HISTORY OF A TSUNAMI — ROBERTS 333 



indicate with any certainty what patterns of earthquake recurrence 

 may exist on the San Andreas, although some geologists have said 

 that great earthquakes may occur somewhere along that zone of 

 weakness at about 60-year intervals. This does not threaten San 

 Francisco with a great jolt at any specific time, for the fault is very 

 long — it extends from an origin in the Gulf of Lower California 

 northward through the San Francisco area to an eventual disappear- 

 ance in the sea off northern California — and an earthquake could oc- 

 cur anywhere along its length. 



The San Andreas, for all its length, is but a detail in one of the 

 world's great belts of seismic activity — an arc extending around much 

 of the Pacific Basin, from the unstable western mountains of South 

 and North America, through southern Alaska where lie the Fair- 

 weathers and Lituya Bay, through the Aleutian Island chain, past 

 Japan and the Philippines, and on southeastward toward New Zea- 

 land. Some of the western Pacific areas on this arc show the greatest 

 concentration of heavy shocks recorded anywhere in the world. The 

 whole great arc is what is known as an orogenic zone — an area of 

 geologically young and changing forms characterized by high moun- 

 tains which have not had time to erode down as have the more mature 

 Appalachians, and deep ocean trenches not yet filled with sediments. 

 The progressive changes are typified by the crustal migrations shown 

 by the geodetic surveys in California, and by the upward growth of 

 southeast Alaska's mountains — movements that become earthquakes 

 when the rocks fail to yield sufficiently fast and end by breaking. 



In the California quake of 1906 two mountains near San Francisco 

 moved 10 feet farther apart at one stroke, and along the San Andreas 

 the horizontal slipping in that one quake amounted in places to no 

 less than 21 feet. Roads, fences, even rows of orchard trees were 

 offset in broken lines where they lay across the fault. In another 

 quake along the southern reaches of the great fault, a unique terri- 

 torial problem was presented when the United States border with 

 Mexico shifted position. There is geological evidence that these move- 

 ments have been repeated so many times that their total accumulation 

 along the San Andreas now amounts to as much as 350 miles or more. 

 This probably took 100 million years, which, though long in himian 

 terms, is but a brief period in geological time. 



In parts of southeast Alaska, particularly north of Icy Strait, the 

 upward growth of the land has been monitored for many years. Long 

 sequences of tide-gage observations east of the Fairweathers have 

 shown how fast this is. At Haines in Lynn Canal the land has risen 

 out of the sea some 5 feet in 60 years ; at Skagway and at Juneau the 

 emergence has amounted to 3 feet in 50 years; and an engineering 

 firm found indications of nearly 6 feet of rise of the land in Excur- 



