Monitoring 'Stealth' Quakes

When the earth's crust snapped deep under Northridge, Calif., in January 1994, the pent-up energy it released shoved the nearby Santa Suzanna Mountains up by a foot and a half.

Since then, the area, which includes the city of Grenada Hills, has crept up another 6 inches in a "stealth" quake. The continuing rise is one of several subtle changes in the region's landscape made visible by Global Positioning System (GPS) satellites.

Last month, the nonprofit Keck Foundation and the National Science Foundation (NSF) contributed nearly $8 million to help pay for 250 satellite receivers to be installed in permanent locations from Ventura County, north of Los Angeles, to the Mexican border.

Using signals from the 24 GPS satellites orbiting earth, researchers say they will be able to measure changes in the locations and heights of the receivers to within millimeters.

These data will allow them to track the buildup of strain, analyze the aftermath of earthquakes, and even identify faults hidden beneath kilometers of sediment filling the Los Angeles Basin's geological bowl.

According to officials at the NSF, when fully installed, the network will be the densest array of receivers anywhere in the world.

Although the idea for a network is about a decade old, the Northridge quake gave it a jump-start. Prior to the quake, researchers had installed less than 20 receivers over a wide area, according to Thomas Henyey, director of the Southern California Earthquake Center at the University of Southern California in Los Angeles.

Future inklings

Geophysicists were trying to see if the GPS, originally designed for US military use, could compete with radio telescopes. The goal: to accurately define the boundary between two of the earth's vast crustal plates, the Pacific Plate and the North American Plate, which grind past each other along the San Andreas Fault.

"The results were encouraging," Dr. Henyey says. After the Northridge disaster, which killed 57 people and did up to $25 billion in damage, researchers proposed a 250-station array of receivers to help find and monitor strain on otherwise hidden "thrust" faults throughout the L.A. Basin and other faults elsewhere in the area.

These faults form as the northward-moving Pacific Plate pushes up against a "roadblock" in the form of a westward jog in the North American Plate. This puts the squeeze on the L.A. Basin.

In hindsight, even the less-concentrated efforts to use GPS data prior to Northridge gave inklings of what was to come.

Two years before the quake, Andrea Donnellan, a geophysicist at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., used GPS data to document increasing strain in an area that encompasses the northern San Fernando Valley, including Northridge, as well as parts of the Ventura Basin.

Since the quake, the growing GPS network, along with other techniques, has helped researchers put the event into perspective.

According to a team led by David Wald, with the US Geological Survey in Pasadena, the Northridge quake was the most damaging US quake since the San Francisco earthquake in 1906. It yielded the largest ground motions ever recorded in a quake striking an urban setting.

The team estimates that as the fault broke, the pent-up energy sent one segment sliding up and along the other at a velocity as high as 3 kilometers a second, or more than 6,700 miles an hour.

Perhaps sobering for emergency planners as the region's population continues to grow, the fault's orientation and the rupture's direction sent most of the energy northward to sparsely populated areas.

Thus, the parts of the valley that experienced the most severe damage were nevertheless spared the brunt of the shaking.

In addition, the network is yielding surprises and, perhaps, a new way of thinking about how some faults in the area form and activate. Detection of the "stealth" earthquake is one surprise, Henyey says.

With the GPS data collected before the quake as a baseline, JPL's Dr. Donnellan and Greg Lyzenga, a geophysicist at Harvey Mudd College in Claremont, Calif., continued to monitor the area's deformation.

Last December at an American Geophysical Union meeting in San Francisco, they reported the 2-inch-a-year growth rate for Grenada Hills. It would take an earthquake with a magnitude of 6 (the Northridge quake registered 6.7) to generate a one-time rise of 6 inches, Dr. Lyzenga says. Acting in a fluid-like manner, the sediment under the Grenada Hills area is continuing to readjust itself.

First-time phenomenon

Noting that this is the first time the phenomenon has been recorded in southern California, Lyzenga says, "We'd like to know if this activity is typical or atypical after earthquakes here. This is giving us a lot of food for thought." At the least, he says, "knowing that there is another account on the [strain] books seriously changes the likelihood of future earthquakes."

As if to underscore that point, JPL geophysicist Michael Heflin took measurements from a single GPS receiver near the lab.

Since the quake, the earth under the JPL receiver has moved as well, but by more than fallout from the Northridge quake could account for.

This suggests, he says, that a fault closer to the lab may be slipping as well. If so, it could mean that "a significant release of strain" can occur without a damaging earthquake.