NON-STEADY STATE SLIP RATES EMERGE AT RESTRAINING BENDS UNDER CONSTANT REMOTE LOADING

Seismic hazard assessments presume that long-term fault slip rates determined from the geologic record can reliably forecast future seismic hazard, which implies that long-term slip rates for active faults are constant. Recent studies, however, provide evidence of slip rate variability over time periods of 10 to 1000 ky. Because field observations alone do not provide enough evidence to distinguish how fault reorganization influences slip rates over time, we use scaled physical experiments in wet kaolin clay to directly observe emergent fault behavior. Scaling relationships between the clay and the crust are such that 1-minute in the experiment scales to 13 ky in the crust and 1 cm on the clay surface is equivalent to 1 - 2 km of crust. Digital Image Correlation provides high-resolution incremental horizontal displacements fields for analyzing slip rates and off-fault deformation.

To assess the degree of slip rate variability that could be revealed in a geologic site investigation, we track fault slip rates at specific locations along the faults that migrate throughout the experiment. The experimental results show that fault sites within the restraining bend exhibit larger variations in slip rates (up to 25%) than sites outside of the bend. The greatest variations in slip rates occur during periods of fault reorganization over time lengths within the experiment that scale to 200 - 930 ky in the crust. We also observe slip rate variations of up to 7% over shorter periods within the experiment that scale to 65 - 150 ky in the crust; current experimental resolution restricts analysis of shorter periods. Contrary to expectations, the growth of new faults sometimes increases strike-slip rates on nearby fault segments. This happens when new thrust faults accommodate previously off-fault contraction; this process unclamps nearby strike-slip segments. The results from this experiment demonstrate that fault system reorganization influences the degree of slip rate variability even when tectonic loading is constant.