This and other photos from the 2006 Yosemite rockfall at
Gravity sucks, and Yosemite National Park has some steep slopes, so the result is rockfall. Mass wasting. Downslope movement. Check out some video of a 2006 rockfall at Yosemite.
The Seismo Blog has a really nice piece on a rockfall in Yosemite Saturday morning, and quotes Greg Stock, the park geologist: the rocks "fell roughly 1800 feet to the floor of Tenaya Canyon, striking ledges along the way. Debris extended well out into the Canyon, knocking down hundreds of trees and burying the southern portion of the Mirror Lake loop trail... Fortunately, due to the event occurring in the early morning, there were no injuries."
Seismo Blog addresses what happens when tons of granite come crashing down to the valley floor. The impact makes the ground vibrate and creates seismic waves very similar to the ones radiated by an earthquake. Seismic stations all over Northern California and Nevada - as far away as 250 miles from Yosemite - registered these waves. The rockfall had the same energy as a magnitude 2.4 earthquake.
The seismic waves of Saturday's rock fall were recorded by many earthquake stations. The 33 depicted here are sorted top to bottom by increasing distance from Yosemite. It takes seismic waves longer to travel further distances, hence the "delay" of almost 60 seconds between the arrivals of the waves at the nearest and most distant stations. (from Seismo Blog)
This probably make more sense than talking about the equivalent Richter magnitude of the Chernobyl nuclear power plant accident!