21. SEISMIC VELOCITIES IN THE NORTHERN BARBADOS RIDGE ACCRETIONARY COMPLEX, SITE 949 ¹

Sheila Peacock,² Graham K. Westbrook,² and Daniel P. Graham ²

ABSTRACT

Two vertical seismic profiles (VSPs) were recorded in Hole 949C of Ocean Drilling Program Leg 156 to determine P- and S-wave velocity structure. The sources were an air gun suspended from the ship at zero offset, and ocean bottom explosive shots. Direct and reflected P-waves and mode-converted S-waves were recorded. Mode-converted S-waves downgoing and upgoing from the zero-offset VSP give an upper bound to S-wave velocities (V_s ) for the wedge. They originate from a thrust fault at 278 m below sea floor (mbsf) and show two low-V_s layers, 492 m/s between 245 and 278 mbsf and 367 m/s between 320 mbsf and the décollement at 398 mbsf, each with a high-velocity cap (greater than 500 m/s). (Similar waves in Hole 948D were misinterpreted as tube waves.) A horizontal-layer model with anisotropy is an alternative to the dipping isotropic model that would explain the presence of mode-converted S-waves in the zero-offset VSP.

S-waves generated by the ocean-bottom shots were not received as intended, because the rapid increase of S-wave velocity with depth bent the raypaths away from the downhole seismometer array. Mode-converted reflected S-waves had a maximum velocity of 520 m/s, whereas water-surface reflected P-waves had maximum velocities no greater than 1730 m/s at the base of the hole at 397 mbsf. A two-dimensional model derived by P-wave raytracing has a low-velocity zone (V_P = 1681 m/s) at the base of the accreted wedge that is overlain by a higher-velocity zone (V_P = 1746 m/s) between 100 and 200 mbsf. An SH component of P-to-S mode-converted reflected waves shows that the waves could have passed through or been reflected from an anisotropic layer with a non-vertical axis of symmetry, either within or just below the accretionary wedge.

The S-wave velocities are higher than laboratory values at zero effective pressure (114-260 m/s). The low velocities at the base of the accretionary wedge indicate high porosity and overpressure, but if the porosity consists of horizontal cracks (of aspect ratio 0.1), the observed velocity reductions for vertically travelling P-waves can be produced by an increase in porosity of 1%.

¹Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results, 156: College Station, TX (Ocean Drilling Program).
² School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B152TT, United Kingdom. s.peacock@bham.ac.uk