Why Study Rock Fall with Instruments?
Rock fall is a natural hazard that is present in steep mountain environments worldwide.
Rock falls present a significant risk to people and infrastructure such as roads, dams,
tunnels, homes, and reservoirs. The steep and dramatic topography of Yosemite National
Park attracts millions of visitors, but is also susceptible to the rock fall hazard.
Almost 600 rock failures were documented during the period 1851 to 2008, resulting in
14 fatalities. Yosemite Valley is therefore an excellent site to study rock fall
processes, where lessons learned can be directly applied to risk management.
Rock falls in Yosemite are currently reported and recorded based on eyewitness reports
only; as such, rock fall processes are not well-understood, and many rock falls likely
go unnoticed. Seismic instruments can detect and differentiate rock falls from other
seismic activity, and provide a complete record of rock falls unbiased by the presence
or lack of eyewitnesses. The main objective of this project is to detect and catalog
rock falls to obtain a better understanding of the rock fall process and causative
events such as freeze-thaw, water seepage, heating and cooling.
Large rock falls are accompanied by loud rumbling noises, dust clouds, and often,
boulders impacting rock ledges and talus slopes. There have been numerous anecdotal
reports of precursors to rock falls in the form of loud cracking noises and a showering
of small pebbles and rocks. The frequency of precursors in Yosemite is entirely unknown,
and searching for them is a major objective of this project.
Instruments are installed on and around the Lower and Middle Brother formation
in Yosemite National Park. Middle Brother has several active rockfall zones, and
is remarkable accessible for instrumentation, even in winter. Currently, we are
monitoring seismic waves chiefly using 4.5 Hz Geophones and Refteks continuously
recording at 1000 sps. We also sometimes use accelerometers
The instruments were installed starting in October, 2008, and are expected to be
in operation through approximately May, 2009. You can check on the status of
the some of the instruments here:
ARGOS Satellite Links
(7329, 7398, 19390, 19391, 19395)
During the winter of 2007-08, two instruments (one geophone and
one accelerometer) were installed on a ledge on 12/11/07
in the vicinity of the Three Brothers Formation in Yosemite National
Cool Stuff about Rock Fall
John Muir was working in the Yosemite Valley in March, 1872 when the "noble" and
Owen's Valley earthquake (magnitude 7.4) shook him out of bed. The Eagle Rock, on
the south wall, collapsed in a "stupendous roaring rock storm". Rock falls continued to
rattle the Yosemite Valley for over 2 months. Of course, John Muir wrote about it.
by John Muir
Park geologist Greg Stock is currently connecting other large rock fall deposits (called
rock avalanches) to much older Owen's Valley Earthquakes. Read about Greg's research on the
El Capitan Rock Avalanche.
Large rock falls have shook seismic instruments at distances up to 400 km in recent past.
1996 Happy Isles rock fall went ballistic and created an air shock wave that
snapped 1000 trees and registered as a magnitude 2.2 seismic event (note: there was no
earthquake - the rock fall created the seismic waves by itself!). The March, 2009 Ahwiyah
Point rockfall also devastated a large area and
registered as a magnitude 2.4 seismic event.
This material is based upon work supported by the National Science Foundation under Grant No. 0840580