Title: What can very small faults in the lab tell us about deformation in the Earth's crust?

Author (Invited): Michele Cooke, UMass Amherst

Abstract:

When assessing where the next earthquake will occur, we need to consider that active faults along plate boundaries of the Earth’s crust are not static nor do that have constant slip rate. Within areas of complex active fault configurations, some large earthquakes happen along new faults and slip records and past slip rates might not predict future slip rates. Because we can’t wait around millions of years to see how active faults evolve within the crust, we document fault evolution within laboratory experiments using materials that have similar properties at the scale of the lab. These experiments show how faults link up to form more efficient geometries that could generate large earthquakes. Our recent experiments explore how loading rate impacts fault growth within wet kaolin, which has strain rate dependent strength and effective viscosity as do geologic materials in the upper portion of the Earth’s crust where earthquakes are generated. We investigate a bend along a strike-slip fault, which provides an irregularity that drives reorganization of the fault system to a more efficient configuration. Under different loading rates, restraining bends with the same initial fault geometry produce differing faulting histories. Slow experiments initiate faults earlier and produce a greater number of faults during the experiment because the strength of the wet kaolin increases with strain rate. For most of the fault evolution, the slow experiment also has greater active fault length and less off-fault deformation. The experimental results suggest that faults within lower strain regions of the Earth’s crust, may have more complex evolution with less stable fault geometry. This could impact how we use past records of faulting to assess future seismic hazard.