WHILE San Franciscans justifiably worry about living near the San Andreas fault, many forget that southern Californians too live in fear of their own 鈥渂ig one鈥�. No major quake has struck the southern San Andreas fault in at least 250 years, and scientists say that the region is now primed for a release of the built-up tension.
A new study, by geophysicist Yuri Fialko of the University of California, San Diego, provides the most precise measurements yet of this accumulated strain 鈥� and it鈥檚 not a pretty picture. Deep within the Earth鈥檚 crust, the west side of the San Andreas fault has moved relative to the east by as much as 8 metres since the region鈥檚 last earthquake. But closer to the surface, the two sides of the fault are jammed against each other, building up ever-increasing strain.
Fialko says that this 8-metre shift is on a par with the maximum movement that the fault has ever experienced between quakes 鈥� and it has packed enough energy in the fault to unleash a magnitude-8 earthquake if the strain were released all at once. 鈥淚f it鈥檚 realised, it鈥檚 going to be a major disaster,鈥� he says.
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Fialko estimated the slip rates along faults in southern California using nine years of high-precision satellite data. Conventional seismic stations can track ground movement very precisely but are typically spaced up to about 10 kilometres apart, on average. Such a coarse grid of data points makes it hard to be certain of what is going on underground. Satellites, on the other hand, can scan the whole region with a resolution down to 20 metres. The fine detail, combined with the use of space-borne radar interferometers and GPS satellites, allowed Fialko to measure ground motions as slow as only a few millimetres per year. 鈥淚t鈥檚 only been a few years since that became possible,鈥� says Fialko (Nature, vol 441, p 968).
鈥淚t鈥檚 certainly the most precise study that I鈥檝e seen,鈥� says Karen Felzer, a geophysicist at the US Geological Survey in Pasadena, California.
These minute movements reveal the deformation of the land surface along the faults, showing for the first time where the strain is accumulating. The tension seems to be divided almost evenly between the parallel San Andreas and San Jacinto faults (see Graphic).
杏吧原创s were also uncertain whether slow movements in the faults at shallow depths were partially relieving the built-up tension. But Fialko鈥檚 analysis shows that this 鈥渟hallow creep鈥� is, unfortunately, trivially small. The southern San Andreas fault may be nearing the end of its quiet period.
