California’s Current Earthquake Hiatus is an Unlikely Pause

Aerial photo of San Andreas Fault looking northwest onto the Carrizo Plain with Soda Lake visible at the upper left. (Image: via wikipedia / CC BY 3.0)

There have been no major ground-rupturing earthquakes along California’s three highest slip rate faults in the past 100 years. A new study published in Seismological Research Letters concludes that this current “hiatus” has no precedent in the past 1,000 years.

U.S. Geological Survey researchers Glenn Biasi and Kate Scharer analyzed long paleoseismic records from the San Andreas, San Jacinto, and Hayward Faults for the past 1000 years to determine how likely it might be to have a 100-year gap in earthquakes across the three faults. They found that the gap was very unlikely — along the lines of a 0.3% chance of occurring, given the seismic record of the past 1,000 years.

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The results emphasize that the hiatus is exceptional, and that the gap isn’t some sort of statistical fluke created by incomplete paleoseismic records, said Biasi. The analysis also indicates that the next 100 years of California earthquakes along these faults could be a busy one. He noted:

Between 1800 and 1918, there were eight large ground-rupturing earthquakes along the faults, including the well-known 1906 earthquake in San Francisco and the similar-sized 1857 rupture of the San Andreas in southern California, but nothing so large since. Biasi said:

The three faults and their major branches analyzed by the researchers accommodate the majority of the slip between the Pacific and North American plate boundary. Paleoseismic records from the faults predict that there would be three to four large ground-rupturing earthquakes (magnitude 6.5 or larger) each century.

Biasi and Scharer examined the best available paleoseismic records from sites along the three faults to determine whether the current gap could be explained by missing data or incorrect radiocarbon dating of past earthquakes. From these data, they calculated the probability that there would be a 100-year gap in ground-rupturing earthquakes across all three faults. Biasi said:

He likened the hiatus to what a person might see if they pulled up a chair alongside a freeway to count passing cars, saying:

The researchers would like more seismologists to focus on the reasons — “the wreck around the bend” — behind the current hiatus. Biasi said:

There may be stronger long-range interactions between the faults than suspected, or there may be unknown features of the mantle and lower crust below the faults that affect the probability of ground-rupturing earthquakes, he noted.

Provided by: Seismological Society of America [Note: Materials may be edited for content and length.]

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