By Martin Kettle in Washington
Guardian
Saturday March 6, 1999
American scientists have discovered an unknown geological fault beneath the centre of Los Angeles capable, they fear, of producing an earthquake bigger than any yet to have hit southern California.
The fracture, christened the Puente Hills fault, is believed to be at least nine miles underground and runs in three broad segments for almost 25 miles under downtown Los Angeles.
Scientists believe that the active fault zone covers an area of around 250 square miles of densely settled urban land. The Puente Hills fault "is clearly a source of major earthquakes and likely could produce much damage to the Los Angeles area", John Shaw, the Harvard University geophysicist whose research on the fracture is published in the latest issue of the journal Science, told the Los Angeles Times.
"This fault system has not been considered in previous hazard assessments." The discovery of the potentially disastrous fault was made possible after geological survey information was given to the researchers by oil companies. It adds to the theory, which has gained weight over the last decade, that Los Angeles is ultimately more seriously threatened by so-called "blind thrust faults" like the Puente Hills fault, than by the more famous, larger and more visible San Andreas fault, which is much closer to the surface.
Recent findings of this kind have led many disaster experts to warn that engineering safety standards and building codes in areas of southern California threatened by earhquakes are inadequate and need strengthening.
The new fault is thought to have caused a 5.9 magnitude quake in Whittier Narrows, in the Los Angeles area, in 1987 which killed eight people and injured 200. The Puente Hills fault is similar in structure to the fault that caused the serious Northridge quake in 1994, which measured 6.7, killed 57 people and left 20,000 people homeless. About $40 billion (£25 billion) worth of damage was caused.
Unlike in that quake, however, a major earth movement from the Puente Hills fault would take place right under heavily populated areas.
At its highest point, the newly discovered fault lies 1.8 miles underneath the stadium of the Los Angeles Dodgers baseball team, which is owned by Rupert Murdoch.
By themselves, each of the fault's three segments could produce a magnitude 6.0-6.5 earthquake, Mr Shaw says. But if all three were to shift simultaneously, the effect would be a quake measuring 7 - catastrophic in a built-up area. Like all of southern California, the Los Angeles basin area is caught in a massive geological vice, in which the enormous Pacific and North American tectonic plates are pressing against one another.
The resulting slippage of rock along a fault line can move at anything from a fraction of an inch in a year to 5,000 miles an hour during major earthquakes. In 1998, southern California recorded more than 11,700 measurable earth movements.
"Finding a new fault doesn't mean that the overall hazard has changed, but that we know more about where an earthquake might occur and how large it is likely to be," a seismologist with the US Geological Survey, David Ward, was reported as saying. "This gives us a much clearer picture of what's down there."