Southcentral Alaska is part of a vast, continuous, seismically active belt that circumscribes the entire Pacific Ocean basin. This region is one of the most seismically active in North America, experiencing thousands of earth shocks each year. As shown in Figure 65, much of Southcentral Alaska falls within seismic zone 4, where structural damage caused by earthquakes is generally greatest. Alaska has a long history of recorded earthquakes dating back to 1788. Figure 66 lists quakes of approximate magnitudes of 5 or larger occurring in Southcentral Alaska from 1970 through 1973.
An earthquake is associated with faulting, which is rock fracturing and displacement. It is the shock that results when rock, distorted beyond its strength, finally ruptures and releases its stored up energy. Earthquakes can disrupt the equilibrium of surrounding rocks, triggering new faults and resultant shocks. The location at which rupture occurs within the earth is known as the focus, or hypocenter, and the point on the ground surface directly above the focus is the epicenter. Aftershocks commonly follow major earthquakes, occurring as rocks stabilize into new positions. Aftershocks generally have different epicenters than the primary shock. The major faults and fault systems of this region are shown on the geology maps, Figures 53a, b, and c.
On March 27, 1964, Southcentral Alaska experienced the many and widespread effects of this geologic phenomenon. This great earthquake and its mechanisms have been recorded in detail in a collection of articles published in 1970, The Great Alaska Earthquake of 1964, eight volumes by the National Academy of Science.
The March 1964 Earthquake
In the late afternoon of March 27, 1964, one of the greatest earthquakes in recorded history struck Southcentral Alaska. One hundred and fifteen lives were lost, and damage was estimated at 330 million dollars. The epicenter was near the head of College Fiord in Prince William Sound, at 61º 06' north latitude and 147º 44' west longitude, with a focus depth between 12 and 31 mi. (20 and 50 km.). The magnitude is believed to have been between 8.4 and 8.6 on the Richter scale. Plafker (1969) suggests that rupture occurred along a fault of considerable length rather than at one specific point because of (1) the abnormally long duration of strong ground motion, (2) the character of the waves generated, and (3) the extensive belt over which 589 recorded aftershocks were distributed (Figure 67). This great geologic event and resultant vibrations manifested themselves through widespread vertical and horizontal displacements, surface faults, ground cracks and fissures, sediment compaction, landslides and subaqueous slides, locally generated sea waves and tsunami. Of these, the tsunami took the most lives, and landslides caused the most damage.
Uplift and Subsidence: Crustal deformation associated with the 1964 earthquake was more extensive than any known deformation related to other quakes. Notable tectonic changes in land level occurred over an area of between 70,000 and 110,000 sq. mi. (180,000 and 285,000 sq. km.). Figure 67 shows the general distribution and direction of displacement in this area. The amount of subsidence averaged 2-1/2 ft. (0.75 m.), with a maximum subsidence of 7-1/2 ft. (2.25 m.) measured along the southwest coast of the Kenai Peninsula. Uplift measurements averaged 6 ft. (1.8 m.) over the broad area along the coast of the Gulf of Alaska, on the adjacent continental shelf, and on parts of the continental slope. The axis of the uplift zone runs along Montague Island, where uplift has been measured at 38 ft. (11.5 m.) and may be as much as 50 ft. (15.25 m.) on the sea floor. The area in and around Prince William Sound experienced vertical and horizontal distortion, which involved land shifts in a seaward direction of about 64 ft. (20 m.).
Landslides: The 1964 earthquake caused four different types of land movement. The most destructive was the translatory (lateral movement) landslides which occurred at Anchorage (Figure 68 ). Following a drastic loss of strength in an already weak layer of sensitive clay, the Bootlegger Cove Clay, large tracts of land slid nearly horizontally seaward, causing extensive damage to the business district (4th Avenue) and to two residential areas (Turnagain and L Street areas).
A second type of land movement resulted in extensive submarine landslides which occurred in thick, steep-fronted sedimentary fills (Valdez Figure 69), and on man-made fill along the coast (Seward). These slides also produced a number of highly destructive waves experienced locally in coastal areas during and immediately after the earthquake. These waves did great damage to the communities of Seward and Valdez. In Valdez, it is estimated that the massive submarine landslide that destroyed the whole waterfront involved approximately 98 million cu. yds. (74.9 million cu. m.) of materials (Coulter and Migliaccio 1966).
Rockslides (Figure 70 ), from cliffs undergoing glacial erosion, of huge volumes of unstable rock was a third type of land movement. These materials moved at high velocities down-slope and spread over large parts of outwash plains. Unstable banks of ice and snow triggered numerous avalanches, but in the case of both the rockslides and the avalanches, little damage was done due to their occurrence in remote, upland regions.
