U.S. Geological Survey
By R.D. Norris1 and F.W. Lester2
The seismic signal generated by the Happy Isles rockfall in Yosemite National Park on July 11, 1996 was widely recorded by regional seismic networks operated by the University of California-Berkeley, the U.S. Geological Survey, and the University of Nevada-Reno. An analysis of short-period seismic data from the Northern California Seismic Network (NCSN) provided an epicentral location that agreed within 3 km of the actual location of the impact point of the rockfall; the closest station was 38 km distant. The nature of its descent and impact was very different from previously recorded rockfalls in the Cascade Range of Washington State and northern California. These rockfalls quickly became debris avalanches that had prolonged contact with the ground, while the Happy Isles rockfall descended mostly in free fall, contacted the ground abruptly, and generated a destructive air blast. The effects of these differences in release and descent on their respective seismic signals will be discussed, along with seismic, visual, and field evidence that the Happy Isles rockfall was generated by two separate failures from its source area.
1 U.S. Geological Survey, Box 351650, Geophysics Program,University of Washington, Seattle, Washington, 98195-1650
2 U.S. Geological Survey, 345 Middlefield Rd., MS-977, Menlo Park, California, 94025-3591