ALERT -- 5.1 Magnitude EARTHQUAKE Strikes LOS ANGELES - Raw Footage

A moderate earthquake that rattled a swath of Southern California forced several dozen people in one community out of their homes after firefighters discovered foundation problems that made the buildings unsafe to enter, authorities said Saturday.

Fire crews red-tagged 20 apartment units in a building in the Orange County city of Fullerton after finding a major foundation crack. Structural woes including broken chimneys and leaning were uncovered in half a dozen single-family houses, which were also deemed as unsafe to occupy until building inspectors clear the structures. The damage displaced 83 residents.

Despite the evacuations, Friday night's magnitude-5.1 quake centred about 25 miles (40 kilometres) south of downtown Los Angeles mostly frayed nerves.

The earthquake question comes up in two out of every three transactions that Eileen Bermingham handles. Demand for San Francisco property has hit new heights in recent years, forcing buyers to offer far above the asking price -- and things don't appear to be slowing, even in the usually sluggish early months of the year. "It's been particularly hectic," confirms Bermingham, an agent with Zephyr Real Estate, which sells houses all over the city.

But the earthquake question is always in the background.

Bermingham gets specific requests from clients for newer homes built under more stringent building codes, houses that have been retrofitted for earthquakes, or specific neighbourhoods that pose less risk. Many parts of the city are built on landfill, and maps highlight large swathes that are at risk of 'liquefaction' -- the soil literally turning from solid to liquid -- in the event of a large earthquake.

"A lot of it comes down to being at least knowledgeable about what you're getting into," Bermingham says of investing in the city. "You don't want to find out later that you're on a liquefaction zone."

And the next "big one" could come any day. Between now and 2038, there's a 99.7% chance of a 6.7-or-larger earthquake striking somewhere in California. That's according to a 2008 report from the US Geological Survey, which estimates that there's a 63% probability of a big earthquake hitting the San Francisco region. The two faults flanking San Francisco -- the northern San Andreas and the Hayward -- have a 21% chance and a 31% chance, respectively. Clearly that's not a sure bet, but if the experts are right, San Francisco will probably be shaking hard again sometime soon. earthquake

The San Andreas Fault is a continental transform fault that extends roughly 810 miles (1,300 km) through California in the United States. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics, and a different degree of earthquake risk. Although the most significant (Southern) segment only dates back about 5 million years, the oldest sections were formed by the subduction of a spreading ridge 30 million years ago.

The fault was first identified in 1895 by Professor of geology Andrew Lawson from UC Berkeley who discovered the northern zone. It is named after a small lake which was formed in a valley between the two plates. Following the 1906 San Francisco Earthquake, Lawson concluded that the fault extended all the way into southern California. In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the San Andreas Fault.[1]

A project called the San Andreas Fault Observatory at Depth (SAFOD) is drilling into the fault to improve recording and prediction of future quakes.