Abstract

Still controversial is the origin of permanent ground deformation and infrastructure damage, including pipeline failures, associated with the M 6.7, 1994 Northridge, CA, earthquake in southern California. Based on site-specific studies, some researchers explain the deformation as failure due to seismic shaking. In contrast, we propose a tectonic origin, consistent with displacement on the primary rupture and with the tectonic geomorphology of the San Fernando Valley. A tectonic origin for the permanent ground deformation is based mainly on regional analyses including: (1) street-by-street mapping of fractures and infrastructure distress above the causative fault; (2) interpretation of post-earthquake aerial photographs; (3) comparison of the type, distribution, and pattern of fractures with early-1900s soil, geologic, and topographic data and 1920s aerial photographs; (4) review of four post-Northridge earthquake site-specific studies; and (5) the number and distribution of small-diameter pipeline failures. We find that the pattern of co-seismic fractures and folds and zones of concentrated pipeline failures all generally coincide with the contraction and resultant dilation and distortion of hanging-wall sediments. We thus conclude that a tectonic origin of the secondary structures and apparent association with local geologic and geomorphic features indicate a high potential for future similar deformation resulting in infrastructure damage and likely failure of critical pipelines.

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