Effective stress, friction, and deep crustal faulting
Effective stress, friction, and deep crustal faulting
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有效应力、摩擦力和深部地壳断层
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发表时间：
2016
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通讯作者：
R. Bürgmann
中科院分区：
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作者：
N. Beeler;G. Hirth;A. Thomas;R. Bürgmann
地壳断层和岩石摩擦的研究总是假设决定摩擦滑动过程中断层剪切阻力的有效法向应力是施加的法向应力减去孔隙压力。在这里，我们提出了脆性转变 (BDT) 附近温度和应力下的有效应力系数 αf 的表达式，该表达式取决于断层上固固接触面积的百分比。 αf 随深度变化，并且当粗糙接触的屈服强度大大超过所施加的法向应力时，αf 仅接近 1。对于静水孔隙压力下的垂直走滑石英断层带，假设摩擦剪切带宽度和韧性剪切剪切带宽度分别为 1mm 和 1km，BDT 约为 13km。接近 1 的 αf 仅限于剪切带狭窄的深度。低于 BDT αf = 0 是由于应变率急剧下降。在这些情况下，摩擦不能通过单独增加孔隙压力而在 BDT 以下重新激活，并且需要局部化。如果孔隙压力增加且断层定位回 1 毫米，则脆性行为可能会发生到约 35 公里的深度。有效应力、接触尺度应变率和孔隙压力之间的相互依赖性可以估计帕克菲尔德附近的圣安地列斯和一些俯冲带发生深部低频地震活动所需的条件。其中的含义是，分离浅层地震和深部低频地震活动的区域中的剪切力是分布的，并且更深的区域涉及升高的孔隙流体压力和局部化。
Studies of crustal faulting and rock friction invariably assume the effective normal stress that determines fault shear resistance during frictional sliding is the applied normal stress minus the pore pressure. Here we propose an expression for the effective stress coefficient αf at temperatures and stresses near the brittle‐ductile transition (BDT) that depends on the percentage of solid‐solid contact area across the fault. αf varies with depth and is only near 1 when the yield strength of asperity contacts greatly exceeds the applied normal stress. For a vertical strike‐slip quartz fault zone at hydrostatic pore pressure and assuming 1 mm and 1 km shear zone widths for friction and ductile shear, respectively, the BDT is at ~13 km. αf near 1 is restricted to depths where the shear zone is narrow. Below the BDT αf = 0 is due to a dramatically decreased strain rate. Under these circumstances friction cannot be reactivated below the BDT by increasing the pore pressure alone and requires localization. If pore pressure increases and the fault localizes back to 1 mm, then brittle behavior can occur to a depth of around 35 km. The interdependencies among effective stress, contact‐scale strain rate, and pore pressure allow estimates of the conditions necessary for deep low‐frequency seismicity seen on the San Andreas near Parkfield and in some subduction zones. Among the implications are that shear in the region separating shallow earthquakes and deep low‐frequency seismicity is distributed and that the deeper zone involves both elevated pore fluid pressure and localization.