The San Francisco Bay Area could be struck by a major earthquake "any day now", says a scientist with the US Geological Survey.
The Hayward fault historically causes a huge earthquake every 140 years and it has been 147 years since the last major quake on the fault.
Hayward caused a magnitude 4.0 earthquake on Tuesday that caused no major damage, but Tom Brocher of the USGS says it will not be long until the next massive quake, which would cause loss of life and economic damage on a large scale.
"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," Mr Brocher told CBS San Francisco.
"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."
The 1868 earthquake killed 30 people and destroyed most property in the area. Population in the Bay Area was less than 265,000 in 1868, now it is more than 7 million.
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