Shear Band Genesis and Frictional Properties, McKinleyville Fault, Humboldt County, California

Understanding the genesis of shear bands and fabric development in rock and sediment is essential to understanding fault formation, propagation, and earthquake mechanics. Previous laboratory experiments and field evidence indicate that shear strength, velocity-dependence of friction, and other host rock properties play key roles in shear band formation. However, we lack laboratory data on undisturbed, naturally formed shear bands. Deformation bands in the footwall of the active McKinleyville thrust fault provide an excellent opportunity to study the effects of shear strain and displacement in natural samples. Samples consist of deformation bands roughly parallel to the McKinleyville Fault, conjugate deformation bands and non-banded material. Shear bands are hosted in late Pleistocene nearshore marine sand and are identified as compacted sand within surrounding less-compacted sand. Shear bands range in thickness from 0.1cm- ~10cm and band parallel displacement from 0.4cm-3.35m. We report on laboratory experiments contrasting displacement and shear band thickness versus particle characteristics, shear strength and velocity-dependence of friction. Experiments were conducted in a servo- controlled apparatus using the single-direct shear configuration. Layers were initially 5 mm thick with nominal contact dimensions of 5 cm x 5 cm. Normal stress was varied from 0.25-2.5 MPa. Velocity stepping experiments were done in an increasing fashion at 1, 3, 10, 30, 100 and 300μm/s. Post shear samples are analyzed under transmitted and SEM light microscopy. Through laboratory experiments we aim to resolve the physical alteration of natural material due to shear by comparing natural shear-band material to laboratory simulated shear bands of non-banded material. Preliminary results on non-banded material yield an average angle of internal friction of 30.4° and cohesion of 93.1 kPa.