In addition to these three types of movement, there was a fourth—horizontal movement of unconsolidated water-saturated soil toward a topographic depression. This phenomenon was responsible for the majority of the ground cracks and for damage to highway and railroad systems in Alaska. The actual process is landspreading on a flat surface toward a free topographic face such as a ditch or a river valley (McCulloch and Bonilla 1970).
The 1964 earthquake also caused damage through sediment compaction. In lowland areas where unstable materials such as silt and clay or man-made fill were present, differential compaction caused collapse in foundations and damage to utilities. Seismic conditions and soil stability are vital considerations in construction planning throughout central Alaska.
Seismic Sea Waves (Tsunami)
Secondary damage effects of the earthquake caused by a tsunami (a train of long waves impulsively generated) reached far beyond Alaska. The tsunami originated on the continental shelf in the Gulf of Alaska and spread rapidly across the Pacific Ocean. Studies indicate that more than one tsunami was associated with the earthquake. These waves were caused by the sudden displacement of water in the Gulf, accompanying the uplift of thousands of square miles of sea floor. The tsunami, which can travel at speeds in excess of 400 mph (645 kph) in open ocean, struck with destructive force all along the Alaskan coast between Cordova and the southern tip of Kodiak Island (Figure 71) and was recorded as far away as Antarctic Peninsula (4-foot maximum), Japan (1-foot waves), and Hilo, Hawaii (7-foot waves). The waves radiated southeastward from their source (Figure 72); wave heights were greater, therefore, along the western coast of North America than they were in the Aleutian Islands.
Over 90 percent of the deaths attributed to the 1964 earthquake were traceable to waves. In addition to the 82 deaths in Alaska associated with tsunami, waves caused the drownings of four people in Oregon and 12 in California. Tsunami waves did extensive damage to Crescent City, California; special topographic features offshore amplified the waves as they moved toward the community. More detailed descriptions of wave mechanisms and effects on communities are available in U. S. Geological Survey Professional Papers 541-546.
Despite land-level shoreline changes resulting from crustal elevation and depression, no significant changes occurred in tidal characteristics or other permanent physical oceanographic effects.
Tsunami and submarine landslides do not account for all the local wave phenomena experienced in the 1964 earthquake. The origin of many waves still remains in doubt; however, from recent studies (Von Huene and Cox 1972), four possible mechanisms may generate local waves: (1) water displacement by crustal tilting; (2) water displacement by large horizontal translations of the land; (3) water displacement by subaqueous landslides; and (4) the resonant coupling of the water with earthquake waves. A good account of the history and significance of the seismic sea waves is given by Van Dorn (1964).
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The Southcentral part of Alaska is one of the most seismically active in the world. The earthquake and its associated effects are potentially the most powerful and destructive of all the natural disasters known to man. The great Alaska earthquake of 1964 has been so thoroughly studied and documented that great strides have been made in man's depth of understanding of this earth phenomenon. Earthquake prediction, once considered impossible, appears on the verge of reality. Scientists believe that by monitoring such things as crustal movements, fluid pressures, magnetic fields, and occurrence of local earthquakes, prediction of certain classes of large and destructive earthquakes will be possible (Scholz et al. 1973). John Kelleher (1970) notes that one area along the Alaska Peninsula has experienced few large earthquakes in recorded history. He suggests that there is a space-time pattern in the distribution of major earthquakes in the Alaskan-Aleutian seismic zone and predicts the approximate location and time of the next major quake. A seismic net of stations has been established in the Shumagin Islands by the U. S. Geological Survey, National Oceanic and Atmospheric Administration, Lamont Observatory, and the University of Alaska. By monitoring the seismic data in the Alaska Peninsula area, this program will be able to obtain a qualitative measure of any changes in activity that might occur prior to any large event.
