Hang on to your hard hats!

LOS ANGELES – Scientists have detected a spike in underground rumblings on a section of California's San Andreas Fault that produced a magnitude-7.8 earthquake in 1857.

What these mysterious vibrations say about future earthquakes is far from certain. But some think the deep tremors suggest underground stress may be building up faster than expected and may indicate an increased risk of a major temblor.

Researchers at the University of California, Berkeley, monitored seismic activity on the fault's central section between July 2001 and February 2009 and recorded more than 2,000 tremors. The tremors lasted mere minutes to nearly half an hour.

Unlike earthquakes, tremors occur deeper below the surface and the shaking lasts longer.

During the study period, two strong earthquakes hit — a magnitude-6.5 in 2003 and a magnitude-6.0 a year later. Scientists noticed the frequency of the tremors doubled after the 2003 quake and jumped six-fold after 2004.

Tremor episodes persist today. Though the frequency of tremors have declined since 2004, scientists are still concerned because they are still at a level that is twice as high as before the 2003 quake.

The team also recorded unusually strong rumblings days before the 2004 temblor.

Results of the research appear in Friday's issue of the journal Science. The work was funded by the U.S. Geological Survey and National Science Foundation.

"The fact that the tremors haven't gone down means the time to the next earthquake may come sooner," said Berkeley seismologist and lead researcher Robert Nadeau.

Bottled water, canned food, emergency numbers, camping equipment....

Bottled water, canned food, batteries

Emergency contacts, flashlights... It might be time to rock and roll, California!:

Scientists are watching closely to see if small faults crossing under the Salton Sea are transferring energy to the larger, more dangerous San Andreas fault after a series of small quake swarms in the area.

The quakes appeared to be tapering off by Monday afternoon, according to the monitoring system run by the U.S. Geological Survey and Caltech. But in a 48-hour period starting Saturday morning, 42 quakes shook just south of Bombay Beach on the Salton Sea. The quakes ranged in magnitude from 0.5 to 3.3, with three larger than 3.0 hitting the area Saturday afternoon.

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Scientists are particularly interested in the area because an earthquake that starts in Bombay Beach and ripples northwest along the San Andreas fault could be the Big One that devastates Los Angeles, said Graham Kent, a research geophysicist at UC San Diego.

These quakes appear to be taking place at the hazy intersection of several recently mapped faults crossing beneath the Salton Sea and the the San Andreas fault.

The worry for scientists comes from a case in 1987, when a magnitude-6.2 earthquake on one of the crossing faults appeared to trigger a 6.6 quake 12 hours later on the Superstition Hills fault to the south. The San Andreas fault is north of these crossing faults and the geometry is similar, Kent said.

This swarm, he said, "is a flashing yellow light that tells you to be prepared and to spend some time trying to understand the significance."

Here's a California earthquake map that notes recent activity.