PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Jim Doyle
FOR IMMEDIATE RELEASE February 4, 1994
Oat Mountain in the Santa Susana range just north of California's San Fernando Valley jumped several centimeters during the 6.6-magnitude earthquake that struck Los Angeles on Jan. 17, said a Jet Propulsion Laboratory scientist.
Other locations, including some communities, also were jerked into new positions, said Dr. Andrea Donnellan, a JPL geophysicist.
The 1,103-meter-high (3,618-foot) mountain jumped up 38 centimeters (14.8 inches). It also moved north 16 centimeters (6.2 inches) and west 14 centimeters (5.5 inches).
JPL has continuously operated stations at Oat Mountain and at California State University, Northridge, since the earthquake. The data suggest that Oat Mountain has risen about 2 to 3 centimeters (about 1 inch) since the quake. The site also moved 3 centimeters (1 inch) back to the south after a 5.0-magnitude aftershock Jan. 29.
"This is mountain building in progress," said Donnellan.
Donnellan and her colleagues measured the movements of the mountain and several other Southern California locations following the devastating earthquake using Global Positioning System (GPS) instruments -- ground receivers that track orbiting navigation satellites.
The Defense Department's GPS network includes 24 satellites orbiting 20,000 kilometers (12,400 miles) above Earth that send microwave transmissions to ground receivers worldwide.
NASA collects data from a global network of 45 stations. The data tell scientists how far Earth's surface has moved in any given period of time. In addition, portable instruments are deployed at other locations around the world.
Donnellan said the 6.6 quake occurred on a fault at the southern and eastern edge of the Ventura Basin, a 100- by 10kilometer (62- by 6-mile) sub-surface feature that stretches from the Pacific Ocean to the San Fernando Valley. At 15 kilometers (9.3 miles) deep, the basin is one of the deepest sedimentary basins on Earth, she said.
Donnellan had been studying the basin since 1987 and came to the conclusion its deep faults were capable of causing a serious earthquake. In a paper she published in the science journal Nature last November, she predicted the basin could suffer an approximately 6.4-magnitude earthquake.
Her studies, using the GPS instruments, indicated the basin was being squeezed from north and south about 7 millimeters (0.3 inches) a year by the movement of the Santa Susana and Santa Ynez ranges.
"It's a north-south closure of the valley," she said. The figures came from analysis of data recorded in the GPS receivers at several locations around the basin.
She said she and her colleagues used computer modeling to look at the faults beneath the basin from a considerable depth up to the surface and saw they were locked -- that is, not slipping to relieve strain. From that model they calculated the potential magnitude of a quake that could strike the region. Although the scientists predicted the locale and size of the earthquake, they could not predict when such a quake might occur.
The Oat Mountain receiver was the location closest to the quake's epicenter and the highest density area of aftershocks. The epicenter was in the valley community of Reseda. The fault, however, is not a single point, but affects a large section of surface ground. The hardest-hit area was the community of Northridge, immediately adjacent to Reseda. Most of Northridge overlies the ruptured fault plane.
Several other communities along the basin also were jerked into new positions. GPS data indicated that the community of Fillmore in Ventura County, which lost much of its downtown section, moved west 5 centimeters (2 inches). The region near Castaic moved southwest 11 centimeters (4.3 inches) and down 9 centimeters (3.5 inches). Santa Paula and Moorpark also were moved westward about 2.5 to 5 centimeters (1 to 2 inches) and the Point Dume area moved due north 4 centimeters (1.5 inches).
In addition to analyzing scientific data from the earthquake, NASA's Airborne Science and Applications program has been conducting surveys of the damage in the area. Data from instruments aboard NASA's C-130 and ER-2 aircraft have been provided to the Federal Emergency Management Administration and local governments to help them assess the damage.
Both JPL's GPS studies and the aircraft surveys, managed by NASA's Ames Research Center, Mountain View, Calif., are funded by NASA's Office of Mission to Planet Earth, Washington, D.C. Mission to Planet Earth is studying how Earth's global environment is changing. Using the unique perspective available from space, NASA is observing, monitoring and assessing large-scale environmental processes, focusing on climate change.