Complex 3-D surface deformation in the 1971 San Fernando, California
earthquake reveals static and dynamic controls on off-fault deformation
Abstract
The shallow 1971 Mw 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 models. We extract its 3-D
coseismic surface displacement field from aerial stereo photographs and
document the amount and width of the vertical and strike-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
68% in both the strike-parallel and vertical components. However, the
magnitude and width of off-fault deformation behave differently between
the vertical and strike-parallel components, which, along with the
rotation in rake near the surface, can be explained by dynamic rupture
effects.