15:18 〜 15:38
[S20-01] [招待講演]地震波トモグラフィーから見た地震火山活動と地球内部ダイナミクス
In the past 30 years I have made seismic tomography studies, which shed new light on seismotectonics, volcanism, and the Earth interior structure and dynamics. In this lecture I introduce the main findings of our tomographic studies made so far.
(1) Subduction zone structure and arc magmatism. We have investigated the 3-D structure of many subduction zones by conducting joint inversion of local and teleseismic data (e.g., Zhao et al., 1992, 1994, 1995, 1997). The subducting slab is imaged clearly as a high-velocity (high-V) zone whose thickness depends on the slab age. Significant low-velocity (low-V) anomalies are revealed in the mantle wedge, which reflect arc magma and fluids associated with corner flow and slab dehydration, leading to the formation of arc volcanoes.
(2) Fluids and earthquakes. A low-V and high Poisson's ratio anomaly was revealed in the source zone of the 1995 Kobe earthquake (M7.2), reflecting a fluid-filled, fractured rock matrix that triggered the rupture of the Nojima fault (Zhao et al., 1996). The fluids in the source zone came from dehydration of the subducting Philippine Sea (PHS) slab under SW Japan (Zhao et al., 2000, 2002). Similar features have been revealed in source zones of various large earthquakes worldwide, suggesting that fluids are probably involved in all types of earthquakes (Zhao, 2021).
(3) Big mantle wedge and intraplate volcanism. East Asian mantle tomography shows that the Pacific slab becomes flat in the mantle transition zone (410-660 km depths) beneath the Korean Peninsula and East China, and a big mantle wedge (BMW) has formed above the slab under East Asia. The intraplate volcanism in NE Asia is caused by hot and wet upwelling flows in the BMW (Zhao et al., 2004, 2009).
(4) Seismic anisotropy and mantle dynamics. The subducting Pacific and PHS slabs exhibit mainly trench-parallel azimuthal anisotropy, which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation, such as normal faults produced at the outer-rise near the trench axis (Zhao et al., 2023). Trench-normal anisotropy is generally revealed in the mantle wedge under the volcanic front and back-arc, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration (Wang & Zhao, 2021; Zhao et al., 2023). Trench-normal anisotropy also occurs in the subslab mantle, reflecting asthenospheric shear deformation associated with the overlying slab subduction (Fan & Zhao, 2021).
(5) Mantle plumes and deep slabs. Global tomography has revealed deeply subducted slabs reaching the core-mantle boundary (CMB). Plume-like, continuous low-V anomalies appear beneath 13 hotspots, suggesting that they are 13 whole-mantle plumes originating from the CMB. These plumes exhibit tilted images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow (Zhao, 2004, 2015).
(6) Lunar tomography and moonquakes. Moonquake arrival-time data recorded by the Apollo seismic network operated during 1969 to 1977 were used to determine the first Vp and Vs tomography beneath the lunar near-side (Zhao et al., 2008, 2012). A correlation between the Vs tomography and the thorium abundance distribution is revealed. The area with high thorium abundance exhibits a distinct low Vs that extends down to ~300 km depth, which may reflect a thermal and compositional anomaly. The distribution of deep moonquakes (700-1300 km depths) shows a correlation with the tomography in the deep lunar mantle. The presence of deep moonquakes and seismic-velocity heterogeneity implies that the interior of the present Moon may still be thermally and dynamically active.
References
Fan, J., D. Zhao (2021). Nature Geoscience 14, 349-353.
Wang, Z., D. Zhao (2021). Science Advances 7, eabc9620.
Zhao, D. et al. (1992). J. Geophys. Res. 97, 19909-19928.
Zhao, D. et al. (1994). J. Geophys. Res. 99, 22313-22329.
Zhao, D. et al. (1995). J. Geophys. Res. 100, 6487-6504.
Zhao, D. et al. (1996). Science 274, 1891-1894.
Zhao, D. et al. (1997). Science 278, 254-257.
Zhao, D. et al. (2000). J. Geophys. Res. 105, 13579-13594.
Zhao, D. et al. (2002). Phys. Earth Planet. Inter. 132, 249-267.
Zhao, D. (2004). Phys. Earth Planet. Inter. 146, 3-34.
Zhao, D., J. Lei, Y. Tang (2004). Chinese Sci. Bull. 49, 1401-1408.
Zhao, D., J. Lei, L. Liu (2008). Chinese Sci. Bull. 53, 3897-3907.
Zhao, D. et al. (2009). Phys. Earth Planet. Inter. 173, 197-206.
Zhao, D. et al. (2012). Global Planet. Change 90, 29-36.
Zhao, D. (2015). Multiscale Seismic Tomography. Springer, 304 pp.
