Sunday marked the 20-year anniversary of the Happy Isles rockfall, one of the most devastating rockfalls in Yosemite’s history.
At 6:52 pm on July 10, 1996, two rockfalls detached in quick succession from the rim of Yosemite Valley east of Glacier Point. Approximately 90,000-180,000 tons of rock slid down a bedrock ramp and went airborne, free-falling 2,200 feet to the base of the cliff. The impact generated ground shaking equivalent to a magnitude 2.1 earthquake, which was detected by seismometers 125 miles away. The impact also generated an airblast with velocities exceeding 240 miles/hour, snapping or toppling more than 1,000 trees. Falling trees in the vicinity of the Nature Center at Happy Isles caused one fatality and several serious injuries. The sky went black for six minutes as a large dust cloud blocked out the evening light, and the area around Happy Isles and the Pines campgrounds was covered in a thick coating of rock dust.
The 1996 Happy Isles rockfall proved to be an important turning point in the study of Yosemite rockfalls. Following this rockfall, the National Park Service established a close working relationship with the U.S. Geological Survey (USGS) to document rockfalls and assess geologic hazards, a relationship that continues to this day. The park now has an active rockfall research program that helps inform management actions to reduce rockfall risk.
You can find more information on Yosemite rockfalls, including the 1996 Happy Isles rockfall, at https://www.nps.gov/yose/learn/nature/rockfall.htm.