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Analysis
of Mechanism of The Giant Tsunami Generation in Lituya Bay
on July 9, 1958
George
Pararas-Carayannis
From
a Paper Presented at the Tsunami Symposium on May 25-27, 1999,
in Honolulu, Hawaii, USA.
© 1999 George
Pararas-Carayannis - All Rights Reserved
ABSTRACT
The giant
waves that rose to a maximum height of 1,720 feet (516 m) at
the head of Lituya Bay, on July 9, 1958, were generated by a
combination of disturbances triggered by a large, 8.3 magnitude
earthquake along the Fairweather fault. Several mechanisms for
the generation of the giant waves have been proposed, none of
which can be conclusively supported by the data on hand. Generative
causes include a combination of tectonic movements associated
with the earthquake, movements of a tidal glacier front, a major
subaerial rockfall in Gilbert Inlet, and the possible sudden
drainage of a subglacial lake on the Lituya Glacier.
Upper Lituya Bay response and associated secondary phenomena
contributing to the giant slushing wave action in Gilbert Inlet,
depended on the earthquake's energy release, proximity to the
epicenter, physical rupture along the fault, propagation path
of surface seismic waves, and the magnitude and duration of the
dynamic, near-field, strong motions. Earthquake ground motions
of high intensity could have resulted in vertical accelerations
of up to 0.75g and horizontal accelerations of as much as 2.0g.
In the absence of adequate data, analogies are drawn from recorded
recent large earthquakes elsewhere for characteristics of near
field ground motions, duration, and of vertical and horizontal
accelerations, that triggered the giant rockfall in Lituya Bay.
Additionally, the tectonic setting of the Fairweather fault is
examined.
The following mechanism can account for the giant 1,720 foot
wave runup at the head and the wave along the main body of Lituya
Bay: The strong earthquake ground motions triggered a giant rockfall
at the headland of the bay. This rockfall acting as a monolith,
and thus resembling an asteroid, impacted with great force the
bottom of Gilbert Inlet. The impact created a radial crater which
displaced and folded recent and Tertiary deposits and sedimentary
 layers.
The displaced water and the folding of sediments broke and uplifted
1,300 feet of ice along the entire front of the Lituya Glacier.
Also, the impact resulted in water splashing action that reached
the 1,720 foot elevation. The rockfall impact, in combination
with the net vertical crustal uplift of about 1 meter and an
overall tilting seaward of the entire crustal block on which
Lituya Bay was situated, generated a solitary gravity wave which
swept the main body of the bay.
U.S.G.S.
Aerial photo of Lituya Bay taken after July 9, 1958 event
An analytical solution based on this proposed impulsive mechanism
can further support the 1,720-foot runup. Mathematical modeling
studies conducted by Dr. Charles Mader, support this mechanism
as there is a sufficient volume and an adequately deep layer
of water in the Lituya Bay inlet to account for the giant wave
runup. Dr. Mader has suggested full Navier-Stokes modeling, as
with asteroid generated tsunami waves.
 Necessary
focus of future research in understanding mega-tsunamis in enclosed
bodies of water, such as Lituya Bay, should be directed towards
the examination and modeling of the elements relative to the
earthquake energy release, the empirical analysis of earthquake
source and seismic energy propagation processes, the near-field
ground motions from finite fault sources of past mega-thrust
earthquake events, and the systematic studies of resulting secondary
effects.
Another
view of Lituya Bay from the head of the Bay looking outwards
showing the effects of the giant waves (University of California
- Berkley photo)
Furthermore,
measurable input and output parameters derived from mathematical
modeling and analysis of the Lituya Bay event can be applied
to models of asteroid tsunami generation for purposes of calibration,
verification and validation.
