Aspect Controls on Rock Weathering and Permeability within the Boulder Creek Critical Zone Observatory

We investigate the controls of slope aspect on rock weathering and permeability within the Boulder Creek Critical Zone Observatory by studying the orientation and density of subsurface fracturing. Based on a series of seismic refraction surveys, we compare the seismic anisotropy of the subsurface soil, saprolite (highly weathered, porous rock), and bedrock with the distribution of fractures observed in 7 borehole logs. In rocky mountainous terrain, weathering typically occurs fastest along rock fractures, which we examine on north- and south-facing slopes via the relationship between fracture orientation and seismic velocity. The south-facing slope demonstrates pronounced seismic anisotropy 2-9m below ground surface, with p-wave velocities of up to 2,000 m/s in the E-W direction, and approximately 1,000 m/s in the N-S direction. By contrast, the north-facing aspect demonstrates slower p-wave velocities and less significant anisotropy in this zone, with velocities ranging from approximately 800-1,500 m/s 2-9m below ground surface. We identify this zone as saprolite and observe its upper and lower boundaries to be consistent on both slopes. In the uppermost 2m of both slopes (soil), p-wave velocities are approximately 500 m/s in all measured directions. Below 9m (bedrock), p-wave velocities are approximately 3,500-4,500 m/s in all measured directions. Borehole logs show conjugate sets of fractures on both slopes striking in a general E-W direction, consistent with the direction of anisotropy observed in the south-facing saprolite. Based on current weathering models, we interpret the slower and more isotropic velocities of the north-facing saprolite to be the result of more intense weathering triggered by a colder average surface temperature and more intense spring runoff than the south-facing slope. Assuming that both slopes experience similar tectonic stress, we interpret the differences in the saprolite velocities of the two slopes as evidence that slope aspect controls rock weathering and permeability.