Predictions of Fine Scale Seismic Structure of the Upper Oceanic Crust

Jack Pelton, Boise State University

Abstract

Seismic velocity in the upper oceanic crust depends largely on the fracture and porosity characteristics of the propagating medium. Significant existing evidence supports the hypothesis of a general increase over time of average upper crustal velocities due to infilling of voids and sealing of fractures. However, little is know about the velocity/porosity relationships of individual lithologic units and how these change over relatively short distances and over time. Heterogeneities in seismic structure can be the result of the juxtaposition of lithologic units with dramatically different cooling and alteration histories, and thus different velocities. We present here the results of outcrop seismic velocity and fracture density measurements obtained from marine basaltic units in the Troodos Ophiolite Complex, Cyprus. These data are used to predict possible fine scale structure of the upper oceanic crust and its effect on VLF seismic propagation.

Sites chose for this study encompass a range of lithologic types, including pillows, pillow breccias, massive and sheet flows, and intermixed hyaloclastites, flows, and dikes, and alteration histories. Fracture systems were characterized in terms of orientation, density and aspect ratio by outcrop mapping on photographic overlays. Velocities from 17 refraction lines were obtained by using a simple reciprocal method and verified in two dimensions using a 2-D tracing inversion method (RAYINVR).

In general, subaerial outcrop velocities seem to depend more on alteration type than strictly on facture density with pillow basalts from low-temperature weathering zone having the highest velocities. Seafloor weathering zone pillows and pillow breccias were found to have the lowest range, possible due to subaerial weathering of the sampled outcrops. Corrections for saturation and seafloor ambient pressure/temperature conditions will be applied to these results for application to the seafloor environment. In addition, upper crustal models constructed from the outcrop results will be presented. Propagation through these models will be compared to VLF results from modern ocean basins.