San Andreas is on the move. This fault, one of the most dangerous in the world, was recently said to be “locked, loaded and ready to go” by a leading seismologist. Now, another team have spotted it actually jutting along, causing both uplift and subsidence.
Although these movements are small and won’t be noticed by most people living along it at the surface, the researchers note in their Nature Geoscience study that the movements are coherent and constant. Every year, across a 200-kilometer (124-mile) stretch of the fault, incremental shifts of about 2 millimeters (0.08 inches) per year, up or down, have been detected by way of GPS measurements.
This movement is undoubtedly being caused by the chaotic, juddering, grinding motion of the Pacific and North American tectonic plates. Small releases of accumulated stress are forcing bits of land surrounding the fault to rise and fall; consequently, the Los Angeles basin is sinking while a part of San Bernardino County is rising, both at the same rate.
These minor shifts provide no immediate danger to the proximal populace. It does demonstrate how dynamic and active the fault is, though, and although it helps to release incremental amounts of stress from San Andreas, it won’t be enough to lessen the blow of the next “Big One.” Massive sections of the fault haven’t moved significantly for 150 years, and other sections have been accumulating stress for more than three centuries.
“Once there is a major event, all of that energy gets released,” lead author Sam Howell, a doctoral candidate in geophysics at the University of Hawaii, told the Los Angeles Times. Comprehending how the fault behaves every time it releases stress, major or minor, will help geoscientists estimate “the effect it’s going to have in the surrounding region” when the next major earthquake happens.
As always, though, it’s impossible to say for sure when the next major event will occur. A magnitude 7.8 tremor in 1906, one of the most powerful in the 20th century, killed 3,000 people in San Francisco, when the northern section of the fault slipped.
However, all eyes are on the southern section: The last time it ruptured was in 1857, when a 360-kilometer (225-mile) segment fractured at the surface, resulting in a magnitude 7.9 earthquake. Terrifying amounts of stress have been accumulating along it ever since.
The general rule is that the longer the time between earthquakes, the more powerful and damaging the next one will be. Although no one would wish for a devastating earthquake to occur along the San Andreas Fault, each year that passes without one increases the likelihood that the future of Southern California will be a dark one.
Image in text: Uplift (red) and subsidence (blue) based on available GPS data (top image) match the predicted movement as determined by computer models (bottom). University of Hawaii