8:45 AM - 9:00 AM
[J03-3-02] Anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release by oceanic intraplate earthquakes
1. Introduction
Oceanic lithosphere experiences thermal contraction, which is up to 3 % in volume, due to cooling after it is generated at oceanic ridges. Thermal stress due to thermal contraction is the main cause of oceanic intraplate earthquakes (OCEQs). One unresolved problem regarding this phenomenon is how freely the oceanic lithsphere contracts in horizontal directions due to thermal contraction. In other words, how is the boundary condition of the oceanic lithosphere? This is important for estimating driving/resistive forces of mantle convection through force equilibrium of lithosphere, and clarifying the cause of orthogonal ridge-transform fault systems. Thus, this study attempted to estimate the horizontal thermal contraction rates of the oceanic lithosphere by analyzing stress release due to observed OCEQs.
2. Method
First, we analyzing each component of stress release due to observed OCEQs in world oceanic lithosphere in 1964-2015. We found strong anisotropy of stress release in young oceanic lithosphere (5-15Ma) that extensional stress release dominates in the ridge-parallel component and compressional stress release dominates in the spreading-directional component. Next, we conducted a numerical simulation of thermal stress evolution of the oceanic lithosphere in order to investigate how large anisotropy of thermal contraction can explain the observed anisotropic stress release.
3. Results and Discussion
By comparing the observed and modeled brittle stress release rates, we found that ridge-parallel contraction rates of young oceanic lithosphere are only 0-30 % of spreading-directional contraction rates; it means strong anisotropic horizontal thermal contraction. This smallness of ridge-parallel contraction produces extensional thermal stress in the ridge-parallel component, which may play a role of weakening of long transform faults by reducing normal stress on the transform faults.
Oceanic lithosphere experiences thermal contraction, which is up to 3 % in volume, due to cooling after it is generated at oceanic ridges. Thermal stress due to thermal contraction is the main cause of oceanic intraplate earthquakes (OCEQs). One unresolved problem regarding this phenomenon is how freely the oceanic lithsphere contracts in horizontal directions due to thermal contraction. In other words, how is the boundary condition of the oceanic lithosphere? This is important for estimating driving/resistive forces of mantle convection through force equilibrium of lithosphere, and clarifying the cause of orthogonal ridge-transform fault systems. Thus, this study attempted to estimate the horizontal thermal contraction rates of the oceanic lithosphere by analyzing stress release due to observed OCEQs.
2. Method
First, we analyzing each component of stress release due to observed OCEQs in world oceanic lithosphere in 1964-2015. We found strong anisotropy of stress release in young oceanic lithosphere (5-15Ma) that extensional stress release dominates in the ridge-parallel component and compressional stress release dominates in the spreading-directional component. Next, we conducted a numerical simulation of thermal stress evolution of the oceanic lithosphere in order to investigate how large anisotropy of thermal contraction can explain the observed anisotropic stress release.
3. Results and Discussion
By comparing the observed and modeled brittle stress release rates, we found that ridge-parallel contraction rates of young oceanic lithosphere are only 0-30 % of spreading-directional contraction rates; it means strong anisotropic horizontal thermal contraction. This smallness of ridge-parallel contraction produces extensional thermal stress in the ridge-parallel component, which may play a role of weakening of long transform faults by reducing normal stress on the transform faults.