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– The first slide at 9:30 p.m. MDT measured 5.1 in surface-wave magnitude, 2.5 in local or Richter magnitude, and 4.2 in duration or “coda” magnitude.
– The second slide at 11:05 p.m. MDT measured 4.9 in surface-wave magnitude, 2.4 in Richter magnitude and 3.5 in coda magnitude.
Pankow said the larger magnitudes more accurately reflect the energy released by the rock avalanches, but the smaller Richter magnitudes better reflect what people felt—or didn’t feel, since the seismograph stations didn’t receive any such reports. That’s because the larger surface-wave magnitudes record low-frequency energy, while Richter and coda magnitudes are based on high-frequency seismic waves that people usually feel during real quakes.
So in terms of ground movements people might feel, the rock avalanches “felt like 2.5,” Pankow said. “If this was a normal tectonic earthquake of magnitude 5, all three magnitude scales would give us similar answers.”
The slides were detected throughout the Utah seismic network, including its most distant station some 250 miles south on the Utah-Arizona border, Pankow said.
The Landslide Triggered 16 Tremors
The second rock avalanche was followed immediately by a real earthquake measuring 2.5 in Richter magnitude and 3.0 in coda magnitude, then three smaller quakes, all less than one-half mile below the bottom of the mine pit.
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.
Later analysis revealed another 12 tiny quakes, measuring from 0.5 to minus 0.8 Richter magnitude. (A minus 1 magnitude has one-tenth the power of a hand grenade.) Six of these tiny tremors occurred between the two parts of the landslide, five happened during the two days after the slide, and one was detected 10 days later, on April 20. No quakes were detected during the 10 days before the double landslide.
“We don’t know of any case until now where landslides have been shown to trigger earthquakes,” Moore said. “It’s quite commonly the reverse.”
A Long, Fast Landslide Runout
The landslide, from top to bottom, fell 2,790 vertical feet, but its runout—the distance the slide traveled—was almost 10,072 feet, or just less than two miles.
“It was a bedrock landslide that had a characteristically fast and long runout, much longer than we would see for smaller rockfalls and rockslides,” Moore said.
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, “we can safely say the material was probably traveling at least 100 mph as it fell down the steepest part of the slope,” Moore said.
The researchers don’t know why the slide happened as two rock avalanches instead of one, but Moore said, “A huge volume like this can fail in one episode or in 10 episodes over hours.”
The seismograph stations also recorded infrasound waves from the landslide, which Pankow said are “sound waves traveling through the atmosphere that we don’t hear” because their frequencies are so low.
Both seismic and infrasound recordings detected differences between the landslide’s two rock avalanches. For example, the first avalanche had stronger peak energy at the end that was lacking in the second slide, Pankow said.
“We’d like to be able to use data like this to understand the physics of these large landslides,” Moore said.
The seismic and infrasound recordings suggest the two rock avalanches were similar in volume, but photos indicate the first slide contained more bedrock, while the second slide contained a higher proportion of mined waste rock, although both avalanches were predominantly bedrock.