Fracture Alignments in Marine Sediments Off Vancouver Island from Ps Splitting Analysis

Alignments of fractures and cracks in marine sediments may be controlled by various mechanisms such as horizontal compaction and extension and basement faulting. The orientation of these alignments can be estimated through analyses of S-wave splitting. If sensors in ocean-bottom observations are deployed through free fall, sensor orientation needs to be determined in order for the recorded data to be used for such analyses. Here, we estimate the sensor orientation from the linear particle motions of P-to-s (Ps) phases converted at the sediment-basement interface and also from T waves that are excited by earthquakes and propagate in the seawater. We examine waveforms of local earthquakes recorded by 32 ocean-bottom seismometers (OBSs) that were deployed through free fall for three months in 2010 off Vancouver Island where the strike-slip Nootka fault zone (NFZ) intersects the deformation front of the Cascadia subduction zone. Because the particle motion of the Ps wave was corrected by estimating splitting parameters, the fast polarization direction, which reflects S-wave anisotropic structure within the sediment, can also be evaluated. Consequently, we could estimate the fast polarization direction at OBSs deployed near the NFZ and west of the deformation front. The obtained fast directions appeared to correspond to alignments of shear fractures in the marine sediments associated with the left-lateral motion of the fault in the basement along the NFZ, margin-normal cracks due to horizontal compression west of and slightly away from the deformation front, and frontal thrust faults within the accretionary prism near the deformation front.