Previously secret oil company data reveal a major active fault system under metropolitan Los Angeles that most likely caused the magnitude 5.9 Whittier Narrows earthquake in 1987, researchers said Thursday.
This buried fracture may be capable of larger and even more damaging earthquakes than the 1994 Northridge quake, according to the research, published today in Science.
The discovery is the latest in a series of recent findings about regional seismic hazards that have some experts questioning whether earthquake engineering safety standards and building codes in Southern California should be strengthened.
The newly found fault "is clearly a source of major earthquakes and likely could produce much damage to the L.A. area," said Harvard University geophysicist John H. Shaw, who conducted the study with Peter M. Shearer at the Scripps Institution of Oceanography in La Jolla. "This fault system has not been considered in previous hazard assessments."
Christened the Puente Hills fault, the newly discovered fault zone runs in three broad segments for almost 25 miles under downtown Los Angeles, through Santa Fe Springs and into the Coyote Hills of northern Orange County. The active fault zone encompasses several hundred square miles of densely settled urban areas.
Invisible from the surface, it is a blind-thrust fault system hidden deep in the broken crockery of the Earth's crust. It is very similar to the fault that caused the magnitude 6.7 Northridge earthquake, which killed 57 people, left 20,000 people homeless and did $40 billion in damage.
Could Produce 7.0 Earthquake
Under the most extreme circumstances, the Puente Hills fault could generate earthquakes as powerful as magnitude 7.0. Its destructive energy probably would most be directed into heavily populated metropolitan areas.
Scientists have long brooded over the possibility that major earthquake faults are hidden under the city. The new research gives the first concrete form to their fears by producing direct images of an active blind-thrust fault concealed beneath the asphalt and steel of urban Los Angeles, according to leading authorities on the geology of the basin.
Even so, the discovery does not seriously increase the overall earthquake risk facing the Los Angeles region, they said, because tectonic energy--and the attendant risk of major earthquakes--is building up in the basin at a relatively constant rate.
