11:30 〜 11:45
[SCG56-10] Importance of heterogeneous rheology model for the postseismic deformation of megathrust earthquake
キーワード：Postseismic deformation, Power-law rheology, Synthetic modeling, 2011 Tohoku earthquake, Depth-dependent rheology, Low-viscosity zone
Introduction
Viscoelastic mantle plays a significant role in relaxing earthquake-induced stress after megathrust earthquakes. By simulating viscoelastic relaxation following megathrust earthquakes, several studies investigated the rheological properties of mantle wedge and oceanic mantle. For example, the linear rheology models that explain the postseismic deformation of the 2011 Tohoku-oki earthquake, often, depict the mantle with uniform viscosities (e.g., Fukuda & Johnson, 2021 JGR). In contrast, several studies showed the geodetic signature of a weak oceanic asthenosphere (Sun et al., 2014 Nature), along-depth viscosity changes (linear Maxwell; Freed et al., 2017 EPSL; Suito, 2017 EPS), and small-scale low-viscosity zone near the volcanic front (linear Burgers; Muto et al., 2016 GRL). Numerous studies that employed power-law Burgers rheology also indicate the heterogeneity in ambient mantle condition (Agata et al., 2019 Nat. Commun.; Muto et al., 2019 Sci. Adv.). Power-law rheology is supported by the constitutive properties of rock derived by laboratory experiments on mantle rocks such as olivine (Bürgmann & Dresen, 2008 Annu. Rev.). These power-law models incorporated rheological heterogeneities based on the two-dimensional thermal structure of mantle-wedge corner flow and implied that the postseismic vertical motion is sensitive to the rheological heterogeneities. Still, how each element of rheological heterogeneity controls vertical movements is poorly understood.
Materials and methods
Here, we employed several two-dimensional synthetic models (Dhar et al., 2023 PEPS) to investigate the role of each heterogeneity including the depth-dependent viscosity, weak oceanic asthenosphere, and low-viscosity zone beneath the volcanic front. We first build a base power-law model (framework from Dhar et al., 2022 GJI) with uniform viscosity for the mantle wedge and oceanic mantle. We then examine the changes in crustal deformation patterns by using several heterogeneous rheology models. We constructed each of the heterogeneities by incorporating the power-law flow law of olivine (Muto et al., 2019 Sci. Adv.).
Results and discussion
Compared with linear rheology, the power-law base model causes a concentration of viscous strain close to the rupture area. The mantle flow of the power-law model displays the same pattern as the linear model but significantly decreases in flow magnitudes. The depth-dependent viscosity suggests that the weak layer in depth range of ~50–100 km largely contributes to the coastal uplift. The amplitude of this coastal uplift is majorly sensitive to the minimum viscosity of the weak layer. Similarly, the oceanic asthenosphere (depth of ~100–180 km) predominantly controls the subsidence in near-trench areas. Furthermore, the low-viscosity zone beneath the volcanic front governs the vertical deformation of the areas from the volcanic front to the forearc in the transient period. Our results delineate the individual contribution of these rheological heterogeneities (Dhar et al., 2023 PEPS), which may help us to quantitatively constrain the subduction zone rheology by using vertical geodetic observations. Moreover, the importance of our results may help build efficient postseismic models to understand the complex interplay of various deformation mechanisms, especially for the postseismic uplift along the Pacific coast of Japan (Iinuma, 2018 JDR).
Conclusion:
Using synthetic modeling, we demonstrated that each element of rheological heterogeneities has a specific role in the postseismic crustal deformation: forearc uplift by weak mantle wedge, volcanic front’s subsidence by low-viscosity zone, and near-trench deformation by oceanic asthenosphere. These significant controls of the viscoelastic mantle may help define the ground deformation pattern in the coming decades.
Viscoelastic mantle plays a significant role in relaxing earthquake-induced stress after megathrust earthquakes. By simulating viscoelastic relaxation following megathrust earthquakes, several studies investigated the rheological properties of mantle wedge and oceanic mantle. For example, the linear rheology models that explain the postseismic deformation of the 2011 Tohoku-oki earthquake, often, depict the mantle with uniform viscosities (e.g., Fukuda & Johnson, 2021 JGR). In contrast, several studies showed the geodetic signature of a weak oceanic asthenosphere (Sun et al., 2014 Nature), along-depth viscosity changes (linear Maxwell; Freed et al., 2017 EPSL; Suito, 2017 EPS), and small-scale low-viscosity zone near the volcanic front (linear Burgers; Muto et al., 2016 GRL). Numerous studies that employed power-law Burgers rheology also indicate the heterogeneity in ambient mantle condition (Agata et al., 2019 Nat. Commun.; Muto et al., 2019 Sci. Adv.). Power-law rheology is supported by the constitutive properties of rock derived by laboratory experiments on mantle rocks such as olivine (Bürgmann & Dresen, 2008 Annu. Rev.). These power-law models incorporated rheological heterogeneities based on the two-dimensional thermal structure of mantle-wedge corner flow and implied that the postseismic vertical motion is sensitive to the rheological heterogeneities. Still, how each element of rheological heterogeneity controls vertical movements is poorly understood.
Materials and methods
Here, we employed several two-dimensional synthetic models (Dhar et al., 2023 PEPS) to investigate the role of each heterogeneity including the depth-dependent viscosity, weak oceanic asthenosphere, and low-viscosity zone beneath the volcanic front. We first build a base power-law model (framework from Dhar et al., 2022 GJI) with uniform viscosity for the mantle wedge and oceanic mantle. We then examine the changes in crustal deformation patterns by using several heterogeneous rheology models. We constructed each of the heterogeneities by incorporating the power-law flow law of olivine (Muto et al., 2019 Sci. Adv.).
Results and discussion
Compared with linear rheology, the power-law base model causes a concentration of viscous strain close to the rupture area. The mantle flow of the power-law model displays the same pattern as the linear model but significantly decreases in flow magnitudes. The depth-dependent viscosity suggests that the weak layer in depth range of ~50–100 km largely contributes to the coastal uplift. The amplitude of this coastal uplift is majorly sensitive to the minimum viscosity of the weak layer. Similarly, the oceanic asthenosphere (depth of ~100–180 km) predominantly controls the subsidence in near-trench areas. Furthermore, the low-viscosity zone beneath the volcanic front governs the vertical deformation of the areas from the volcanic front to the forearc in the transient period. Our results delineate the individual contribution of these rheological heterogeneities (Dhar et al., 2023 PEPS), which may help us to quantitatively constrain the subduction zone rheology by using vertical geodetic observations. Moreover, the importance of our results may help build efficient postseismic models to understand the complex interplay of various deformation mechanisms, especially for the postseismic uplift along the Pacific coast of Japan (Iinuma, 2018 JDR).
Conclusion:
Using synthetic modeling, we demonstrated that each element of rheological heterogeneities has a specific role in the postseismic crustal deformation: forearc uplift by weak mantle wedge, volcanic front’s subsidence by low-viscosity zone, and near-trench deformation by oceanic asthenosphere. These significant controls of the viscoelastic mantle may help define the ground deformation pattern in the coming decades.