Gravitational
Spreading (sackung) studies at GEO-HAZ
(Updated
Jan. 16, 2008)
Photo of a prominent sackung scarp (at center,
with red circles) near Gillett Pass, Alaska, formed during the 3 Nov. 2002 M7.9 Denali, Alaska
earthquake. Summit elevation of the ridge in foreground is about 6500 feet
(1980 m), with the base elevation (below photo) about 3500 feet (1070 m),
yielding a local relief of 3000 feet (915 m) on the failed ridge. Cross-cutting
relationships (red circles) indicate that the sackung scarp formed before the
rock and snow avalanches. Photograph by Peter Haeussler, U.S. Geological Survey; annotations
by Jim McCalpin, GEO-HAZ.
Landslides and Gravitational
Spreading in Tertiary Volcanic Rocks of the Eastern Traverse
Range, Salt
Lake and Utah Counties, Utah
The Eastern Traverse Range is a footwall salient
of the Wasatch Range at the boundary between the Salt Lake City segment and
Provo segment of the Wasatch fault zone. Altered Tertiary volcanic rocks have
been relatively uplifted on this salient and now lie on a ridge about 1000 feet
above the valley floors of adjacent Salt
Lake and Utah Valleys.
The Wasatch fault zone dips westward at about 35 degrees beneath the ridge and
experiences M>7 earthquakes about every 1400 years. Due to the combination
of weak rock mass characteristics, steep slopes (particularly on the north
side), and periodic strong ground shaking, the Range periodically undergoes
deep-seated gravitational spreading. Surface landforms include linear and
curved scarps and bench-and-riser topography. Most prominent spreading
landforms (sackung) appear to be using pre-existing Tertiary normal faults.
Ridgetop
Spreading in California:
Contributions Toward Understanding a Significant Seismic Hazard
In cooperation with Earl Hart (ex-CGS), GEO-HAZ
contributed 3 papers to this award-winning 2 CD-ROM set (Holdredge Award
winner, AEG, 2005; http://devaegweb.i4a.com/i4a/pages/Index.cfm?pageID=3755).
The papers document gravitational
spreading and cracking of ridges (as well as related landsliding) due to strong
earthquake shaking, primarily in the San Gabriel Mountains and Santa
Susana Mountains
of southern California.
The CDs contain high-resolution scans of 25 mapped 7.5’ quadrangles. For
more information, go to:
http://www.conservation.ca.gov/cgs/information/publications/release_statements/Documents/CD2003_05.pdf
Origin and Seismic
Significance of Antislope Scarps on Pilgrim
Peak, Central
Idaho
The U.S. Bureau of Reclamation, Denver,
Colorado, retained GEO-HAZ to assess the
origin and seismic significance of a series of 8 antislope (uphill-facing)
scarps near the Bureau’s Deadwood Dam in central Idaho,
USA.
Based on 1 week of fieldwork and photogeologic mapping, we concluded that these
scarps were formed by deep-seated gravitational spreading (sackung), and not by
coseismic, tectonic surface rupture. With the elimination of these scarps as
tectonic, there is now no evidence for Holocene or late Pleistocene
displacement on faults near Deadwood Dam. A surprising result of our study was
the widespread nature of deep-seated gravitational failure of granitic rocks of
the Idaho
batholith, and the generally poor rock quality of much of the batholith. There
was a nearly continuous spectrum of failures ranging from sackung scarps, to
large toppled blocks of granite, to complexes of detached blocks, to typical
landslide deposits, with each stage representing a progressively greater amount
of downslope movement.
1998-99:
Ridgetop spreading, splitting, and shattering related to earthquakes in
southern California:
