We measured the spatiotemporal evolution of pore pressure on- and off-fault during failure and slip in initially intact Westerly granite under triaxial conditions. The pore pressure perturbations in the fault zone and the surrounding bulk presented opposite signs upon shear failure, resulting in large pore pressure gradients over small distances (up to 10 MPa/cm). The on-fault pore pressure dropped due to localized fault dilation associated with fracture coalescence and fault slip, and the off-fault pore pressure increased due to bulk compaction resulting from the closure of dilatant microcracks mostly parallel to the maximum compression axis. We show that a reduction in bulk porosity and relatively undrained conditions during failure are necessary for the presence of the off-fault pore pressure elevation. Considering this phenomenon as a consequence of a main shock, we further show that off-fault pore pressure increase has the potential to trigger neighboring fault instabilities. In nature, we expect the phenomenon of off-fault pore pressure increase to be most relevant for misoriented faults, where the pre-rupture stresses can be large enough to reach the dilatancy threshold in the wall rocks.

中文翻译：

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实验室模拟地震断层内外孔隙压力的相反变化


我们测量了三轴条件下最初完整的西风花岗岩在破坏和滑移过程中断层内外孔隙压力的时空演化。断裂带和周围体中的孔隙压力扰动在剪切破坏时表现出相反的迹象，导致小距离内出现大的孔隙压力梯度（高达 10 MPa/cm）。由于与裂缝合并和断层滑移相关的局部断层扩张，断层内孔隙压力下降，而断层外孔隙压力由于大多平行于最大压缩轴的膨胀微裂缝闭合导致的整体压实而增加。我们表明，故障期间体积孔隙率的降低和相对不排水的条件对于断层外孔隙压力升高的存在是必要的。考虑到这种现象是主震的结果，我们进一步表明断层外孔隙压力的增加有可能引发邻近断层的不稳定。在自然界中，我们预计断层外孔隙压力增加的现象与定向错误的断层最相关，其中预破裂应力可能大到足以达到围岩中的剪胀阈值。