Two massive landslides that occurred at Utah’s Bingham Canyon copper mine last 10 April moved about 65 million cubic meters of earth, enough to bury New York City’s Central Park 20 meters deep. That figure makes them the largest nonvolcanic slides known in North America in centuries—although the landslide that triggered the 1980 eruption of Mount St. Helens in Washington state was about 57 times as voluminous. Coming 95 minutes apart, the two slumps raced down the 970-meter-tall northern wall of the open-pit mine at speeds topping 160 kilometers per hour, researchers report in the January issue of GSA Today, a publication of the Geological Society of America. Nearby seismometers revealed that the events released energy equivalent to earthquakes of magnitudes 5.1 and 4.9 and triggered 16 tiny quakes over 10 days, the smallest of which packed less pop than a hand grenade. The team is now trying to identify the precise mechanism that triggered the aftershocks, which are the first ever to be recorded in the wake of a landslide.

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