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November 7, 2002 Flight Observations of M7.9 Denali Fault Earthquake
On November 7, 2002
Dennis Trabant and
Rod March (US Geological Survey,
Fairbanks, AK), and Dr. Martin Truffer
(University of Alaska, Geophysical Institute, Fairbanks, AK) flew by fixed wing Cessna 206 to document the effects of the Nov. 3, 2002
magnitude 7.9 Denali Fault earthquake. We were able to fly and photograph the fault trench
from near Cantwell to just NW of Mentasta Lake and additionally south of the fault trench
between the Richardson Highway and Susitna Glacier to observe the areas around Eureka,
Maclaren, and East Fork Glaciers. The north flank of the central and eastern Alaska Range was
obscured by clouds and was not investigated. Deep shadows due to low sun angles made some
areas difficult to see. However, digital enhancement of the photos overcomes this obstacle and
reveals significant features not observed in the field. Features observed and photographed
included numerous rock, snow, and ice avalanches, single linear faults, sub-parallel linear
fault pairs, shear zones with multiple sub parallel faults, long curved faults that appear to
follow glacial structures, zigzag faults, at least one significant fault perhaps 1 km south of
the Denali Fault that was not parallel to the Denali Fault trench, odd looking holes on a
glacier surface along the main fault that may be surrounded by blast debris, avalanche-debris
dammed lakes both on and off glaciers, numerous small glacier surficial lakes that had
drained, cracking in lake ice, glacier cracks that indicate drainage of subglacial lakes
(these may be pre-quake features), dark rims around lakes that may have been evidence of
seiches. The most dramatic features by far were the very large mostly rock avalanches onto
Black Rapids and the very large glacier collapse/avalanche onto Gakona Glacier.
The following is a chronological description of our flight observations.
In this interest of timeliness, this was extracted from an email and has not been carefully
edited.
We began near the magnitude 7.9 epicenter adjacent to the lower West
Fork Glacier. Clouds north of the Alaska Range largely obscured our view during the approach.
However, near the crest of the Ak Range we saw mixed snow and rock avalanches in increasing
numbers and size as we approached the epicenter. The upper Yanert Glacier near Mt. Deborah was
severely avalanched. The Yanert ended a surge during the summer of 2001, so its surface was
very rough, but that roughness was not caused by the earthquake. We saw no linear fault trace
on the surface near the epicenter or to the east across the main branch of the West Fork
Glacier. There were many avalanches, the largest that we saw began of the east side of the
West Fork Glacier below peak 7628 and ran out onto the surface of West Fork Glacier 100 meters
of more. That avalanche, like most, was a mixture of rock and snow. In general, the snow cover
in the Ak Range is very light for this time of year at lower altitudes, but well above normal
at higher altitudes based on early Oct. measurements on Gulkana Glacier, just east of the
Richardson Highway. Most large seracs in the icefall where the 1st tributary on the NW side of
West fork Glacier enters the main stream had fallen giving the ice fall an unusual appearance
in that almost no "flat-topped" seracs remained standing. Above the icefall, many snow bridges
over crevasses and bergschrunds had collapsed.
The first identified surficial rift was on the hills between the West
Fork and Susitna Glaciers in T16S R3E (Fbx Meridian). At this location, there was little
evidence of vertical displacement accompanying the lateral displacement (the rift is on a
north facing hill slope so any judgment about vertical displacement is poorly founded). There
was no way to estimate the magnitude of the lateral displacement from the air. We were able to
track the surficial rift almost continuously from near the east side of West Fork Glacier onto
the Susitna Glacier. On the lower Susitna Glacier, in an area of post-surge deflation, some
vertical differential between the two sides was noticeable due to the low-angled November
light. The north side of the rift was higher than the south side. Between the West Fork
Glacier and the Black Rapids divide, the observed rift is a single, ragged crack. On the
Susitna Glacier, the rift was located about 20 percent of the glacier-width from the northern
edge of the glacier along a medial moraine. Sometimes near the southern base of the moraine
and sometimes near the crest of the moraine. The rift was continuous to about 5000 feet on
Susitna Glacier,
In the saddle between Susitna and Black Rapids Glaciers and below the
tributary that enters the north edge of the main glaciers west of Aurora Peak, the rift
lineament was not obvious on the surface. The lineament trend in that area was crossed, almost
perpendicularly, by a few freshly opened cracks (very similar in appearance to the rift
cracks) that were as much as several hundred meters long and separated by several hundred
meters (these values are from memory only and can be revised when the photos are available for
reference/analysis). In this pass area, some of the "pot hole" lakes were still filled with
water and others were empty. There was no sign that any of the lakes had had any recent
changes; i.e., filling, seiching, or emptying. Those that contained water had continuous,
uncracked ice covers with no signs of distortions at their edges. Those that were empty had
their locally typical, light snow covers, undisturbed. The rift lineament was identifiable on
the upper Black Rapids for several kilometers east of the Susitna/ Blk Rapids saddle favoring
the northern 15-20% of the glacier width near medial and lateral moraine edges and a second
lineament was identified in a surface drainage channel. This second lineament continued
through a series of odd holes that resembled neither moulins nor entrances to crevasses; two
look a little like they might be surrounded by blast debris. The, 5 or 6 holes were above the
snow line and are difficult to understand. They were the only such features we observed though
much of the flight may have been too high to see such features.
