Off-Fault Deformation Due to Changes in Slip Dynamics on the San Andreas Fault: An Experimental Approach

Abstract:

The San Andreas fault is the most well-developed of the strike-slip faults in the coastal plate boundary system. In central California, the fault has a relatively simple geometry, but the slip behavior changes along strike. The creeping segment, with rates as high as ~28 mm/yr, lies between two locked segments that have interseismic periods lasting over 100 years. The southern transition from creeping to locked behavior near Parkfield, CA, coincides with changes in the strain field derived from GPS data. Because there are indications that creep has been a long-lived process, we hypothesize that off-fault structures could reflect these changes in the strain field.

In order to isolate the effect of the locked-to-creeping transition on off-fault deformation, we model the system using physical experiments. To mimic the slip behavior in central California, our model system includes a locked section and a creeping section. Deformation in the creeping segment is imposed on a simple shear table with one side stationary and one side moving at a constant rate of 0.1 mm/sec, while deformation of the locked portion is modeled by distributed simple shear. A slab of silicone is placed on the table surface and cut and lubricated along the creeping segment, creating a discrete transition between the creeping and locked section of the fault. Time-lapse photography and photogrammetry of the models over time allow for quantitative analysis in 2D and 3D.

In the experiments we observe the formation of compressional and extensional zones on either side of the fault, which correlate to strain field patterns observed in nature and predicted by simple elastic models. More specific landscape features may also be explained by this change in slip behavior. These results show a clear connection between off-fault deformation and the change in slip along a strike slip fault such as the San Andreas fault.