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John H. Shaw(1*) and Peter M. Shearer(2)
Seismic reflection profiles, petroleum wells, and relocated earthquakes reveal the presence of an active blind-thrust fault beneath metropolitan Los Angeles. A segment of this fault likely caused the 1987 Whittier Narrows (magnitude 6.0) earthquake. Mapped sizes of other fault segments suggest that the system is capable of much larger (magnitude 6.5 to 7) and more destructive earthquakes.
Damages exceeding $35 billion from the 1994 Northridge [magnitude (M) 6.7] earthquake (1), combined with recent evidence of larger (>M 7) events in the geologic record (2), have focused attention on the hazards posed by thrust faults to metropolitan Los Angeles. Efforts to assess and mitigate these hazards are complicated, however, because thrust faults beneath the city are typically blind, meaning that they lie concealed beneath Earth's surface (3, 4). Here, we used high-resolution, subsurface images acquired by the petroleum industry (Fig. 1) and relocated seismicity to map a large blindthrust system lying directly beneath the metropolitan area (Fig. 2).
Many blind thrusts produce near-surface folds that grow during repeated earthquakes (3-5) in response to motions through bends in fault planes or above propagating fault tips, and their shapes reflect underlying fault geometries and slip (6, 7). Seismic reflection profiles and information from oil wells reveal a series of these anticlines extending from downtown Los Angeles to the Coyote Hills (8-10) in northern Orange County (Fig. 2). These folds grew in the Quaternary while sediments were deposited above them, yielding patterns of deformed strata that record fold growth and fault slip (11). These deformed strata are particularly well imaged in the Santa Fe Springs anticline (Fig. 1).
On the basis of fault-related folding theories (11), the shape of the growth fold at Santa Fe Springs suggests that an underlying fault, which we call the Puente Hills thrust, dips to the north and extends upward into the Pliocene Fernando Formation. This fault shape and position are consistent with a north-dipping reflection beneath the anticline that cuts across bedding. This reflection is observed on more than 10 seismic profiles and persists through a range of processing steps aimed to remove noise and artifacts (12). Thus, we interpret this reflection as an image of the fault surface, caused by velocity and density...