INTRODUCTION
The
Lituya Bay Earthquake of July 9, 1958
On July
9, 1958, a large earthquake caused by tectonic movements along
the Fairweather Fault struck Southeastern Alaska. Its epicenter
was at lat 58.6'N., long 137.1`W., at a point near the Fairweather
Range about 7.5 miles (12 km) east of the surface trace of the
Fairweather fault and 13 miles (20.8 km) southeast of the head
of Lituya Bay (Fig.1). The earthquake had a magnitude of 7.9,
on the Richter Scale, although some sources have reported it
to be as much as 8.3. (Brazee & Cloud, 1960).
This was the strongest earthquake in the region since the September
4, 1899, 8.2 magnitude, Cape Yakataga earthquake. The shock was
felt at all cities in southeastern Alaska over an area of 400,000
square miles, and as far south as Seattle in the state of Washington,
and as far eastward as Whitchorse, Y.T., Canada.
Ground displacements of 3.5 feet (1.05 m) upward and 21 feet
(6.3 m) in the horizontal plane were measured on surface breaks
along the Fairweather fault 6 to 10 miles southeast of Lituya
Bay's Crillon Inlet (Tocher and Miller, 1959). It is presumed
that similar displacements occurred along the Crillon and Gilbert
inlets at the head of Lituya Bay.
Map
of Lituya Bay showing setting and effects of 1958 giant wave.
(Modified after Miller, 1960)
The
Giant Waves
Almost
immediately, the earthquake of July 9, 1958, was followed by
a massive wave that splashed to a maximum height of 1,720 feet
on the southeast spur of Gilbert Inlet at the headland of Lituya
Bay, then by a wave that wiped everything in its path over an
area of about 4 square miles (10.4 sq. kms) on either side of
the Bay.
There were three fishing boats anchored near the entrance of
Lituya Bay on the day the giant waves occurred. One boat was
sunk and the two people on board lost their lives. The other
two boats were able to ride the waves. Among the survivors were
William A. Swanson, and Howard G. Ulrich, who provided accounts
of what they observed. Miller (1960) documented in great detail
all accounts, measurements, and observations related to the giant
waves in Lituya Bay. Waves of cataclysmic proportions have repeatedly
occurred in Lituya Bay in the past (Miller, 1954). Because of
the unique geologic and tectonic conditions of Lituya Bay, giant
waves will undoubtedly occur again in the future.
Tectonic
Setting
The Pacific
and the North American tectonic plates move in complex, irregular
patterns resulting in earthquakes with faulting that differs
along their boundaries. The Fairweather fault in Southeastern
Alaska marks one of these boundaries. To the south, in the vicinity
of California, the boundary is marked primarily by a large transform
fault system which is the San Andreas and the numerous secondary
faults. The San Andreas fault is also the boundary between the
Mendocino fault separating the Gorda and Pacific plates.
Immediately north of this area is the Cascadia subduction zone
which marks the boundary between the Gorda and Juan de Fuca plates
offshore and the North American plate. The Gorda plate is the
block being subducted beneath the North American plate. However,
a thrust fault of this type slopes gently relative to the earth's
surface. Earthquakes along such a thrust fault push the rock
above the ramp up and over the rock beneath it. In very active
subduction zones, the boundary between the plates resembles a
giant thrust fault, which usually extends for hundreds of miles
in length. The locked part of the subduction interface is known
as the megathrust. All of the worlds greatest earthquakes (with
moment magnitude of 8.5 and larger) which have produced Pacific-wide
tsunamis, are associated with ruptures of megathrusts along steeper
angles.
The Fairweather fault in the vicinity of Lituya Bay, differs.
It is not acting as a thrust fault but as a transform fault,
but with substantial vertical movement of the oceanic crustal
block upward. The great 1899 earthquake on the Fairweather, caused
some dramatic vertical changes. Both the Crillon and Gilbert
inlets at the head of Lituya Bay, and their extensions covered
by glaciers on either side for a total distance of 12 miles,
have been formed by trenching action along the Fairweather fault.
The inlets themselves and the entire Bay are part of the oceanic
plate, which actually rose by about 3.5 feet in this particular
area, as a result of the July 9, 1958, earthquake. The fault
line traverses the entire head of the Bay on the northeastern
side of the inlets.