Recommendations made by the Committee on the Alaska Earthquake of the National Academy of Sciences in Toward Reduction of Losses from Earthquakes (1969) include:
Most of Alaska's active volcanoes occur along an arcuate belt extending over 1,500 mi. (2,500 km.) from Mt. Spurr opposite Anchorage to the island of Buldir at the tip of the Aleutian Islands. The only exceptions are the Wrangell volcanoes and Mt. Edgecumbe in Southeast Alaska. This belt is part of the poetically named Ring of Fire that rims the entire Pacific Basin—a narrow zone of active tectonism, seismicity, and volcanism occurring landward of deep-sea trenches (Figure 81). Recent plate tectonic theories suggest that this activity is mainly related to the diving of one crustal plate beneath another along a subduction or Benioff zone as shown in Figure 52 (Bird and lsacks 1972). Pacific rim lavas are probably generated by the partial melting of oceanic crust and upper mantle rocks as they descend under the adjacent plate (Coats 1962).
|Figure 81. Pacific Rim Belt of Active Tectonism,
and Volcanism—The Ring of Fire
The Alaska portion of the Ring of Fire contains about 60 volcanic centers that have erupted since the last major glacial period or during the Holocene Epoch, the past 10,000 years. Over 40 of these centers are known to be steaming or have been reported as active since 1760 (Figure 82). Some type of activity has been noted almost every year somewhere along the belt. The most recent eruption was on Great Sitkin Volcano, Adak, on February 19, 1974.
|Figure 82. Alaskan Volcanoes|
The Southcentral Region contains 28 volcanic centers ranging from Mt. Veniaminof on the south to Mt. Spurr and the Wrangell Mountains on the north (Figures 82 and 83 ). Fifteen have been active since 1760. Any of these 28 centers are potentially eruptive.
Volcanic centers form most of the high peaks along the Alaska Peninsula and the Aleutian Range and provide spectacular scenery as well as fine examples of volcanic features—cones, craters, calderas, etc. Perhaps most spectacular are the Katmai Volcanoes, which already have been included in a National Monument.
Most Pacific rim volcanoes are called andesitic volcanoes because they produce molten material that is relatively high in silica and quite viscous. This high viscosity tends to make these volcanoes more explosive and thus potentially more hazardous to nearby activities. Some of the more characteristic eruptive phenomena associated with the andesitic volcanoes include:
During these eruptions finer pyroclastic material may be carried for great distances depending on the frothiness or fineness of the material and the direction and intensity of the wind.
Secondary but potentially hazardous phenomena often associated with these eruptions include:
All of these primary and secondary phenomena have occurred in the region during historic times.
Most eruptions of Alaskan volcanoes have been relatively moderate with respect to their explosivity and intensity. The 1942 and 1953 eruptions of Mounts Trident and Spurr are good examples of these more moderate eruptions, which are generally characterized by the first three primary eruptive phenomena described above (Snyder 1954, Juhle and Coulter 1955, Wilcox 1959). These eruptions consisted of bursts of ash and gas that rose vertically as high as 70,000 ft. (approximately 20,000 m.) accompanied by small, blocky lava flows at the vent area. The turbulent cloud accompanying the Spurr eruption rose to 70,000 feet in 40 minutes and could have been hazardous to aircraft operating in the area. This eruption also showered ash on Anchorage causing damage to aircraft and necessitating an expensive clean-up.
Occasionally, Alaskan volcanoes produce large, violent eruptions characterized by the latter two primary eruptive phenomena described above. The best historic example is the 1912 eruption of Mt. Katmai, one of the world's greatest. More than 6 cu. mi. (25 cu. km.) of material was ejected followed by collapse of the summit crater. As much as one ft. (0.3 m.) of ash fell on Kodiak Island over 100 miles away and caused extreme damage to buildings and crops (Figure 85) (Wilcox 1959). A large, glowing avalanche flowed northwest from the avalanche summit area and filled the Valley of 10,000 Smokes. This glowing avalanche could have caused damage to nearby villages if the blast had been directed differently. Large, locally felt earthquakes also accompanied the eruption and corrosive acid rains occurred at Seward and Cordova. A summary of the volcanic activity in the Katmai region is contained in a book soon to be published by the University of Washington Press (Forbes, in Press).
The eruption of Augustine Volcano in 1883 is another example of a large, violent eruption that created a large summit crater (Forbes and Kienle 1971). Associated with this eruption was a massive mudslide on the north flank of the volcano that apparently reached the sea and caused a large seawave. This seawave struck English Bay with a maximum amplitude of 20 to 30 ft. (7 to 9 m.), but fortunately the tide was low and no lives were lost. Since 1883 Augustine has built three new lava domes, the latest dome during eruptions in 1963 and 1964 to a height of 755 ft. (230 m.). Other violent eruptions are likely to occur at any time. The marine setting and proximity to transportation routes perhaps make Augustine the most hazardous of any volcano in the region.
Other examples of secondary eruptive phenomena occurred during the 1966 eruption of Mt. Redoubt. This eruption caused a flash flood that threatened the pipeline terminal at Drift River and necessitated a helicopter evacuation of a seismic party from the Drift River valley.