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，以及确定基底剪切牵引力和附加俯冲板块对板块施加的力的内在困难。本项目采用简单的显式定义，将平衡力矩分为4个分量：(1)静石压力力矩；(2)侧强扭矩；(3)基底阻力力矩；(4)净板拉扭矩。它应用复杂的薄壳建模工具，将真实地形、可变热流、可变地壳和岩石圈厚度、真实的非线性流变学和断层结合起来。计算各板块上的扭矩平衡分以下几个步骤：(1)建立由52个板块和13个造山带组成的全球岩石圈的高分辨率模型；(2)对每个板块施加正确的速度，无论是在与俯冲板块相连的边缘（如果有的话），还是在强板块内部的点上；(3)计算反力，并将其相加，求出每块板上必须由基底剪切力和净板拉力提供的总扭矩；(4)将这些力矩作为基底剪切力重新分配到没有板块的板块基底上。在板块速度没有明显变化的情况下，重新计算了全局动态平衡，得到了更真实的板块和造山带内部的应力和应变率分布。为了考虑对板块边界断层有效摩擦的依赖性（许多先前的研究表明这种依赖性很低），正在对具有不同有效断层摩擦系数的单参数模型套件进行计算。正在根据(a)某些无板下地震各向异性的方位角进行预测；(b)大地速度；(c)海底扩张速度；(d)断层的地质滑动速率；(e)板内应力方向。基底剪切牵引力和净板牵引力的总和被分解为每个附板板的两个分量。没有附板的板块上的基底牵引力表明基底牵引力的一般水平。已知的上地幔地震各向异性给出了基底剪切牵引的两个可能方向。这些约束条件允许进行全局（52板块x 3扭矩分量）反演研究，以确定净板块拉力如何取决于已知的相对板块速度、绝对海沟速度、板块年龄和/或海沟深度等因素。确定（许多）板块上基底剪切牵引的方向和意义将约束全球对流模型，从而使其他人能够确定地幔其余部分的粘度和密度异常结构。了解净板拉力如何取决于上述因素，将有助于理解板块速度在地质历史上发生变化的原因。最终的最佳板内动应力平衡模型将给出板内非弹性应变率的全局预测，这将用于地震危险性估计。

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.