Thrust-Fault Mapping Effort Comes Up Short

An elaborate attempt to map thrust faults buried under the Los Angeles Basin, such as the one that ruptured in the 1994 Northridge earthquake, has produced only indistinct images, quake scientists reported this week.

The results are a disappointment to researchers, who had hoped to gain valuable insight into the complex web of faults underlying the Southland.

The $600,000 federally funded experiment involved setting off 60 subterranean ammonium nitrate explosions on land, as well as firing thousands of small bursts of compressed air from a research vessel off Long Beach, in the fall of 1994.

Hundreds of seismographic instruments recorded waves from the blasts, the strongest of which were equivalent to a magnitude 2.5 earthquake, in the attempt to map seismic features six to 15 miles underground.

The results, described at a Caltech earthquake briefing, show how difficult it is to obtain a precise picture of the potentially destructive underground faults, particularly with explosions small enough to be acceptable to the public.

Background urban noise impeded the experiment, the scientists said.

“Things are never as good as the ideal case,” said Jim Mori, scientist in charge of the U.S. Geological Survey’s Pasadena field office. “Originally, there was thought that we would be able to image a lot of faults under Los Angeles, and that turned out not to be the case.

“It would not have been appropriate for us to be shaking the ground hard under Los Angeles in the months following Northridge,” he said.

“But, still, the images we got were sometimes exciting. Mapping thrust faults wasn’t the only purpose of the experiment. We were able to see the bottom of the sediments in the basin and observe possible links between fault systems.”

For instance, there were tenuous indications of a possible master fault under the San Gabriel Mountains linking the San Andreas fault with the thrust fault system of the basin.

The project director, Gary Fuis of the Geological Survey’s Menlo Park office, cited “indirect images” of the San Andreas and Sierra Madre faults at considerable depth, and a “seismic bright layer” 15 miles down that possibly links them.

Fuis described the layer as seeming to form a boundary under which earthquake activity in the Los Angeles region stops.

“So it may represent a master fault beneath the San Gabriel Mountains and the northern Los Angeles Basin that feeds displacement upward into the blind thrust fault system of the basin,” he said.

An alternate explanation is that the scientists are seeing an older formation than either the San Andreas or Sierra Madre faults, he said.

Fuis said the study’s inconclusive results are partly the result of the amount of explosives used. “In the L.A. Basin, we might have been able to see more had we used larger explosives,” he said, “but we went just as big as we could without getting to the threshold of making people nervous.”

Some of the explosions under the mountains and the Mojave Desert farther north used 4,000 pounds of ammonium nitrate, but only 900 pounds was used under the basin.

“We didn’t get a good reflection image” in the basin, Fuis said.

Tom Henyey, director of the Southern California Earthquake Center at USC, described the results of the experiment as “moderately decent, giving us some information we didn’t have before.”

“Science moves forward often in little pieces,” he said. “We have some tantalizing information now that may lead to further study. You don’t get everything all at once.”

With an extensive seismic hazard mapping project underway at the state Division of Mines and Geology, officials there have expressed frustration at the difficulty of showing where all the hazards are when some buried thrust faults have not been located.

There have been two important thrust earthquakes in the Los Angeles Basin in the last decade, Whittier Narrows and Northridge. Both occurred on faults that did not intersect the surface. In the Northridge case, the precise alignment of the fault remains unknown.

At the same time, scientists believe there are other thrust faults under the basin where no earthquakes have been recorded since the area was settled.