5:15 PM - 7:15 PM
[SCG61-P02] Crust/uppermantle deformation structure across the central NE Japan arc - Integrated interpretation for the 2019 onshore seismic profiling -
Keywords:island arc crust/upper mantle structure, NE Japan arc, seismic expedition with controlled seismic source, crustal deformation, earthquake activity, backarc spreading of the Sea of Japan
Introduction : A series of intensive analyses for the 2019 onshore seismic expedition data revealed various aspects on the crust and uppermantle structure across the central NE Japan arc (Fig. 1). In this presentation, we perform integrated interpretation based on the obtained seismic velocity model (Iwasaki et al., 2024) and reflection sections (Sato et al., 2021a,b, Kurashimo et al., 2022) together with geophysical/geological information. Our profile line, 155 km in length, crosses the Shonai Plain, Ouu backbone range, Kitakami river valley (KRV) and Kitakami Mts. from the west. Geologically, the eastern and central parts of the profile correspond to the units of backarc basin basalt (BBB) and felsic caldera complex (FCC), respectively, while the easternmost part belongs to the southern Kitakami massif (SKM).
Upper crust : The uppermost crust is characterized as highly deformed layers of sedimentary and volcaniclastic rocks with P-wave velocities (Vp) of 1.6~5.5 km/s. Along the profile, their configuration is well correlated with fault and caldera systems formed under the back arc spreading of the Sea of Japan and subsequent island arc volcanism. Beneath the BBB, these layers have larger velocity contrasts (0.5-1 km/s), indicating a different formation process from that further east. Except for the SKM, the 3-5 km thick uppermost crystalline crust consists of two parts. The Vp in upper part is 5.65~5.8 to 5.8-5.9 km/s with gradual decrease from the BBB to the FCC, while 6.0-6.2 km/s in the lower part. In the reflection section, this upper crystalline crust from the BBB to the FCC is partly reflective, showing a marked contrast with rather transparent feature under the KRV and SKM.
In the eastern part of the profile, a higher Vp (5.8~5.9-6.3 km/s) crystalline block of the SKM gently descends under the KRV to form a several-km thick westward dipping boundary zone. In the backarc spreading, several westward dipping normal faults were formed west of the SKM. With the domino block rotation, faults in the western part of the block were deformed to have gentler inclination than in the eastern part, as expressed by our boundary zone. Another possible interpretation for the gentle boundary is the 3-D geometry of fault systems around the KRV. The high Vp block shows high seismic activity including the 2003 Northern Miyagi earthquake (Mj=6.4) occurring along the Sue fault, which was originally formed as a normal fault by the backarc spreading. Around the boundary zone, another seismic activity is recognized down to about 15-km depth. Contrarily, the crystalline crust under the KRV is characterized by a lower Vp (5.8-6.1 km/s) and very few seismicity. This suggests that the high Vp block of the SKM has different properties from the upper crust under the KRV not only in seismic wave velocities but in mechanical behaviour. Earthquakes under the FCC are concentrated at shallower depths (< 7-8 km). Our wide-angle data indicate the existence of local reflectors around at 5-km depth. These features are explained by the island arc volcanic activity in this area. The seismic activity beneath the BBB extends to ~20 km depth. This area corresponds to the edge of the failed rift zone associated with the backarc spreading. The deep seismicity together with slightly high Vp in the crystalline crust strongly indicates the existence of the intruded basalt with a higher melting point, which formed the deep brittle-ductile transition zone.
Middle/Lower crust : The middle crust in our model is expressed as a 6-7 km thick layer with a velocity of 6.3-6.5 km/s. The lower crust beneath the central part of the profile is generally reflective and composed of three layers with velocities of 6.6 to 7.1 km/s. At the bottom of the lower crust, there exists a 2-km thick high velocity gradient zone of 1.5- 2 s-1, probably representing a transition zone from the uppermost mantle to the lower crustal material.
Uppermost Mantle : The Moho is situated at 30.5-32 km depth. The Pn velocity is about 7.7 km/s. Due to the transition zone mentioned above, the velocity contrast at the Moho becomes only 0.2~0.3 km/s. The uppermost mantle contains two velocity discontinuities with a contrast of 0.1~0.2 km/s at depths of 38 and 46 km. Low frequency earthquakes are occurring in the reflective lower crust and uppermost mantle, just above the mantle velocity discontinuities. This strongly indicates that the upward fluid migration from the mantle controls the occurrence of low frequency events.
