"LARSE II" Earthquake Hazard Study, Oct 20-24, 1999

A joint project of the United States Geological Survey (USGS) and the
Southern California Earthquake Center (SCEC)

After each large earthquake, residents of southern California ask whether a strong earthquake can occur near their home and how the ground will shake due to any large earthquake in the region. Scientists are working to answer these questions so that damage or injury due to earthquakes can be reduced. A crucial step is getting an accurate picture of the network of active faults and other structures that underlie the Los Angeles region. The Los Angeles Region Seismic Experiment II (LARSE II) was designed in order to create this picture.

LARSE II was scheduled for over two years, and was not a response to the Hector Mines Earthquake (October 16, 1999). The project was located 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 1994 in response to the Northridge earthquake, and will provide information about many underground structures. These structures include "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 that will be imaged are sedimentary basins-- large valleys filled with sand, clay and other erosional deposits-- 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, since deeper basins result in stronger shaking at the surface.

LARSE II used sound waves traveling beneath the Earth's surface to produce images of these structures, similar to the method used to create an ultrasound image. Sound waves were generated by 93 charges detonated over 60 feet beneath the surface in specially-drilled boreholes, located approximately 1 kilometer (0.6 miles) apart. The sound waves were received by over 1400 portable seismographs spaced every 100 meters (approximately 100 yards) along the same line. LARSE also made use of seismic waves from natural earthquakes.

Because the amount of blasting agent for each borehole was small and buried deep below the surface, the detonations did not cause property damage. Great efforts were made to ensure that the detonations did not disturb people, however people in the immediate vicinity may have felt a small bump. The instruments used were very sensitive and detected the unnoticeable vibrations further away. There was no chance that the detonations will trigger an earthquake, since earthquakes originate several kilometers beneath the surface. Other daily activities throughout southern California such as construction, mining, or even jets landing at airports vibrate the ground much more than these detonations and never have caused earthquakes. Furthermore, this procedure has been conducted hundreds of times, around the world for the last 30 years, and has never caused earthquakes. Because of the Hector Mines earthquake, more aftershocks than usual occured in southern California, none of which were the result of LARSE II. In fact, LARSE II seismographs recorded many of these aftershocks, adding more detail to the final "picture" of underground structures.

Frequently Asked Questions

LARSE II Fact Sheet in pdf format (1 MB)

LARSE II fact sheet, online

LARSE Links

Exploratorium Faultline Project: http://www.exploratorium.org/faultline/index.html

LARSE I home page - http://www.data.scec.org/larse/larse.html

LARSE I News Release, January 1, 1997 - http://quake.wr.usgs.gov/study/asig/EHZ/larsepr_1_16_97.html