If brittle fault strength depends only on friction, slip instability is discouraged at low effective normal stress, σ. Stress drop and the critical stiffness necessary for unstable sliding both vanish with σ; small earthquakes cannot occur. Very low σ is inferred in the source region of low-frequency earthquakes (LFEs) on the San Andreas fault (SAF). Moreover, if pore pressure, p, is undrained at low σ, then instabilities are prevented at all scales. This is due to dilatant strengthening which arises due to a dependence of porosity on strain rate. Dilatant strengthening is σ-independent and dominates at low σ. Undrained p is inferred over time scales of less than a few days for the SAF LFEs. Based on experiments that measure rapid contact overgrowth between 350 and 530°C at very low σ, fault failure controlled by time-dependent cementation is invoked as an explanation for the SAF LFEs. Because this “cohesion” is σ-independent, stress drops can occur at σ = 0. If in addition cohesion exceeds any dilatant strengthening during slip, cohesion dominates strength at low σ. Dilatancy measured in prior faulting and shear experiments indicate that at all stress levels steady-state porosity depends on σ in addition to strain rate. Moreover, porosity at low σ depends elastically on the confining and differential stresses. A model with these additional pore pressure effects, friction, and time-dependent cohesion, applied to the SAF LFEs produces stress drops, slip speeds, and durations that are consistent with the observations, when the shear-induced dilatancy is not extreme.

中文翻译：

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高温下的脆性断层和消失的有效应力

如果脆性断层强度仅取决于摩擦，则在低有效正应力σ下不鼓励滑动不稳定性。不稳定滑动所需的应力降和临界刚度均随σ消失；小地震不可能发生。在圣安德烈亚斯断层 (SAF) 的低频地震 (LFE) 源区推断出非常低的σ 。此外，如果孔隙压力p在低σ时不排水，那么在所有尺度上都可以防止不稳定性。这是由于孔隙率对应变率的依赖性而产生的膨胀强化。膨胀强化与σ无关，在低σ时占主导地位。不排水p对于 SAF LFE，在不到几天的时间范围内推断。基于在 350 和 530°C 之间在非常低的σ下测量快速接触过度生长的实验，由时间相关的胶结控制的断层失效被用作对 SAF LFE 的解释。因为这种“内聚力”与σ无关，所以在σ = 0时会发生应力下降 。此外，如果在滑移过程中内聚力超过任何剪胀强化，则内聚力在低σ时支配强度。在先前的断层和剪切实验中测量的剪胀性表明，在所有应力水平下，稳态孔隙度除应变率外还取决于σ 。此外，低σ的孔隙率弹性地取决于限制应力和微分应力。当剪切引起的剪胀不是极端时，应用于 SAF LFE 的具有这些额外孔隙压力效应、摩擦和时间相关内聚力的模型会产生与观察结果一致的应力降、滑移速度和持续时间。