Split shear waves are highly sensitive to pore-fluid pressure. Measurement shear-wave splitting could provide a powerful way to monitor subtle changes of pore-fluid pressure if accuracy is high enough. This is particularly difficult when done with marine data, however; when the target is thin, or deep, the shear-wave splitting signal easily becomes overwhelmed by the effects of heterogeneity or P-wave anisotropy. We illustrate the potential for making highly-precise measurement of polarisation directions and time delays of split PS waves using an array of OBS deployed near the Storegga slide offshore Norway. We find a 50-m-thick zone about 150 mbsf, under 950 m of water, in which S-wave polarisation orientations are consistently flipped by 90-degrees. Data from a nearby borehole and results of traveltime inversion of PP and PS phases indicate that this zone is over-pressured. A reflection image across the OBS array shows clear evidence for slope failure detaching on planes within the overpressured zone. We suggest that high-precision measurement of S-wave splitting could be generally applied to the assessment and monitoring of submarine slope stability, with clear application to geotechnical engineering and development of seabed infrastructure.


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