And with it goes the wait for the Big One - the disastrous quake that experts have been promising for decades.

Each time there is a series of quakes, scientists strive to see signs that the Big One is on the way, many families stock up on emergency supplies of water and food and there is a sudden surge in earthquake insurance contracts.

But California is used to living on the San Andreas fault, with what scientists say is a 70 percent chance of a major quake over the next 30 years.

The latest series of quakes started last Sunday when a temblor measuring 5.5 on the open-ended Richter scale was felt from Los Angeles down to San Diego, near the Mexican border.

The epicentre was at Anza, in the desert northeast of Los Angeles.

The most recent temblor, in northern California, was a relatively mild, 2.0 magnitude quake at about 5:00 am local time (1200 GMT) Sunday, centered some 116 kilometers (72 miles) northwest of the California capital city of Sacramento.

The quake, determined to be an aftershock, had a depth of some 2.3 kilometers (1.4 miles) according to the US Geological Survey, which tracks seismic activity.

On Tuesday, there was a 7.2 quake in the sea off San Francisco, which provoked a brief tsunami alert along the US Pacific coast. People evacuated Crescent City, which was hit by a giant wave in 1964, in search of higher ground.

Two days later, a quake of 4.9 hit the same area as the first on Sunday, and was felt from Ventura, north of Los Angeles, to San Diego. One person was hurt and a few windows cracked.

A few hours later, a quake measuring 6.5 on the Richter scale shook the sea bed again off the northern Californian coast. It was probably an aftershock from Tuesday's sea quake.

Experts say they are looking for links between the first and third quakes but are not yet saying there will be a major rupture of the San Andreas fault where the Pacific and North American tectonic plates meet.

It was a break in the San Andreas that caused the San Francisco earthquake in 1960, flattening or burning much of the city, with the loss of more than 2,000 lives.

"Any time there's an earthquake, there's a small chance that it is a foreshock. It is about one out of 20 or five percent. That means five out of 100 earthquakes turn out to be followed by something larger," said Kate Hutton of the Californian Institute of Technology.

Lucy Jones of the US Geological Survey's office in Pasadena, California, said when "you have two (magnitude) Fives a few days apart, you start wondering. But we've done this before and not had it lead on to anything else."

She said southern California generally averages two to three quakes of more than 5.0 on the Richter Scale. But, unusually, there have been only four or five since 2000.

California authorities have still been reminding the public to make sure they have supplies of food, water and blankets at home in the office and even in the car.

One California insurance broker told AFP that on Thursday he saw a 30 percent increase in calls from people wanting an earthquake damage policy.

"Based on the past, any earthquake increases the interest in earthquake insurance," said Stan Devereux, director of public and government affairs for the California Earthquake Authority.

The authority is a publicly managed but privately funded insurance fund founded in 1996 after the 1994 quake in Los Angeles that killed 37 people and caused an estimated 40 billion dollars of damage.

According to Devereux, only 14 percent of Californians have earthquake insurance. The state has only imposed quake resistant norms for new buildings since 1975.

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