Abstract
We developed a forward model using the Trishear module in MOVE to better understand the structure of the northwestern San Fernando Valley and the relationship among the Santa Susana, Hospital, Mission Hills and Northridge Hills faults. This study was motivated by the 1971 San Fernando earthquake and previous work that inferred a high slip rate on the Santa Susana fault, which is in apparent contrast to the lack of significant geomorphic expression of the fault in the Sylmar Basin region. We trenched the Mission Hills anticline from the crest to the base of slope and demonstrate that the Mission Hills anticline is an actively growing fault propagation fold. The associated thrust tip is either deeper than 15 m or sufficiently far to the south that the fault was not encountered in large diameter borings, but the minimum structural relief across the Mission Hills fault since the late Pleistocene is on the order of 37 m, suggesting a minimum uplift rate of 0.5 mm/yr. Our work presents a structural analysis that demonstrates how the Santa Susana fault system evolved in time, with the frontal thrust progressively migrating southward to the Mission Hills fault, and farther south to the Northridge Hills blind thrust. The progression of faulting towards the direction of vergence is compatible with the observed thrust front migration in the western Transverse Ranges of California, and other trust belts around the world.
Highlights
Estimating the fault structure, and identifying earthquake sources in tectonically active regions is essential for hazard assessment and reliable ground motion modeling
Our proposed model for the structural evolution of the Sylmar basin demonstrates the likely connectivity of the north dipping reverse and thrust faults in the subsurface, and helps explain how slip can be distributed from the Santa Susana/Hospital/Sierra Madre fault system to the Mission Hills and Northridge Hills faults south of it
The line of evidence we included in combination with our interpretation of regional pattern of southward migration of the locus of deformation along the Western and Central Transverse Ranges, as demonstrated by our forward model, supports the lower estimates (
Summary
Estimating the fault structure, and identifying earthquake sources in tectonically active regions is essential for hazard assessment and reliable ground motion modeling. Two of the most devastating earthquakes in southern California occurred in the San Fernando Valley: the 1971 Mw 6.7 San Fernando and 1994 Mw 6.7 Northridge earthquakes. Each of these events resulted in many millions of dollars in damaged infrastructure along with 60 causalities and thousands were wounded. The 1971 San Fernando earthquake ruptured the ground surface around the Sylmar basin, which is located in the northern part of the San Fernando Valley (Figures 1, 2) (Oakeshott, 1975). No surface rupture was produced as a result of the Northridge earthquake, and the little ground deformation that was reported was located on a secondary structure or due to shaking effects (Woods and Seiple, 1995)
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