Geologic
Setting
The entire
Lituya Bay represents a valley carved by glaciers which begun
retreating when the Wisconsin interglacial period begun, nearly
ten thousand years ago. The U-shaped floor at the head of the
Bay is underlain by recent terminal moraine deposits as well
as from deposits of previous glaciation during the Tertiary period.
The entrance to the bay is marked by a long spit, La Chaussee
Spit, which is the remnant of an arcuate terminal moraine from
the last period of glaciation .
Bathymetry
Bathymetric
surveys made in 1926 and 1940 (U.S. Coast and Geodetic Survey,
1942), show the head of Lituya Bay to be a pronounced U-shaped
trench with steep walls and a broad, flat floor sloping gently
downward from the head of the bay to a maximum depth of 720 feet
just south of Cenotaph Island. From there, the slope rises toward
the outer part of the Bay. At the entrance to the Bay, the minimum
depth is only 33 feet at mean lower low water. The outer portion
of Lituya Bay is enclosed by La Chaussee Spit, with only a very
narrow entrance of about 700-800 feet kept open by tidal currents.
The tide in the bay is diurnal, with a mean range of 7 feet and
a maximum range of about 15 feet (U.S. Coast and Geodetic Survey,
1957). The U-shape of the bay and the flatness of its floor indicate
that extensive sedimentation has taken place, but the thickness
of the sedimentary layers is not known.
ANALYSIS
OF SOURCE MECHANISMS
It has been
well documented in the scientific literature that waves with
large energy content are generated impulsively by different mechanisms
related to large earthquakes in regions of subduction, to volcanic
and nuclear explosions, to landslides, and to large masses of
water added suddenly to a body of water. To these we must also
add the impulsive impacts from large rockfalls or from asteroids
and comets falling on a body of water on earth. The characteristics
of waves generated by such impulsive mechanisms will depend upon
the disturbing force and the rate at which the force is applied.
Resulting water waves may be oscillatory in character, nearly
solitary in form, a complex non-linear wave existing entirely
above the initial undisturbed water surface, or a bore (Prins,
1958a, 1958b).
The giant 1958 wave that rose to a maximum of 1,720 feet at the
head of of Lituya Bay. and the subsequent huge wave along the
main body the Bay, were caused by an impulsive event with a very
large energy content. The mechanism that generated the giant
wave runup of 1,720 feet above sea level has been a mystery that
has baffled scientists. That such a giant wave is possible has
been extensively doubted on theoretical grounds. Several mechanisms
have been proposed, none of which can be conclusively supported
by the data.
The giant wave must have been generated by a combination of disturbances
triggered by the large earthquake. Factors that contributed were
the result of cumulative effects rather than those from a single
source. Generative causes included a combination of tectonic
movements associated with the earthquake, movements of a tidal
glacier front, the possible sudden drainage of a subglacial lake
on the Lituya Glacier, but primarily as this study proposes,
a major subaerial rockfall into Gilbert Inlet. In this paper
we shall review and comment on all such impulsive mechanisms.
Landslide
Mechanism
Landslides
are not very effective mechanisms for tsunami generation. The
energy imparted to the water body is about 4% of the total energy.
No known landslide ever produced a wave that would approach the
magnitude of the Lituya Bay event. The runup of 1,720 feet is
more than 8 times the maximum height reached by the largest of
the slide-generated waves in Norway. Simple displacement of water
by material of an ordinary landslide cannot account for the 1720
foot runup observed on the other side of Gilbert inlet. Dr. Mader's
modeling studies confirm that such high runup from such mechanism
was not possible.
Tectonic
Mechanism
Similarly,
fault displacement could not have been an important contributing
mechanism to the generation of the giant wave that reached the
1,720 ft. elevation at the spur of Gilbert Bay. As indicated
previously, the Fairweather fault line in the vicinity of Lituya
Bay, lies near the northeast side of Gilbert and Crillon Inlets.
