Faulting apparently related to the 1994 Northridge, California, earthquakeand possible co-seismic origin of surface cracks in Potrero canyon, Los Angeles County, California

Apparent southward-dipping, reverse-fault zones are imaged to depths of abo ut 1.5 km beneath Potrero Canyon, Los Angeles County, California. Based on their orientation and projection to the surface, we suggest that the imaged fault zones are extensions of the Oak Ridge fault. Geologic mapping by oth ers and correlations with seismicity studies suggest that the Oak Ridge fau lt is the causative fault of the 17 January 1994 Northridge earthquake (Nor thridge fault). Our seismically imaged faults may be among several faults t hat collectively comprise the Northridge thrust fault system. Unusually str ong shaking in Potrero Canyon during the Northridge earthquake may have res ulted from focusing of seismic energy or co-seismic movement along existing , related shallow-depth faults. The strong shaking produced ground-surface cracks and sand blows distributed along the length of the canyon, Seismic r eflection and refraction images show that shallow-depth faults may underlie some of the observed surface cracks. The relationship between observed sur face cracks and imaged faults indicates that some of the surface cracks may have developed from nontectonic alluvial movement, but others may be fault related. Immediately beneath the surface cracks, P-wave velocities are unu sually low (<400 m/sec), and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m. On the basis of velocity data, we suggest that unconsolidated soils (<800 m /sec) extend to depths of about 15 to 20 m beneath our datum (<25 m below g round surface), The underlying rocks range in velocity from about 1000 to 5 000 m/sec in the upper 100 m, This study illustrates the utility of high-re solution seismic imaging in assessing local and regional seismic hazards.