FAULT ZONES AND PORE PRESSURES

In fault zones the release of earthquakes is governed by shearing stresses around the zones and pore pressures inside the zone. Increase in either of these can trigger earthquakes. Where large earthquakes are triggered and how large they are is governed by local conditions, f.ex. length of faults. It is concluded on various reasons  Rice:1992 that high pore pressure inside fault zones has significant influence on the stress field at the fault, i.e. to create compression across the fault so the axis of largest compression approaches being perpendicular to it, and thus seemingly to lock the fault. An example of this is the San Andreas fault. However, these faults have strike-slip motion now and then, aseismic or seismic. The reason is that the pore fluids which keep up the subnormal locking pressure lower the breaking strength of the material of the expanding zone locally because of lowering the normal pressure or lubricating faults. But they are in general the unstable component of the system, because they flow around and into cracks which are created. So the locking is unstable and relatively small stress changes caused by earthquake or fluid intrusion outside or inside the system may disrupt the balance of the system by reorientation of stresses and cause earthquakes or lead to internal processes that may by time lead to large earthquakes.