396 E. 8. Holden — Earthquakes in California. 



early as fifteen seconds before the zero second adopted, but 

 the amplitude of E. and W. tremors is never more than -fa 

 of a millimeter during the whole shock and the time of their 

 beginning cannot be fixed. I presume we have here a case 

 where the normal vibrations were strictly^ in an E. and W. 

 plane. The transverse vibrations which arrived later are there- 

 fore N. and S. and of their full size in the diagram. We may 

 then dismiss all further consideration of the E. and W. wave. 

 It had scarcely a measurable amplitude. At seconds the N. 

 and S. tremors begin to show ; the whole record of the vertical 

 component is lost till 17 seconds. 



At 3 sec. the earth moved S. of the neutral line l mm 



5 " N. " 1 



6 " S. " 1 

 9 " S. " 1 



10 " N. " 1 



\\\ " S. " 1 



13 " N. " 1 



15 " S. " I 



16 " N. " 1 

 18 " S. " \ 

 10 " N. " % 



and small tremors with a double amplitude of about % mm (on 

 the trace) continue till 66 seconds. 



The vertical component as recorded by the machine is given 

 below : 



At 18 sec. the earth moved above the neutral line l mm 

 19 " below " i 



2l£ " above " £ 



23 " below " 1 



and tremors of not more than |- mm continue on the trace till 

 about 56 seconds. 



We may assume for a basis of computation : 



Number of waves in 10 seconds = 4, 

 Period, about 2 '5 seconds =T, 

 Amplitude magnified, l mm , a = 0-3 mm , 



Velocity of projection :=V= — ^- = 0*75, 



V 2 

 Intensity = — =1-90, 



which corresponds to about I on the Rossi-Forel scale. The 

 period of these waves is very slow. 



April 28, 8:48 p. m. — Reno (Nevada), a smart shock : three 

 waves in 3 sec, followed by a general trembling for 10 sec. 



