NEW EARTHQUAKE-DATING TECHNIQUE SHARPENS PICTURE OF WHEN NEXT MAJOR EARTHQUAKES IN CALIFORNIA, NEW ZEALAND COULD OCCUR
Contact: Lori Stiles, UA News Services
Phone: 520-621-1877
Email: [email protected]
-- Sent Feb. 6, 1998 --
(Contact: William B. Bull, 520-297-2175, [email protected])
A University of Arizona geologist and a Yale University colleague have found an unlikely new source of information about prehistoric earthquakes -- rock-loving lichens. Their lichen studies suggest major earthquakes along the San Andreas fault in California and the Alpine fault in New Zealand ruptured more often than previously known, which implies that future big earthquakes may come sooner than expected.
At first, fellow scientists said the lichen earthquake-dating technique "looked too good to be true," said William B. Bull, UA professor emeritus of geosciences. He began developing "lichenometry" as a method of dating large earthquakes nine years ago. "But it turns out, this method is better than even we thought it was," he added.
Bull and Mark Brandon of Yale University published a major essay on the methodology of using lichens as an important new tool for analyzing earthquake hazards in the January Bulletin of the Geological Society of America. They use lichens growing on rock fall deposits to determine the prehistoric record of major earthquakes and the extent of seismic shaking caused by the events. Their research is supported by the National Science Foundation and the National Geographic Society.
This approach already has had important implications for understanding earthquake recurrence on the San Andreas fault of California and the Alpine fault of New Zealand, which is another active San Andreas-like fault. Bull and New Zealand colleagues discovered from 1996 field research that the Alpine fault is likely to rupture in a major earthquake in the next decade or two. The Alpine fault has not ruptured since Europeans colonized the South Island in 1840.
At present, the most widely used method for evaluating earthquake potential on highly active plate boundary fault zones is to trench fault scarps and to date disturbance events found in sedimentary deposits within and adjacent to the fault zone. Researchers then use the radiocarbon method to date wood or other organic material deposited in layers below or above a disturbed zone created by abrupt slip along the fault.
Bull began developing "lichenometry" as an earthquake-dating tool in 1989, when, he said, he realized that the largest earthquakes generate synchronous pulses of rockfalls throughout a region. Earthquakes larger than magnitude 7 commonly generate numerous rock avalanches and landslides over the entire region affected by strong seismic shaking. Soon after the event, lichens colonize the fresh surfaces of blocks in the rock fall deposits.
Lichens are a distinctive small flat-lying plant, commonly green, yellow or black in color, that grow on open rocky surfaces. It has been known for some time that a specific variety of lichens will grow at a fairly well determine rate. Sierra Nevada lichens grow about 9.5 millimeters (or about three-eighths of an inch) every century. Lichens growing on substrates exposed 1,000 years ago would measure about 95 millimeters, or 3.75 inches across, for example. Lichens grow faster in the wetter Southern Alps of New Zealand, about 15 millimeters every century, according to Brandon.
The Bull-Brandon method uses the lichens to determine the ages and distribution of rockfall events over a large seismically active region. They show that regional synchronous rockfall events in New Zealand correlate closely in timing and size with the age, location, and magnitude of ten large historic earthquakes. By measuring the largest lichen on thousands of blocks or rock, they are able to date the times of many earthquake-generated rockfall events back to 1200 AD and older.
Locating and dating prehistoric earthquakes with this technique requires careful site selection and accurate calibration of lichen growth rates, Bull said. 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.
Using lichens to date earthquake-generated landslides has several advantages to the traditional approach, the geologists said. Many radiocarbon dates of the past 300 years are of little value. Research by 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.
Also, most radiocarbon dating of earthquakes requires accumulation of materials in depressions created next to fault scarps, but earthquake-generated landslides can occur hundreds of kilometers away from an earthquake epicenter. By measuring lichens in mountains that are sensitive to seismic shaking, geologists can study distant earthquakes. These include great subduction zone earthquakes beneath the sea, such as in the Pacific Northwest, and earthquakes caused by 'blind' thrust faults that never reach the surface to create a fault scarp." The Northridge, California, earthquake of January 1994 is an example of an earthquake on a blind thrust fault.
Bull and Brandon actually map the nature of seismic shaking associated with either historic or prehistoric earthquakes according to the decreasing abundance of rockfalls away from an earthquake epicenter. Brandon notes that lichenometry ages are precise to about 10 to 20 years for earthquakes that have occurred within the past thousand years, whereas radiocarbon dates are precise to about 40 years.
Application of the Bull-Brandon method has already changed the outlook for earthquakes on two major fault zones. Bull's work on regional rockfall events generated by the San Andreas fault northeast of Los Angeles identified a 1690 earthquake. Geologists using conventional radiocarbon dating technique missed the 1690 earthquake at the classic Pallett Creek site because no deposits of wood or other organic matter had accumulated in layers of earth above or below the disturbed horizon.
The lichen study bolsters other seismologists who argue that earthquakes along the San Andreas fault have occurred more regularly than previously thought and that the next "big one" may come sooner than supposed.
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 lichen dates agree with radiocarbon dates and other scientific evidence such as forest disturbance events. Bull and New Zealand colleagues say 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.
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