Implications of an Elastic Analysis of In Situ Stress Measurements Near the San Andreas Fault
Implications of an Elastic Analysis of In Situ Stress Measurements Near the San Andreas Fault
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DOI：
10.1029/jb087ib09p07797
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发表时间：
1982-09
影响因子：
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通讯作者：
A. Mcgarr;M. Zoback;T. Hanks
中科院分区：
文献类型：
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作者：
A. Mcgarr;M. Zoback;T. Hanks
加州棕榈谷附近用水力压裂技术获得的29个地应力测量结果，是对圣安德烈亚斯断层附近莫哈韦沙漠应力状态进行弹性分析的基础。测量的深度从80米到849米，距离断层2到34公里。弹性解表明了大陆典型的偏应力状态，因为推断的最大剪应力的深度梯度约为7.9兆帕/千米。外推得到地壳上部14公里处的平均剪应力约为56兆帕，这一结果高于根据热流数据所估计的圣安德烈亚斯断裂的平均剪应力。然而，如果偏应力的区域状态受到断裂带强度的限制，这一发现与基于圣安德烈亚斯断层泥样品摩擦系数的实验室测定的断层强度估计是一致的。如果是这样，那么从应力场结果推断的圣安德烈亚斯断裂带的摩擦系数约为0.45。应力状态似乎没有随着距离圣安德烈亚斯断层的距离而有系统的变化，尽管观察到了相当大的局部变化。观测结果表明，垂直于圣安德烈亚斯断裂方向的最大剪应力水平梯度的上限约为0.1兆帕/千米，这意味着对孕震层底部施加的剪力约为1.4兆帕。最后，我们展示了地应力数据在直接评估累积滑移方面的潜在应用，这些累积滑移可以在大地震中释放。我们表明，基于一个包含闭锁断层的模型，延伸约22公里，闭锁部分以下的总断层滑动不到13m。一组更全面的应力数据可以允许估计更低的界限。
Twenty-nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.