Satellites show L.A. area's yearly rise, fall

Geologists say the Los Angeles area experiences seasonal rises and falls.

Menlo Park, California (CNN) -- Geologists say they have proof that much of the Metropolitan Los Angeles area literally rises and falls with the seasons.

A team led by Gerald Bawden at the United States Geological Survey in Menlo Park, California, used data from 250 Global Positioning Satellite monitoring stations to conclude that seasonal rises and falls in groundwater levels cause the ground level to rise each winter and sink each summer in Los Angeles. In some areas, the seasonal difference can be as much as 12 centimeters (just under five inches).

The USGS team measured the phenomenon in a roughly oval-shaped area from downtown Los Angeles on the north, to Compton and Long Beach on the west, south to Newport Beach and then east to the Santa Ana Mountains.

The researchers say the new discovery of this seasonal activity may actually complicate their main task of monitoring for earthquakes. The GPS network is designed to search for possible hints of upcoming earthquakes by detecting minuscule movements in the earth's crust. Now, with the earth's surface apparently moving for other reasons, advance warnings of some earthquakes may be more difficult to deliver. Human activity -- the pumping out of groundwater used for drinking water in Orange County and elsewhere in the Los Angeles area, and oil drilling south of the city -- may also contribute to changing the ground level, further clouding the earthquake monitoring data.

The USGS team published their findings Wednesday in a peer-reviewed letter to the journal Nature.