Blind Faults

It's not my fault: New study shakes up interest in earthquakes
Purdue News, by Xan Davidson, October 30, 1998

A new look at the information on a 1971 California earthquake shows that several nearby faults were activated during the temblor, supporting evidence from more recent earthquakes that nearby faults may contribute to earthquake damage. Xan Davidson, a graduate student working with Purdue University earthquake expert Arvid Johnson, found that surface fractures and other strains actually were caused by two faults other than the main fault, and contributed to the damage to streets, highways and buildings in the 1971 San Fernando earthquake. Her study also found that because of blind faults, or faults that cannot be seen at the surface, the San Fernando fault zone is 11 miles long instead of 8 miles long, as it previously was assessed.

Results of the study, presented Oct. 29 at the annual meeting of the Geological Society of America in Toronto, suggest that estimates of potential earthquake damage should include areas surrounding nearby faults, as well as damage caused by the main rupture and ground shaking. "Prior to 1989, scientists thought that all of the damage from an earthquake was the result of a single fault," Davidson says. "Since 1989, we have found evidence that more than one fault can be activated during an earthquake." This is the first study that ties such an event to an earthquake prior to 1981, Davidson says. "If, in fact, further research supports the notion of coactive faults during earthquakes, then predictions of earthquake damage must change focus from the main fault and the ground shaking caused by it, to include faults that might slip and move coactively with the main fault," she says.

Davidson says damage produced by blind faults and coactive faults was not recognized until studies of the 1989 Loma Prieta, 1992 Landers and 1994 Northridge earthquakes. Those studies were carried out by Johnson and Bob Fleming of the U.S. Geological Survey. To see if nearby faults were activated during the 1971 San Fernando earthquake, Davidson used survey data, from 1972 and earlier, to determine damage to streets, highways, office buildings and dwellings in areas the size of city blocks. Her study showed that patterns of energy release -- illustrated by ground ruptures and other damage -- indicate that two nearby faults were activated during the earthquake. "Damage in these areas could not be attributed to the ground shaking caused by the main fault," she says. Davidson and Johnson now are working with others to study how coactive faults are activated during an earthquake, and how fault lines may be interconnected.