SEISMOLOGICAL NOTES. 



By harry fielding REID. 



(Read April 24, 1909.) 



(a) Conditions Preceding and Leading to Tectonic 

 Earthquakes. 



There are two classes of earthquakes : Volcanic and Tectonic ; 

 the former, connected with volcanic outbursts, seem to be due to 

 explosions or to the sudden liberation of steam ; the latter are due 

 to ruptures of the rock. It is only the latter class that we shall 

 consider at present. 



Rock, like all solids, is elastic, and when subjected to external 

 forces it suffers an elastic strain; if this strain is too great for the 

 strength of the rock to withstand a rupture occurs; but it is never 

 possible for a rupture to take place until the rock has been deformed 

 or stretched beyond its elastic limit. When the rupture occurs, the 

 two sides spring apart under the elastic forces and come to positions 

 of equilibrium, free of elastic strains. The following experiments 

 have been made to illustrate these conditions. Two short pieces of 

 wood were connected by a sheet of stiff jelly i cm, thick, 4 cm. wide 

 and about 6 cm. long, as shown in Fig, i. The jelly was cut 

 through along the line, tt' , by a sharp knife and a straight line, AC, 

 was drawn in ink on its surface. The left piece of wood was then 

 shifted about i cm. in the direction of t' , and a gentle pressure was 

 applied to prevent the jelly from slipping on the cut surface. The 

 jelly was sheared elastically and the line took the position AC shown 

 in Fig. 2. On relieving the pressure so that the friction w^as no 

 longer sufficient to keep the jelly strained, the two sides slipped along 

 the surface tt' and the line AC broke into the two parts AE and DC. 

 At the time of the slip A and C remained stationary, and the amount 

 of the slip, DE, equalled the shift which A had originally experi- 

 enced. A straight line, A'C, w^as drawn on the jelly after the left 

 side had been shifted, but before the jelly slipped along tt'. At the 



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