The Hector Mine earthquake that occurred Oct. 16, 1999, in the Mojave Desert east of Los Angeles, woke a lot of people up but injured no one and caused a minimal amount of property damage. It was, however, the perfect earthquake at the perfect time for scientists from the U.S. Geological Survey (USGS) and the Southern California Earthquake Center (SCEC), who were gearing up for "LARSE II," the Los Angeles Region Seismic Experiment II, which got underway on October 20.

Dr. Gary Fuis, one of the lead scientists on that project will describe the components of that experiment and its importance to the Los Angeles community, in a poster session at the annual meeting of the Seismological Society of America, at the Hanalei Hotel, in San Diego, April 9-12. Fuis’s poster presentation will be from 8:30 a.m. to 12:30 p.m., Monday, April 10.

After each large earthquake, many residents of southern California ask whether a strong earthquake could occur near their home and how much the ground will shake due to any large earthquake in the region. "We can’t answer either of those questions until we have an accurate picture of the network of active faults and other structures that underlie the Los Angeles region," Fuis said, "and the best way to can get that picture is to obtain seismic images of the earth’s crust, similar to 'ultrasound' and 'CAT-scan' images that are made as part of medical tests. To obtain these types of seismic images we need to create small sources of underground sound-wave energy. In LARSE II we used underground explosions."

LARSE II is the second phase of a project that began in 1994, following the Northridge earthquake of January 17, that year. During the experiment, sound waves were generated by 93 explosive charges, detonated over 60 feet beneath the surface in specially-drilled boreholes, located approximately 1 kilometer (0.6 miles) apart. The sound waves produced by the detonations were received by over 1400 portable seismographs spaced every 100 meters (approximately 100 yards) along a line from Pacific Palisades to the San Fernando Valley, then northward across the Mojave Desert to the Tehachapi Mountains.

The line was chosen in the hopes that the resulting data would yield information about many underground structures, including "blind" thrust faults-- buried ramp-like faults that do not extend to the surface, such as the fault on which the Northridge earthquake occurred. Knowing the configuration of buried faults is crucial to understanding how the entire earthquake-producing mechanism works in the Los Angeles region. Other important structures are sedimentary basins-- large valleys filled with sand, clay and gravel eroded from nearby mountains-- such as the San Fernando Valley. "Information on the thickness and shape of sedimentary basins is essential for predicting how the ground will shake in future earthquakes," Fuis said, "since deeper basins result in stronger shaking at the surface."

Preparing for LARSE II involved more than just drilling some holes and setting out portable seismographs to record the vibrations created by the explosions. For two years prior to the experiment, Fuis and other scientists associated with the project attended public hearings and informational meetings to explain the project and to assure residents and community leaders that the detonations would neither trigger an earthquake nor cause enough surface shaking to damage property. Permits were obtained from both government and private entities, for more than 40 shotpoints and the placement of nearly 400 portable seismographs. "Our efforts paid off," said Fuis, "as the data quality is good, and seismic energy from some of those small shots in public areas propagated waves for 50 to 80 kilometers (30-50 miles), to the western Mojave Desert."

The occurrence of the Hector Mines earthquake, said Fuis, was just the "icing on the cake." "Even though the earthquake occurred prior to the beginning of LARSE II, many of the aftershocks from that earthquake were recorded by the portable seismographs, adding more detail to our final 'picture' of underground structures."

"We are still analyzing data from LARSE II," Fuis said, "and looking for discoveries similar to those that were revealed in LARSE I. Preliminary images are expected in the next few months, but final images are more than a year away. The two phases of LARSE will make significant dents in our ignorance of the subsurface of the Los Angeles region, but unfortunately, these two surveys address only parts of the LA region, and decades of work will be required to cover it all."

As the Nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the Nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources.

This press release and in-depth information about USGS programs may be found on the USGS home page: http://www.usgs.gov. To receive the latest USGS news releases automatically by email, send a request to listproc@listserver.usgs.gov. Specify the listserver(s) of interest from the following names: water-pr: geologic-hazards-pr; biological-pr; mapping-pr; products-pr; lecture-pr. In the body of the message write: subscribe (name of listserver) (your name). Example: subscribe water-pr joe smith.