THE
GREAT ALASKA EARTHQUAKE AND TSUNAMI OF MARCH 28, 1964
Source
Mechanism Study
George
Pararas-Carayannis
Excerpts from studies of the
earthquake and tsunami undertaken under contract with the National
Science Foundation under the U S - Japan program for cooperative
research in the Pacific and for the U.S. Navy, Office of Naval
Research - published in the University of Hawaii Institute of
Geophysics Reports, in Pacific Science, and in the Volume on
Seismology and Geodesy on the Great Alaska Earthquake of 1964,
by the National Academy of Sciences).
 (
ABSTRACT
The greatest earthquake
of the 20th century in the northern hemisphere occurred in the
Prince William Sound of Alaska on March 28, 1964. It was felt
over a large area of Alaska and in parts of Western Yukon Territory
and British Columbia, Canada. Earthquake - induced landslides
triggered numerous local destructive tsunamis within the Prince
William Sound. A Pacific-wide tsunami was generated by the earthquake
in the Gulf of Alaska. The following is a brief summary of the
geologic history and general geomorphology of the area affected
by the great earthquake and of the source mechanism of the major
tsunami that was generated in the Gulf of Alaska. A brief discussion
is provided on the volume of crustal displacements, on the dimensions
of the tsunami-generating area - as estimated by an inverse tsunami
wave refraction method - and on the estimates of earthquake and
tsunami energies. The near and far field effects of the Pacific-wide
tsunami are briefly discussed. Additional links to further studies
are provided.
Introduction
On March 28, 1964,
(UTC date, March 27, local date) the largest earthquake of the
20th century in the northern hemisphere, occurred in the Prince
William Sound and the Gulf of Alaska. Earthquake- induced landslides
triggered numerous local destructive tsunamis within the Prince
William Sound area. Large scale crustal movements in the Gulf
of Alaska generated a Pacific-wide tsunami that devasted many
towns along the Gulf of Alaska and caused serious damage at Alberni
and Port Alberni, Canada, along the West Coast of the United
States, in California (Crescent City
mainly), and in the
Hawaiian Islands.
Earthquake
Origin Time, Epicenter, Magnitude, Focal Depth, Aftershocks and
Felt Motions
 Origin Time -
3/28/64 03:36:14.0 GMT (3/27/64, 05:36:14.0 p.m., local time)
Epicenter - Lat. 61.04 N. - Long. 147.73
W., near the east shore of Unakwik Inlet in northern Prince William
Sound, about 10 km east of the mouth of College Fiord, about
90 km west of Valdez, and about 120 km east of Anchorage.
Magnitude - The Surface-wave magnitude was
8.4 on the Richter scale, which was greater than the 1906 San
Francisco earthquake (8.3). A Moment Magnitude of 9.2 was later
assigned to this event, which made it the second largest earthquake
recorded in the 20th Century. The largest was the 9.5 earthquake
in Chile in 1960.
Focal Depth - Shallow (23-25 km.)
Duration of rupture
- It took approximately
4 minutes (240 sec.) for the earthquake's rupture to propagate
a distance of about 800 km.
Aftershocks - In the first day of the main
earthquake there were 11 aftershocks with magnitude greater than
6.0 on the Richter scale. The largest of these had a magnitude
of 6.7. In the next three weeks there were 52 more aftershocks,
9 of which were large. Thousands more smaller aftershocks were
recorded in the months following the main event. The aftershocks
continued for more than a year. They extended in an area from
about 15 km north of Valdez to about 55 km. south of Trinity
Islands and were heavily concentrated on the northeast and the
southwest of the uplifted region (USC&GS, 1964), The aftershock
zone was about 250 km wide and extended about 800 km. - closely
corresponding to the tsunami generating region.
Felt Motions
The earthquake ruptured
the earth's crust from near Valdez to south of Kodiak Island
in the Gulf of Alaska. The ground motions lasted for about four
minutes.The strong motions triggered many landslides and avalanches
which in turn generated many destructive local tsunamis within
Prince William Sound. The earthquake was felt over an area of
about 1,300,000 km2 extending from Alaska to parts of western
Yukon Territory and British Columbia, Canada, and as far south
as the State of Washington. Long period seismic waves traveled
around the earth for several weeks. Basically the whole earth
vibrated like a church bell. Vertical motions of up to 5 to 10
cm were reported from as far away as Texas and Florida. There
were reports of water level fluctuations at distant wells and
lakes.
