The Giant Lituya Bay Tsunami of July 9, 1958 (original) (raw)

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.

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. 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.

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)

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.

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)

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.

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.

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.

The Giant Lituya Bay Tsunami of July 9, 1958 (original) (raw)

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