A Study of the Source Mechanism of the Alaska Earthquake and ...

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308 PACIFIC SCIENCE, Vol. XXI, July 1967be the highest, at least within the generatingarea. At Uzinki, Kodiak City, W omen's Bay,and elsewhere on the island of Kodiak, however,the third and fourth waves were the highest.A theory of generation from a singlepattern of crustal deformation is therefore notsatisfactory here. Such factors as reflection fromcoastal boundaries , wave interaction, and resonanceshould be taken into consideration.Slumps or avalanches, similar to the ones thatoccurred in Prince William Sound , are usuallylocalized; they can produce no large tsunamisthat would travel across wide portions of theocean. According to Wiegel (195 4), not morethan 2% of the potential energy of a fallin g orsliding body is converted into wave energy. InPrince William Sound, however, slumping andsliding when added to tectonic movements createdtsunami waves of very large energy, buttheir effect was catastrophic only locally; verylittle of the energy escaped the Sound .SUMMARY AND CONCLUSIONSThe Alaska earthquake of March 27, 1964affected an area of approximately 215,000 krn-,extending from the W rangell Mountains at thenortheast to the Trinity Islands in the southwest,and from the west side of Shelikof Straitand Cook Inlet east to the vicinity of the BeringGlacier.....'110"Geologic evidence has revealed a dipole patternof positive and negative tectonic movementsresulting from this earthquake. The areaof subsidence covers approximately 110,000 km 2and the volume of crust that has been depressedbelow its pre-earthquake level is about 115 km".The area of uplift covers about 105,000 km 2and includes the southern and eastern parts ofPrince William Sound, the coastal area as fareast as the Bering Glacier, and a great part ofthe continental shelf and slope bordering theGulf of Alaska.The seaward limits of the area affected bythe Alaska earthquake and the tsunami-generatingarea were determined by means of a seriesof wave refraction diagrams as shown in Figure5, based on Snell's Law of Refract ion. Thetsunami-generating area covers 140,000 km 2and includes the whole of the region of upliftand part of the region of subsidence. It extendsfrom the Trinity Islands to the Bering Glacierand includes Shelikof Strait, Cook Inle t, andthe continental shelf bordering the Gulf ofAlaska to a depth of approximately 200 m. Thetotal volume of displaced material in the tsunami-generatingarea was estimated to be 120krn", and the energy associated with the tsunamiwas calculated to be in the order of 6 X 10 21ergs.As a result of this work the following conclusionsare drawn:ee·I--1----I--~___1---_1_---___t_;~~""--'----_j_-__:;~~M1r+t__\HHf--4-4---I_i'i-____i".ATTU l~:;,........"..,...,...FIG. 5. Di agram of wave front's ief~acted toward the earthquake area from Attu Island (dashed line),Adak Island (solid line) , and ,Unalaska Island (d otted line) .
Alaska Earthquake and Tsunami, I-PARARAS-CARAYANNIS 3091. Two main tsunami-generating areas canbe distinguished: one along the continentalshelf bordering the Gulf of Alaska; the otherin Prince William Sound .2. The main generating area in the Gulf ofAlaska roughly corresponds to the geographicdistribution of the major aftershocks.3, The energy of the tsunamis gener ated inPrince William Sound was expended inside theSound ; not much energy escaped this closedregion.4. The long period of the waves generatedin the Gulf of Alaska is related to the longseiche period of the shallow shelf.5. The preferential radiation of energy towardthe southeast is attributed to the orientationof the tectonic displacements along thecontinental shelf of the Gulf of Alaska.6. The waves arriving at Cape Yakataga hadtheir origin in .the shallow coastal area near theBering Glacier, whereas the waves arriving atYakatat traveled through the deeper waters.BERINGAf TU l"..~ 1'~~' Ws ' , « r >AL E U T \ A ~ •• ;10 ~A LAS KA, ..• .~ ..o-... AN DRE AN OF IS .7. In Prince William Sound two major tsunamiswere distinguished: one had its originnear the west coast of Montague Island, theother originated in the Port of Valdez.8. Two types of tsunami-generating mechanismswere associated with the Alaska earthquake: (a) waves generated directly by tectonicmovements of the sea floor, and (b) wavesgenerated indirectly from landslides, mudflows,and slumping of alluvial deposits.9. In Prince W illiam Sound both generationmechanisms were evident , while in the generatingarea along the Gulf of Alaska, the generatedtsunami was the direct result of tectonic movements.ACKNOWLEDGMENTSThe work on which this paper is based wassupp orted in part by the National Science Foundationunder the United States-Japan programfor cooperative research in the Pacific, throughgrant N o. GF-1 53, and in part by the Officeo'?~
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