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Posted by Canie on November 07, 2002 at 22:31:57:
Subject: USGS NEWS RELEASE: Alaska Interior Reveals Scars and Ruptures From 7.9
Denali Fault Quake
11/07/2002 01:04:40 PM News Release Address:
U.S. Department of the Interior 4200 University Dr.
U.S. Geological Survey Anchorage, Alaska 99508 For Release: Contact:
November 7, 2002 Peter Haeussler Alaska Interior Reveals Scars and Ruptures from 7.9 Denali Fault Quake NOTE TO NEWS EDITORS: Reproducible photos are available. Please see end of
press release for photo descriptions and urls. Sunday's magnitude 7.9 earthquake in central Alaska created a scar across
the landscape for more than 145 miles, according to surveys conducted the
past two days by geologists from the U.S. Geological Survey and the Alaska
Division of Geological and Geophysical Survey. The geologists, who followed the earthquake rupture by helicopter through
valleys, across streams, and along glaciers, measured a maximum horizontal
offset of 22 feet across the Tok Highway Cutoff, a road that goes from Tok
to Glenallen and intersects with the Alaska Highway. This earthquake, one of the largest ever recorded on U.S. soil, occurred on
the Denali fault system, one of the longest strike-slip fault systems in
the world, rivaling in size California's famed San Andreas strike-slip
fault system. Peter Haeussler, the USGS scientist helping lead the surveys, noted that
when the Trans-Alaska Pipeline was constructed, no clear surface features
existed that revealed the exact position of the Denali fault where it
crosses the pipeline corridor. New surface ruptures after Sunday's quake
demonstrate that even though it is now clear that the structures engineered
to accommodate fault movement are largely north of the fault trace, the
pipeline engineering design was robust, functioned well, and resulted in
minimal earthquake damage. The survey also revealed other visible evidence of the earthquake,
including clearly developed scarps and cracks where the faults pass beneath
glaciers. Geologists have long noted that glaciers follow fault zones.
Rocks in the faults have been ground up by sliding past each other, and the
glaciers follow those lines of weakness. "But this is the first recorded
earthquake beneath a glacier, and it's amazing how clear the details of the
fault traces show up," said Haeussler. "Some cracks are easily large enough
to fit a bus, where the fault pulled ice blocks away from each other." Along the valley slopes near the fault trace, there were large rock and
snow slides involved blocks as big as houses, with some slides traveling
large distances. On the Black Rapids glacier, rock slides released from
south-facing slopes crossed the one and a half mile-wide valley and flowed
part way up the opposite slope. In a few areas, rock and snow slides
dammed creeks, creating small lakes. Near Mentasta Lake, a village that experienced some of the worst damage in
the quake, scientists discovered that the surface scar turned from the
Denali fault to the adjacent Totschunda fault, which trends toward more
southeasterly down toward the Canadian border. Researchers documented other evidence of the earthquake from upper Black
Rapids Glacier, west of the Richardson Highway, to an area east the village
of Nabesna, where both the Denali fault and the related Totschunda fault
broke the surface. Overall, the geologists found that measurable scarps indicate that the
north side of the Denali fault moved to the east and vertically up relative
to the south. Maximum offsets on the Denali fault were 22 feet at the Tok
Highway cutoff and were 6.5 feet on the Totschunda fault. The largest
offsets were in the region between the Richardson Highway and the Tok
Cutoff Highway. These observations are consistent with what seismologists are learning
about how the earthquake progressed, Haeussler said. The mainshock started
at the western end of the rupture zone and then traveled eastward along the
fault, where most of the seismic energy was released. Scientists will use
measurements of fault offsets to calibrate models of how faults slip during
earthquakes, and to help them better understand the earthquake hazards
associated with large faults. To address these concerns and further enhance
earthquake mitigation, the USGS is building a modern network of
sophisticated ground-shaking measurement systems, both on the ground and in
buildings, called the Advanced National Seismic System (ANSS). ANSS will
become the first line of defense for earthquake hazards, with the ultimate
end result being public safety, lives saved, and major losses to the
economy avoided. The November 3 earthquake was the largest seismic event ever recorded on
the Denali fault system, and was larger than the 1906 "San Francisco"
earthquake, which had a magnitude of 7.8. Both fault systems are
strike-slip systems that cut through continents. The 1964 Alaskan
earthquake was centered offshore and generated by a different type of fault
separating oceanic and continental rocks; at magnitude 9.2, it was the
second largest earthquake ever recorded on earth. The epicenter of the Nov. 3 trembler was about 75 miles north of Anchorage,
causing multiple landslides and road closures, but minimal damage and
amazingly few injuries and no deaths. This was the second major earthquake
along this portion of the fault in less than two weeks. A magnitude 6.7
earthquake occurred a few miles to the west on Oct. 23 and is now believed
to be a foreshock of Sunday's massive quake. The USGS serves the nation by providing reliable scientific information to
describe and understand the Earth; minimize loss of life and property from
natural disasters; manage water, biological, energy, and mineral
resources;and enhance and protect our quality of life.