The earthquake resulted in tectonic displacements which were
primarily in the horizontal plane. There was an upward movement
of 3.5 feet and a horizontal movement of 21 feet.
Even if we assume that nearly the entire area under water at
the head of Lituya bay moved relatively northwestward and up
by 3.5 feet, such tectonic movement could not have displaced
enough water to generate the extreme runup or the wave observed
subsequently in the Bay. The wave motion resulting from such
tectonic displacement should have been directed toward the northwest
and southeast side of the bay and (or) toward the head of the
bay. Vertical displacement of the bottom of the bay along the
Fairweather fault would have generated waves as a line source
across the head of the bay. However, according to eyewitnesses
reports, this was not the case as there was a lapse ranging from
1 to 2.5 minutes between the onset of the earthquake and the
first sighting of the wave at the head of the bay. Also, the
eyewitness accounts and the subsequent observations indicated
a wave source mechanism that resulted in a radial pattern of
propagation from a point source in Gilbert Inlet. In conclusion,
a tectonic mechanism alone could not displace sufficient volume
of water to account for either the extreme runup at the head
or the subsequent wave inundation in Lituya Bay. Also, Dr. Mader's
modeling studies confirm it.
Sudden Glacial
Lake Drainage Mechanism
A partly
subglacial lake exists just northwest of the sharp bend in the
Lituya Glacier at the head of Lituya Bay . Following the earthquake
of July 9, 1958, an observation was made that the level of the
lake had dropped by about 100 feet. Therefore a mechanism of
sudden drainage of a large volume of water from this glacial
lake has been s considered as the cause of the giant 1958 wave.
However, such mechanism would also be unlikely for the following
reasons. To hypothesize the great 1720 ft. runup from such mechanism,
not only a great volume of water would need to be ponded in a
chamber at an elevation high enough to produce the necessary
hydraulic head, but a strong triggering mechanism would be needed
to cause its sudden drainage into Gilbert Inlet.
Certainly the earthquake displacements and ground motions were
sufficient to perhaps trigger such an event. Therefore, the remaining
questions are: a) was there enough water drained to cause the
1720 ft. wave? b) was the hydraulic head high enough and the
rate of drainage sudden and fast enough to account for the large
runup? c) did the water roll down the face of the glacier or
was it suddenly released beneath the glacier or through an ice
tunnel below sea level in the front of Gilbert inlet?, and d)
did subsequent wave inundation of the coast line in Gilbert and
Crillon inlets as well as in the Lituya Bay validate such mechanism?
In answer to these questions the following can be said. The hydraulic
head was high enough. However, there was no physical evidence
that sudden drainage of the lake on the surface of Lituya glacier
itself occurred. Since the water level was 100 feet lower following
the earthquake, it is quite possible that a fairly large volume
of water drained from the glacial lake through some glacial tunnel
and resulted in some sudden up welling immediately in front of
the glacier. It is believed that neither the volume of water
nor the rate of drainage would have been sufficiently high to
account for the 1720 ft. wave or to justify the subsequent wave
observed in the Bay. Finally, given that such drainage would
have occurred in front of Lituya Glacier, maximum runup would
have been expected on the opposite side in Crillon inlet, rather
than at the spur on the southwestern corner of Gilbert inlet.
In view of these considerations, it can be concluded that sudden
glacial drainage was not the mechanism that produced the extreme
giant wave in Lituya Bay. There was not sufficient volume of
water and the drainage was not sufficiently impulsive. Dr. Mader's
modeling studies confirm also that this could not have been the
mechanism.
Impulsive
Rockfall Impact Mechanism
The giant
wave runup of 1720 feet at the head of the Bay and the subsequent
huge wave along the main body of Lituya Bay were caused primarily
by the enormous subaerial rockfall into Gilbert Inlet (Fig. 3).
