High Sierra Topix

Scientists Seek to Predict Future Rockfalls

UC Berkeley scientists are using seismographic records from an ongoing rockfall in Yosemite National Park to research a model for predicting large and often dangerous slides.

Since the fall began in March, about 43,000 cubic meters of rock have broken from a cliff at Yosemite's Ahwiyah Point, toppling nearly 1,000 trees and covering part of a popular hiking trail.

At its most violent point, the fall produced a magnitude 2.4 tremor, recorded on a seismograph that scientists placed about four kilometers away in Yosemite Village, said Valerie Zimmer, a civil and environmental engineering department graduate student who is currently studying the event.

In this specific rockfall-which resulted from rock on the cliff eroding over time-a large chunk of the cliff broke off and cracked into smaller pieces, she said.

Because the seismograph was located relatively close to the fall, scientists were able to collect accurate recordings of shaking before and after the tremor, according to Zimmer.

The records of shaking before the tremor, known as precursor events, may help the scientific community better understand how to predict rock falls, she said.

"We're searching for these precursors, so we can we detect a major rockfall before it happens and basically evacuate the area," Zimmer said. "We can do this with seismic instruments that are close to the source, and look for precursors."

She said that she had instruments running continuously and gathering data in Yosemite for the past two winters, so she was able to capture this rockfall from the beginning to the end.

Just as the scientists are searching for precursor events in the seismographs that may indicate a rockfall's future occurrence, people in the area have reported hearing loud, cracking noises and seeing smaller rocks fall before a fall of this magnitude, indicating the need for a warning system, Zimmer said.

She said that she hopes to have more instruments to collect data in the future in order to eventually develop a successful alarm system for rockfalls.

Wayne Pennington, professor and chair of the department of geological and mining engineering and sciences at Michigan Technological University, said the study will help scientists better understand how rocks fall and improve predictions to increase safety near slide-prone areas.

"If they saw features on the seismogram a day or an hour in advance that a rockfall was likely to happen, then that is extremely important information," Pennington said. "These (rockfall) events can happen at quite different time scales, and scientists and engineers are working hard to understand the mechanisms better."

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