and a direction of the line of rupture, S 30° W. This direction of 

 rupture he finds to be some 5° at variance from the hinge line of zero 

 deformation - a difference he considers to be within the limits of error. 

 Ben-Menahem and Toksoz previously had concluded on the same basis (see 

 Press, 1965) that the fault length was 65O kilometers and that the near 

 vertical fault-plane was uniquely determined. 



It was shown by Benioff, Press, and Smith (1961), and by Press, Ben- 

 Menahem, and Toksoz (1961) that the main shock of an earthquake represents 

 the rupture that starts at the hypocenter and runs the length of the fault 

 at the rate of about 3 to 3.5 kilometers/second, the speed of shear waves 

 in crustal rock. Since most observers' estimates of the duration time 

 for the main Alaskan earthquake range from 2 1/2 to 8 minutes and average 

 about 5 minutes (cf. Chance, I966), the fault length may be estimated on 

 this basis to have been as much as 900 kilometers. We favor the fault 

 length L = 8OO kilometers proposed by Press and Jackson (1965); Press 

 (1965); and Furumoto (1965) over the shorter lengths of 6OO and 65O 

 kilometers proposed by some seismologists. Fault length data for the 

 Alaskan earthquake are found to be in reasonable accord with a statis- 

 tical relationship proposed by Tocher (1963), (see also Press and 

 Brace, I966), connecting length L with earthquake magnitude M. This 

 relationship 



M = 6.6 + log-LQ L (2) 



(for L in kilometers) is shown in Figure 23. The data and curve fitting 

 of lida (1959) included in Figure 23 yield a rather different relation- 

 ship, which clearly overestimates the fault length for large earthquake 

 magnitudes. Tocher's result must be considered to supersede a result 

 given by Wilson, et al (1962), Wilson (196U) which was based only on 

 lida's data. 



For convenience we summarize now in Table I some of the essential 

 characteristics of the earthquake as cited by various authorities. 



6. Character of the Source Inferred from Tsunami Waves 



The Alaskan earthquake initiated trains of seismic sea waves 

 or tsunamis which lashed the shores adjacent to the generation area and 

 penetrated to every part of the Pacific Ocean. A discernine,.study of the /i 

 source mechanism of the tsunami has been made by Van Dorn C<L96|i', 19S^~) — o 

 who reasoned that the entire vertical earth motions (Figure-~-fiO occurred 

 within the time (2-6 minutes) of ground shaking during the main shock of 

 the earthquake. The essential dipole character of earth movement, revealed 

 in Figures lU and 22, was recognized by Van Dorn as being responsible also 

 for an initial dipole surface disturbance of the sea and of the atmos- 

 phere; the latter was clearly registered as a pressure disturbance on a 

 microbarograph at La Jolla, California (Figure 2H). A similar trace was 

 recorded on a microbarograph at Rice University, Houston, Texas, soon 

 after the earthquake was registered there (Houston Chronicle, March 29, 

 I96H), and other traces may have been recorded elsewhere. 



28 