Effects
of the Earthquake and of the Tsunami
Deaths and
Damage
Major structural damage
occurred in many of the major cities of Alaska. Damage was heavy
in many towns, including Anchorage, Valdez, Whittier, Seward,
Seldovia, Homer, Chitina, Moose Pass, Portage, Sterling, Wasilla,
Glennallen, Hope, Kasilof, Kenai and Kodiak. However, the death
toll from both the tsunami and the earthquake were extremely
small considering the magnitude and extent of the area affected.
The reason was Alaska's low population density and the fact that
the earthquake occurred on a holiday (Good Friday). Also, most
of the homes and buildings in Alaska were of light wood construction.
The earthquake and
the tsunamis were responsible for the total loss of 125 lives
(tsunami 110, earthquake 15 in Alaska; 16 from tsunami in Oregon
and California), and for about $311 million (1964 dollars) in
property damage. Much of the damage and most of the lives lost
were due to the effects of tsunami waves.
Anchorage: At
Anchorage, which was about 120 kilometers northwest of the epicenter,
there was severe damage to property. About 30 blocks of homes
and commercial buildings in the downtown area were severely damaged
or destroyed. Among them were: the J.C. Penny Company building
(which was damaged beyond repair), the Four Seasons apartment
building, a new six-story structure (which collapsed). Many other
multistory buildings were damaged heavily. The schools in the
Anchorage were almost devastated. The Government Hill Grade School
was almost a total loss. Anchorage High School and Denali Grade
School were damaged severely.
Landslides and ground
subsidence were responsible for the heavy damage. Huge landslides
occurred in the downtown business section, and at Government
Hill area. The largest of the landslides occurred at Turnagain
Heights where an area of about 130 acres was completely devastated.
About 75 homes were destroyed. Water mains, gas, sewer, telephone,
and electrical systems were disrupted throughout the area.
Near and
Far Field Tsunami and Seiche Effects
Tsunamis in Prince
William Sound
- The shallow continental
shelf and the islands bordering the southern side of Prince William
Sound, as well as the pattern of crustal displacements, confined
the waves generated in this area to the Sound itself; very little
energy escaped this closed region. Most of the energy was expended
in the narrow, deep fjords of the Sound, creating catastrophic
waves and setting up resonating oscillations and surges that
lasted for hours.
( Aerial photo of
Valdez, Alaska, showing extent of inundation).
In certain places
maximum inundation occurred five or six hours later, at high
tide. At Valdez, for example, the third wave came in at 2300,
March 27, and the fourth one at 0145, March 28 (Brown, 1964).
This last wave took the form of a tidal bore and inundated the
downtown section of Valdez, ruining almost all the merchandise
in the stores. These waves could not have come from the generating
area outside Prince William Sound because if this were so, it
would have taken them only 34 minutes to reach Valdez. It is
more likely, then, that the waves at Valdez arrived in resonance
at high tide, from the immediate area of Port Valdez.
 Maximum
positive crustal displacement in Prince William Sound occurred
along the northwest coast of Montague Island and in the area
offshore. These earth movements caused a gradient in hydrostatic
level and the resulting short-period wave raced through Knight
Island Passage within 10 minutes and on toward Chenega Island,
inundating the village of Chenega to an elevation of 15.5 m and
completely destroying it. This same wave continued north through
Knight Island Passage and inundated Perry and Naked Islands,
but to lesser heights (Berg et al,, in prep.).
(Aerial view of Seward,
Alaska showing extent of inundation and damage to oil storage
tank ).
Bathymetric surveys
by the USC&GS (1964) in the area off Montague Island and
at the north end of Latouche Island revealed a number of large
submarine slides. It is possible therefore, that the combination
of submarine slides and the tilting of the ocean floor due to
uplift created the solitary wave reported at Chenega village
and at Perry and Naked Islands.
A second wave about
40 meters high (125 feet) was reported coming out of the Valdez
Narrows and spreading across the Sound (Plafker and Mayo, 1965).
The maximum tsunami wave height recorded was 67 meters at Valdez
Inlet. This wave was caused by slumping of the glacial deltas
in Port Valdez which had been shaken loose by the force of the
earthquake .
The effects of local
tsunamis in the Prince William Sound area and of the Pacific
wide tsunami generated in the Gulf of Alaska are further discussed
separately (see links below).