The triggering mechanism of this rockfall and the effects that
it produced were significantly different from those of subaerial
or submarine landslides. This was not a gradual process as with
a landslide, but a very sudden event. The giant rockfall was
triggered impulsively. Thus, the term rockfall rather than rockslide
or landslide, is used to distinguish this particular type of
phenomenon and to explain the subsequent effects of its impulsive
impact. In some respects, corrected for scale factors of mass,
terminal velocity and angle of entry, the impact of this rockfall
into Gilbert Inlet could be considered analogous to that of an
asteroid falling on earth. To explain the impulsive mechanism
of wave generation from such impact we must first examine the
time history of events immediately following the onset of the
earthquake and the intense ground motions and accelerations that
triggered the rockfall.
Strong Ground Motions
Little is
known about the ground motions in the immediate area at the head
of the Bay. There were no strong motion recordings of this event.
However, because of the proximity of the upper Lituya Bay to
the epicenter and because of the geometric orientation with the
Fairweather fault, the surface waves and the strong ground motions
begun almost immediately after the onset of the earthquake. For
an earthquake of this magnitude, it would be expected that the
strong ground motions lasted anywhere from 40-60 seconds or even
90 seconds, perhaps with some interruption, but probably peaking
at about 20-25 seconds after the beginning of the quake.
Detailed
map of head of Lituya Bay, showing site of the rockfall, landslides,
changes in the shoreline (heavy dotted line), and extent of wave
inundation (light dotted line) from the 1958 earthquake and the
giant wave it triggered. Lighter barred line depicts shoreline
just prior to the earthquake and wave (Modified after Miller,
1960)
Intensities and Accelerations
 The ground motions
associated with the earthquake were of very high intensities.
Eyewitness accounts confirm it. Survivor Swanson situated on
a boat anchored near La Chaussee Spit close to the bay entrance,
reported seeing the whole Lituya Glacier moving up and down.
This may have been an optical illusion as the Lituya Glacier
was out of his line of sight. However what he probably observed
could have been happening on the other side of Gilbert inlet
where a giant rockfall was triggered, or could have been ice
going over the spur on the southwest wall of the inlet when the
1720 foot splash occurred.
An isosmeismal map of the U.S. Geological Survey indicates a
distribution of high earthquake intensities from which we can
infer very strong ground motions during the earthquake (Fig.
4). Maximum intensity of XI was reported in the main part of
the Bay, although closer to the fault, at the head of the Bay,
an intensity of XII is very possible. Earthquake ground motions
of such high intensity (XI, XII on the Modified Mercalli scale)
could have resulted in vertical accelerations of up to 0.75g
and horizontal accelerations of as much as 2.0g. Such ground
accelerations would have caused the movement of ice observed
by Swanson.
Isosmeismal
map of the earthquake of July 9, 1958 showing distribution of
intensities from which very strong ground motions can be inferred
for Lituya Bay (Modified after a U.S. Geological Survey map).
In the absence of adequate data for the Lituya Bay event to support
these assumptions, analogies can be drawn from recorded recent
large earthquakes elsewhere. For example, such high horizontal
and vertical accelerations were associated with the 17 January
1994, Northridge earthquake in California. This earthquake, although
of moderate 6.7 magnitude, produced vertical accelerations of
as much as .75 g, horizontal accelerations of 2.0 g. and caused
extreme and unexpected damage in San Fernando Valley (Fig. 5).
The Northridge earthquake occurred along the White Wolf fault
in the Transverse Ranges north of Los Angeles which, in contrast
to other segments of the San Andreas fault system, is characterized
primarily by transform faulting, similar to what occurs along
the Fairweather fault.
Scenario
and Time History of Events
The 8.3 magnitude earthquake of July 9, 1958 in Lituya Bay
was associated with ground motions of high intensity which, as
with the Northridge earthquake, could have resulted in very high
ground accelerations near the head of the Bay. Such strong motions
and accelerations must have been present to trigger the extreme
events which subsequently occurred, almost immediately following
the earthquake. Eyewitness accounts and subsequent measurements
support the following scenario of events and impulsive rockfall
impact mechanism.
