Indication of high pore-fluid pressures in a seismically-active fault zone

Polarisations of seismic shear-wave splitting observed above small earthquakes in Iceland are typically approximately NE to SW, parallel to the direction of maximum horizontal stress. In contrast, the polarisations of shear-waves at three new stations sited over the Húsavík–Flatey Fault, a major seismically-active transform fault in northern Iceland, are approximately NW to SE, orthogonal to the stress-aligned polarisations elsewhere. Modelling suggests that these 90°-flips in polarisations are caused by propagation through cracks containing fluids at high pore-fluid pressures within one or two MPa of the critical stress. These observations suggest that high pore-fluid pressures, which play a key role in earthquake source mechanisms, can be monitored by analysing shear-wave splitting above seismically-active fault planes.