‘Locked, loaded and ready to roll’: San Andreas fault danger zones (Source cosmosmagazine.com)
A series of small earthquakes up to magnitude 4 started popping off right next to the San Andreas fault at the end of September, giving Californian seismologists the jitters. This swarm of more than 200 mini-quakes radiated from faults under the Salton Sea, right down at the southern end of the San Andreas fault. And although the small quakes only released tiny amounts of energy, the fear was that this fidgeting could be enough to trigger an earthquake on the big fault. . “Any time there is significant seismic activity in the vicinity of the San Andreas fault, we seismologists get nervous,” said Thomas Jordan, director of the Southern California Earthquake Centre in Los Angeles.
Because despite a plethora of sensitive instruments, satellite measurements and powerful computer models, no-one can predict when the next big one will rattle the Golden State. Slicing through 1,300 kilometres of Californian landscape from Cape Mendocino in the north-west all the way to the Mexican border in the south-east, the San Andreas fault makes itself known. Rivers and mountain ranges – and even fences and roads – are offset by the horizontal movement of this “transform” fault, where the Pacific Ocean plate to the west meets the North American plate to the east. The fault moves an average of around 3.5 centimetres each year, but the movement comes in fits and starts. Large earthquakes doing most of the work, punctuating long periods of building pressure.The fault divides roughly into three segments, each of which tends to produce a big quake every 150 to 200 years. The last time the northern segment (from Cape Mendocino to Juan Bautista, south of San Francisco) released stress was during the devastating magnitude-7.8 San Francisco Bay quake in 1906, which killed thousands and destroyed around 80% of San Francisco.
Meanwhile, the central section, from Parkfield to San Bernardino, has been quiet for longer still, with its last significant quake in 1857, when a magnitude-7.9 erupted underneath Fort Tejon.
But most worrying of all is the southern portion (from San Bernardino southwards through the Coachella Valley), which last ruptured in the late 1600s. With more than 300 years of accumulated strain, it is this segment that seismologists view as the most hazardous.
“It looks like it is locked, loaded and ready to roll,” Jordan announced at the National Earthquake Conference in Long Beach in May 2016..