Beginning at about 10:16 p.m. on July 9, 1958, within 15-20 seconds
following the onset of the earthquake, the southwest side and
probably most of the bottom of Gilbert and Crillon Inlets begun
to move northwestward and up relative to the northeast shore
at the head of Lituya Bay, on the opposite side of the Fairweather
fault. Because of the proximity to the epicenter and to the fault,
strong ground motions peaked within 25-30 seconds. Within 50
to 60 seconds, net tectonic displacements had pushed the entire
inlet and its extensions along the Crillon and Lituya Glaciers
by 3.5 feet upward and 21 feet in the horizontal plane, tilting
the entire Bay in a seaward direction. These tectonic displacements
are supported by observations of the surface breakage along the
Fairweather fault 6 to 10 miles southeast of Crillon Inlet (Tocher
and Miller, 1959).
Intense shaking in Lituya Bay continued for at least 1 minute
according to the account of Swanson, and possibly as much as
4 minutes according to Ulrich. However, it is doubtful that the
earthquake shaking could have lasted as long as 4 minutes as
Ulrich reported.
During the first 50-60 seconds, the tectonic displacements, in
combination with the stronger ground motions and high vertical
and horizontal accelerations of surface seismic waves, weakened
a large slab of rock on the precipitous northeast shore at the
head of Lituya Bay. Both Ulrich's and Swanson's accounts, indicate
almost certainly that the rockfall was triggered by the earthquake.
According to eyewitness Ulrich, a deafening crash, resembling
an explosion, was heard at the head of the bay approximately
2.5 minutes after the earthquake was first felt. He also reported
that the wave definitely started in Gilbert Inlet, just before
the end of the quake. According to him the water did not go up
to the 1,720 foot elevation, but splashed to that elevation.
However, the timing of the explosion sound and the appearance
of the wave are somewhat inconsistent in his account. As it was
indicated above, for an earthquake of that size, the ground motions
would not have lasted more than 60-90 seconds. A wave would not
have appeared before the explosion sound. The other eyewitness,
Bill Swanson, reported seeing the glacier riding high into sight
from behind the western mountain, followed by a great wave of
water washing over its steep face.
In spite of some uncertainty in the chronology of events, the
accounts support the following conclusions: No less than 50-60
seconds and no more than 150 seconds after the earthquake begun,
a large mass of rock material along the very steep mountain side
on the northeast side of Gilbert Inlet at the head of Lituya
Bay, on the other side of the Fairweather fault, cleaved and
ruptured. The giant rock mass had more than 40 million cubic
yards of material and extended as high as 3,000 feet, with a
center of gravity at about 2,000 feet above sea level. Driven
by gravity force of almost 1g, this rock mass plunged practically
as a monolithic unit into Gilbert Inlet at a very steep angle
of perhaps as much as 75-80 degrees, as the sides of the Bay
were truly precipitous. The rockfall left a giant scar on the
mountain. The impact of the large rockfall on the surface of
the water was the explosion-like sound heard by Ulrich. The impact
of this mass of rock, not only displaced with great force the
water but struck the bottom of Gilbert inlet and created a large
radial crater, displacing and folding an equivalent volume of
recent glacial sediments and deeper semi consolidated Tertiary
layers, to an arcual distance estimated to be least 800 feet
out from the front of the precipitous shore.
U.S.G.S
photograph showing an aerial view of Gilbert Inlet taken after
the earthquake of July 9, 1958, showing the Lituya Glacier,and
the effect of the giant wave runup of 1720 feet at the southeastern
spur in clearing all trees and vegetation.
 The sudden rockfall impact, the displaced water,
and the folding of the bottom sediments, in combination with
the dynamic ground motions, sheared 1300 feet of ice from the
entire Lituya Glacier front, leaving a vertical wall of ice almost
normal to the trend of Gilbert inlet. Also, the rockfall impact
generated a non-linear wave existing entirely above the initial
undisturbed water surface, which splashed as a sheet of water
to the 1720 foot elevation on the other side of Gilbert inlet,
three times the water depth.
