Major San Francisco Quake Expected 'Any Day Now'

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."

The Hayward fault runs about 43 miles (70km) along the foot of the East Bay Hills and through built-up areas such as Berkeley, Oakland and San Jose.

Its last major incident was on 21 October 1868 when a 6.8-magnitude earthquake killed 30 and caused damage to many San Francisco buildings.

But Mr Brocher warns that massive urban development means many more lives are now at risk.

He says the population around the fault has rocketed from 24,000 to around two-and-a-half million, making it the "single most urbanised earthquake fault in the United States".

"The population is now 100 times bigger in the East Bay, so we have many more people that will be impacted," he told CBS.

The seismologist warned in a 2008 report that the "tectonic time bomb" could cause hundreds of deaths and leave thousand homeless.

An even more alarming analysis by the Earthquake Engineering Research Institute in 1996 said the number of homeless could be as high as several hundred thousand.

San Francisco's most devastating quake was the infamous 1906 disaster, caused by the San Andreas fault.

The fires that broke out after the 7.8-magnitude quake destroyed most of the city and led to some 3,000 deaths.