HR: 1330h
AN: B12A-0734    [PDF]
TI: Shallow (Layer 2A) Crustal Shear Velocity Structure Near the 9-50'N Vent Fields, East Pacific Rise
AU: * Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Reves-Sohn, R A
EM: rsohn@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: In 1994, we exploded 16 explosive shots on the seafloor to generate seismic arrivals that were detected by 8 ocean bottom seismometers in the vicinity of the 9-50 N vent fields on the East Pacific Rise. The shots generated high frequency (0.5-15 Hz) surfacewaves (also called Stoneley waves) which traveled primarily in the low velocity waveguide formed by the top of the extrusives layer. Waveform modeling of the surface wave trains recorded provides strong constraints on the shear wave velocity in the uppermost 200m in very young oceanic crust. The results demonstrate shear velocity increases rapidly from values below 200m/s in the uppermost 20m, to values near 500 m/s near the bottom of the extrusives layer. This rapid increase in velocity must primarily be associated with lower porosity and higher confining pressures at depth within the layer. Variations in shear velocity around the vent fields may represent the effects of hydrothermal cementation and changes in porosity due to mineralization. The body waves arrivals generated by these shots were analyzed using tomography to generate models of P and S wave velocity in layers 2A and 2B. This work is described elsewhere in this volume (Sohn et al., 2003).
DE: 5102 Acoustic properties
DE: 5114 Permeability and porosity
DE: 7220 Oceanic crust
DE: 7255 Surface waves and free oscillations
SC: Biogeosciences [B]
MN: 2003 Fall Meeting