Complex 3-D Surface Deformation in the 1971 San Fernando, California Earthquake Reveals Static and Dynamic Controls on Off-Fault Deformation

The shallow 1971 M-W 6.6 San Fernando, California earthquake involved a complex rupture process on an immature thrust fault with a non-planar geometry, and is notable for having a higher component of left-lateral surface slip than expected from seismic source models. We extract its 3-D coseismic surface displacement field from aerial stereo photographs and document the amount and width of the vertical and fault trace-parallel components of distributed deformation along strike. The results confirm the significant left-lateral surface offsets, suggesting a slip vector rotation at shallow depths. Comparing our offsets against field measurements of fault slip, we observe that most of the offset was accommodated in the damage zone, with off-fault deformation averaging 69% in both the fault trace-parallel and vertical components. However, the magnitude and width of off-fault deformation behave differently between the vertical and fault trace-parallel components, which, along with the rotation in rake near the surface, can be explained by dynamic rupture effects.