This system is generally known as the San Andreas Fault System, after the San Andreas Fault, which runs the length of California.

The San Andreas Fault runs along the length of San Andreas Lake

The presence of the San Andreas Fault was brought dramatically to the world's attention on 18 April 1906, when sudden surface movement along the fault caused the great San Francisco earthquake and fire.

More than 700 people died and millions of dollars of damage were caused.

The Earth's crust is not a solid, uninterrupted structure. It is divided into a series of plates that have been moving very slowly over the Earth's surface for millions of years.

Two plates colliding

At the San Andreas Fault, two major plates meet - the Pacific Plate and the North American Plate. They move against each other, causing regular earthquakes.

The entire fault system is more than 1,200 km (800 miles) long and extends to depths of at least 16km (10 miles) within the Earth, according to the United States Geological Survey.

Older buildings are particularly vulnerable

In a report on the San Andreas Fault system, the USGS says that the "sudden offset that initiates a great earthquake occurs on only one section of the fault at a time."

Sections remain locked after minor earthquakes and strain builds up, often over hundreds of years.

The accumulated strain is then suddenly released as a major earthquake.

'Thousands of quakes'

Thousands of small earthquakes occur in California each year, according to the USGS report.

The largest recorded earthquakes along the San Andreas Fault occurred in 1857 and 1906.

The earthquake of 9 January, 1857, in Southern California, was of about the same magnitude as the San Francisco earthquake of 1906, says the USGS.

The 1906 earthquake was felt as far away as Oregon and central Nevada. It has been estimated at a magnitude 8.3 on the Richter Scale.

In 1989, the Loma Prieta earthquake in Northern California killed nearly 70 people and caused $6 bn in damage.

Waiting for the big one

The 1989 quake caused massive damage

In a report marking the10th anniversary of Loma Prieta, the USGS predicts that the San Francisco area has a 70% chance of being hit by a major earthquake in the next 30 years.

But a great earthquake is unlikely to occur unannounced. It would probably be preceded by smaller shocks, scientists say.

"Before the next large earthquake, seismologists also expect to record changes in the Earth's surface, such as a shortening of survey lines across the fault, changes in elevation, and effects on strainmeters in wells," say USCG scientists Sandra Schulz and Robert Wallace.

Defence

The US authorities follow a three-line strategy in their defence against earthquakes.