by Robert Sanders
The rock fall in Yosemite National Park Aug. 14 shook the Sierra Nevada like a Richter magnitude 2.15 earthquake at exactly 6:52:54 p.m. PDT, according to a Berkeley analysis of data from nearby seismometers.
The accuracy with which the time and magnitude of the July 10 event can be determined is a tribute to the new broadband seismometers the Berkeley Seismographic Station has been placing around the northern part of the state to monitor quakes.
The rock fall produced a temblor of a size that typically can be felt several miles away, said seismologist Robert Uhrhammer, who analyzed the recordings. He hopes to use the data from the rockfall to perfect his models of the Sierra crust, which in turn will be used to understand earthquake propagation throughout the region.
The National Park Service estimates the size of the granite slab that gave way at about 31,500 tons. It dropped about 2,200 feet to the floor of Yosemite Valley near Happy Isles, creating an air blast that knocked over trees like they were bowling pins. One person was killed.
Uhrhammer estimates that the slab took about 12 seconds to drop 2,200 feet, hitting the ground at a speed of about 230 miles per hour. The largest amplitude seismic waves it generated traveled outward at about 2.4 miles per second.
Seismometers within 150 miles of Yosemite recorded the rock fall, but the analysis involved only three that were within 50 miles of the park. The nearest was at Kaiser Creek in a Southern California Edison facility 31 miles to the south-southeast.