities ranging from 6.6 to 7.1 km/s, is generally reflective with less seismic activity.
(8) The Moho is situated at 30.5-32 km depth. The 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). The resultant velocity contrast at the Moho becomes only 0.2~0.3 km/s. The uppermost part of the mantle contains two velocity discontinuities at depths of 38 and 46 km with a contrast of 0.1~0.2 km/s.
(9) Low frequency earthquakes are occurring at depths of 17-40 km just within the reflective zones in the lower crust and upper mantle in our model, suggesting upward fluid migration.
References
Iwasaki, 2002. EPS, 54, 663-677. Iwasaki et al., 2023a, JpGU Meeting, SCG62-P02. Iwasaki et al., 2023b. Fall Meeting of SSJ, S06-03. Kato et al., 2004, EPS, 56, 1369-1374. Sato et al., 2020a. 2JpGU-AGU Joint Meeting, MIS03-P05. Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.