High-resolution stereo optical images reveal complex near-field displacements of the Mw 6.6 1971 San Fernando, California earthquake

Abstract:

In February of 1971, a shallow M_w 6.6 earthquake caused approximately 64 deaths and over $500 million in damages in the San Fernando Valley, an urban area located within the Greater Los Angeles region. This event occurred on a structurally complex segment of the Sierra Madre Fault Zone, an active fault system opposite the Great Bend of the San Andreas Fault that was unknown prior to this earthquake. We use high-resolution stereo optical images from the 1960s and 1970s to create digital elevation models (DEMs) and use optical image correlation and DEM differencing to quantify coseismic displacement near the surface rupture. The near-field surface displacements have significant left-lateral and vertical components, particularly in the western Sylmar segment, contrasting with the earthquake’s almost purely thrust focal mechanism, and seismic and elastic dislocation models that indicate that the main rupture occurred as a simple oblique thrust fault with ~45° dip. However, the results support previous observations from decades-old detailed field and levelling surveys that indicate that near-surface faulting on this segment is likely dominated by slip on secondary faults that are much steeper than the main rupture plane, enhancing the vertical and left-lateral slip near the toe of the fault. These results illustrate the importance of using both near-field and far-field data for characterizing complex fault geometries and assessing seismic hazard. Furthermore, this study shows some important limitations when creating high-precision DEMs using historical imagery.