POISSONS RATIO STRUCTURE OF YOUNG OCEANIC-CRUST

We have applied full waveform inversion to wide-aperture seismic refle ction data from the southern East Pacific Rise near 14 degrees S. The data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the P and S -wave velocities (Vp, Vs, and hence Poisson's ratio sigma) and seismic attenuation (Qp , Qs) structure of seismic layer 2. Layer 2A is highly attenuating (Qp =18-30 and Qs=8-15) and layer 2B is moderately attenuating (Qp=30-50 a nd Qs=20-25). Our results show high sigma at the seafloor and in layer 2A (sigma=0.48). Across the top of the 2A/B transition the rapid incr ease in Vp is accompanied by a sharp drop in sigma to 0.25 within just 200 m of the seafloor. We perform simple calculations to gain an insi ght into the porosity and crack distribution with depth. These calcula tions suggest that porosity is in excess of 30% in layer 2A but reduce s to 6-7% at the top of the 2A/B transition and to about 5% at a depth of 600 m below seafloor within layer 2B. Our results suggest that the re is an increase in the average aspect ratio with depth across the 2A /B transition. The most likely explanation is that numerous thin crack s either mechanically close or are infilled at depth. Our results show there to be an abrupt change in the pore structure across the 2A/B tr ansition which;is consistent with a lithologic transition from extrusi ves to dykes but is equally consistent with a transition (mechanical o r hydrothermal) within the extrusive pile.