A major earthquake could hit the San Francisco Bay Area "any day now", according to a scientist from the US Geological Survey (USGS).
The Hayward fault, less well known than California's famous San Andreas fault, caused a minor 4.0-magnitude tremor on Tuesday.
There was no major damage when it hit the border of Fremont and Union City, but Tom Brocher from the USGS said the fault is a ticking time bomb.
"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."
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Major San Francisco Quake Expected 'Any Day Now'