Zhao, D. (2021). Earth-Science Reviews 214, 103507.
Zhao, D. et al. (2023). Surveys in Geophysics 44, 947-982.
(1) Subduction zone structure and arc magmatism. We have investigated the 3-D structure of many subduction zones by conducting joint inversion of local and teleseismic data (e.g., Zhao et al., 1992, 1994, 1995, 1997). The subducting slab is imaged clearly as a high-velocity (high-V) zone whose thickness depends on the slab age. Significant low-velocity (low-V) anomalies are revealed in the mantle wedge, which reflect arc magma and fluids associated with corner flow and slab dehydration, leading to the formation of arc volcanoes.
(2) Fluids and earthquakes. A low-V and high Poisson's ratio anomaly was revealed in the source zone of the 1995 Kobe earthquake (M7.2), reflecting a fluid-filled, fractured rock matrix that triggered the rupture of the Nojima fault (Zhao et al., 1996). The fluids in the source zone came from dehydration of the subducting Philippine Sea (PHS) slab under SW Japan (Zhao et al., 2000, 2002). Similar features have been revealed in source zones of various large earthquakes worldwide, suggesting that fluids are probably involved in all types of earthquakes (Zhao, 2021).
(3) Big mantle wedge and intraplate volcanism. East Asian mantle tomography shows that the Pacific slab becomes flat in the mantle transition zone (410-660 km depths) beneath the Korean Peninsula and East China, and a big mantle wedge (BMW) has formed above the slab under East Asia. The intraplate volcanism in NE Asia is caused by hot and wet upwelling flows in the BMW (Zhao et al., 2004, 2009).
(4) Seismic anisotropy and mantle dynamics. The subducting Pacific and PHS slabs exhibit mainly trench-parallel azimuthal anisotropy, which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation, such as normal faults produced at the outer-rise near the trench axis (Zhao et al., 2023). Trench-normal anisotropy is generally revealed in the mantle wedge under the volcanic front and back-arc, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration (Wang & Zhao, 2021; Zhao et al., 2023). Trench-normal anisotropy also occurs in the subslab mantle, reflecting asthenospheric shear deformation associated with the overlying slab subduction (Fan & Zhao, 2021).
(5) Mantle plumes and deep slabs. Global tomography has revealed deeply subducted slabs reaching the core-mantle boundary (CMB). Plume-like, continuous low-V anomalies appear beneath 13 hotspots, suggesting that they are 13 whole-mantle plumes originating from the CMB. These plumes exhibit tilted images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow (Zhao, 2004, 2015).
(6) Lunar tomography and moonquakes. Moonquake arrival-time data recorded by the Apollo seismic network operated during 1969 to 1977 were used to determine the first Vp and Vs tomography beneath the lunar near-side (Zhao et al., 2008, 2012). A correlation between the Vs tomography and the thorium abundance distribution is revealed. The area with high thorium abundance exhibits a distinct low Vs that extends down to ~300 km depth, which may reflect a thermal and compositional anomaly. The distribution of deep moonquakes (700-1300 km depths) shows a correlation with the tomography in the deep lunar mantle. The presence of deep moonquakes and seismic-velocity heterogeneity implies that the interior of the present Moon may still be thermally and dynamically active.
References
Fan, J., D. Zhao (2021). Nature Geoscience 14, 349-353.
Wang, Z., D. Zhao (2021). Science Advances 7, eabc9620.
Zhao, D. et al. (1992). J. Geophys. Res. 97, 19909-19928.
Zhao, D. et al. (1994). J. Geophys. Res. 99, 22313-22329.
Zhao, D. et al. (1995). J. Geophys. Res. 100, 6487-6504.
Zhao, D. et al. (1996). Science 274, 1891-1894.
Zhao, D. et al. (1997). Science 278, 254-257.
Zhao, D. et al. (2000). J. Geophys. Res. 105, 13579-13594.
Zhao, D. et al. (2002). Phys. Earth Planet. Inter. 132, 249-267.
Zhao, D. (2004). Phys. Earth Planet. Inter. 146, 3-34.
Zhao, D., J. Lei, Y. Tang (2004). Chinese Sci. Bull. 49, 1401-1408.
Zhao, D., J. Lei, L. Liu (2008). Chinese Sci. Bull. 53, 3897-3907.
Zhao, D. et al. (2009). Phys. Earth Planet. Inter. 173, 197-206.
Zhao, D. et al. (2012). Global Planet. Change 90, 29-36.
Zhao, D. (2015). Multiscale Seismic Tomography. Springer, 304 pp.
Zhao, D. (2021). Earth-Science Reviews 214, 103507.
Zhao, D. et al. (2023). Surveys in Geophysics 44, 947-982.