HR: 1330h
AN: S52D-0161    [PDF]
TI: Joint Seismic and Compliance Constraints on S-Wave Structure
AU: Hulme, T
EM: hulme@ipgp.jussieu.fr
AF: Insitut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005 France
AU: Crawford, W C
EM: crawford@ipgp.jussieu.fr
AF: Insitut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005 France
AU: * Singh, S C
EM: singh@ipgp.jussieu.fr
AF: Insitut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005 France
AB: Seafloor compliance -- the pressure--displacement transfer function of the seafloor under forcing from long period ocean waves -- provides a useful complement to conventional marine seismic techniques. Synthetic modeling of compliance shows that it is principally sensitive to regions of low shear modulus, and inversions of compliance measurements can therefore be used to calculate S-wave structure. Existing inversions of compliance data assume a one-dimensional velocity model, whereas in reality there may be significant lateral velocity variation. The effect of this two-dimensional structure on compliance measurements and subsequent inversions can be investigated by calculating (quasi-analytically) the compliance over a cylinder. The compliance displays the same sensitivity to regions of low shear modulus as is seen for one-dimensional situations, generating a strong peak over the low-velocity zone. The width of this peak, which is a measure of the horizontal resolution of compliance, is approximately proportional to the depth of the cylinder, while its amplitude is controlled both by the cylinder's radius, its depth and the background velocity. Minimum structure one-dimensional inversions of the compliance measured above a fluid cylinder recover distinct low velocity zones, although the recovered shear wave velocity is much greater than zero, and the low velocity zone is smeared out over a greater depth range than that occupied by the cylinder. Including \textit{a priori} information from seismic reflection and refraction studies provides important extra constraints on the shear wave velocity structure. Furthermore, a joint analysis of seismic and compliance data can also be used to provide information about intrinsic attenuation structure: the seismic waves recorded by compliance have frequencies several orders of magnitude lower than those used in conventional reflection and refraction experiments, and are thus sensitive to velocity dispersion produced by attenuation. This effect is demonstrated by an example from 9$^\circ$33'N on the East Pacific Rise, where the differences in on-axis S-wave structures derived from seismic and compliance data can be explained by attenuation due to fluid inclusions in the fractured upper crust.
DE: 3210 Modeling
DE: 7220 Oceanic crust
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting