The Lavic Lake fault: a long term cumulative slip analysis via combined field work and thermal hyperspectral airborne remote sensing

Rebecca A. Witkosky, Joann M. Stock, David M. Tratt, Kerry N. Buckland, Paul M. Adams, Patrick D. Johnson, David K. Lynch, & Francis J. Sousa

In Preparation October 8, 2018, SCEC Contribution #8898

The 1999 Mw 7.1 Hector Mine earthquake was a major surface-rupturing event, with coseismic right-lateral slip in excess of 5 m along the Lavic Lake fault. The Lavic Lake fault is a major structure within the Mojave Desert’s Eastern California Shear Zone (ECSZ). The fault’s cumulative long-term bedrock offset and geologic slip rate are not well-defined, which inhibits tectonic reconstructions. Access to the Lavic Lake fault is restricted, which complicates field work to study the fault’s geological history. We gained access to the area in 2012 and 2014, to collect new field data. New thermal hyperspectral airborne imagery with a 2 m pixel size was collected in 2013. With our new field data and additional data from older geologic maps, we created lithologic maps of the area using supervised and unsupervised classifications of the remote sensing imagery. The high spatial and spectral resolution of the airborne sensor, along with a lack of significant vegetation cover, allowed us to optimize a data processing sequence for supervised classifications over a small test area, resulting in lithologic maps with an overall accuracy of 71 ± 1%. To identify bedrock offset by the fault, we produced a lithologic unsupervised classification map over the complete image swath. A boundary between units in our unsupervised classification map correlated very well with a lithologic boundary from a previously published geologic map. This lithologic boundary, a displaced depositional contact between two volcanic units, pre-dates the Lavic Lake fault, thus capturing the entire cumulative long-term offset. We used the displaced depositional contact to calculate the vertical component of slip along the main Lavic Lake fault, and an older cross fault, to measure the horizontal component of slip along the main fault. We then combined the vertical and horizontal components to derive a net fault slip of 960 +70/-40 m. We interpret that value as a maximum due to uncertainty in the structural geometry. Our value is significantly less than a previous estimate of cumulative offset that was based on an offset magnetic feature, which we posit may be due to off-fault deformation along proximal smaller ECSZ structures. The magnitude of displacement that we calculate is much less than reasonably required for a long-term slip rate that would contribute significantly to the total ECSZ geologic slip rate. This supports the hypothesis that the discrepancy between the ECSZ’s total geologic and current geodetic slip rates reveals a true transient nature of the system that has been suggested previously.

Witkosky, R. A., Stock, J. M., Tratt, D. M., Buckland, K. N., Adams, P. M., Johnson, P. D., Lynch, D. K., & Sousa, F. J. (2018). The Lavic Lake fault: a long term cumulative slip analysis via combined field work and thermal hyperspectral airborne remote sensing. Journal of Geophysical Research, (in preparation).