There were other far
field effects caused by seiche action in rivers, lakes, bayous,
and protected harbors and waterways along the Gulf Coast of Louisiana
and Texas and minor damage was reported. Waves were was also
recorded on tide gages in Cuba and Puerto Rico. These waves were
caused by strong surface seismic waves traveling around the earth,
rather by the tsunami generated in the Gulf of Alaska.
Tectonic
Setting of the Eastern Segment of the Aleutian Trench and Arc
 The Aleutian Island Arc and the
Aleutian Trench extend for 2,800 km from Kamchatka to south-central
Alaska along remarkably smooth curves which are convex toward
the south (Fig. 1). The Arc forms the Alaska Peninsula, and according
to Wilson (1954), intersects north of Cook Inlet a second tectonic
arc that extends northward from the vicinity of the Wrangell
Mountains. However, Plafker (1965) regards this second segment
as a continuation of the Aleutian Arc.
Where the trench impinges
on Alaska, it loses its identity, although an offshore range
of seamounts suggests that it may have once extended around to
the south, to parallel the continental slope as postulated by
Menard and Dietz (l951). Concavity in the former shape of the
trench on its eastern segment is also suggested by the sedimentary
arc defined by Wilson (1954) - which embraces Kodiak Island and
the Kenai Peninsula. As indicated by Wilson, such concavity is
to be expected where two arcs meet at an acute angle, as is well
exemplified where the Aleutian end Kuril-Kamchatha arcs intersect.
It is also quite possible that large horizontal movements of
crustal blocks have helped change the shape of the Aleutian Trench
and Arc on their eastern segments. However no such evidence was
found in a field study that followed the Good Friday earthquake
(Berg et al., personal communication).
 The nature of termination of
the eastern segment of the Aleutian Trench is obscured by thick
sediments washed in from the continental shelf against which
it abuts offshore from Cape Suckling. The sediments are of geosynclinal
dimensions in the sedimentary arc on Kodiak Island (Menard and
Dietz, 1951) and as shown by drilling on the Kenai Peninsula.
Woollard et al. (1960) show that there is geophysical evidence
for at least 7 km of sediments in Cook Inlet, a graben separating
the primary arc from the offshore sedimentary arc. Sediment is
about 2 km thick off Kodiak Island along the Aleutian Trench,
thinning out to about 0.7 km south of Unimak Island in the deep
water area, according to seismic measurements by Shor (1962).
The March 28, 1964
earthquake and its aftershocks occurred along a low-angle thrust
fault on the eastern segment of the subduction boundary of the
Pacific and North American plates, delineated by the Aleutian
Island Arc and the Aleutian Trench. The northwestward motion
of the Pacific plate in this segment is estimated to be about
5 to 7 cm per year. This motion causes the crust of southern
Alaska to be compressed and warped, with some areas along the
coast being depressed while other areas inland are being uplifted.
After periods of tens to hundreds of years, the compression which
is accomodated elastically, is relieved by the sudden southeastward
motion of portions of coastal Alaska.- thus restoring temporary
equilibrium in the region. The end result of the great earthquake
of March 28, 1964 was the sudden movement of the Pacific plate
under the North American plate by about 9 meters on the average.
SOURCE
MECHANISMOF THE MARCH 28, 1964 ALASKA TSUNAMI
The
Generating Area of the 1964 Tsunami in the Gulf of Alaska
The generating area
of the March 28, 1964 tsunami corresponded to the region of vertical
and lateral tectonic dislocations of the earthquake in the Gulf
of Alaska. Based on surveys of accessible areas on land, it was
established that the tectonic dislocations ranged over a distance
of 800 km from the upper portion of Prince William Sound to southwest
of the Trinity Islands (Van Dorn, 1964). However, the seaward
limits of the tectonic displacements could not be determined.
The present study was undertaken to define the seaward extent
of the earthquake's tectonic displacements from indirect ocenographic
analysis of the tsunami waves that it generated (using inverse
wave refraction) and the characteristic wave patterns recorded
at distant tide gauge stations. The tsunami refraction studies
described here, strongly indicated that the tsunami-generating
area was mainly in the belt of uplift and included a large segment
of the continental shelf and slope. This vertical deformation
affected an area of approximately 250,000 km2 (100,000 square
miles) - most of it under water.
 Extent of Area Affected
by Crustal Uplift and Subsidence - Extent of Tectonic Displacements.
