Unprecedented hiatus of major California earthquakes could mean flurry of big ones this century

Scientists studying California’s earthquake patterns said there have been no massive ground-rupturing quakes along that states’ three major faults since 1919 – an “exceptional hiatus” that has seen no precedent in the past 1,000 years.

The result of this overly long quiet period could mean that the next century of California earthquakes along these faults could be a busy one, concluded researchers whose work was published in April’s Seismological Research Letters.

 “If our work is correct, the next century isn’t going to be like the last one, but could be more like the century that ended in 1918,” U.S. Geological Survey researchers Glenn Biasi said in a Seismological Society of America news release.

Biasi and colleague Kate Scharer analyzed paleoseismic records from the San Andreas, San Jacinto and Hayward Faults since 1800. From that time to 1918, the researchers found there were eight large ground-rupturing earthquakes along the faults, including the well-known 1906 earthquake in San Francisco, which registered with a magnitude of 7.9, and the similar-sized 1857 rupture of the San Andreas in Southern California, but nothing so large since.

As for the reason? The researchers aren’t completely sure, but one theory is that the large flurry of earthquakes up until 1918 “wrung out” the system.

 “It’s possible that among them they just wrung out—in the sense of wringing out a dishrag—a tremendous amount of energy out the system,” Biasi said.

The study looked at earthquakes that broke the surface and left a geologic record over the past 1,000 years.   The 6.9-magnitude Loma Prieta earthquake, which struck Northern California on Oct. 17, 1989, was different, and Biasi said there is no way to know whether or how often similar events might occur.  The 1989 earthquake did relieve stress locally, he added, which amounts for a very small fraction of the fault system.   

“We know these big faults have to carry most of the [tectonic] motion in California, and sooner or later they have to slip,” Biasi said. “The only questions are how they’re going to let go and when.”