Why California megaquake could come sooner than forecast

A new report from the United States Geological Survey points to how scientists are refining earthquake predictions as huge volumes of new data allow them to understand more deeply how fault networks work.

The report found that the probability of a magnitude 8.0 or greater earthquake in California increased slightly from 4.7 percent to 7 percent. Meanwhile, the likelihood of moderate-size earthquakes (magnitude 6.5 to 7.5) decreased by about 30 percent, from an average of one per 4.8 years to about one per 6.3 years.

California homeowners shouldn't be any more concerned than usual, the report's authors say.

"For the average citizen it’s nothing more than a reminder that this is earthquake country and big earthquakes can occur anywhere in the state," says Ned Field, a USGS seismologist and lead author of the report. "Going from one [major earthquake] every 600 years to one every 500 years, that’s a pretty rare thing being changed to still a pretty rare thing."

Scientists believe the last mega-quake to hit Southern California was a magnitude 7.9 earthquake in 1857.

The prediction changes came as scientists had access to vastly more data than they had in previous reports. The USGS forecast released Wednesday – known as the third Uniform California Earthquake Rupture Forecast, or UCERF3 – used calculations from more than 350 earthquake faults around the state. The 2007 UCERF2 forecast only considered about 200 faults, and the original 1988 report only used 16.

That larger data set has helped scientists gain insights into tectonic activity that had previously been less understood.

In particular, Dr. Field's team was able to better account for a phenomenon where big earthquakes often "jump" from one distinct fault line to another.

In the past, Field said, scientists assumed that the many faults throughout the state are separated and isolated, and that they ruptured independent of each other as well.

"As we’ve added more and more faults based on new science in California, we've come to discover it’s really an interconnected fault system," Field said.

The increasing likelihood of large multifault earthquakes "consequently reduces the likelihood of moderate-sized events," the report read.

The report also noted that three of the most-recent, largest earthquakes in California followed the fault-jumping pattern, rupturing past the named boundaries of prominent fault lines. These were the 1992 magnitude 7.3 Landers, the 1999 magnitude 7.2 Hector Mine, and the 2010 magnitude 7.2 El Mayor-Cucapah earthquakes. The 2011 magnitude 9.0 Tohoku earthquake off the coast of Japan, contributing to the deadly tsunami and Fukushima nuclear disaster, also followed this pattern, according to the USGS report.

When the UCERF2 forecast was published in 2007, Field said many in his team were frustrated they were unable to factor fault-jumping behavior into their models.

"I really wanted to see that problem solved," Field said.

The answer was to use supercomputers to crunch the vast amounts of data Field's team had to process. The analysis would have taken eight years to complete on a single computer, Field said.

In terms of how their findings could impact earthquake preparedness around the state, the report concluded that, while a major earthquake may be slightly more likely in the next few decades, homeowners may feel less of an impact than others in the state since individual homes are more vulnerable to moderate earthquakes (magnitude 6.5 to 7.5), which are forecast to become less common in the coming years.

"For a tall building or a large bridge this might be bad news because they’re more vulnerable to large earthquakes," Field said. "If you’re concerned about your own home, I would say for the most part this is probably good news because you're probably more vulnerable to having moderate-sized earthquakes than larger ones."

The public-safety implications of the findings are likely to vary across the state, Field said. 

"Building codes, earthquake insurance products, emergency plans, and other risk-mitigation efforts will be updated accordingly," he added in a fact sheet released with the report.