The zone between the
known areas of tectonic uplift closely corresponded to a major
low-angle thrust fault defined by crustal seismic measurements
conducted by the Department of Terrestrial Magnetism of the Carnegie
Institution of Washington (Woollard et al., 1960). In view of
the shallowness of the earthquake (23 km), it was concluded that
the crustal dislocations occurred alongside a zone of tilting
or a surface rupture (Grantz et al., 1964). A survey of the area,
however, failed to identify such a feature. The focal depth correspond
though, to the base of the granitic layer defined by Woollard's
analysis of the Carnegie Institute crustal measurements.
Fig. 1 Generating
area of the Alaska tsunami. Crosshatched area indicates (-) area
of subsidence and (+) area of uplift. Heavy dashed lines indicate
the backwards refracted tsunami wave fronts. Solid line marked
by a zero is the axis of rotation (no elevation change) Other
solid lines are titled as tectonic axes.
Dipole Pattern
of Crustal Displacements
The study of tsunami
source mechanism required a closer examination of crustal displacements
on land and extrapolation of estimated displacements on the continental
shelf in the Gulf of Alaska. Also examined were the patterns
of uplift and subsidence which had been slowly developing prior
to the earthquake. When the earthquake struck, these patterns
were suddenly reversed.
Also, from the geologic
evidence on land areas, it was established that the dislocations
caused by the great earthquake followed a dipole pattern of positive
and negative displacements on either side of a zero-line (line
of no vertical change separating zones of uplift and subsidence).
This zero line intersected the east coast of Kodiak Island and
continued northeast to the western side of Prince William Sound.
There, changing direction, it ran east along the upper part of
the Sound. The zero line roughly paralleled the Aleutian Trench
axis and it separated the Kodiak geosyncline from the shelf geanticline.
As illustrated in
the map, the areas north and west of this zero line of displacement
underwent negative elevation changes (as a result of the earthquake),
whereas the areas east and south underwent positive changes.
Also, there were significant lateral movements. For example,
Latouche Island moved about 18 meters to the southeast.
Extent of
Crustal Uplift
The earthquake's vertical
uplift ranged up to 15 meters. This maximum uplift on land was
off the southwest end of Montague Island, where there was absolute
vertical displacement of about 13 - 15 meters (but suspected
to have been considerably more offshore). The major area of uplift
trended northeast from southern Kodiak Island to Prince William
Sound and trended east-west to the east of the Sound. Also, large
positive displacements were observed as far south as Middleton
Island and southwest to Sitkinak Island. Uplift occurred along
the extreme southeastern coasts of Kodiak and Sitkalidak islands
and part or all of Sitkinak Island. The estimated uplift of Sitkinak
Island was from 0.35 to o.65 m and possibly as much as 1.5 m
(according to Plafker, 1965). An extensive pattern of positive
surface dislocations under the sea was suspected to lie east
of the island of Kodiak and along the continental shelf bordering
the Gulf of Alaska.
In brief, the area
of uplift was estimated to cover about 105,000 km2 (sq. Km) and
extended from southern Kodiak Island northeast to Prince William
Sound. It included the southern and eastern parts of Prince William
Sound, the coastal area as far east as the Bering Glacier, the
continental shelf and part of the continental slope to a depth
contour of approximately 200 meters.
Extent of
Crustal Subsidence
The area that subsided
included the northern and western parts of Prince William Sound,
the western segment of the Chugach Mountains, portions of the
lowlands north of them, most of the Kenai Peninsula and almost
all of the Kodiak Island group. This area of subsidence is about
800 km long and 150 km wide. This zone of subsidence covered
about 285,000 square kilometers. However Plafker (1965) estimated
that the volume of crust that has been depressed below its pre-earthquake
level was about 115 km3 (cubic km). An average of about bout
2.3 meters of subsidence relative to sea level occurred throughout
this region.
Seaward
limits of Crustal Displacements
Geological investigations
defined the land areas affected by the earthquake. To the east,
the zone of deformation appeared to die out between the Bering
Glacier and Cape Yakataga. The northwestern limit of tectonic
changes extended at least to the west side of Shelikof Strait
and Cook Inlet (Plafker, 1965). The north inland limit was known
only along the highway connecting Valdez and Fairbanks; it appeared
to extend in a northeasterly direction to the vicinity of the
Wrangell Mountains, and quite possibly into the Alaska Range.
 Inverse Tsunami Wave
Refraction Used to Determine Seaward Limits of Tectonic Dislocations
and the Tsunami Generating Area
The seaward limits
of crustal displacements could not be determined by conventional
geologic surveys. Therefore, the seaward limits of the earthquake
and the tsunami-generating area were determined by means of a
series of wave refraction diagrams based on Snell's Law of Retraction
using the velocity equation for shallow water waves:
C (tsunami wave velocity)
= Square Root of gxd,
where "g"
is the earth's gravitational acceleration at that latitude,
and "d"
is the changing depth of the ocean .
