EARTHQUAKES IN NORTH AMERICA — GUTENBERG 



313 



Study of a number of earthquake sequences occurring in all the 

 active regions of the world has revealed that most of them form creep 

 series. Many types of creep are represented, such as constant velocity 

 creep, exponential velocity creep, elastic creep, and elastic flow creep. 

 Individual sequences may have linear extents of 20° to 30° of latitude, 

 as in the case of the South American and Tonga sequence, and the 

 evidence strongly suggests that they are derived from movements 

 of single mechanical units. The deep-focus Tonga sequence exhibits 



JAN. I. I90S JAN. I. 1915 JAN. 1. 1925 



70<IO" I I t I I I I I I t I t I I I t I I I I I I I 



JAN. I, 1935 JAN. 1. 1945 



I I I I 1 I I I I I I I I I 1 I I I I I I 1 I I 



riGUBE 5. — Strain release characteristic, San Andreas fault system (as indicated 

 in the inserted map). Symbols for magnitude refer to the main figure only. 

 (After H. Beniofl, unpublished.) 



elastic creep which continued some 25 years, thus demonstrating that 

 at depths of 650 kilometers rock masses can support elastic creep 

 stresses without appreciable flow for many years. Unfortunately, 

 data available for this type of research cover too short a period to 

 permit conclusions to be drawn as to the exact processes involved. 

 In some of the series investigated by Dr. Benioff, discontinuities in 

 the rate of movement were observed within the short interval of time 

 during which instrumental records are available. Other sequences 

 appear to exhibit no evidence of a discontinuous change in rate since 

 1904, 



