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LOS ANGELES — It took a decade, but the last of 250 GPS monitoring stations was installed this week, allowing scientists to record, with unprecedented precision, the minute movements of the Earth associated with earthquakes, seismologists said Friday. 

Unlike traditional networks of seismometers, which record ground shaking, the global positioning system units will track the subtle creep of the Earth’s crust as strain builds on faults – only to be released later as quakes. 

Standing on spindly legs, and painted a dull gray, the stations pepper a wide swath of Southern California and the Baja California peninsula in Mexico. 

Seismologists began building the Southern California Integrated GPS Network, or SCIGN, a decade ago; the 250th station was installed Monday. 

“We have in Southern California over half of the nation’s earthquake risk, and we are applying GPS technology in new ways to assess this risk,” said Ken Hudnut, of the U.S. Geological Survey and SCIGN’s chairman. 

Linked to an orbiting cluster of satellites, the GPS stations will provide continuous data – for 50 years or more – about otherwise imperceptible shifts in the Earth’s crust. 

The network is so precise it can record as little as .04 inches of distortion of the ground or movement along a fault.  

The movement typically occurs steadily, but slowly, without the ground-shaking associated with earthquakes. 

The buildup of strain is directly tied to earthquake potential.  

Tracking it will help scientists create hazard assessments. 

“Now with SCIGN, Southern California is ’wired’ like no place else in the U.S. Never before has a network like SCIGN been built,” said John Filson, national program coordinator of the USGS’s earthquake hazards office. 

When a significant earthquake does occur, the network will measure the release of strain and the deformation that follows, often for months afterward. 

Scientists positioned the bulk of the stations in and around Los Angeles because of its large population and significant seismic risk. 

The stations sit on private property, alongside freeways, atop dams and, in at least one case, on an oil drilling platform. 

Past measurements have shown that the Los Angeles basin is being compressed in a north-south direction, shrinking by about .03 inches a year thanks to the clash of the Pacific and North American plates. 

SCIGN is a project of the Southern California Earthquake Center.  

The National Aeronautics and Space Administration’s Jet Propulsion Laboratory, the Scripps Institution of Oceanography and USGS are the program’s main participants. 

On the Net: http://www.scign.org/