  We apply a deep learning model to segment and identify rock characteristics
based on a Structure-from-Motion orthomap and digital elevation model of a
rocky fault scarp in the Volcanic Tablelands, eastern California. By
post-processing the deep learning results, we build a semantic rock map and
analyze rock trait distributions. The resulting semantic map contains nearly
230,000 rocks with effective diameters ranging from 2 cm to 250 cm. Rock trait
distributions provide a new perspective on rocky fault scarp development and
extend past research on scarp geometry including slope, height, and length.
Heatmaps indicate rock size spatial distributions on the fault scarp and
surrounding topographic flats. Median grain size changes perpendicular to the
fault scarp trace with the largest rocks exposed on and downslope from the
scarp footwall. Correlation analyses of the segmented fault scarp illustrate
the relationship between rock trait statistics and fault scarp geomorphology.
Local fault scarp height correlates with median grain size (R2 of 0.6), the
mean grain size of the largest rocks (R2 of 0.76), and the ratio of the number
of small to large rocks (R2 of 0.40). The positive correlation (R2 of 0.81)
between local fault scarp height and standard deviation of grain size suggests
that rocks on a higher fault scarp are less well sorted. The correlation
analysis between fault scarp height and rock orientation statistics supports a
particle transportation model in which locally higher fault scarps have
relatively more rocks with long axes parallel to fault scarp trace because
rocks have a larger distance to roll and orient the long axes. Our work
demonstrates a data-driven approach to geomorphology based on rock trait
distributions, promising a greater understanding of fault scarp formation, as
well as many other applications for which granulometry is an indicator of
process.
