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 were installed on and around the Lower and Middle Brother formation in Yosemite National Park. Middle Brother has several active rockfall zones, and is remarkably accessible for instrumentation, even in winter. Primarily, we monitor seismic waves using 4.5 Hz Geophones and Refteks continuously recording at 1000 sps. We also sometimes use accelerometers and microphones for monitoring.
Interesting Facts about Rock Fall in Yosemite
John Muir was working in the Yosemite Valley in March, 1872 when the "noble" and "tremendous" 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.
The Earthquake, 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 been recorded by seismic instruments at distances up to 400 km in recent past. The 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.
Publications related to this work
Zimmer, V.L., Collins, B.D., Stock, G.M. and Sitar, N. (2012, in press.) Rock fall dynamics and deposition: an integrated analysis from the 2009 Ahwiyah Point rock fall, Yosemite National Park, USA. Earth Surface Processes and Landforms, special issue on Rock Slope Erosion.
National Science Foundation Civil, Mechanical, and Manufacturing Innovation (CMMI) Grant #0840580
Instruments Loaned By:
Defense Threat Reduction Agency (DTRA) in Albuquerque, NM
PASSCAL Instrument Center in Socorro, NM
Microphones from the InfraVolc research group at New Mexico Tech