Jun 22, 2006
Anyone who has seen the San Andreas fault will be familiar with that section of fencing where the farmer hammered in the posts, put up the slats and then moved 20 feet sideways to do the next section.
Or, maybe it was an earthquake that moved the fence posts. Yes, that’s it. So when is the next big fence moving going to happen? Seismologists really don’t know. There has been no major earthquake on the southern section of the fault, running to the east of Los Angeles, during the recorded history of European settlement in western California (the past 250 years).
How much longer can the strain on this part of the fault build up without rupturing?
In this week’s Nature, Yuri Fialko of the Scripps Institution of Oceanography, La Jolla, CA, attempts to answer that question with the help of data on the fault movements collected between 1985 and 2005. These radar and position (for example, GPS) measurements show how much the two sides of the fault have been moving past one another – the so-called slip rate. The bigger the average slip rate along a fault line, the more stress might be expected to be building up on parts of the fault that remain locked together, where a sudden rupture and release of the accumulated strain will happen in an earthquake.
Estimating the chance of such quakes is complicated, because movement across the fault does not necessarily have to lead to a build-up of stress. The stress might be getting released gradually rather than accumulating for a catastrophic event, owing to small movements called creep on the fault. Or it might be relaxed by compensating movements on other geological faults that branch off from the San Andreas fault, such as the San Jacinto fault to the south. Fialko found that the slip rates and accumulated stresses on the southern part of the San Andreas fault are indeed substantial: creep is not helping the fault to relax. The resulting strain is divided roughly equally between this and the San Jacinto fault – so the latter is not taking up most of the stress. Fialko concludes that there is a real likelihood that the long-dormant southern San Andreas fault could undergo a big earthquake before very much longer.