Sometimes it takes an earthquake to know where the fault lies

October 25, 1999

The recent 7.1 earthquake at the newly named Lavic Lake fault in Southern California is a good reminder that even geologists aren't always sure which faults are active until there is an earthquake. Lucy Jones, a scientist with the United States Geological Survey Western Earthquake Hazards Team, says many faults in Southern California are labeled "inactive", sometimes simply because there is not enough evidence to determine otherwise.

The problem facing scientists in studying faults like Lavic Lake, is that not all faults are equal when it comes to looking for evidence of activity. Some faults are very defined (you can see the San Andreas from the air), while signs of activity from others are subtle, and can be nearly invisible.

To study the recent activity of a fault, geologist look at something called "offset features". These are rocks (or other geologic material) which can be dated and are separated by some distance, indicating that the fault has moved at some time in the past. Jones says the Lavic Lake fault was labeled inactive because "the one geologist who mapped it did not find an obvious offset feature."

A fault's recent activity is important, says Jones, because it helps researchers identify which faults are likely to move again in the relatively near future. "We work from a paradigm that this decade's earthquakes will be near those in the historic record, or those that show very recent motion." However, Jones points out that the example of Lavic Lake shows that just because a fault line doesn't show recent activity, doesn't mean it's sleeping for eternity.

Ruth Ludwin
Research Scientist
Pacific Northwest Seismograph Network
University of Washington
(206) 543-4292

Linda Curtis
Western Earthquake Hazards Team
U.S. Geological Survey
(626) 583-7817

American Institute of Physics

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