SCEC Award Number 21128 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title The 1971 San Fernando Earthquake: Off-Fault Deformation Measured From 3-D Coseismic Displacements
Name Organization
Gareth Funning University of California, Riverside
Other Participants Élyse Gaudreau
Edwin Nissen
James Hollingsworth
SCEC Priorities 3e, 2e, 2d SCEC Groups Geodesy, Geology, Seismology
Report Due Date 03/15/2022 Date Report Submitted 03/30/2022
Project Abstract
The goal of this project was to use historic stereo air photos of the epicentral region of the 1971 San Fernando earthquake to investigate the coseismic deformation field and the degree to which the deformation was distributed on or off of the faults that ruptured the surface in the earthquake. A secondary task in the proposed work was to visit and analyze the surface ruptures in the field, particularly sites where we identify significant surface offsets in our imagery analysis that were not mapped extensively in the post-earthquake response.

Overall, we confirm the results of contemporary field investigations and geodetic surveys, finding significant left-lateral displacements and surface offsets, and differing from seismic first motion studies that show a dominant thrust mechanism for the event – implying a change in rake to more left-lateral slip at shallow depths. Compared to contemporary field offset measurements, our estimates of surface offset from fault-perpendicular profiles through our deformation field show imply most of the offset was accommodated in the damage zone, rather than on the fault itself, with off-fault deformation averaging 71% in the strike-parallel component and 64% in the vertical component. In detail, the magnitude and width of off-fault deformation vary between the vertical and strike-parallel components, and also vary along-strike. We propose that this variation, as well as the rotation in rake near the surface, can be explained by near-surface dynamic rupture effects.
Intellectual Merit Understanding of the controls on near-surface rupture dynamics is a key goal of earthquake science. The San Fernando earthquake was a rare case of a surface-rupturing thrust earthquake, and the main ruptures were mapped and measured in detail in the days and weeks following the mainshock. Recent developments in photogrammetry and optical image correlation, allowed us to develop a new work flow to apply to airphotos that were acquired before and after the event, and thus to allow us to take a fresh look at the earthquake with modern techniques. We find that vertical off-fault deformation differs significantly between segments of the rupture and may indicate the influence of fault maturity on rupture propagation.
Broader Impacts The 1971, M6.6 San Fernando earthquake was a landmark event for our understanding of seismic hazard and treatment of seismic risk in California, leading to significant public policy changes and modification to building codes. Understanding the links between near-surface geology, fault maturity and rupture behavior can lead to lessons we can apply elsewhere in southern California, especially in the Los Angeles and Ventura basins. The majority of the work for the project was conducted by a female graduate student, a new SCEC participant.
Exemplary Figure Figure 1: Displacement field of the 1971 San Fernando earthquake, estimated from image correlation of orthorectified airphotos (East-West and North-South components) and differencing of warped digital elevation models estimated from stereo photogrammetry of airphotos (vertical). Note the similar amplitudes of East-West and vertical displacements, and the strong left-lateral shear sense of motion in the horizontal displacements along both the Sylmar and Tujunga segments of the fault (solid black lines).