Seismic refraction data from Gordon Gulch, Boulder Creek Critical Zone Observatory

We investigate the relationship between slope aspect, subsurface hydrology, and critical zone (CZ) structure in a montane watershed by examining the orientations of foliation and fracturing and thicknesses of weathered material on north- and south-facing aspects. Weathering models predict that north-facing slopes will have thicker and more porous saprolite due to colder, wetter conditions, which exacerbate frost damage and weathering along open fractures. Using borehole imaging and seismic refraction, we compare the seismic velocity and anisotropy of north- and south-facing slopes with the orientation of fracturing. Fracturing occurs in the same dominant orientations across slopes, but the north-facing slope has more developed and slightly thicker soil as predicted, while the south-facing slope has thicker and more intact saprolite that is highly anisotropic in the direction of fracturing. Our data support hypotheses that subsurface flow is matrix-driven on north-facing slopes and preferential on south-facing slopes. We attribute thicker saprolite on south-facing slopes to heterogeneity induced by competition between infiltration, topographic stress, and permafrost during Pleistocene glaciation. We provide new constraints on subsurface architecture to inform future models of CZ evolution.