Such a method of preparing
refraction diagrams has shown good results, especially if carried
out on large-scale charts with detailed bathymetry (Johnson,
O'Brien, and Isaacs, 1948 ).
In constructing the
refraction diagrams for the Alaska tsunami, the marigrams of
different tide gage stations around the Pacific were consulted
and the total travel tine of the first wave at each station was
determined.
Then refraction diagrams
were constructed toward the earthquake area from each tide gage
station in lengths of time equal to the calculated travel time
for that station.
It was assumed that
the last wave front in each refraction diagram would correspond
to a point on the boundary of the generating ares, and if enough
refracted wave fronts from different stations were plotted, an
envelope defining the tsunami-generating area could be drawn.
Tsunami
Wave Refraction Results
Wave fronts were refracted
from Yakatat, Cape Yakataga, Seward, Uzinki, Kodiak, Old Harbor,
Unalaska, Adak, Attu, and Honolulu. See diagram of wave fronts refracted towards
the earthquake area from Attu Island (dashed lines), Adak Island
(solid lines) and d Unalaska Island (dotted lines).
The last front of
each of the retracted waves is shown by a heavy dashed line in
Figure 1. The seaward boundary of the generating area is near
the 200-m depth contour which defines the edge of the continental
shelf. Maximum displacements of the ocean floor occurred along
the continental shelf, from an area southeast of Kodiak Island,
to an area close to Cape St. Elias south of the island of Kayak
(see Fig. 1). Geologic evidence, however,
has shown positive land displacements as far north as Cape Suckling
and as far east as the Bering Glacier. It is quite probable,
therefore, that the tsunami-generating area extended farther
to the northeast although waves generated in such shallow water
would reach tide gauges much later and their origin would not
be identifiable.
Unfortunately this
same wave retraction technique could not be used to define the
northern and western boundaries of the main tsunami-generating
area, because conditions in Prince William Sound and elsewhere
along the coast of Alaska were further complicated by local tsunamis,
oscillations, and surge. In addition, no tide gauge stations
were operating in the area, and personal accounts were conflicting
as to the arrival times of the different tsunami waves.
The northward limit
is assumed to be restricted by the land boundaries, and the western
limit to extend to the west side of Shelikof Strait and Cook
Inlet.
Corrections
for Earthquake Rupture Travel Time
In estimating the
travel time of the tsunami waves, corrections were made for the
delay at the island of Kodiak in the arrival of the ground motions
and rupture from Prince William Sound. These corrections ranged
from 1 minute to 6 minutes and were based on the fact that the
Navy Weather Central on the island of Kodiak listed the time
of the principal shock in Prince William Sound as 6 minutes later
than the time listed by the USC&GS. This would imply that
the wave front generated on the northeast side of the disturbance
area had a 6-minute head-start on the wave front generated southeast
of Kodiak Island.
Tsunami
Generating Area
Based on the tsunami
wave refraction study described here and the initial geologic
measurements of vertical displacements, the total area affected
by the Alaska earthquake of March, 28, 1964 was estimated to
be approximately 215,000 km2 (square kilometers). However, this
estimate was later revised to about 520,00 square kilometers,
when more extensive surveys were completed.This is the largest
area known to be associated with a single earthquake within historic
times.
The tsunami-generating
area covers an area 700 km long by 150 km wide, a total of about
out 105,000 km2 (sq. km). The volume of the uplifted crust along
the continental shelf is about out 96 km3 (cubic km). The energy
associated with the tsunami has been estimated by Van Dorn (1964)
to be on the order of 2.3 x 10 (raised to the 21 power) ergs.
This estimate is based on the source dimensions of an area 240
nautical miles by 100 nautical miles and an uplift of 1.8 m (6
feet) at the northeastern end of this area and zero at the southwestern
end. This estimate however, is considered low because the generating
area had dimensions that were much larger than those estimated
by Van Dorn.
Tsunami
Generating Area and Earthquake Afterschock Distribution 
According to Japanese
seismologists ( Iida, 1958), the generating area of tsunami waves
roughly corresponds to the distribution of the major aftershocks
. This appears to indeed be the case in the Gulf of Alaska.
