HR: 14:55h
AN: S43C-06    [Abstracts]
TI: Azimuthal Seismic Anisotropy From a Multi-Component PS-wave Experiment West of Svalbard.
AU: * Haacke, R R
EM: rrh826@bham.ac.uk
AF: Earth Sciences, School of Geography, Earth and Environmental Sciences University of Birmingham, Edgbaston, B15 2TT United Kingdom
AU: Westbrook, G K
S43C-06 AF: Earth Sciences, School of Geography, Earth and Environmental Sciences University of Birmingham, Edgbaston, B15 2TT United Kingdom
AU: Peacock, S
S43C-06 AF: Earth Sciences, School of Geography, Earth and Environmental Sciences University of Birmingham, Edgbaston, B15 2TT United Kingdom
AU: Long, C
S43C-06 AF: Earth Sciences, School of Geography, Earth and Environmental Sciences University of Birmingham, Edgbaston, B15 2TT United Kingdom
AB: A high-resolution seismic experiment, using an array of closely spaced ocean-bottom seismometers, was conducted by the HYDRATECH project at a site near the intersection of the Knipovich ridge and the Molloy transform at the base of the continental slope west of Svalbard. A series of novel techniques were used to detect and characterise azimuthal anisotropy from PS waves generated on reflection to reveal the presence of horizontal transverse isotropy with depth-variable symmetry orientation. Two independent techniques for layer-stripping converted-wave data to derive the lag and orientation of the split S waves were developed, one of them an adaptation of the Alford rotation technique normally used with VSPs. The techniques were tested using synthetic anisotropic waveforms generated with the Aniseis software. These techniques were successfully applied to the experimental data, and revealed two primary anisotropic zones; an upper zone (from 30-140 mbsf) with the fast symmetry plane oriented at an azimuth of 130-140°, parallel to the local strike of the dipping seabed and which appears likely to be caused by down-slope gravitational stresses, and a lower zone (from 180 mbsf to the limit of the sampled depths) with fast symmetry plane oriented at 50-60°, near-parallel to the maximum principal stress of the tectonic regime associated with the nearby Knipovich ridge and Molloy transform zone. The shear-wave splitting accumulated at a rate of ~200 ms/km in the upper zone and at a rate of ~120 ms/km in the lower zone. Because of the nearly 90° difference in orientation of the symmetry axes of the two anisotropic layers, the net shear-wave splitting observed at the seabed increased to a peak at around 160 mbsf before decreasing again in the lower anisotropic zone as the dominant stress regime changed. A region with no azimuthal anisotropy (but which may still have VTI symmetry) centred at 160 mbsf, separates the competing stress regimes dominant in the two anisotropic zones. If the anisotropy is caused by aligned vertical cracks then the crack density in both zones is around 0.05.
UR: http://www.hydratech.bham.ac.uk
DE: 7245 Mid-ocean ridges
DE: 7250 Transform faults
DE: 7260 Theory
SC: Seismology [S]
MN: Fall Meeting 2005