Research Uncovers New Fault in Earthquake-Prone Los Angeles
By: Cynthia Long
In Los Angeles, earthquakes are as much a part of life as the sun, the smog and the Santa Ana winds. But many live in silent fear of the fabled "Big One"—the long-predicted, massive earthquake that experts believe could transform the City of Angels into the City of Ruins.
According to new research, that fear may be more justified than ever. An active, major blind-thrust fault system has been discovered directly under the Los Angeles metropolitan area. Given the right circumstances, the fault could potentially generate what seismologists classify as a "major" earthquake (magnitude 7.0 or higher on the Richter scale).
Named the Puente Hills fault, the system is comprised of three sections and runs 12.5 miles wide and nearly 25 miles long under downtown Los Angeles, through Santa Fe Springs and into the Coyote Hills of northern Orange County.
According to the research, published in the current issue of Science, the fault probably caused the 1987 Whittier Narrows quake—a magnitude 5.9 temblor that resulted in eight deaths, 200 injuries, and $358 million in property damage.
Geologists long have recognized that the Los Angeles Basin, and all of southern California, is constantly being compressed in a kind of immense tectonic shoving match between the Pacific and North American plates, with the latter grudgingly conceding about eight millimeters of territory per year.
During the compression, fractured blocks of the earth's mantle thrust each other upward, resulting in hills and mountain ranges. It is the stress from this ongoing collision that has resulted both in the web of faults that crisscross the Los Angeles region, and also the temblors that occasionally rock the area.
While thrust faults that reach the surface (such as the infamous San Andreas) are more easily studied, blind-thrust faults—buried faults that terminate underground before breaking the surface—are no less important to geologists and have been responsible for a recent string of destructive quakes.
The hidden Northridge thrust fault, for example, generated the magnitude 6.7 Northridge earthquake of 1994, which killed 57 people, left 20,000 homeless, and resulted in $40 billion in damage. Northridge was one of the most expensive natural disasters in US history, and reminded seismologists and the general public of the importance of these hidden faults.
Until now seismologists blamed the Elysian Park fault, an 11-mile system also running below the city, for the Whittier Narrows quake. But with rarely obtained data gathered by oil drilling companies, researchers John Shaw of Harvard University and Peter M. Shearer of the Scripps Institution of Oceanography, were able to use measurements of how sound waves travel through the rocks to create an image of the distinct Puente Hills fault system.
The high-resolution images were created in much the way doctors make a sonogram of an unborn child. "This is an important earthquake source for Los Angeles," Shaw said, "and one that we've been able to establish beyond inference."
The newly mapped fault encompasses several hundred square miles of heavily settled urban areas. Its lowest known point is more than nine miles deep, in the area north of Whittier. Its highest point extends to 1.8 miles below Dodger Stadium and the skyscrapers of downtown Bunker Hill.
If the three sections of the Puente Hills fault ruptured at once, the fault system could generate a magnitude -7.0 or stronger earthquake, devastating much of the metropolitan area. Fortunately, the scientists said an earthquake involving all three segments would occur perhaps once every 500 to 2,000 years.
Cynthia Long is staff writer at DisasterRelief.org, where the article first appeared.