The vast area of tectonic
movements indicates that wave crests were generated along one
or more line sources from the region of maximum uplift. Thus
the shores of the Kenai Peninsula were struck within 23 minutes
after the start of the earthquake, and those of Kodiak Island'
within 34 minutes.
Unfortunately the
violence of the earthquake left south-central Alaska without
a tide gage in operation. The only reliable record from the generating
area is the one that was obtained by personnel of the U. S. Navy
Fleet Weather Station at Kodiak: it is shown in adjacent Figure
( Diagram of wave
activity at Women's Bay, Kodiak Island - reconstructed from visual
observations made at Marginal Pier, Nyman Peninsula ).
This record has been
corrected by Berg et al. for the 1.7 m ((5.6-foot)) submergence
of the area as determined from tide gages of USC&GS (1964).
Outside the immediate
generating area., the record of Cape Yakataga, as constructed
from the personal account of C. R. Bilderback, a resident of
the area, is the next most reliable record. This record is the
only one obtained outside the immediate generating area that
shows an initial drop in the water level (Berg et al., in prep.).
Withdrawal of the water immediately following the earthquake
has been reported from Kayak, Middleton, and Hinchinbrook Islands,
as well as Rocky Bay and Nuka Bay, at the end of the Kenai Peninsula,
but these islands are inside the generating area. (See adjacent diagram of wave activity at Cape
Yakataga.)
Yakatat, a coastal
town 170 km southeast of Cape Yakataga, had a tide gage in operation,
and the marigram shows that a positive wave arrived first
Marigram of wave activity
at Yakutat.
It is quite possible
therefore that the first waves to arrive at Cape Yakataga had
a different origin from that of the first waves to arrive at
Yakatat. It could very well be that the Cape Yakataga waves traveled
aver the shallow portion of the shelf, whereas the Yakatat waves
came from the open ocean.
An interesting aspect
of these two records is that of the difference in amplitude and
period of the first waves to arrive at these two sites- which
also supports the hypothesis of different origin s.
Estimated
Energy of the Tsunami of 28 March 1964
Using the above source
dimensions, and assuming that the total energy was equal to the
potential energy of the uplifted volume of water, the total energy
for the tsunami in the Gulf of Alaska was calculated as follows:
Total Energy, Et =
1/6 pxgxh(rasised to the 2nd power)x A = 1/6 x(1.03)x (.980)x
(10raised to the 3rd power) x(lO raised to the 4rth power)x (1.83
raised to the 2nd power) x(1.5 x 10 raised to the 7th power)
x(7 x 10 raised to the 7th power) = 5.88 x 10 raised to the 21st
power (ergs)
where Et = Ep (potential
energy) = total energy
p = 1.03 gm/cm39(cubic)
= density of water
g = 980 cm/sec2(square)
= gravitational acceleration
h = height of crustal
displacement .
A = Area
1 erg = gr cm2(sq.)
sec (raised to the - 2 power)
The waves generated
in the Gulf of Alaska were of unusually long period on the order
of an hour or more. Their energy radiation was preferentially
directed towards the south east and this is why more damage was
done to the North American coast than anywhere else east or south
of the generating area. This preferential directivity of energy
radiation can be attributed to the orientation of the tectonic
displacements along the continental shelf of the Gulf of Alaska,
and the long peiod ot the waves can be related to the long seiche
period of the shallow shelf.
TSUNAMI
MECHANISM
Most tsunamis result
from earthquakes having focal depths of less than 60 kilometers.
Iida (1958) has derived an empirical relation giving the maximum
focal depth H (in km) for an earthquake of magnitude M which
has resulted in a detectable tsunami:
M- 6.42 + 0.01 H (1)
where M is the Richter
magnitude given by
log E(ergs) - 11.8
+ 1.5 M (2)
The focal depth of
the Alaska earthquake was about 20 kilometers . This was shallow
enough to create tsunami waves even though the epicenter of the
main shock was as much as 100 km inland from the coast. A number
of shallower aftershocks over a large area ranging from Hinchinbrook
Island to Southeast Kodiak indicate that crustal movements over
a wide area were involved. Undoubtedly these shallow aftershocks
created smaller waves that could not be separated out, in the
tide gage records, from reflections of the initial tsunami.
If the tsunami waves
that hit the island of Kodiak were only the result of crustal
movements, then the first wave could be expected to be the highest,
at least within the generating area. At Uzinki, Kodiak City,
Women's Bay and elsewhere on the island of Kodiak, however, the
third and fourth waves were the highest. A theory of generation
from a single pattern of crustal deformation is therefore not
satisfactory here. Such factors as reflection from coastal boundaries,
wave interaction, and resonance should be taken into consideration.