References : Iwasaki et al., 2024, Fall Meeting of SSJ, S06-02; Kurashimo et al., 2022. JpGU Meeting, SCG50-03; Sato et al., 2020a. 2JpGU-AGU Joint Meeting, MIS03-P05; Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.
Upper crust : The uppermost crust is characterized as highly deformed layers of sedimentary and volcaniclastic rocks with P-wave velocities (Vp) of 1.6~5.5 km/s. Along the profile, their configuration is well correlated with fault and caldera systems formed under the back arc spreading of the Sea of Japan and subsequent island arc volcanism. Beneath the BBB, these layers have larger velocity contrasts (0.5-1 km/s), indicating a different formation process from that further east. Except for the SKM, the 3-5 km thick uppermost crystalline crust consists of two parts. The Vp in upper part is 5.65~5.8 to 5.8-5.9 km/s with gradual decrease from the BBB to the FCC, while 6.0-6.2 km/s in the lower part. In the reflection section, this upper crystalline crust from the BBB to the FCC is partly reflective, showing a marked contrast with rather transparent feature under the KRV and SKM.
In the eastern part of the profile, a higher Vp (5.8~5.9-6.3 km/s) crystalline block of the SKM gently descends under the KRV to form a several-km thick westward dipping boundary zone. In the backarc spreading, several westward dipping normal faults were formed west of the SKM. With the domino block rotation, faults in the western part of the block were deformed to have gentler inclination than in the eastern part, as expressed by our boundary zone. Another possible interpretation for the gentle boundary is the 3-D geometry of fault systems around the KRV. The high Vp block shows high seismic activity including the 2003 Northern Miyagi earthquake (Mj=6.4) occurring along the Sue fault, which was originally formed as a normal fault by the backarc spreading. Around the boundary zone, another seismic activity is recognized down to about 15-km depth. Contrarily, the crystalline crust under the KRV is characterized by a lower Vp (5.8-6.1 km/s) and very few seismicity. This suggests that the high Vp block of the SKM has different properties from the upper crust under the KRV not only in seismic wave velocities but in mechanical behaviour. Earthquakes under the FCC are concentrated at shallower depths (< 7-8 km). Our wide-angle data indicate the existence of local reflectors around at 5-km depth. These features are explained by the island arc volcanic activity in this area. The seismic activity beneath the BBB extends to ~20 km depth. This area corresponds to the edge of the failed rift zone associated with the backarc spreading. The deep seismicity together with slightly high Vp in the crystalline crust strongly indicates the existence of the intruded basalt with a higher melting point, which formed the deep brittle-ductile transition zone.
Middle/Lower crust : The middle crust in our model is expressed as a 6-7 km thick layer with a velocity of 6.3-6.5 km/s. The lower crust beneath the central part of the profile is generally reflective and composed of three layers with velocities of 6.6 to 7.1 km/s. At the bottom of the lower crust, there exists a 2-km thick high velocity gradient zone of 1.5- 2 s-1, probably representing a transition zone from the uppermost mantle to the lower crustal material.
Uppermost Mantle : The Moho is situated at 30.5-32 km depth. The Pn velocity is about 7.7 km/s. Due to the transition zone mentioned above, the velocity contrast at the Moho becomes only 0.2~0.3 km/s. The uppermost mantle contains two velocity discontinuities with a contrast of 0.1~0.2 km/s at depths of 38 and 46 km. Low frequency earthquakes are occurring in the reflective lower crust and uppermost mantle, just above the mantle velocity discontinuities. This strongly indicates that the upward fluid migration from the mantle controls the occurrence of low frequency events.
References : Iwasaki et al., 2024, Fall Meeting of SSJ, S06-02; Kurashimo et al., 2022. JpGU Meeting, SCG50-03; Sato et al., 2020a. 2JpGU-AGU Joint Meeting, MIS03-P05; Sato et al., 2020b. 2020 Spring Meeting of JAPT, 016.