122 EARTHQUAKES ON THE PACIFIC COAST 



At 113^ sec. the earth moved S. of the ueutral line 1 mm. 

 13 

 15 

 16 

 18 

 19 



and small tremors with a double amplitude of about 1/2 rum. (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 1 mm. 



19 " below " 1/2 " 



211/2 " above " Vg " 



23 " below " 1 " 



and tremors of not more than 1/2 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, 1 mm. a = 0.3 mm. 



2^ 

 Velocity of projection — V := -?p ■=. 0.75. 



ys 

 Intensity = — zz 1.90, 



a 



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

 period of these waves is very slow. 



1888. April 28; 8:48 p. m. 



Eeno (Nevada), a smart shock: three waves in 3 sec, followed by a 

 general trembling for 10 sec. The time of the third and severest 

 shock was 8h. 48m. 38s. p. m. Direction S. to N. (letter from U. 

 S. Surveyor-General Irish). Two other observers say W. to E — 

 Grass Valley: felt in the Idaho mine below the 1600 ft. level, 

 Alta, May 2d. Very heavy, lasting 5 sec, from E. to W. (Chronicle, 

 April 30). — Grass Valley: the Orleans mine was flooded. The 

 shock was at 8.45 p. m. and very heavy -(VII). It was preceded 

 by a loud noise. The duration was about 5 sec, and the wave 

 was E. to W. Clocks stopped, plastering fell, and also tops of 

 chimneys. — Nevada City: walls of courthouse cracked (VIII). — 

 Wellington (Nev.), two slight shocks in quick succession, 8.55 

 p. m. — at Marysville, Downieville, Truckee, Colfax and Sacra- 

 mento the shock was very strong (G. V. Tidings, April 30, May 2). 

 —Nevada City: two severe shocks at 8.48 p. m. preceded by aj 

 deep rumbling sound. Direction N. — Dutch Flat; 8.46 p. m,, 

 severe from S. to N. People were badly frightened. — Stockton: 

 four shocks at 8.40, from N. to S. — Dixon, 8.45 p. m. — Biggs: heavy j 

 shock "lasting 75 (?) seconds" [seven to five? E. S. H.], at 8.45] 

 (VII), plastering cracked, etc. — Santa Eosa: slight shock at 8.45, 



