In order to detect the evidence of extreme ground motion in the past, we have begun to catalog geomorphic characteristics that distinguish slope deposits strongly influenced by extreme ground motion from deposits primarily influenced by climate processes. Underground nuclear explosions (UNEs) of yields between 200 kilotons and 1.3 megatons were conducted under Pahute Mesa at the Nevada Test site from 1962 to 1992. The primary surface effects from these tests were surface cracks, triggered earthquakes, offsets on pre-existing faults, and changes in land surface topography. Rockfall and rock spall were observed along cliffs after a few nuclear tests; however, few observations of accumulations of shattered rock were documented. A large volume of rockfall located along a 1.5-km¬-long cliff of welded ash-flow tuff resulted from extreme ground motions from two nearby UNEs. In 1968 UNE Rickey released maximum ground motions of 500 cm/s peak ground velocity (PGV) at the closest cliff face and PGV decreased to about 300 cm/s at the north end of the cliff. Large boulders with 1-3-m average diameters were shaken loose from fracture planes and cooling joints to form a stack of jumbled boulders at the base of the cliff. Very few large boulders rolled to the base of the hillslope. Subsequently, in 1976, UNE Pool induced 300-350 cm/s PGV along the same cliff. A significant volume of rock, also released along fractures and joints, was added to the coarse boulder colluvium shaken loose in 1968. Ground motion from Pool also rearranged the hillslope boulders from UNE Rickey, but did not cause many boulders to roll downslope. Extreme ground motions from these two UNEs resulted in 1.5-3.0 m of physical erosion to the cliff face. Rockfall from less welded ash-flow tuff units situated above and below the cliff produced significantly less boulder colluvium. Our observations indicate that boulder size and rockfall volume from a cliff or ridge crest due to extreme ground motion are strongly influenced by physical properties of the rock, by unit thickness of the source rock, by pre-existing fracture patterns, and by weathering history. Boulder distribution on volcanic hillslopes governed by climate processes is significantly different from the concentrated piles of large boulders generated by extreme ground motion. Unshaken cliffs of welded ash-flow tuff are undercut by accelerated erosion of underlying non-welded and partially to moderately welded tuffs. As the undercut cliff recedes boulders are gradually released along weathered joints, which results in a pattern of large boulders distributed more or less evenly downslope except where debris flows and hillslope channels concentrate clasts. Be-10 and Cl-36 exposure ages of welded tuff boulders indicate that large boulders on southern Nevada volcanic hillslopes have been preserved for 200,000 to greater than 500,000 years. Thus, coarse rockfall deposits that resulted from past earthquakes characterized by extreme ground motion may persist on this volcanic landscape for several hundreds of thousands of years and may be distinguishable from climate- generated hillslope colluvium.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 7212 Earthquake ground motions and engineering seismology;
- 7221 Paleoseismology (8036);
- 8175 Tectonics and landscape evolution