Characterizing emissivity spectra from geomorphic surfaces along the southern San Andreas Fault

Ryan D. Witkosky, Paul M. Adams, Kerry Buckland, Kenneth W. Hudnut, Patrick D. Johnson, David K. Lynch, Katherine M. Scharer, Joann M. Stock, & David M. Tratt

Published September 15, 2016, SCEC Contribution #6653, 2016 SCEC Annual Meeting Poster #085 (PDF)

Geologic mapping and cosmogenic exposure ages of alluvial surfaces along the southern San Andreas Fault provide independent data sets to evaluate corresponding emissivity spectra from thermal hyperspectral airborne imagery. We use new 1-m pixel resolution data from 2015 covering the Mission Creek strand between Thousand Palms Oasis and Pushawalla Canyon, where a set of alluvial surfaces are progressively offset by the fault. After converting the image data from at-sensor radiance to emissivity, we examined spectra from each of four dated surfaces that span ~10 to 90 ka (Blisniuk and Sharp, 2014). This comparison reveals that the magnitude of an absorption feature located between 9-10 micron wavelengths generally increases with age of the surface. We hypothesize that the depth of this absorption feature varies by exposure age as clay minerals accumulate in coatings of desert varnish on the surface. Lab spectra for clays commonly found in desert varnish (e.g. illite and montmorillonite) show a significant absorption feature between 9-10 micron wavelengths caused by vibrational stretching in the silicon-oxygen tetrahedra that comprise the crystal lattice. In combination, these results suggest that as these geomorphic surfaces alter with age, accretion of desert varnish results in a greater contribution to the spectrum for each pixel. Using varnish development as an age proxy for relative dating is common in the literature but results have generally been inconclusive or difficult to apply more broadly due to regional and temporal changes in surface development. We plan to further analyze the emissivity spectra at specific sites to determine whether the observed spectral absorption feature behaves consistently or if sampling strategies alter the results. If the preliminary observations are robust, this technique may provide a tool for expanding existing age data, or reconnaissance evaluation of surface ages in arid locales that are conducive to varnish development, facilitating slip rate studies and guiding dating efforts for these surfaces.

Key Words
Southern San Andreas Fault, desert varnish, thermal hyperspectral airborne imagery

Citation
Witkosky, R. D., Adams, P. M., Buckland, K., Hudnut, K. W., Johnson, P. D., Lynch, D. K., Scharer, K. M., Stock, J. M., & Tratt, D. M. (2016, 09). Characterizing emissivity spectra from geomorphic surfaces along the southern San Andreas Fault. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology