10:45 〜 12:15
[SCG58-P16] Arc-parallel changes of water content inferred from the postseismic deformation of the 2011 Tohoku-oki earthquake
キーワード：Postseismic deformation, Power-law rheology, 2011 Tohoku earthquake, Water content, Rheology of mantle wedge, Northeastern Japan
Introduction
Water content is a significant rheological element for viscoelastic earth that controls the subduction zone dynamics from day to millennial scale. Researchers often approximate water contents by either mineralogical analysis of mantle rocks or numerical modeling of geophysical observations. In past decades, several studies recognized a pronounced rheological contrast (Wada & Wang, 2009 Geochem. Geophys. Geosyst.; Muto et al., 2019 Sci. Adv.; Luo & Wang, 2021 Nat. Geosci.), and seismic anisotropy contrast (Uchida et al., 2020 Nat. Commun.) between the sub-arc and forearc mantle wedge. Furthermore, geodetic observations after the 2011 Tohoku-oki earthquake indicate significant changes in crustal deformation patterns between the Miyagi-Yamagata and Fukushima-Niigata areas (Dhar et al., 2022 GJI). Such variations in crustal deformations (particularly, vertical motions; Muto et al., 2016 GRL) are influenced by rheological heterogeneities of the mantle wedge beneath northeastern Japan. These rheological heterogeneities may provide information on the three-dimensional variations of water content across the mantle wedge.
Materials and methods
Here, we employed a three-dimensional rheological model (Dhar et al., 2022 GJI) based on the lab-derived constitutive properties of olivine mineral (Muto et al., 2019 Sci. Adv.) and simulated the five years of geodetic observations following the 2011 Tohoku-oki earthquake across Miyagi-Yamagata and Fukushima-Niigata areas. We used a one-dimensional thermal profile and optimized the water content and afterslip magnitude (aseismic slip on plate interface) using neighbourhood algorithm (Masuti et al., 2016 Nature).
Results and discussion
Our model explains the horizontal and vertical observations (both in time and space) in Miyagi-Yamagata and Fukushima-Niigata areas. The model depicts the oceanic mantle with the effective viscosity of 1019-1020 Pas, which may explain the local purturbation in shear-wave seismic velocity and attenuation tomography (Behn et al., 2009 EPSL). The effective viscosity of the mantle wedge is estimated to be lower than that of the oceanic mantle by one to two orders of magnitude, indicating a higher water content on average. Moreover, the water content of the Fukushima mantle wedge is estimated to be substantially lower than that of the Miyagi mantle wedge. The reduction of water content causes the increase of average viscosity by several orders of magnitude beneath the Fukushima prefectures. This arc-parallel changes in the forearc rheology can also be substantiated by a depressed geothermal anomaly (Tanaka et al., 2004 EPS), deepening of D90 depth (above which 90% earthquake occurs; Omuralieva et al., 2012 Tectonophysics), and seismic attenuation tomography (e.g., Liu et al., 2014 JGR). These arc-parallel changes in water content could be caused by variations in buoyancy-driven fluid migration (Nakajima et al., 2013 JGR) or changes in corner-flow patterns due to the local disparity in subduction obliquity (Wada, 2021 J. Geodyn.). Further analysis of arc-parallel heterogeneity in water content may intrigue future research through laboratory experiments and thermomechanical models (e.g., Horiuchi & Iwamori, 2016 JGR).
Conclusion
Based on the geodetic observations in the postseismic of the 2011 Tohoku-oki earthquake, we inferred an arc-parallel change of mantle-wedge water content between Miyagi-Yamagata and Fukushima-Niigata areas. These lateral changes in water content cause significant rheology changes between these two transects which may explain the previous thermal and seismotectonic observations
Water content is a significant rheological element for viscoelastic earth that controls the subduction zone dynamics from day to millennial scale. Researchers often approximate water contents by either mineralogical analysis of mantle rocks or numerical modeling of geophysical observations. In past decades, several studies recognized a pronounced rheological contrast (Wada & Wang, 2009 Geochem. Geophys. Geosyst.; Muto et al., 2019 Sci. Adv.; Luo & Wang, 2021 Nat. Geosci.), and seismic anisotropy contrast (Uchida et al., 2020 Nat. Commun.) between the sub-arc and forearc mantle wedge. Furthermore, geodetic observations after the 2011 Tohoku-oki earthquake indicate significant changes in crustal deformation patterns between the Miyagi-Yamagata and Fukushima-Niigata areas (Dhar et al., 2022 GJI). Such variations in crustal deformations (particularly, vertical motions; Muto et al., 2016 GRL) are influenced by rheological heterogeneities of the mantle wedge beneath northeastern Japan. These rheological heterogeneities may provide information on the three-dimensional variations of water content across the mantle wedge.
Materials and methods
Here, we employed a three-dimensional rheological model (Dhar et al., 2022 GJI) based on the lab-derived constitutive properties of olivine mineral (Muto et al., 2019 Sci. Adv.) and simulated the five years of geodetic observations following the 2011 Tohoku-oki earthquake across Miyagi-Yamagata and Fukushima-Niigata areas. We used a one-dimensional thermal profile and optimized the water content and afterslip magnitude (aseismic slip on plate interface) using neighbourhood algorithm (Masuti et al., 2016 Nature).
Results and discussion
Our model explains the horizontal and vertical observations (both in time and space) in Miyagi-Yamagata and Fukushima-Niigata areas. The model depicts the oceanic mantle with the effective viscosity of 1019-1020 Pas, which may explain the local purturbation in shear-wave seismic velocity and attenuation tomography (Behn et al., 2009 EPSL). The effective viscosity of the mantle wedge is estimated to be lower than that of the oceanic mantle by one to two orders of magnitude, indicating a higher water content on average. Moreover, the water content of the Fukushima mantle wedge is estimated to be substantially lower than that of the Miyagi mantle wedge. The reduction of water content causes the increase of average viscosity by several orders of magnitude beneath the Fukushima prefectures. This arc-parallel changes in the forearc rheology can also be substantiated by a depressed geothermal anomaly (Tanaka et al., 2004 EPS), deepening of D90 depth (above which 90% earthquake occurs; Omuralieva et al., 2012 Tectonophysics), and seismic attenuation tomography (e.g., Liu et al., 2014 JGR). These arc-parallel changes in water content could be caused by variations in buoyancy-driven fluid migration (Nakajima et al., 2013 JGR) or changes in corner-flow patterns due to the local disparity in subduction obliquity (Wada, 2021 J. Geodyn.). Further analysis of arc-parallel heterogeneity in water content may intrigue future research through laboratory experiments and thermomechanical models (e.g., Horiuchi & Iwamori, 2016 JGR).
Conclusion
Based on the geodetic observations in the postseismic of the 2011 Tohoku-oki earthquake, we inferred an arc-parallel change of mantle-wedge water content between Miyagi-Yamagata and Fukushima-Niigata areas. These lateral changes in water content cause significant rheology changes between these two transects which may explain the previous thermal and seismotectonic observations