Slumps or avalanches,
similar to the one a that occurred in Prince William Sound, are
usually localized; no large tsunamis can be produced that would
travel across wide portions of the ocean. According to Wiegel
(1954) not more than 2 per cent of the potential energy of a
falling or sliding body is converted into wave energy. In Prince
William Sound, however, slumping and sliding when added to tectonic
movements created tsunami waves of very large energy, but their
effect was catastrophic only locally; very little of the energy
escaped the Sound.
SUMMARY
AND CONCLUSIONS
The Alaska earthquake
of 27 March 1964 affected an area of approximately 215,000 km2
(sq. km), extending from the Wrangell Mountains at the northeast
to the Trinity Islands to the southwest and from the west side
of Shelikof Strait and Cook Inlet east to the vicinity of Bering
Glacier.
Geologic evidence
has revealed a dipole pattern of positive and negative tectonic
movements resulting from this earthquake. The area of subsidence
covers approximately 110,000 km2 (sq. km) end the volume of crust
that has been depressed below its pre-earthquake level is about
115 km3 (cubic km).
The area of uplift
covers about 105,000 km2 (sq. km) and includes the southern and
eastern parts of Prince William Sound, the coastal area as far
east as the Bering Glacier, and a great part of the continental
shelf and slope bordering the Gulf of Alaska.
The seaward limits
of the area affected by the Alaska earthquake and the tsunami-generating
area were determined by means of a series of wave refraction
diagrams as shown in Figure 5, based on Snell's Law of Refraction.
The tsunami-generating area covers140,000 km2 (sq. Km) and includes
the whole of the region of uplift and part of the region of subsidence.
It extends from the Trinity Islands to the Bering Glacier and
includes Shelikof Strait, Cook Inlet and the continental shelf
bordering the Gulf of Alaska to a depth of approximately 200
meters. The total volume of displaced material in the tsunami-generating
area was estimated to be 120 km3 (cubic km), and the energy associated
with the tsunami was calculated to be in the order 6x10(raised
to the 21st power) ergs.
As a result of this
work the following conclusions are drawn:
1. Two main tsunami-generating
areas can be distinguished: one along the continental shelf bordering
the Gulf of Alaska; the other, in Prince William Sound.
2. The main generating
area in the Gulf of Alaska roughly corresponds to the geographic
distribution of the major aftershocks.
3. The energy of the
tsunamis generated in Prince William Sound was expended inside
the Sound:, not much energy escaped this closed region.
4. The long period
of the waves generated in the Gulf of Alaska is related to the
long seiche period of the shallow shelf.
5. The preferential
radiation of energy towards the southeast is attributed to the
orientation of the tectonic displacements along the continental
shelf of the Gulf of Alaska.
6. The wares arriving
at Cape Yakataga had their origin in the shallow coastal area
near Bering Glacier, whereas the waves arriving at Yakatat traveled
through the deeper waters.
7. In Prince William
Sound two major tsunamis were distinguished: one had its origin
near the west coast of Montague Island the other originated in
the Port of Valdez.
8. Two types of tsunami
generation mechanisms were associated with the Alaska earthquake:
(a) waves generated
directly by tectonic movements of the sea floor , and
(b) waves generated
indirectly from landslides, mud flows, and slumping of alluvial
deposits.
9. In Prince William
Sound both generation mechanisms were evident, while in the generating
area along the Gulf of Alaska the generated tsunami was the direct
result of tectonic movements.
Acknowledgements
The initial work on
was supported in part by the National Science Foundation under
the U. S. -Japan program for cooperative research in the Pacific
through grant No. GF-153, and in part by the Office of Naval
Research through contract Nonr-3748(03). For this support I am
grateful.
I am particularly
indebted to professors D. C. Cox, W. M. Adams, and G. P. Woollard
for their advice and constructive criticism.
Also, I would also
like to acknowledge with appreciation the advice, comments. and
suggestions given to me by Professors A. S. Furumoto, K. Kajiura,
a. W.. Groves, H. G. Loomis, a. R. Miller and Robert Harvey.
I also thank the members
of the U. S..-Japan Cooperative Field Survey ( E. Berg, D. C.
Cox,, A. S. Furumoto K. Kajiura, H. Kawasumi, arid E. Shima)
for permission to use data from their initial report prior to
publication.
