Seismologists say southern California and Los Angeles are facing an imminent threat of a major earthquake.
True, that scenario has been marketed for some years now, but scientists say considerable strain is building in the San Andreas fault, the lower portion of which has not had an earthquake for at least 250 years.
Recent measurements indicate the immediate Los Angeles area could feel the effects of a devastating quake within a few years.
It could be tomorrow; it could be 10 years from now, said Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography at San Diego, but it appears unlikely to accumulate another few hundred years of strain.
According to Nature magazine, researchers employed satellite data to measure the amount of movement between the North American and Pacific plates. Their boundary in this country is known as the San Andreas fault. It extends from below the Mexican border to well beyond San Francisco.
The magazine said researchers found evidence of substantial movement of the plates, indicating pressure may soon be released in an earthquake. The more movement along a fault line, the greater chance of a major quake.
Professor Fialko said: The observed strain rates confirm that the southern section of the San Andreas fault may be approaching the end of the interseismic phase of the earthquake cycle.
Some scientists think the fault line could be releasing its strain very slowly over a period of time, by inching forward along its length, or by putting some of the strain on neighboring fault lines. The new study suggests that neither fast nor slow movement is happening in any substantial amount.
Fialko, in making this study, collected eight years worth of data from European Space Agency satellites, which measure ground movement in detail. He then added 20 years worth of information from global-positioning measurements on the ground.
Taken together, he said, the measurements suggest the two plates on either side of the southern San Andreas fault are slipping past one another at about 25 millimeters per year. With no earthquake to release the pressure, the fault line, which has long been static, has built up between 5.5 and 7 meters of slip deficit.
If that pressure were released all at once, it could produce an earthquake equal to the Fort Tejon quake of 1857, which measured 8.0 on the Richter scale.
Such a quake in populous Southern California would be even more devastating than the 1906 quake in San Francisco, which measured 7.8 on the scale.
One of the most surprising findings of the study was that the strain on either side of the fault is building differently. The North American plate side appears to be moving in a more flexible fashion, while the Pacific plate side is more rigid, according to Fialko.
Ray Weldon, a geologist at the University of Oregon in Eugene, said that sort of asymmetry is only now being recognized as an important phenomenon.
In the past, Weldon said, weve sort of thought that elastic properties of the rock dont vary around faults. This is showing that there are quite large variations.
The study also indicates, according to an article on firstname.lastname@example.org, that other fault lines that branch off the San Andreasknown as the San Jacintopresent a greater earthquake threat than previously thought. The San Jacinto runs west of the San Andreas, passing near the cities of Riverside, Hemet and Borrego Springs. Fialkos computations show strain along that fault is growing at a rate twice that expected.
Fialko said the new study will aid in pinpointing where the earthquake problems are in California.
The southern San Andreas, Fialko said, is fully loaded for the next event.
From the July 5-11, 2006, issue