Seven miles beneath the San Gabriel Mountains, a long-dormant fault ruptured Friday, continuing an ancient process that has slowly pushed the rugged peaks almost two miles above the Los Angeles Basin.
The 7:43 a.m. jolt, which measured 6.0 on the Richter scale, probably lifted the mountains 2 or 3 inches higher, said Egill Hauksson, a seismologist at the California Institute of Technology."The mountains are there because there are faults pushing them up," said Lucy Jones, a seismologist with the U.S. Geological Survey.
The same kind of "thrust" earthquake was responsible for the Whittier Narrows earthquakes, which measured 5.9 and 5.3 and killed eight people in October 1987, and for the magnitude-6.4 San Fernando quake, which killed 65 people on Feb. 9, 1971.
A thrust quake occurs when one block of the ground moves up and over another block of the ground.
Meanwhile, Jones said there is a one-in-three chance that by next Friday, there will be a sizable aftershock measuring 5 or more on the Richter scale.
There is a 4 percent chance by early Monday of a quake as big or stronger than Friday's quake, said Jones, who bases her forecasts on historical records of quakes and their aftershocks.
Friday's quake was centered 8 miles north of Monrovia and 7 miles under the San Gabriel Mountains, probably on the Sierra Madre Fault, which runs roughly 50 miles from Upland to near San Fernando, Jones said.
Scientists have measured quakes in the region for only 60 years, but geologic evidence shows the fault hasn't produced a significant quake in 10,000 years, she added.
The San Fernando Fault, part of the same system of faults, was responsible for the 1971 San Fernando quake, Hauksson said.
The mountains rise sharply from the Los Angeles Basin to 10,064-foot Mount San Antonio, commonly called Old Baldy Peak.
The San Gabriels are part of a chain of mountains called the Transverse Ranges. The chain parallels Southern California's coast, stretching from the Santa Ynez Mountains near former President Ronald Reagan's ranch southeastward through the Santa Monica, San Gabriel, San Bernardino and San Jacinto mountains.
Southern California straddles the boundary of two great plates that make up Earth's crust - the Pacific Plate, which is under the ocean and part of Southern California, and the North American Plate, which is under the rest of the continent.
As the plates slide past each other, quakes occur on numerous "strike-slip" faults, such as the San Andreas, on which ground slides horizontally past ground on the other side of the fault.
But the two gigantic plates also squeeze together, creating the Transverse Ranges and a series of thrust faults that parallel the mountains and run along their southern edge.
6.0 quake kills 2 and injures 50
The strongest earthquake to hit Los Angeles in 20 years is now being blamed for killing two people, injuring at least 50 others and damaging more than 250 buildings.
The quake, which measured 6.0 on the Richter scale, caused severe damage to homes, stores and office buildings and sent frightened people pouring into streets.