There was a 4.0 earthquake the other day in California yesterday morning.  The 2:41 a.m. earthquake on the border of Fremont and Union City occurred on the Hayward Fault at a depth of 5 miles.  Fremont and Union City are in the San Francisco and Oakland area.

That’s not the news.  The news is this: Tom Brocher, a scientist with the USGS, told CBS that the Hayward Fault is due to produce a major earthquake “any day now.”

Per CBS:

“The population is now 100 times bigger in the East Bay, so we have many more people that will be impacted,” said Tom Brocher, a research geophysicist with the USGS.

“We keep a close eye on the Hayward Fault because it does sit in the heart of the Bay Area and when we do get a big earthquake on it, it’s going to have a big impact on the entire Bay Area,” Brocher said.

While a 2008 report put the probability of a 6.7-magnitude or larger earthquake on the Hayward-Rodgers Creek Fault system over the next 30 years at 31 percent, Brocher said the reality is a major quake is expected on the fault “any day now.”

“The past five major earthquakes on the fault have been about 140 years apart, and now we’re 147 years from that 1868 earthquake, so we definitely feel that could happen any time,” Brocher said.

Why isn’t everyone fraking out?  Why isn’t this front page news everywhere?

Two reasons.  Earthquake prediction is horrible.  The earth’s crust moves at a geologically slow rate.  Geologically slow.  So trying to predict when a major quake will come is tricky.  It has never been done with success.

The other reason? Bocher has been sounding the alarm on the Hayward Fault for a while now. In 2008, he was the lead author of a USGS report that described the fault as a “tectonic time bomb” and warned that a 6.8-7.0 magnitude quake could “could cause hundreds of deaths, leave thousands homeless, and devastate the region’s economy.” Among the factors that lead the report’s authors to suggest that the Hayward Fault is the country’s most dangerous one are the facts that it is the “single most urbanized earthquake fault in the United States” and that “critical regional gas and water pipelines and electrical transmission lines cross.”  You can read the full report here [pdf].

So when he says “any day now”,  you have to remember he is a geologist.  And to a guy who thinks in geological terms, “any day” doesn’t necessarily mean “within the next week”.  It means, literally, “any day”.  Could be tomorrow or maybe a couple of centuries from now.