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

304limit to extend to the west side of ShelikofStrait and Cook Inlet.In estimating the travel time of the tsunami,corrections were made for the delay at the islandof Kodiak in the arrival of the groundshocks from Prince William Sound. These correctionsranged from 1 minute to 6 minutes andwere based on the fact that the Navy W eatherCentr al on the island of Kodiak listed the timeof the principal shock in Prince W illiam Soundas 6 minutes later than the time listed by theU. S. Coast and Geodetic Survey. This wouldimply that the wave front generated on thenortheast side of the disturbance area had a 6­minute head start on the wave front generatedsoutheast of Kodiak.The tsunami-generating area covers an area700 km long by 150 km wide, a total of about105,000 krn-, The volume of the uplifted crustalong the continental shelf is about 96 krn". Theenergy associated with the tsunami has beenestimated by Van Dorn ( 1964) to be of theorder of 2.3 X 10 21 ergs. This estimate is basedon the source dimensions of an area 240 nauticalmiles by 100 nautical miles and an upliftof 1.8 m ( 6 ft ) at the northeastern end of thisarea and zero at the southwestern end. Thisestimate, however, is considered low because thegenerating area had dimensions that were largerthan those estimated by Van Dorn.Using our source dimensions, and assumingthat the total energy was equal to the potent ialenergy of the uplifted volume of water, thetotal energy for the tsunami in the Gu lf ofAlaska was calculated as follows:1E t = - pgh 2 A61= - (1.03) ( .980) (1 0 3 ) (10 4 ) ( 1.8 3 2 ) (1.56X 10 7 ) (7 X 10 7 ) = 5.88 X 10 21 ergswhereE, = E" = total energyp = 1.03 g/cm 3 = densityg = 980 cm/sec 2h = height of displacement = 1.83 mA = area1 erg = g cm 2 sec- 2The waves generated in the Gulf of Alaskawere of an unusually long period, on the orderPACIFIC SCIENCE, Vol. XXI, July 1967of an hour or more:' Th eir energy radiation waspreferentially directed toward the southeast andthis is why more damage was done to the N orthAmerican coast than anywhere else east or southof the generating area. This preferential directivityof energy radiation can be attributed tothe orientation of the tectonic displacementsalong the continental shelf of the Gulf ofAlaska, and the long period of the waves canbe related to the long seiche period of theshallow shelf.According to Japanese seismologists (Iida,1958), the generating area of a tsunami roughlycorresponds to the distribution of the majoraftershocks. This appears to be indeed the casein the Gulf of Alaska.There were 52 aftershocks of the Alaskaearthquake. The largest had a magnitud e of 6.7.The aftershocks occurred in an area from about15 km north of Vald ez to about 55 km south ofTrinity Islands, and were heavily concentratedon the northeast and the southwest of the upliftedregion (USCGS, 1964), which also wasthe main tsunami-generating area.The vast area of tectonic movements indicatesthat wave crests were generated along one ormore line sources from the region of maximumupli ft. Thus, the shores of the Kenai Penin sulawere struck within 20 minutes after the start ofthe earthquake, and those of Kodiak Island ,within 34 minutes.Unfortun ately, the violence of the earthquakeleft south-cent ral Alaska without a tide gaugein operation. The only reliable record from thegenerating area is the one that was obtained bypersonnel of the U. S. N avy Fleet W eatherStation at Kodiak; it is shown in Figure 2. Thisrecord has been corrected for the 1.7-m (5 .6-ft)submergence of the area.Outside the immediate generating area, therecord of Cape Yakataga, as constructed fromthe personal account of C. R. Bilderback, a residentof the area, is the next most reliable record.This record is the only one obtained outsidethe generating area that shows an initial dropin the water level (Berg et aI., in preparation ) .Withdrawal of the water immediately followingthe earthquake has been reported from Kayak,Middl eton, and Hinchinbrook islands, as wellas from Rocky Bay and N uka Bay, at the end