CALCULATION AND IMPLICATIONS OF POISSON'S RATIO FOR UPPER CRUSTAL STRUCTURE AND COMPOSITION IN THE SOUTHWEST CASCADES, WASHINGTON

Data from a wide-angle seismic refraction profile that extends from the coast across the High Cascades to the Columbia Plateau of southwest Washington State contains sufficient S-wave energy that they can be used to produce both P- and S-wave velocity models, and thus a model for Poisson’s ratio of the upper crust. The data, which come from a 320 km long profile collected in 1995 by the U. S. Geological Survey and collaborators, consist of 17 shots fired into receivers deployed at 1600 stations at a spacing of ca. 200 m. Three-component recorders deployed in the central portion of the line recorded strong S-wave energy.

Here we present results from modeling P- and S-wave arrival times to obtain velocity structure for the upper 10 km of the crust. These models were then combined to calculate Poisson’s ratio, a more reliable indicator of rock type than either P- or S-wave velocity alone. The results show a strong correlation between Poisson’s ratio and known structure and stratigraphy in the region. Structures such as Morton and Skate Mountain Anticlines are clearly seen in the Poisson’s ratio model. Surficial deposits have Poisson’s ratios of 0.23 to 0.24 and are interpreted to represent sedimentary and volcanic units of the Puget Group and younger deposits. Beneath this lies a zone exhibiting Poisson’s ratio of 0.29 to 0.3 that we interpret to be an extension of the Summit Creek basalt. This zone appears to lie directly on top of the Southern Washington Cascades Conductor (SWCC), a major low-resistivity feature in the region that has previously been interpreted to be largely composed of lower Eocene marine sedimentary rocks. Our model shows the SWCC to have a Poisson’s ratio of 0.26, a value too high for young marine sedimentary rocks. We suggest that alternative interpretations for the SWCC including the presence of graphitic sedimentary rocks or the occurrence of hydrothermal fluids at depth are a better choice for the origin of this feature.