NASA develops radar for underground quakes

A fault that runs near Yuma may be long overdue for a mega-earthquake.

That warning concerns the southernmost section of the San Andreas Fault and stems from new earthquake research being conducted by NASA and scientists from the Jet Propulsion Lab.

"There are very few earthquakes along this section of the fault," said Eric Fielding, a research scientist at JPL in Pasadena. "The fault has been locked and building up stress over the years. It could rupture at any moment."

The San Andreas Fault runs just west of Yuma.

Fielding said this section of the fault has not produced a major earthquake in more than 300 years and is thought to be capable of a magnitude 8 event, as large as any known California earthquake.

"The consensus from scientists is there is a 67 percent chance that a major earthquake will occur along this fault sometime within the next 30 years," Fielding said.

Predicting exactly when the next "big one" might happen is beyond scientists' ability. However, some space-age technology they are now using may help them know where it may happen.

Fielding said scientists at JPL are using a very precise 3-D airborne radar they developed. It is strapped to the bottom of a flying jet to measure exact surface elevations along the southernmost section of the San Andreas fault from North of San Francisco to Yuma.

Called the Uninhabited Aerial Vehicle Synthetic Aperture Radar, or UAVSAR, the L-band wavelength radar uses pulses of microwave energy to see below the surface to detect and measure very subtle buildup and releases of strain along faults, Fielding said.

UAVSAR works like this: When flying over a fault at an altitude of 45,000 feet, the radar collects data over the selected region. It then flies over the same region again, minutes to months later, using the aircraft's advanced navigation system to precisely fly over the same path to an accuracy of within 4.6 meters (15 feet).

By comparing these camera-like images, called interferograms, over time, scientists can measure the slow surface deformations involved with the buildup and release of strain along earthquake faults.

"It is a technique for comparing two 3-D images," Fielding said. "By comparing the images, we can measure the ground motion to a fraction of an inch."

This southernmost section of the San Andreas Fault last erupted in 1690, producing an estimated 7.7-magnitude quake but causing little injury or damage because hardly anyone lived in the area at the time.

Scientists, Fielding said, know very little about this 100-mile dormant segment, which slices through Southern California from San Bernardino, east of Los Angeles, to near the Mexican border.

Formed 15 to 20 million years ago, the 800-mile-long San Andreas Fault has defined California's seismic history and dramatically altered its landscape.

The fault caused the 1906 San Francisco earthquake that led to about 3,000 deaths. It also serves as the boundary between the two massive tectonic plates under California: the Pacific and North American plates.

Paleoseismological studies dating back 1,500 years have shown that large earthquakes occur on the southern section of the San Andreas about every 250 to 300 years, on average.

Because of the stress has been building up for such a long period of time on the southernmost section of the fault, scientists think it is overdue for a mega-earthquake. And if it does erupt, it could inflict severe damage in Los Angeles, San Diego and even as far east as Phoenix.

This multiyear mapping project will for the first time give scientists an extremely accurate picture of the Earth's surface near the San Andreas and other other California faults, Fielding said.

As earthquake-inducing stress builds up, scientists expect to be able to detect changes in elevations by flying over the area, taking new measurements.

"We won't be able to say when it will happen, but what we will be able to forecast is where it is going to happen," Fielding said.

Fielding added that while UAVSAR will help scientists forecast where an eruption will happen, they still won't know which direction it will occur.

For example, if the rupture happens from north to south, its energy will be focused on the Imperial Valley and the Yuma area. If the rupture happens from the south to the north, the energy will be focused on the Los Angeles area.

Scientists already use a broad of array of tools to "listen" to the San Andreas and other faults in an effort to detect changes on or below the Earth's surface.

They dig trenches and place instruments such as seismographs, creep meters and stress meters into the ground.

Scientists have long known that Los Angeles, San Diego and Riverside are creeping north at 1.4 inches per year past San Bernardino, Lancaster and the rest of North America. This slippage occurs regularly along some parts of the San Andreas.

Studies have also already indicated that strands of the fault in the Salton Sea area are extremely overstressed. A recent chain of small earthquakes east of Borrego Springs and north of El Centro was feared to have overstressed the main fault, according to JPL.

Last November, JPL scientists began conducting a series of UAVSAR flights over regions of Northern and Southern California that are actively deforming and are marked by frequent earthquakes.

About every six months for the next three years, JPL scientists will precisely repeat the same flight paths to produce interferograms.

From this data, 3-D maps will be created for regions of interest, including the San Andreas and other California faults.

In May, the scientists completed their first full map of the San Andreas.

By comparing these repeat-pass radar observations, scientists will have a better understanding of regional earthquake hazards in California.

In doing so, scientists will estimate the total displacement occurring in each region. As more observations are collected, they expect to be able to determine how strain is partitioned between individual faults.

They'll also be able to measure ground signals caused by human activities, such as pumping water into or out of the ground or drilling for oil.

These UAVSAR flights will then serve as a baseline for pre-earthquake activity. Should earthquakes occur during the course of this project, scientists will measure the deformation at the time of the earthquakes to determine the distribution of slip on the faults, and then monitor longer-term motions after the earthquakes to learn more about fault zone properties.

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The NASA Jet Propulsion Lab in Pasadena contributed to this article. James Gilbert can be reached at jgilbert@yumasun.com or 539-6854.