31 March 2009

Yosemite rockfall – seismic signal

Posted by Dave Petley

Improvements in the quality of seismic monitoring systems, in addition to increases in the numbers of instruments and better processing techniques have meant that in the last few years it has become increasingly common to detect large rockfall events from their seismic signature. A nice example of this occurred this week. On 28th March a large rockfall occurred from Ahwiyah Point near Half Dome in Yosemite. This is a pretty large lump of granite:

There is a very nice image of the rockfall scar and track available on Flicker, but it is not possible to import the file. Take a look. This is a less good image from the National Park Service – you do get a pretty good idea of what was involved:

The National Park Service description of this event is as follows:
“[the rocks] fell roughly [600 m] 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.” They are also reporting that there was a small air blast, but no-one was killed.

Of course, seismic monitoring in the part of the USA is a bit of a fine art, so it is unsurprising that the event was picked up on multiple instruments. There is a nice image on Seismoblog of the recorded seismic events from a range of instruments located an increasing distance from the site:

Based on the sesmic data the USGS were quickly able to locate the fall – pretty impressive!