OF THE INTERIOR U . S . GEOLOGICAL SURVEY GROUND DEFORMATION IN GRANADA HILLS AND MISSION HILLS RESULTING FROM THE JANUARY 17 , 1994 , NORTHRIDGE , CALIFORNIA , EARTHQUAKE by

The M 6.7 Northridge earthquake of January 17,1994, produced concentrated ground deformation and associated damage in a 5-km-long linear belt within suburban Granada Hills and Mission Hills in the northern San Fernando Valley. Small-displacement cracks comprise discrete zones with characteristics that reflect control by local physiographic and near-surface conditions. Several semi-arcuate zones of tension cracks and a zone of compression (expressed by crumpled pipelines and crushed pavement) formed in Holocene alluvium on the gently sloping piedmont west of Bull Canyon. These observations, along with evidence from trenches and surveys of road deformation, point to shallow mass movement as the primary cause of deformation in the area. A combination of tension cracks and compressional buckles developed along the pre-development course of Bull Creek in lower Bull Canyon and above filled tributary drainages farther east. The relation of this deformation to stream channels and, locally, to small sand boils implicates liquefaction, sediment compaction, and/or lurching of surficial deposits as the likely cause(s) of deformation. Tension cracks also developed on the moderately steep flank of the Mission Hills anticline at the east end of the deformation belt. Locally, these features are clearly related to slope failure, but cracks in the largest set also follow downslope-dipping bedding and thus could be an expression of slip accommodation related to possible coselsmic folding in the anticline. The various characteristics of deformation observed in the Granada Hills-Mission Hills belt are consistent with one or more types of shallow-seated phenomena and are not consistent with a tectonic faulting origin. The nature of these phenomena implicates strong ground shaking as a primary driving mechanism for deformation. The distribution of ground deformation strongly reflects the distribution of damage (to roads, utility lines, and structures) in the Granada Hills-Mission Hills-area. This correlation points to the value of understanding factors that control the location of shaking-induced ground failures (such as location of concealed thrust faults), as well as how frequently these failures occur. Understanding the risk posed by this hazard requires more work, however; both locally, to better establish recurrence times, and regionally, to identify the extent of the hazard.