By far, the most dramatic mass failures we saw emanated from the highest
peaks along the south walls above Black Rapids Glacier east of the Lockit tributary. Three,
largely rock debris avalanches crossed almost the entire, 1-mile width of Black Rapids
Glacier, overriding the monster medial moraine that is 10's of meters high and leaving a
blanket of rock debris that is probably thick enough to inhibit ablation, in places. Two
avalanches came from the peak between the Lokit tributary and the next tributary to the east.
The third, and probably most voluminous, came from a peak in the next easterly separator
between tributary entrants. The total glacier area covered with rock debris is about 13 km2
(an average of the three observers independent map drudles). The thickness of the deposit is
the largest uncertainty in a volume estimate, but bounding the estimates are: a minimum
average thickness of 1 meter; gives a volume of 0.01 km3, and a large average thickness of 5
meters equates to a volume of 0.07 km3 (NOTE: these are very crude values which desperately
need verification). The rift lineament was not identifiable through the avalanche area.
The lineament was again identified where the Denali Fault deviates from
Black Rapids trench and approaches the Augustana Creek drainage from the northwest. The rift
was discernible above the snowline, but could not be identified below the snowline in the
vegetated slopes or fluvial sediments of the Delta River.
The Trans Alaska Pipeline crossing and lower Cantwell Glacier rifts have
previously been described. We flew up the Eel Glacier and found comparatively minor
avalanching and surface disturbances. There was a large mixed rock and snow avalanche off
M'Ladies Mountain on to the upper Castner Glacier. A graben, estimated to be 20 meters between
bounding rifts and possibly one hundred meters long, was seen high on Gulkana Glacier south
east of Cony Mountain, on the Gulkana side of the ridge crest between Gulkana and Canwell
Glaciers. There were several sizeable avalanches onto Gulkana Glacier, one very near one of
our long-term measurement sites below the Moore Icefall.
The next most intensively shattered ridge is the SE wall of the Gakona
Glacier along the Denali Fault between the Canwell-Gakona pass and where Gakona Glacier leaves
the Denali trench. There, one very large (possibly larger than those on Black Rapids) ice and
snow dominated avalanche was released. Most of the deposit was deposited less than half way
across the glacier width, but a considerable volume was spread across nearly the full width of
the glacier. There were many smaller avalanches off that same ridge, most of which did not
cross more than one half of the glacier width. Some made curiously curved run out paths. The
surface rift is almost continuously visible in the parts of the Canwell, Gakona, West Fork,
and Chistochina Glaciers that lie on the Denali Fault. For most of this route, the rift is
confined to the northern 20-25% of the glacier widths and is a single ragged crack. However,
in the ice divide between Gakona and West Fork of Chistochina the lineament is two sub
parallel traces separated by a couple of hundred meters (or more) approximately centered in
the glacier's width. On the West Fork of the Chistochina Glacier only a single ragged
lineament was seen and it became "Z"ed (c/ 100-m arms) for a few hundred meters then straight
for a while then "Z"ed again for a ways. The ice on a glacier dammed lake between the main
branch of the West Fort of the Chistochina Glacier and its most easterly tributary was lightly
shattered ? the rift lineament ran directly through the lake. There was no strong evidence for
the lake level changing.
East of the Chistochina's east terminus, the fault lineament rises above
the valley floor and parallels the valley floor varying in height above the valley floor up to
1/3 of the wall height, along the northern valley wall. The lineament was lost in the upper
Slana River drainage. However, a couple of kilometers of strong lineament was seen about 1 km
east of peak 6565 (near the head of Canyon Creek, a tributary to Mankomen Lake). The strike of
the lineament is north-northwest ? south-southeast. Extension in either direction was not
found.
A small debris-avalanche-dammed lake was seen in the Lost Creek drainage
(NW of Mentasta Lk ~10 miles). There was definitely fresh debris added to the dam. However, we
judged that the lake had been dammed by a previous landslide because the lake surface was
frozen, therefore probably had not formed in the last few days, although the level might have
changed.
Cloud cover prevented investigation of the large glaciers along the north
flank of the Alaska Range. General impressions from returning along the south flank of the Ak
Range (south of the southern crest flanking the Denali Fault) were that the intensity of
avalanching and surface disruption diminishes quite rapidly laterally away from the rift
valley to the south though this might partly be due to gentler topography. Indeed, we remarked
among ourselves that flying along the south flank bare miles south of he main rift valley,
that one would little expect to see the carnage we had just seen over the next ridge north.
West of the epicenter and back in the Denali Fault valley system, we saw no new rifting,
although the ancestral rift was evident in a few places. Just a few miles west of the terminus
of the Yanert Glacier, but back in the Denali Fault trench, there were frozen lakes with
unbroken ice and little or no evidence of seiching or even edge cracking. Again, it is
surprising how quickly the extreme surface disturbances are reduced by distance.
If, by extension, the rapid decrease in avalanching laterally away from
the main rift valley applies to the large glaciers on the north flank of the Alaska Range,
then there may be few large avalanches or glacier disturbances that were not documented due to
the cloud obscuration on the north flank.
In no case was there any evidence of unusual glacier flow having been
stimulated by the recent seismicity. However, the avalanche deposits will affect the mass
balance and flow of Black Rapids, Canwell, and Gakona Glaciers for decades.
Maintainer: Rod
March
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Last update:
Wednesday, January 03, 2007 12:46 PM
URL:
http://ak.water.usgs.gov/glaciology/m7.9_quake/nov_7_flight_desc.htm
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