The rockfall impact, with some contribution from the net vertical
crustal uplift of about 3.5 feet, and from the overall tilting
seaward of the entire crustal block on which Lituya Bay was situated,
generated a solitary gravity wave. This huge wave originated
in Gilbert inlet and propagated outward the head of the Bay where
its height was estimated at 100 feet or even much greater by
Ulrich. Because of its point origin and initial orientation the
wave moved in a southerly direction striking first against the
steep cliffs on the south side of the main bay in the vicinity
of Mudslide Creek where maximum runup occurred. Then the wave
reflected and refracted toward the north shore into the main
portion of Lituya Bay, and again back to the south shore near
the vicinity of Coal Creek. Time estimates by eyewitnesses Ulrich
and Swanson of the time elapsed from the first sighting of the
wave at the head of the bay until it reached their boats, indicate
that the wave must have been traveling at an average speed ranging
between 97 and 130 miles per hour, at least in the deeper portion
of the bay south of Cenotaph Island.
 Mathematical Modeling - Navier-Stokes
Verification of the Lituya Bay Impulsive Rockfall Source Mechanism
- Asteroid Model Validation
An analytical
solution of this impulsive rockfall mechanism can further support
the 1720-foot runup at the spur of Gilbert inlet and the giant
wave in Lituya Bay. Preliminary modeling by Dr. Charles Mader
shows that there is a sufficient volume and an adequately deep
layer of water to account for the giant wave runup and the subsequent
inundation. Dr. Mader suggested full Navier-Stokes modeling,
as with asteroid generated tsunami waves.
Because of the similarity of wave generation to that of asteroid
impact, full Navier-Stokes modeling of this impulsive rockfall
mechanism may be useful also in the validation of the asteroid
model. With proper scale corrections, analogies can be drawn
between the impulsive impact of the Lituya Bay rockfall to asteroid
impact on ocean floor sediments and on wave generation. Although,
the trajectory angle, terminal velocity and total mass and density
of material of an asteroid would be significantly different,
these can be scaled and adjusted for the purpose of validating
the model. For example, an asteroid would be expected to approach
the earth at a much lower angle of perhaps only 15 degrees from
horizontal and may impact the ocean with a terminal speed which
may be 20 km/second or more. Because of differences in mass,
trajectory angle, and speed at impact, the effects on the ocean
floor will be markedly different, but these too could be scaled.
For example, even a small asteroid of perhaps the same dimensions
and mass would be expected to disturb the ocean sediments to
a far greater extent than the gravity driven rockfall of Lituya
Bay. A small asteroid of only 1/3 mile in diameter falling in
the ocean at 20 km /second at a low angle of entry, would be
expected to carve a path of at least twelve miles on the ocean
floor and to create a much larger cavity which would be cylindrical
rather than radial. Horizontal and vertical accelerations of
seismic waves from asteroid impact may be much greater. However,
because of the lower trajectory angle of entry, wave generation
and splashing action to a nearby shoreline will not be as great
as that caused by the Lituya Bay rockfall. Also, a hard basalt
ocean bottom with a thin layer of sediment may not cause the
same effect as the Lituya rockfall on softer sediment layers.
Yet, in spite of differences, analogies could be drawn. Known
input and wave runup output parameters of the rockfall can be
used, first to calibrate the Lituya Bay model, then to validate
the asteroid model.
Wave generation based on simulating the time history, large energy
content, and other input parameters of the Lituya Bay rockfall,
corrected for scale factors of volume, trajectory path, terminal
impact velocity, water depth and energy imparted to the water
body, can provide meaningful initial conditions to determine
and separate the nonlinear portion from the mathematical solutions
which use the Navier-Stokes equations to describe the gravity
wave portion of an asteroid-generated tsunami - at least in its
propagative phase, following impact, as it travels in the ocean.
