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Bursting Rocks and Trembling Earth
While most of the Bay Area was rattled on Monday morning around 10:40 am by a magnitude 4.3 earthquake near Morgan Hill, another earthshaking event went almost unnoticed by the public - unless you were in Yosemite over the weekend. Early Saturday morning, a huge mass of rock came crashing down from Ahwiyah Point near Half Dome. Greg Stock, the Park Geologist at Yosemite, writes that the rocks "fell roughly 1800 feet to the floor of Tenaya Canyon, striking ledges along the way. Debris extended well out into Tenaya 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."
But what happens when tons and tons of granite come crashing down unto the valley floor? Such an impact makes the ground vibrate and thereby creates seismic waves very similar to the ones being radiated by an earthquake. Indeed, on Saturday morning seismic stations all over Northern California and Nevada - as far away as 250 miles from Yosemite - registered these waves. The automatic earthquake location computer for Northern California at the offices of the USGS in Menlo Park picked up the recordings and calcuated an epicenter just half a mile to the northwest of Half Dome - which is actually pretty close to Ahwiyah Point. The program even computed a magnitude for the rock fall: Its impact had the same energy as a magnitude 2.4 earthquake.
While the seismic waves generated by a rock fall can be mistaken for the rumblings of an earthquake, the physics behind the two phenomena is completely different. Most earthquakes are the result of tectonic stress, which has accumulated in the rocks due to the movement of the lithospheric plates. A rock fall happens when the rock has been weakened by weathering. Water, which accumulates in cracks, freezes during the winter frosts. As ice occupies a larger volume as the same mass of liquid water, the freezing ice makes the rock expand and burst - similiar to a water bottle left in the freezer for too long. If such cycles of freezing and thawing are repeated often enough, the rock becomes loose and can break.
These rock bursts are by no means rare in granite world of Yosemite. Last October two rock falls hit some of the tents and cabins in Curry Village. In July 1996 more than 162,000 tons of rock cascaded down more than 2,000 feet, killing one visitor and crushing 500 trees. This blast was also recorded on many seismic stations, although it was somewhat smaller than Saturday's rock fall. After the 1996 event, BSL's Bob Uhrhammer analysed the seismic data carefully and reconstructed the details of the fall.