REFERENCES
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Pararas-Carayannis,
George. A Study of
the Source Mechanism of the Alaska Earthquake and Tsunami of
March 27, 1964, Water Waves.
in Contributions of the H.I.G. University of Hawaii for the Year
1967. Honolulu: s.n., 1967, pp. 237.cont. No. IR4
Pararas-Carayannis,
George. Source Mechanism
Study of the Alaska Earthquake and Tsunami of 27 March 1964,
The Water Waves.
Pacific Science. Vol. XXI, No. 3, July 1967.
Pararas-Carayannis,
George. Catalog of
Tsunami in the Hawaiian Islands.
World Data Center A- Tsunami U.S. Dept. of Commerce Environmental
Science Service Administration Coast and Geodetic Survey, May
1969.
Pararas-Carayannis,
George, and D.C. Cox. A
Catalog of Tsunamis in Alaska.
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Pararas-Carayannis,
George, "Source
Mechanism of the Water Waves Produced." Reprinted from Pacific Science, Vol. 21,
No. 3, "A Study
of the Source Mechanism of the Alaska Earthquake and Tsunami
of March 27, 1964."
Volume on Seismology and Geodesy on the Great Alaska Earthquake
of 1964, National Academy of Sciences, Washington D.C., pp 249-258,
1972.
ORIGINAL BIBLIOGRAPHY
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28 March 1964 in Alaska.
Hawaii Inst. Geophysics report series.
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Tsunamic activity
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Alaska. 20 pp.
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and R. Kachadoorian. 1964. Alaska's
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R. S., Dietz. 1951. Submarine
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R. Mayo. 1965. Tectonic
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U. S. Coast and Geodetic
Survey. 1964. Preliminary
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Source mechanism
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Laboratory studies
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REFERENCES - Prince
William Tsunami Waves
BERG, E., D.C. Cox,
A. Furumoto, K. Kajiura, H. Kawasumi, and E. Shima, Field
Survey of the Tsunami of 28 March 1964 in Alaska, Hawaii Inst. Geophys. Rpt.
BROWN, D.L., 1964. Tsunami
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ms., 20 pp.
COULTER, H. W. and
R. R. Migliaccio, 1966. Effects of the March 27, 1964
Earthquake at Valdez, Alaska,
U.S. Geol. Survey Prof. Paper 542-C, 36 pp.
COX, D.C. and Pararas-Carayannis,
George. A Catalog of Tsunamis in Alaska. World Data Center A- Tsunami
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Preliminary Catalog
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Whittler, Alaska,
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G., 1965. Source Mechanism Study of the Alaska Earthquake
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PARARAS-CARAYANNIS,
G., 1967. A Study of the Source Mechanism of the Alaska
Earthquake and Tsunami of March 27, 1964, Water Waves. in Contributions of the H.I.G.
University of Hawaii for the Year 1967. Honolulu: s.n., 1967,
pp. 237.cont. No. IR4
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and Tsunami of 27 March 1964, The Water Waves. Pacific Science. Vol. XXI, No. 3, July 1967.
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G. 1972. "Source
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of the Alaska Earthquake and Tsunami of March 27, 1964." Volume on Seismology and
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PLAEKER, G. and R.
Kachadoorian, 1966. Geologic Effects of the March
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U.S. Geol. Survey Prof. Paper 543-D, 46 pp.
PLAFKER, G. and L.R.
Mayo, 1965. Tectonic Deformation, Subaqueous Slides and Destructive
Waves Associated with Alaskan March 27, 1964 Earthquake: An Interim Geologic Evaluation,
U.S. Geol. Survey, Menlo Park, Calif., 21 pp
Stover Carl W. and
Jerry L. Coffman, 1993, Seismicity
of the United States, 1568-1989
(Revised), U.S. Geological Survey Professional Paper 1527, United
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VAN DORN, W.G., 1964. Source
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Coast and Geodetic Survey, No. 10-3, 263 pp.
See also:
The
March 27, 1964, Great Alaska Earthquake
Source
Mechanism of the March 27, 1964, Great Alaska Earthquake and
Tsunami
The
March 27, 1964 Tsunami in the Gulf of Alaska
The
March 27, 1964 Tsunami Waves in Prince William Sound, Alaska
The
Effects of the March 27, 1964 Alaska Tsunami in Canada
The
Effects of the March 27, 1964 Alaska Tsunami In California
The
Effects of the March 27, 1964 Alaska Tsunami in the Hawaiian
Islands
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