High pore-fluid pressures at Bhuj, inferred from 90°-flips in shear-wave polarizations

Abstract

SUMMARY Shear-wave splitting above local earthquakes in the aftershock zone of the 2001, M w7.6, Bhuj earthquake display a spatial variation of anisotropy related to crack orientation. At most seismic stations, the fast polarization directions are approximately NNE‐SSW, parallel to the inferred directions of maximum horizontal stress, suggesting the dilatancy-induced anisotropy resulting from approximately stress-aligned parallel vertical microcracks. In contrast, the fast polarization directions at Station Samkhyali (SKL), very close to the epicentre of the main shock, are fault parallel, approximately EW and almost orthogonal to the stress-aligned polarizations elsewhere. APE modelling and observations elsewhere suggest that these are 90 ◦ flips in shearwave polarizations, caused by propagation through microcracks containing fluids at critically high pore-fluid pressures surrounding the seismically active fault plane. The presence of high pore-fluid pressures on all seismogenic faults also explains the high scatter in shear-wave time delays. The average path-normalized delay time varies from 2 to 4 ms km −1 , except at Station SKL, where higher delays of up to 13 ms km −1 are found. The large scatter in measured time delays is caused by cracks saturated with high-pressure pore fluids. These observations can be explained by the presence of both normal stress-aligned and 90 ◦ -flipped, shear-wave polarizations at Bhuj. This suggests that the Kutch Mainland Fault is a major tectonic feature.