312 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



in Japan, in the Philippines, and in New Zealand the prevailing 

 movement is such that the continental side is also moving southward 

 relative to the Pacific side. However, the data are insufficient for a 

 more general conclusion as to movements of the continents relative 

 to the Pacific block. 



Additional information on processes leading to earthquakes can 

 be expected from investigations on the relation of earthquakes and 

 aftershock sequences (fig. 4) to rock creep, wliich Hugo Benioff is 



II X lo" (er^s)^^ 



1 — I I 1 1 1 II 



S2 = [2.50 + 7.40{l-exp(-I.I5T'^2)}]xl0" 

 T= t -0.135 



.01 t DAYS 0.1 1.0 



FiGUEB 4. — Strain release characteristic, Long Beach aftershock sequence, 1933. 

 (After H. BeniofC, 1951a.) 



undertaking (Benioff, 1949, 1951a). On the basis of the elastic re- 

 bound theory, the fault rock strain relief which produces an earth- 

 quake is proportional to the square root of the energy. Consequently, 

 in a sequence derived from a single fault system the square root of the 

 energy of each shock represents a strain release (or increase) incre- 

 ment, and a plot of the accumulated sum of such increments against 

 time represents the motion of a fault as a function of time (fig. 5). 

 The method thus appears to provide a means for observing tectonic 

 movements in progress. The energy is derived from magnitudes 

 of earthquakes as determined by Gutenberg and Richter (1949). In 

 the case of aftershock sequences. Dr. Benioff has found that creep 

 curves exhibit either simple compressional elastic creep of the fault 

 rock or compressional elastic creep release followed by shearing elastic 

 creep release. 



