new trend is perhaps not surprising since previous data are probatly 

 incomplete and do not incorporate, in particular, accurate information 

 on horizontal components of displacement. 



5. Seismic Evidence for Fault-Plane Mechanism 



The zone of aftershocks following the main quake of March 27 is 

 indicated in Figure T and in much greater detail in Figure l8. More than 

 7,500 aftershocks were detected instrumentally ; they covered a belt about 

 800 kilometers long and 250 kilometers wide (Press, I965). This shot- 

 scatter of aftershocks following a great earthquake provides significant 

 information on the general extent of the faulting, which usually runs 

 along the long axis of the approximately elliptical area covered by the 

 aftershocks. On this basis alone the fault can be prescribed as being 

 approximately 8OO kilometers long (Press and Jackson, 1965)- 



The average focal depth for some 200 aftershocks for which determi- 

 nations were made was found to be about 20 kilometers (Press, I965). The 

 deepest quake of these occurred at 60 kilometers. However, according to 

 Press, specially sensitive seismographs were subsequently installed in 

 Alaska and measured hundreds of aftershocks , some of which occurred to 

 depths as great as 200 kilometers. 



Investigators, using some of the latest tools of seismology (cf. 

 Stauder, 1962; Hodgson and Stevens, 196U), have employed seismic P-wave 

 and S-wave data for the determination of the fault-plane mechanism. 

 Essentially, the methods determine two possible fault planes at right 

 angles to each other, one of which is auxiliary to the other. However, 

 which of the two planes is actually the fault plane is not uniquely de- 

 termined unless both P- and S-wave data are used in the analysis. Most 

 recording seismographs at suitable range were rendered inoperative after 

 arrival of the P-waves from the main shock of March 27, I96U. As a result 

 there is a difference of opinions as to which of the two fault planes 

 found was responsible for the earthquake. Berg (196H) and Algermissen 

 (1966), respectively, find one well-defined plane with a strike N 72° E 

 (N 61° E) and a dip of 89° NW (82° SE), which is close to being vertical 

 and in the general direction of the hinge axis of zero movement (Figure 7)' 



Press and Jackson (1965) and Press (1965) reported the results of ap- 

 plying dislocation theory, assuming the fault to be a vertical rectangular 

 dislocation sheet in a half-space. The vertical surface displacements 

 calculated for their three models are shown in Figure 19a, which plots 

 also measured vertical displacements across a section normal to the belt 

 of epicenters. These authors favor Model 3 (with a vertical dip slip 

 Au = 9m, fault length L = 8OO kilometers, and a dislocation between depths 

 16 kilometers and 200 kilometers) as being in reasonable accord with the 

 measurements. This fault model is one without an intersection at the 

 surface. 



24 



