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[SCG43-06] P-wave anisotropy and intraslab earthquakes in the Tohoku forearc
Keywords:seismic anisotropy, intraslab earthquake, subduction zone
We used P-wave arrival times of local earthquakes recorded at 462 onshore network stations and 150 offshore S-net stations to conduct Vp anisotropic tomography in NE Japan. To focus on the forearc region, we only used the data at those stations where the Pacific slab depth is less than 120 km. We selected an optimal set of 22,908 local events (M 0.0-7.2) from the JMA unified earthquake catalog containing more than 3 million earthquakes that occurred since June 2002. These events are selected to be uniformly distributed in the study volume. Among these events, 6060 earthquakes contain at least one P-wave arrival time recorded at the S-net stations. Our final data set contains a total of 513,519 P-wave arrival times, and about 10% of them are recorded at the S-net stations. In this work, 3-D transverse isotropy with freely-orientated symmetry axis (a special kind of anisotropy) is considered in our model by applying the method of P-wave tilting-axis anisotropic tomography (Wang & Zhao, 2021), which was developed from the isotropic tomographic method of Zhao et al. (1992). By inverting our P-wave arrival-time data, 3-D isotropic Vp variations and anisotropy of the study volume are determined simultaneously (Wang et al., 2022). We adopt the assumption of fast-velocity plane (FVP) with a tilting slow-velocity axis to express the 3-D anisotropic structure of the crust, mantle wedge and the subducted slab (Wang & Zhao, 2021).
In the mantle wedge beneath the forearc between the volcanic arc and the Pacific coast, the obtained FVPs are generally upright and normal to the trench, which indicates vertical and trench-normal fast velocity directions. This feature is well consistent with upwellings associated with slab dehydration and corner flow driven by the plate subduction. The trench-normal fast velocity directions derived from the FVPs in the mantle wedge can also explain the trench-normal fast directions revealed by shear-wave splitting measurements (e.g., Huang et al., 2011). In the upper portion of the subducted Pacific slab, both isotropic Vp and Vs are lower as compared with the core of the slab, whereas Vp/Vs and Poisson’s ratio are relatively higher, implying weak oceanic crust and/or hydrated oceanic mantle. In the Vp anisotropic image, a set of aligned FVPs are revealed, which are parallel to the trench (mostly < 20°) and intersect the slab upper surface at high angles (45°-90°, generally > 60°). The average amplitude of the intraslab anisotropy is ~3%, which is significant as compared with the other parts of the subduction zone. The aligned FVPs indicate trench-parallel fast directions in the slab, which are consistent with previous results of 3-D azimuthal anisotropic tomography. We think that hydrated normal faults formed in the outer-rise subducting with the oceanic plate dominate the intraslab anisotropy in the upper portion of the slab. Other causes for the slab anisotropy such as slab deformation are more likely to influence the core portion of the slab. To confirm this interpretation, we compare the intraslab FVPs with two intraslab earthquakes whose focal mechanisms are considered to also reflect the hydrated faults. About one month after the great 2011 Tohoku-oki earthquake (Mw 9.0), a large intraslab earthquake (Mw 7.1) occurred on April 7, 2011 in the NE Japan forearc. On February 13, 2021, ten years after the great 2011 Tohoku-oki earthquake, another similar intraslab earthquake (Mw 7.1) occurred close to the hypocenter of the 2011 intraslab event. Both of the intraslab events have trench-parallel fault planes that are obviously oblique to the slab surface. Their trench-parallel fault planes are highly consistent with the nearest FVPs (<15°), which is a good verification for the hypothesis of hydrated-faults causing intraslab anisotropy. Ruptures of the hydrated faults may cause the large intraslab earthquakes.
References
Huang, Z., D. Zhao, L. Wang (2011). Shear-wave anisotropy in the crust, mantle wedge and the subducting Pacific slab under Northeast Japan. Geochem. Geophys. Geosyst. 12, Q01002.
Wang, ZW., D. Zhao (2021). 3D anisotropic structure of the Japan subduction zone. Science Advances 7, eabc9620.
Wang, ZW., D. Zhao, X. Chen (2022). Seismic anisotropy and intraslab hydrated faults beneath the NE Japan forearc. Geophys. Res. Lett. 49, e2021GL097266.
Zhao, D., A. Hasegawa, S. Horiuchi (1992). Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909–19928.
In the mantle wedge beneath the forearc between the volcanic arc and the Pacific coast, the obtained FVPs are generally upright and normal to the trench, which indicates vertical and trench-normal fast velocity directions. This feature is well consistent with upwellings associated with slab dehydration and corner flow driven by the plate subduction. The trench-normal fast velocity directions derived from the FVPs in the mantle wedge can also explain the trench-normal fast directions revealed by shear-wave splitting measurements (e.g., Huang et al., 2011). In the upper portion of the subducted Pacific slab, both isotropic Vp and Vs are lower as compared with the core of the slab, whereas Vp/Vs and Poisson’s ratio are relatively higher, implying weak oceanic crust and/or hydrated oceanic mantle. In the Vp anisotropic image, a set of aligned FVPs are revealed, which are parallel to the trench (mostly < 20°) and intersect the slab upper surface at high angles (45°-90°, generally > 60°). The average amplitude of the intraslab anisotropy is ~3%, which is significant as compared with the other parts of the subduction zone. The aligned FVPs indicate trench-parallel fast directions in the slab, which are consistent with previous results of 3-D azimuthal anisotropic tomography. We think that hydrated normal faults formed in the outer-rise subducting with the oceanic plate dominate the intraslab anisotropy in the upper portion of the slab. Other causes for the slab anisotropy such as slab deformation are more likely to influence the core portion of the slab. To confirm this interpretation, we compare the intraslab FVPs with two intraslab earthquakes whose focal mechanisms are considered to also reflect the hydrated faults. About one month after the great 2011 Tohoku-oki earthquake (Mw 9.0), a large intraslab earthquake (Mw 7.1) occurred on April 7, 2011 in the NE Japan forearc. On February 13, 2021, ten years after the great 2011 Tohoku-oki earthquake, another similar intraslab earthquake (Mw 7.1) occurred close to the hypocenter of the 2011 intraslab event. Both of the intraslab events have trench-parallel fault planes that are obviously oblique to the slab surface. Their trench-parallel fault planes are highly consistent with the nearest FVPs (<15°), which is a good verification for the hypothesis of hydrated-faults causing intraslab anisotropy. Ruptures of the hydrated faults may cause the large intraslab earthquakes.
References
Huang, Z., D. Zhao, L. Wang (2011). Shear-wave anisotropy in the crust, mantle wedge and the subducting Pacific slab under Northeast Japan. Geochem. Geophys. Geosyst. 12, Q01002.
Wang, ZW., D. Zhao (2021). 3D anisotropic structure of the Japan subduction zone. Science Advances 7, eabc9620.
Wang, ZW., D. Zhao, X. Chen (2022). Seismic anisotropy and intraslab hydrated faults beneath the NE Japan forearc. Geophys. Res. Lett. 49, e2021GL097266.
Zhao, D., A. Hasegawa, S. Horiuchi (1992). Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909–19928.