SALT LAKE CITY — The gargantuan awe-inspiring landslide at Kennecott's Bingham Canyon mine last April was so stunning, the "firsts" and "mosts" it accomplished are something wild to ponder.
The April 10 landslide was likely the largest nonvolcanic slide in North America in the modern age and the first known to trigger earthquakes — 16 small ones in total. It is also among the best recorded in history and may be the most expensive, possibly eclipsing the costs of the 1983 Thistle slide in Utah.
Details about the slide and the knowledge gained from subsequent research were published in the January 2014 cover study in the Geological Society of America magazine GSA Today.
“This is really a geotechnical monitoring success story,” says the new study’s first author, Kris Pankow, associate director of the University of Utah Seismograph Stations and a research associate professor of geology and geophysics. “No one was killed, and yet now we have this rich dataset to learn more about landslides.”
University of Utah researchers say the landslide was two rock avalanches that happened 90 minutes apart. The slide moved at an average of nearly 70 mph and reached estimated speeds of at least 100 mph.
Researchers noted that it left a deposit so large it “would cover New York’s Central Park with about 66 feet of debris. By local comparison, it would have deposited 545 feet of earthen material over Salt Lake's Liberty Park.
The slide occurred in the form of two huge rock avalanches at 9:30 p.m. and 11:05 p.m. at Rio Tinto-Kennecott Utah Copper’s open-pit Bingham Canyon Mine, with each rock avalanche lasting about 90 seconds.
Although the slides were technically not quakes, they were picked up by seismic scales as having magnitudes up to 5.1 and 4.9, respectively. Researchers say the subsequent real quakes were smaller.
Pankow said the quakes were like aftershocks.
"They tell us how the crust was adjusting from the movement of that much material," she said. "From a science perspective they are very interesting."
There were no injuries or fatalities due to the slide, but it damaged or destroyed 14 haul trucks, three shovel trucks and closed the mine's main access ramp until November. Mine production was slashed 50 percent as a result of the slide, and about 100 workers were laid off.
Researchers said there have been much bigger human-caused landslides on other continents and much bigger prehistoric slides in North America, including one about five times larger than Bingham Canyon about 8,000 years ago at the mouth of Utah’s Zion Canyon.
Kennecott has estimated the landslide weighed 165 million tons. The new study estimated the slide came from a volume of rock roughly 55 million cubic meters (1.9 billion cubic feet). Rock in a landslide breaks up and expands, so researchers estimated the landslide deposit had a volume of 65 million cubic meters (2.3 billion cubic feet).
Co-author Jeff Moore, an assistant professor of geology and geophysics at the University of Utah, said the material not only would have buried Central Park 60 times over, but was equivalent to the amount of material in 21 of Egypt's great pyramids of Giza.
The Utah researchers sped up recorded seismic data by 30 times to create an audio file in which the second part of the slide is heard as a deep rumbling, followed by sharp gunshot-like bangs from three of the subsequent quakes. While no one was present to measure the speed, rock avalanches typically move about 70 mph to 110 mph, while the fastest moved a quickly as 220 mph.
So at Bingham Canyon, Moore said the material was moving about 100 mph down the steepest part of the slope.
Pankow and Moore conducted the study with several colleagues from the university’s College of Mines and Earth Sciences: J. Mark Hale, an information specialist at the Seismograph Stations; Keith Koper, director of the Seismograph Stations; Tex Kubacki, a graduate student in mining engineering; Katherine Whidden, a research seismologist; and Michael K. McCarter, professor of mining engineering.
The study was funded by state of Utah support of the University of Utah Seismograph Stations and by the U.S. Geological Survey.