Earthquakes along the San Andreas fault northeast of Los Angeles have occurred more regularly than previously thought, and the next "big one" may come sooner than supposed, say two geologists who are using their knowledge of lichens to pinpoint the dates of previously unknown ancient earthquakes.
Using a new method called "lichenometry," William B. Bull of the University of Arizona and Mark T. Brandon of Yale University examined numerous rock avalanches and landslides caused by major historic and prehistoric earthquakes on New Zealand's South Island occurring since A.D. 1200. The new study tests the precision of their method for dating ancient earthquakes and supports previous lichenometry work by Bull that identified a major unknown quake near Los Angeles that occurred in 1690.
"Earthquakes larger than magnitude 7 commonly generate numerous rock avalanches over an entire region as far as 400 kilometers from the epicenter. Soon after a quake, lichens began to colonize the fresh rock surfaces. Because of their predictable growth rate, lichens make it possible for us to pinpoint within 10 to 20 years when an earthquake in the last thousand years occurred," said Brandon, associate professor of geology at Yale.
Lichenometry could provide a valuable new tool for revealing the seismic history of earthquake-prone areas and predicting the chances of future earthquakes, Brandon said. The New Zealand studies, which were reported in January in the Bulletin of the Geological Society of America, were funded by the National Science Foundation and the National Geographic Society.
Prehistoric earthquakes on the Alpine fault in New Zealand appear to have occurred about every 260 years, with the most recent major earthquake about 248 years ago, the geologists noted. The lichen dates agree with radiocarbon dates and other scientific evidence of earthquakes, such as forest disturbances. "This regular occurrence of earthquakes cannot be used as a forecast, but it does suggest a high probability for a major earthquake in the South Island of New Zealand in the next two decades," Bull said.
The most widely used method for studying highly active faults, such as the San Andreas and New Zealand Alpine faults, is to take samples from sedimentary deposits within and adjacent to visible fault lines. Radiocarbon dating -- a method that measures the decay rate of naturally occurring radioactive carbon 14 -- is used to determine the age of wood or other organic material deposited in layers surrounding an abrupt slip along a fault.
Lichenometry is more accurate than radiocarbon-dating, which has an error margin of plus or minus 40 years, Brandon said, and has the added advantage of being able to detect quakes hundreds of kilometers away in subduction zones beneath the sea floor. The method also can detect "blind thrust faults" that never reach the surface to create visible faults, such as the devastating Northridge, Calif., earthquake on Jan. 17, 1994. Such quakes leave few clues for scientists using standard methods, Brandon said.
Citing further reasons for using lichenometry, Bull said research at the University of Arizona Laboratory of Tree Ring Research has shown that the amount of radiocarbon in the atmosphere, which is produced by cosmic rays in the upper atmosphere, has varied greatly during the past three centuries. Therefore, many radiocarbon readings during that period are of little value.
Well-determined Growth Rates
Lichens are small, flat-lying plants, commonly green, yellow or black in color, that grow on open rocky surfaces. Scientists have known for some time that specific varieties of lichens grow at fairly well-determined rates. For example, lichens in California's Sierra Nevada Mountains near Los Angeles grow about 9.5 millimeters (or about three-eighths of an inch) every century. Lichens growing on rock surfaces exposed 1,000 years ago would measure about 95 millimeters, or 3.75 inches across. Lichens grow faster in the wetter Southern Alps of New Zealand -- about 15 millimeters every century, Brandon said.
Bull began developing "lichenometry" as a method of dating large earthquakes in 1989 when he realized that the largest earthquakes generate "synchronous pulses" of rockfalls throughout a region. The Bull-Brandon method uses lichens to determine the ages and distribution of rockfall events over a large, seismically active region in mountains that are sensitive to seismic shaking.
The geologists found that regional, synchronous rockfalls in New Zealand correlate closely in timing and size with the age, location and magnitude of 10 large historic earthquakes. By measuring the largest lichen on thousands of rocks, they were able to date the times of many earthquake-generated rockfalls back to A.D. 1200 and earlier. They mapped the amount of seismic shaking associated with either historic or prehistoric earthquakes according to the decreasing abundance of rockfalls at greater distances from an earthquake epicenter.
"Locating and dating prehistoric earthquakes with this technique requires careful site selection and accurate calibration of lichen growth rates," said Brandon, who added that the method is best used for earthquakes occurring in the last 500 years. "Sites must be selected to minimize the influence of snow avalanches and debris flows. Conditions like shade and wind that promote faster lichen growth must be factored into the growth rate."
Application of the Bull-Brandon method has already changed the outlook for earthquakes along the San Andreas fault. In 1996, Bull first reported lichen evidence of a previously unknown major earthquake in 1690 near Los Angeles -- evidence the New Zealand studies corroborate. Geologists using conventional radiocarbon dating at the site on Pallett Creek in the Sierra Nevada Mountains missed the quake because no deposits of wood or other organic matter had accumulated in earth layers above or below the disturbed zone.
The discovery of the 1690 quake bolsters other seismologists who argue that major earthquakes along the San Andreas fault have occurred more regularly than previously thought. A study of the seismic history of the fault using lichenometry could help seismologists more accurately estimate the probabilities of future quakes, Brandon said.
Note to Editors: Mark T. Brandon, associate professor and director of undergraduate studies, Yale department of geology and geophysics, can be reached for interviews at (203) 432-3135. E-mail: email@example.com or view his Web site at: http://www.yale.edu/geology. William B. Bull, geosciences department, University of Arizona, is available for interviews at (520) 297-2175, e-mail firstname.lastname@example.org.