Scientists Gauge Earthquake Hazards Through Study Of Precariously Balance Rocks

Their findings are shared in the January-February issue of American Scientist magazine. Scientists believe that zones of precarious rocks -- rocks that have come close but haven't tipped over in the wake of a major seismic event -- provide important information about seismic risk, its magnitude and its frequency.

"There's really no long-term data to test seismic hazards other than precarious rocks," said Matthew Purvance, a postdoctoral scholar in geophysics at the University, who authored the article along with James Brune, professor in the Department of Geological Sciences and past director of the Nevada Seismological Laboratory, and Rasool Anooshehpoor, research professor in the Nevada Seismological Laboratory.

"By studying precariously balanced rocks, it can serve as an indicator that an earthquake of a sufficient size to topple a tippy rock has not occurred ... at least for a very long time. We think this is a fundamental story that gives fundamental information on seismic hazards that has never been done before."

The data from the study is important, as it not only tests ground-motion probability, but can help further refine United States Geological Survey hazard methodologies that are used by engineers to formulate building codes. Purvance explained that seismologists and engineers since the late 1960s have increasingly followed a method known as probabilistic seismic-hazard analysis in trying to get a more firm grasp on earthquake probability. This analysis allows researchers to determine the number and magnitude of earthquakes on relevant faults. The study of precarious rocks, which act as "witnesses" to strong seismic events throughout history, has provided scientists an important research window to test the predictions of probability, Purvance said.

The team tested massive rocks of up to 1,000 pounds and more than 10,000 years old, measuring the force and angle it would take to tip them over. One of the more interesting aspects of the study was a technique used by Anooshehpoor, which measured the restoring force that has allowed the rock to remain upright through centuries of wear and the force of past strong seismic events.

Anooshehpoor's technique allowed the team to measure a tipping boulder's restoring force with a digital load cell and the rock's tilt with an inclineometer. The work wasn't easy. By pushing and pulling on the massive, bus-sized rocks with a series of wire cables, nylon straps, chains, pulleys, winches, hydraulic pistons, ground anchors and 4 by 4 blocks of wood, the team was able to record data for precarious rocks that had never been tested before.

"It gives us very useful information about the precarious rocks and further adds to the knowledge of gauging earthquake hazards," Purvance said, noting that it was work by Brune in the early 1990s with precarious rocks in southern California that led to the rocks becoming more widely recognized as an accurate barometer of seismic force and occurrence. "These measurements help better explain the story of how the rock has managed to withstand some of the forces of time and nature."

Added Anooshehpoor: "The rocks that we have studied are from large earthquakes and are so rare. If throughout history the world had tons of instruments and recorded many of these earthquakes, we probably wouldn't have the need to study precarious rocks. The lack of data has been a major problem in estimating ground motion. With this study, we've been provided with another opportunity to give the engineers the right information they need."