Computer simulations of California tectonics confirm very low strength of major faults
Computer simulations of California tectonics confirm very low strength of major faults
复制标题
加州构造的计算机模拟证实主要断层的强度非常低
DOI：
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发表时间：
1994
期刊：
影响因子：
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通讯作者：
X. Kong
中科院分区：
文献类型：
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作者：
P. Bird;X. Kong
最近改进的技术建模断层网络与薄板有限元产生完全收敛的解决方案，没有任何妥协的代表性的摩擦流变学的上地壳或过渡到位错蠕变在下地壳。
我们应用这些技术来模拟加州地区，将所有故障估计滑动速率超过1毫米/年，以及海拔，热流和地壳厚度的变化。模型两侧的速度边界条件基于NUVEL-1板块模型和大盆地变形的近似值。地壳的摩擦和位错蠕变流变常数经过校准，以再现在350-410 °C的模型温度下发生的地震活动最大深度的观测变化。这留下了两个自由参数：断层上的（时间平均）摩擦系数和下地壳蠕变的表观活化能。
这些参数分别在三组10、81和64个实验中系统地变化。每个模型的预测进行了测试，对三个已公布的数据集：一组79个地质限制断层的平均滑动速率，一组221个主应力方向，和一组841个长期率的大地测量基线（三边测量和VLBI [甚长基线干涉测量]）。模型得分的模式表明，主要断层的时均摩擦系数仅为0.17-0.25，仅为居间块体摩擦系数（0.85）的20%~ 30%. (The单独的激光三边测量数据将表明对高应力模型的偏好，但是该模型是具有非常小的断层滑动和地震活动性的模型，这可以基于其他理由而被拒绝。）在最后的三参数集的64个模型，我们测试是否异常薄弱的故障是均匀的或成比例的净滑移。与滑相关的削弱，有一个普遍的减少预测误差，这支持了假设。地质数据和大地测量数据一致表明，断层软弱（即摩擦力为0.17）是普遍的，圣安德烈亚斯还有30%的滑动相关弱化（摩擦力为0.12，或正常值的14%）。然而，如果所有的弱点都是滑动相关的（实际上，只有圣安德烈亚斯是弱的），那么应力数据是最好的。因此，不应拒绝所有弱化都与滑移有关的假设。
最好的模型预测所有可用的数据与RMS（均方根）失配低至3毫米/年（相对板块速度的6%），所以他们的预测可能是有用的地震危险性估计，至少沿着故障没有数据。
这项研究扩展了发现非常低的摩擦从俯冲带，在那里它是以前记录的，一个占主导地位的走滑系统，其中下冲的湿沉积物是不普遍的。先前俯冲的水从地幔中返回，并以细粒泥的形式进行化学或物理结合，可能是摩擦力非常低的原因。
Recent improvements in the technique of modeling fault networks with thin-plate finite elements yield full convergence of the solutions, without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust.
We apply these techniques to model the California region, incorporating all faults with estimated slip rate over 1 mm/yr, as well as variations in elevation, heat flow, and crustal thickness. Velocity boundary conditions on the model sides are based on the NUVEL-1 plate model and an approximation of deformation in the Great Basin. The frictional and dislocation-creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity, which occurs at model temperatures of 350-410 °C. This leaves two free parameters: the (time-averaged) coefficient of friction on faults, and the apparent activation energy for creep in the lower crust.
These parameters are systematically varied in three sets of 10, 81, and 64 experiments, respectively. The predictions of each model are tested against three published data sets: a set of 79 geologic limits on average slip rate on faults, a set of 221 principal stress directions, and a set of 841 secular rates of geodetic baselines (both trilateration and VLBI [very long baseline interferometry]). The patterns of model scores indicate that the time-averaged friction coefficient of major faults is only 0.17-0.25, or only 20%-30% of the value (0.85) that is assumed for the friction in the intervening blocks. (The laser trilateration data taken alone would indicate a preference for a high-stress model, but this model is one with very little fault slip and seismicity, which can be rejected on other grounds.) In the final three-parameter set of 64 models, we test whether the anomalous weakness of faults is uniform or proportional to net slip. With slip-dependent weakening, there is a general reduction in prediction errors, which supports the hypothesis. The geologic data and geodetic data agree in implying that fault weakness (that is, friction of 0.17) is general, with an additional 30% slip-dependent weakening of the San Andreas (to friction of 0.12, or 14% of normal). The stress data, however, are fit best if all weakness is slip dependent (effectively, only the San Andreas is weak). Thus, the hypothesis that all weakening is slip dependent should not be rejected.
The best models predict all available data with RMS (root mean square) mismatch of as little as 3 mm/yr (6% of the relative plate velocity), so their predictions may be useful for seismic hazard estimation, at least along faults where no data are available.
This study extends the finding of very low friction from subduction zones, where it was previously documented, to a dominantly strike-slip system in which underthrusting of wet sediment is not widespread. Return of previously subducted water from the mantle and its chemical or physical binding in fine-grained gouge are possible explanations for the very low friction.