Active landsliding and rock strength controls along Big Sur Coast, CA

The Santa Lucia Mountains of the Big Sur coast, California contain abundant active landslides. To evaluate the hypothesis that rock strength, as influenced by lithology, fracture density, and weathering extent modulate deep-seated (10’s m thick) rockslide susceptibility, we quantified intact-rock strength, fracture characteristics, and rock mass strength along 140-km of coastline at 73 outcrops in 12 geologic units including: deep-water trench deposits of the Franciscan complex, the plutonic arc and metamorphic belt of the Salinian block, and Cretaceous sediments. Rock hardness was characterized using a type-L Schmidt hammer (SH) and a point-load strength tester (Is50) was used as a proxy for uniaxial compressive strength. Least-squares regressions between Is50 and SH, regardless of intact rock type, produce robust relations (R2≈0.8). To semi-quantitatively describe the structure and conditions of discontinuities, we calculated rock mass strength (RMS) and rock quality designation (RQD) using 3-D scanlines. Derivative values of friction angle and cohesion were calculated using the Hoek and Brown failure criterion. The lowest calculated strength parameters are within the highly fractured argillite and serpentine of the Franciscan complex which in turn correlate with the highest landslide densities indicating that rock type and field-estimated strength exert a primary control on landslide densities.

Schmidt K (2021). Active landsliding and rock strength controls along the Big Sur Coast, California. USGS Landslide Hazards Program Seminar Series, 25 August 2021.