k. Horizontal Earth Movement during the Alaskan Earthquake 



In addition to subsidence and uplift, substantial horizontal 

 movement of the land in the earthquake area has been reported (cf. 

 Parkin, 1966). The U. S. Coast and Geodetic Survey has undertaken an 

 extensive postquake triangulation survey encompassing a large area of 

 south central Alaska. The Survey has compared these measurements vith 

 prequake triangulations on the assumption that station FISHHOOK, about 

 ^5 miles northeast of Anchorage near Palmer (Figure l), suffered no hori- 

 zontal displacements. This base of reference appears justified because 

 results show the station to be on an axis of zero horizontal movement, 

 roughly in line -with the Knik Arm of Cook Inlet, north of which displace- 

 ments were tending to the northwest, and south of which displacements 

 were to the south-southeast. 



The essential outcome of the triangulation is given in Figure 15 

 (Parkin, I966) from which contours of horizontal movement and streamlines 

 of flow have been developed in Figure 16 for purposes of this study. A 

 reading for Middleton Island, near the edge of the Continental Shelf, 

 permits a useful degree of extrapolation over the shelf southwest of 

 Montague Island. This extrapolation suggests that horizontal movements 

 as great as 80 feet may have developed in a narrow area approximately 

 along the axis of maximum uplift and that here the thrust was directed 

 south-southwest. Montague Island shifted horizontally a distance of kO 

 to 50 feet. The contotirs on Figure 16 show that whereas the northwest 

 side of Montague Island displaced horizontally through 50 feet, La Touche 

 Island to the northwest (Figure ll) moved from 60 to TO feet, thus narrow- 

 ing the straits by some 10 to 20 feet. A comparison of surveys made in 

 1933 and 196i+ shows the straits to have become shorter by 15 to 20 feet 

 (Parkin, 1966). A typical cross-section AA (Figiure I6) normal to the 

 hinge line for vertical movement shows the vectors of horizontal displace- 

 ment, and suggests that a dilatation of the crust occurred on the north- 

 west side of the hinge line and a compression on the southeast side. 



Parkin (1966) has pointed out that most of the triangulation stations 

 occupied in the s\irvey were on peaks at elevations of 1,500 to 3,000 feet 

 above sea level. Because of this and the possibilities of earth tilt, the 

 apparent horizontal displacements may not be entirely meaningful since a 

 tilt of one degree in a block of the earth's crust containing a peak could 

 yield a differential horizontal movement of 50 feet between peak and sea 

 level. However, since the crustal tilt over most of the area surveyed 

 would favor sea level displacements in excess of those found, we must 

 conclude that the recorded horizontal movements are minimal. 



Assuming a conservative figure for maximum horizontal ground move- 

 ment of say 5 = 70 feet and a maximimi value of vertical mo vement of 

 C = 50 feet, the resultant maximum ground displacement A(=/£'^ + c,^) is 

 found to be 86 feet or 26 meters. Plotted against earthquake magnitude 

 M in Figure IT, this value suggests that the statistical relationship 

 between A and M needs some revision from that formerly given by Wilson 

 (196U). The fact that data for other large earthquakes now underlie the 



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