Jarring notice of California quake dangers.

Jarring notice of California quake dangers

The magnitude 6.1 earthquake that rattled Los Angeles on Oct. 1 was a jolting reminder of the hazards of living in earthquake country. Along with its aftershocks, the quake was responsible for at least six deaths and more than $100 million in damage, giving Angelinos a tiny taste of a much larger quake expected to hit the San Andreas fault in the next several decades.

But even though the Oct. 1 quake sprang from the same forces straining the San Andreas, seismologists say it was too far from the San Andreas to release that fault's strain or alter the likelihood of the predicted great quake. The recent shaker was centered in Rosemead, 55 kilometers west of the San Andreas.

California earthquakes result from the Pacific plate's northwest drive past the North American plate at a rate of 5.6 centimeters a year. Near Rosemead, the San Andreas slides only 2.5 cm/yr, leaving the rest of the motion for nearby faults.

The recent earthquake, however, was only "a little detail' of this larger motion, says seismologist Lucille Jones at the U.S. Geological Survey in Pasadena, Calif. The quake's main scientific significance, she says, is that it may help researchers understand the processes by which faults end at sedimentary basins. It broke at the tip of the northwest-trending Whittier fault, which, like most California faults, is mostly strike-slip, with the blocks sliding past one another horizontally. This earthquake, however, was a thrust quake, with one block being pushed up over the other at the end of the Whittier fault.

It was the region's strongest quake in 16 years. Essentially, says Jones, it was not predicted. However, she adds, the day before the quake she had noticed that 11 months had passed since there had been an earthquake of magnitude 4 or greater. This was the longest such "quiescence' period for the area in 55 years of record keeping. The previous quiescence period had lasted nine months and ended untheatrically with a magnitude 4 quake.

The interesting thing about the more recent quiescent period, says Jones, is that southern California was quiet except that Los Angeles was rocked by more magnitude 3 quakes than usual. The problem now, she says, "is that we don't have any idea of how often something like that is followed by a major event. But maybe in the next years, this will seem significant and we might be able to use it to predict future earthquakes.'

In the meantime, residents will have to prepare for the inevitable. Magnitude 6 quakes occur in southern California every four or five years, and the San Andreas's "big one' still lies in wait.