Direct Computation and Systematic Interpretation of Plate Driving Forces Worldwide

由于以下几个原因，人们对驱动地球板块的力的动态平衡知之甚少：未定义的术语、过于简化的模型，以及确定底部剪切力和附加的俯冲板块对板块施加的力的内在困难。本项目使用简单明确的定义，将平衡扭矩分为4个部分：(1)静压扭矩；(2)侧向强度扭矩；(3)基本阻力扭矩；(4)净拉力扭矩。它应用了复杂的薄壳建模工具，结合了真实的地形、可变的热流、可变的地壳和岩石圈厚度、逼真的非线性流变性和断层。每个板块上的扭矩平衡是分步骤计算的：(1)建立一个由52个板块和13个造山带组成的高分辨率全球岩石圈模型；(2)在每个板块上施加正确的速度，无论是在与俯冲板块相连的边缘(如果有)，还是在强板块内部的点上；(3)计算反作用力并求和，以求出每个板块上必须由基面剪力和板片净拉力提供的总扭矩；(4)在没有板块的板块的基础上，将这些扭矩作为基面剪力重新分配。全球动态平衡被重新计算，以获得更真实的板块和造山带内的应力和应变率分布，而不会显著改变板块速度。为了考虑对板块边界断层的有效摩擦的依赖性(许多以前的研究表明有效摩擦系数很低)，正在对具有不同有效断层摩擦系数的一组单参数模型进行计算。预测的依据是：(A)某些无板块板块下地震各向异性的方位；(B)大地速度；(C)海底扩展速率；(D)断层上的地质滑动速率；(E)板内应力方向。对于每个带有附着板的板，底部剪力和净板拉力矩之和被分解为两个分量(S)。没有附着板的板上的基面牵引力表示基面牵引力的总体水平。在已知的情况下，上地幔的地震各向异性为基底剪切牵引提供了两个可能的方向。这些约束允许进行全球(52个板块x 3个扭矩分量)反演研究，以确定净板拉力如何取决于相对板块速度、绝对沟槽速度、板龄和/或沟槽深度等已知因素。确定(许多)板块上的底部剪切牵引力的方向和感觉将约束全球对流模型，使其他模型能够确定地球地幔其余部分的粘性和密度异常结构。了解板块净拉力是如何依赖于上述因素的，将有助于理解板块速度在地质历史中发生变化的原因。最终的最优动态应力平衡模型将给出板块内部滞弹性应变率的全局预测，这将用于地震危险性评估。

The dynamic balance of forces that drive the Earth's plates is poorly understood for several reasons: undefined terms, oversimplified models, and the intrinsic difficulty of determining the forces exerted on plates by basal shear tractions and attached subducting slabs. This project uses simple explicit definitions that divide the balanced torques into 4 components: (1) lithostatic pressure torque; (2) side-strength torque; (3) basal drag torque; and (4) net slab pull torque . It applies sophisticated thin-shell modeling tools that incorporate real topography, variable heat-flow, variable crust and lithosphere thickness, realistic nonlinear rheologies, and faults. The balance of torques on each plate are being computed in steps: (1) construct a high-resolution model of the global lithosphere with 52 plates and 13 orogens; (2) impose the correct velocity on each plate, either at edges connected to subducting slabs (if any), or at points in the strong plate interior; (3) compute the reaction forces, and sum them, to find the total torque on each plate that must be supplied by basal shear tractions and net slab pull; (4) redistribute these torques as basal shear tractions on the bases of plates with no slabs. The global dynamic balance is recomputed to obtain more realistic stress and strain rate distributions within the plates and orogens without significant changes in plate velocities. To account for dependence on the effective friction of plate boundary faults (which many previous studies have shown to be low) computations are being carried out for a one-parameter suite of models with different effective fault friction coefficients. Predictions are being scored against (a) azimuths of seismic anisotropy under certain slab-free plates; (b) geodetic velocities, (c) seafloor spreading rates; (d) geologic slip rates on faults; and (e) intraplate stress directions. The sum of basal shear traction and net slab pull torques is being decomposed into its two components for each of the plates with attached slab(s). Basal tractions on plates with no attached slabs indicate the general level of basal tractions. Seismic anisotropy of the upper mantle, where known, gives two possible directions for basal shear traction. These constraints permit a global (52 plates x 3 torque components) inversion study to determine how net slab pull depends on known factors of relative plate velocity, absolute trench velocity, slab age, and/or trench depth. Determining the direction and sense of basal shear traction on (many) plates will constrain global convection models, allowing others to determine the viscosity and density-anomaly structures in the rest of Earth's mantle. Understanding how net slab pull depends on the factors listed above will help understand why plate velocities have changed in geologic history. The final best model of dynamic stress balance in the plates will give global predictions of anelastic strain rates in plate interiors, which will be used in seismic hazard estimation.