UA prof is walking seismograph

Photo Courtesy of Mary Bull UA geologist William Bull measures lichen on a rock to figure out dates from past earthquakes. Bull has found that the steady growth rate of lichens can help determine the intervals between earthquakes.

A University of Arizona geologist has found a way to help scientists better predict when the next big earthquake will hit.

William Bull has been studying lichens in California and New Zealand for nine years to figure out the dates of past earthquakes. He and Yale University geologist Mark Brandon determine the intervals between earthquakes by examining the size of lichens in the quake zone.

"It's an important tool, and an important step," said Paul Sheppard, a UA geosciences professor.

The rocks exposed after an earthquake have fresh surfaces which attract lichens, small plants similar to mosses, that live for about 1,000 years. By measuring the size of the lichens in an area, Bull can determine their age and thus estimate the date of the earthquake.

Lichens are ideal not only because they grow on rocks but because their rate of growth is so steady, Bull said.

"We've found that the rate of growth was consistent regardless of factors like rainfall, wind and elevation," he said.

The technique, dubbed "lichenometry" by Bull and Brandon, has scientists thinking that the San Andreas fault could be due for a major earthquake much sooner than previously thought.

Bull, however, said he was reluctant to set an estimated date for another California quake because the fault is "so erratic.

"It's all over the place," he said.

Bull identified an earthquake that shook California in 1690 using lichenometry. The quake was missed by the prevailing method of radiocarbon dating.

Bull has had less trouble finding an interval for the Alpine fault in New Zealand.

His work in New Zealand, combined with radiocarbon dating and tree-ring dating, has set an interval of 260 years between major earthquakes. The last major quake in New Zealand came 248 years ago, so scientists are now predicting another event there in roughly the next decade.

The UA professor added that pinpointing the actual date of a future earthquake is basically impossible.

"Are they going to predict whether or not it's going to rain in Tucson in three weeks? Of course not," Bull said. "Predicting the actual dates of earthquakes is just as uncertain."