Additionally, since the incompressible Navier-Stokes equations
are used to describe tsunami propagation in deep water following
the impact of an asteroid on the ocean, and since these equations
have limited direct application in shallow water and no application
at all when turbulent, chaotic processes are encountered, the
Lituya Bay rockfall and its subsequent wave generation can be
used to further refine, calibrate and validate a model where
turbulent flow and friction are significant factors in determining
the extent of inundation. For example, based on the measured
parameters of inundation, speed, and water particle velocities
of the giant 1958 Lituya Bay waves, coefficients of friction
can be derived empirically. These coefficients can be used to
estimate more realistically wave attenuation over a land mass,
of an asteroid-generated tsunami as it travels chaotically past
the sea-land boundary.
SUMMARY
AND CONCLUSIONS
The giant
wave runup of 1720 feet at the head of the Bay and the subsequent
huge wave along the main body of Lituya Bay which occurred on
July 9, 1958, were caused primarily by an enormous subaerial
rockfall into Gilbert Inlet at the head of Lituya Bay, triggered
by dynamic earthquake ground motions. The large mass of rock,
acting as a monolith and thus resembling an asteroid, impacted
with great force the bottom of the inlet. The impact created
a crater which displaced and folded recent and Tertiary deposits
and sedimentary layers. The displaced water and the folding of
sediments broke and uplifted 1300 feet of ice along the entire
front of the Lituya Glacier. Also, the impact resulted in water
splashing action that reached the 1720 foot elevation on the
other side of the inlet. The same rockfall impact, in combination
with strong ground movements, the net vertical crustal uplift
of about 3.5 feet, and an overall tilting seaward of the entire
crustal block on which Lituya Bay was situated, generated the
giant solitary gravity wave which swept the main body of the
bay.
Mathematical modeling studies conducted by Dr. Charles Mader,
support this mechanism as there is a sufficient volume and an
adequately deep layer of water in the Lituya Bay inlet to account
for the giant wave runup and subsequent inundation. Because of
the similarity to asteroid generated tsunami waves, full Navier-Stokes
modeling, as suggested by Dr. Mader, could further verify this
impulsive rockfall mechanism. Measurable output parameters derived
from mathematical modeling and analysis of the Lituya Bay event,
adjusted for scale, can be applied to the calibration, verification
and validation of asteroid models of tsunami generation. Based
on measured parameters of inundation, speed, and water particle
velocities of the giant 1958 Lituya Bay waves, coefficients of
friction can be derived empirically which may be used to estimate
more realistically attenuation over a land mass, of an asteroid-generated
tsunami as it travels chaotically past the sea-land boundary.
REFERENCES
Brazee &
Cloud, 1960, "U.S.
Earthquakes 1958", U.S. Dept. of Com. Coast & Geodetic Survey
76 pp.
Mader Charles L., 1999, "Modeling the 1958 Lituya Bay Mega-Tsunami" (Personal Communication)
Miller Don J., 1960, "Giant
Waves in Lituya Bay, Alaska" Geological Survey Professional Paper
354-C, U.S. Government Printing Office, Washington (1960)
Miller Don J., 1954, "Cataclysmic
Flood Waves in Lituya Bay, Alaska", Bull. Geol. Soc. Am. 65, 1346
Prins, J.E., 1958a, "Characteristics
of waves generated by a local disturbance", Am. Geophys. Union Trans., v. 39,
p. 865-874.
Prins, J.E. 1958b, "
Water waves due to a local disturbance", Proc. 6th Conf,. Coastal Engineering,
Council Wave Research, Eng. Found., Berkeley,Calif., p. 147-162.
Tocher, Don, and Miller, D.J, 1959, "Field observations on effects
of Alaska earthquake of 10 July, 1958", Science, v. 129, no. 3346, p. 394-395.
U.S. Coast and Geodetic Survey, 1942, Chart 8505, Lituya Bay.
U.S. Coast and Geodetic Survey, 1957, "Tide Tables, West coast of North
and South America", 1958, p.120, 182.
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