***USGS***
This press release and in-depth information about USGS programs may be
found on the USGS home page: http://www.usgs.gov. To receive the latest
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PHOTO INFORMATION AND URLS: All photos are copyright-free and should be credited to Peter Haeussler,
U.S. Geological Survey. The photos were taken on a survey along and near
the Denali fault after Alaska's Nov. 3 7.9 magnitude earthquake. To preview
photos in a thumbnail picture, you should replace the "3" near the end of
each url with a "1." http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/augustanack3.jpg
- Fault scarp near Augustana Creek, just west of Delta River. The offset
was approximately 16 feet. Some cracks were up to 9 feet deep. A pressure
ridge is visible in the background. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/brglslide3.jpg
- These two rockslides flowed a mile northward over the Black Rapids
Glacier. They were the largest observed. They are about 12 miles west of
the Richardson Highway. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/cglacier3.jpg
- Trace of the Denali Fault running up the north side of the Canwell
Glacier about 10 miles east of the Richardson Highway. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/cglacierb3.jpg
- Farther up the Canwell glacier the fault trace zig-zags across the snow
and ice surface. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/valley3.jpg
? A nicely visible fault trace near Chistochina River.
http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/faultintrees3.jpg
? A fault trace visible in trees southeast of Tok Cutoff Highway.
http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/fieldinglake3.jpg
? Liquefaction, spreading and settling at the north end of Fielding Lake,
which is about 11 miles south of the Denali fault. Note the tilted
buildings. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/offsetstreambank3.jpg
? Near the head of Slate Creek, a stream bank offset of 3.3 m is visible;
larger offset including drag along fault was roughly 11 feet. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/richardsonhwy3.jpg
- Richardson Highway offset 8.5 feet in right-lateral sense. This
location is near where supports to the Trans Alaska Pipeline sustained
damage. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/rhnpipeline.jpg
- Richardson Highway on right, looking north. The Alaska Pipeline is on
the left. Road offset reveals Denali fault location. The part of the
pipeline designed to withstand movement along the Denali fault is in the
middle part of the photograph. The fault runs beneath the pipeline near the
left edge of the photograph. Some damage to the pipeline occurred at this
location. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/siavalanches3.jpg
- Snow and ice avalanches on the Gakona Glacier. A fault trace is dimly
visible in foreground. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/tokhighway3.jpg
? Fault offset of Tok cutoff highway. On the ground researchers estimated
offset as 23 feet. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/totschundabreak3.jpg
- Totschunda fault trace. A significant finding of the initial surveys
was that the Totschunda fault ruptured during the earthquake. The
Totschunda fault connects with Denali fault, is located at the southeastern
extent of the rupture zone, and the fault has a more southeasterly trend
than the Denali fault. http://earthquake.usgs.gov/eqinthenews/uslbbl/photos/pr071102/totschundathrust3.jpg
? A view northwest along the Totschunda fault at the SE most extent.
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