UNDULATIONS OF SHEAR SURFACES CONTROL STRENGTH OF FAULT GOUGE AND MAY AID CATASTROPHIC LANDSLIDE MOTION

Landslides present significant hazards and modify landscapes. Both of these effects were illustrated when a large landslide near the town of Oso, Washington buried a floodplain, dammed a river, destroyed a neighborhood, and killed 43 people on March 22, 2014. Most landslides shear within gouge comprising mixtures of clay and granular material, as do tectonic faults. Previous studies of gouge shear resistance have focused on particle-scale mechanisms; however, field studies reveal that gouge is commonly permeated by undulatory shear surfaces, which also may influence shear strength by creating geometrical interference to displacement in excess of particle-scale interference. We studied silt-clay mineral mixtures that formed the majority of the Oso landslide using a specialized ring shear device to improve understanding of mechanisms controlling fault (shear) gouge strength and identify how large strain behavior of these materials might have influenced landslide motion. Unlike most, the apparatus we used develops gouge sufficiently thick to form undulatory shear surfaces.

We found that shear-surface undulations critically controlled shear resistance, often in ways contrary to particle-scale controls and with diverging effects as clay and silt contents varied. Shearing along undulations resulted in persistently elevated pore-water pressures and caused dilative weakening. These previously unknown mechanisms significantly weakened siltier material because elevated pore-water pressure reduces granular particle contact stresses and dilation reduces particle-scale geometrical interference. However, dilative weakening and elevated pore pressures were relatively unimportant in controlling strength of clayier material as double-layer water precludes most true particle contacts in clay and platy clay particles align well during shearing. Our findings reveal that shear-surface undulations fundamentally control shear resistance and require consideration during studies of landslide and tectonic fault strength. With respect to the Oso landslide, strength loss in the silt-clay deposit aided its catastrophic motion; however, other geological materials involved in the landslide also affected its mobility.