In a search for possibly destructive hidden thrust faults beneath Southern California, earthquake scientists plan to set off 60 small explosions in October along a 100-mile line extending from Seal Beach through the San Gabriel Valley to the Mojave Desert.
The blasts, which each will use 250 to 4,000 pounds of ammonium nitrate, are to be detonated underground mostly on public property belonging to the military, the federal Bureau of Land Management, the U.S. Forest Service and Los Angeles County.
Authorities said the explosions will be felt only by people in the immediate vicinity and then very weakly.
The blasts will send out energy waves that, when analyzed by more than 600 specially implanted seismographs, should tell scientists a great deal about the geological structure underlying parts of the Los Angeles Basin.
In addition to discovering the existence of unknown faults, scientists hope to develop estimates of potential shaking intensities, ultimately leading to new construction codes for areas likely to be subject to the strongest shaking in future quakes.
Similar experiments have been conducted in the San Francisco Bay Area, Alaska and elsewhere in recent years without triggering earthquakes or causing damage, according to project coordinator Gary Fuis of the U.S. Geological Survey, and Jim Mori, scientist in charge of the agency's Pasadena office.
"These can best be compared to blasting done for construction projects or in mining," Fuis said. Mori said they would be equivalent to no more than magnitude 2.5 earthquakes, which are recorded at the rate of 40 to 50 a day in Southern California.
But Mori also said that blasts near populated areas would be set off at somewhat greater depths, perhaps 180 feet rather than 50 feet, and involve less explosives than those conducted under sparsely settled mountains and deserts.
Ammonium nitrate, Fuis said, cannot be set off "by anybody who isn't an expert."
The Southern California Earthquake Center at USC, also involving professors from Caltech, UCLA, UC San Diego, UC Santa Cruz, UC Santa Barbara and Columbia University, will help staff the $600,000 government-funded project. Fuis said the first results will be released within a year of the explosions.
Several damaging earthquakes that have occurred in California in the last decade, including temblors in Coalinga, Whittier Narrows and Sierra Madre, happened on so-called "blind thrust" faults that were not previously known. In quakes on thrust faults, one side rises. The jolts were centered deep beneath the surface and created no surface ruptures.
Energy waves generated by the explosions will move through different kinds of rocks at different speeds. The faults they pass through will be reflected on the instruments, and the researchers expect to develop a much more comprehensive view of what lies under the Los Angeles Basin, the San Gabriel Mountains and the Mojave Desert.
Fuis said several factors contributed to the decision to use the Seal Beach-to-Mojave Desert route for the project.
"It crosses the Los Angeles Basin in an area somewhat less populated than other areas," he said. "It crosses the San Gabriel Mountains along an accessible route, Highway 39. It passes perpendicular to the epicentral regions of the 1933 Long Beach, the 1987 Whittier Narrows and the 1991 Sierra Madre earthquakes, which had magnitudes in the 5 and 6 range.
"And it's near a route along which oil company reflection data has been collected that we can also use to improve our image of the uppermost part of the Earth's crust in this area."
Another Geological Survey scientist, Lucile M. Jones, recently told a meeting of the state's Earthquake Prediction Evaluation Council that the Sierra Madre fault zone, along the southern base of the San Gabriel Mountains, appears to constitute the greatest destructive quake danger to the Los Angeles Basin.
Fuis said there are secondary goals of the study:
* By studying the geological structure beneath the Los Angeles Basin, there is hope that the mechanisms that produce earthquakes can be better understood. Scientists are debating whether quakes are triggered by movements of liquids deep beneath the surface or by accumulated stress in rocks.
* By examining the way energy waves strike rocks as deep as 15 miles underground, scientists may be able to determine areas where energy from earthquakes is most apt to reflect back to the surface far from the epicenter, as occurred in parts of San Francisco in the 1989 Loma Prieta quake.
* By determining how fast waves travel through various sediments, scientists may be able to determine more precisely where quakes occur. At present, particularly with deeply buried thrust faults, the epicenter is subject to different interpretations.
The overriding purpose of such projects is to mitigate earthquake dangers, he added.
"Ultimately, we hope to be able to communicate more accurate earthquake hazards information to the public," Fuis said. "If we can get the proper ground-shaking information into the hands of engineers, city and county relief organizations and ordinary citizens, we may go a long way to replacing worry with preparedness."
The Search for Buried Faults
In a search for hidden thrust faults, scientists from the Southern California Earthquake Center at USC and the U.S. Geological Survey this fall will set off 60 small blasts at least 50 feet underground along a line extending about 100 miles northeast from Seal Beach through the San Gabriel Mountains to the Mojave Desert.
Source: U.S. Geological Survey