ROCK STRENGTH CONTROLS ON LANDSLIDING ALONG THE BIG SUR COAST, CA

We hypothesize that rock strength, influenced by lithology, fracture density, and weathering extent, modulates deep-seated (10's m thick) rockslide susceptibility within coastal drainages of the Santa Lucia Mountains along the Big Sur Coast, CA.  The region is composed of widely differing rocks: deep-water trench deposits of the Franciscan Complex, the plutonic arc and metamorphic belt of the Salinian Block, and the Great Valley turbiditic sedimentary sequence.  To ascertain the influence of rock type on landslide occurrence, we quantified intact-rock strength, fracture characteristics, and rock mass strength along 140-km of coastline at 65 outcrops in 12 geologic units.  We characterized rock hardness, or elasticity, at all 65 outcrops by over 5800 field measurements using a type-L Schmidt hammer (SH).  Point-load strength index (I_s), a proxy for uniaxial compressive strength, was determined from 551 measurements on a subset of 24 outcrops with irregularly shaped intact samples. 

Statistical correlations between I_s and SH data indicate a robust linear relation (R²=0.82) for weak and moderately strong rocks (<4 MPa).  For strong rocks (Salinian block quartz diorite and Franciscan Complex greenstone and greywacke), however, the relation is weak (R²=0.24), even when SH values are adjusted by specific gravity (R²=0.25).  Linear, power, and exponential curve fits on the entire data set regardless of lithology produce moderately robust relations (R²=0.62 - 0.77). 

From the measured intact-rock strengths and fracture densities, we calculated rock mass strength (RMS) and rock quality designation (RQD) at the 65 field sites.  Salinian Block rocks have the lowest density of deep-seated landslides (<2% by area), and exhibit high RMS, RQD, SH, and I_s values.  In contrast, some rocks of the Franciscan Complex, particularly argillite and serpentine units, have the lowest strength values.  The Franciscan Complex also has the highest density of deep-seated landslides, up to 53% by area south of the Sur-Nacimiento fault.  Other Franciscan Complex rocks with similar strengths and fracture densities, exposed near Pfeiffer Point to the north, have landslide densities of only 27%.  This contrast likely arises from differences in topographic relief and gradient caused by differential tectonic uplift across the Sur-Nacimiento fault.