The earthquake had its main effects in the Pacific Motintain region 

 in the south central area and the spread of perceptible effects from the 

 source throughout Alaska is illustrated in Figure 2b (cf . Townshend and 

 Cloud, 196U). The substructure of the coastal area mainly affected is 

 underlain by cretaceous sediments of graywacke which have been folded and 

 warped into the system of geanticlines and geosynclines already mentioned. 

 Within this system lies a series of arcuate faults which parallel the 

 structural features of the region, as also the coast and the well-known 

 Aleutian Trench (Figure 3). Of the faults shown, only the Chugach- 

 St. Elias fault appears to comprise a boundary between radically dif- 

 ferent rock types. The dip and sense of movement along the faults is 

 not yet well established. (Wood, et al, 1966), nor are all the faults 

 fully recognized or located. As we shall see, the seismic evidence sug- 

 gests that the fault planes giving rise to the Alaskan earthquake are 

 apparently buried and have no intersections with the free surface. 

 Certain localized faults, however, notably the Manning Bay and Patton 

 Bay faults on Montague Island, became visible during the earthquake, but 

 these apparently have only secondary connection with the submerged faults 

 already mentioned (Plafker, 1965). 



2. Historical Distribution of Earthquakes in Alaska 



The South Alaskan seaboard is one of the most active seismic re- 

 gions in the world. It forms part of the circum-Pacific belt of seismicity 

 that gives a well-defined geographical distribution to the occurrence of 

 earthquakes and volcanic eruptions in the Pacific arena. 



The positions of the epicenters of major Alaskan earthquakes occur- 

 ring in the interval from 1898 to I96I are shown in Figure 3 (from Hansen, 

 et al, 1966). These epicenters are scattered along the island arc of the 

 Aleutian Islands and concentrated in Cook Inlet, on Kenai Peninsula, and 

 at the head of Prince William Sound. A corresponding distribution for 

 strain-energy release covering the period 190i+-196i+ and incorporating the 

 196k earthquake (Berg, 196^), is shown by the contoiirs in Figiire h, which 

 indicate that Prince William Sound has now become one of the focal areas 

 of high energy concentration. 



A detailed listing of earthquakes in the Alaskan region from I786 to 

 196k has been compiled by Wood, et al (1966). From data such as these. 

 Berg (196U) has plotted the frequency of occurrence of Alaskan earthquakes 

 versus their magnitude M. Compared with Japanese and World data (Figiire 5)j 

 the trend is foiand to be similar and the mean annual number of earthquakes 

 Wg^ for Alaska is somewhat higher than for Japan. According to this, an 

 earthquake of magnitude M = 8.5, as that of March I96U, has about a 1-in- 

 30 year frequency of occurrence within the Alaskan Aleutian arc region. 

 An empirical equation relating N^^ and M, of the form used by Gutenberg 

 and Richter (195^), is 



logio K = 0.99(8-M) - 1.0 (1) 



and is essentially that given by Berg (196U). 



Text resiimes on page 9 



