124 



EARTHQUAKES OM" THE PACIFIC COAST 



1888. September 17; 3:51 a. ni. 



Lick Observatory: The seismograph gives the following records 

 (magnified 1.6 times for the vertical, 3.3 times for the horizontal 

 components). At 3 seconds after an assumed zero second, the 

 vertical component began its trace with a wave of period about 

 1^2 seconds. The amplitude (on the trace) is hard to estimate, 

 but is probably not less than 5 mm. for the first semi-wave, then 

 about 1 mm. for a full wave, and after this mere tremors until 

 about 40 seconds. The N. and S. component (magnified) was as 

 follows: 



At 4.3 seconds the earth moved S. of the neutral line 5 mm. 

 5.7 

 5.9 

 6.1 

 6.4 . 

 6.9 

 7.5 

 8.9 



and tremors occasionallj^ as large as % mm. continued until about 

 40 seconds. 



The E. and W. component (magnified) was as follows: 



At 4.3 seconds there was strong movement of the earth west of 

 about 3 mm.; this was followed by a wave of period about 1 

 second double amplitude 2 mm.; and this again by another of 

 period % second double amplitude 1 mm. After this tremors 

 continue for about 30 seconds. 



The strata of which Mt. Hamiltoti is composed lie at a high angle 

 to the horizon and the direction of the stratification is nearer N. and S. 

 than E. and W. The earthquake instruments are at the very 

 summit of the mountain. This may account for the fact that 

 (at least for the shocks so far observed) the vertical component 

 is relatively large, and that the N. and S. component (in the 

 general direction of the stratification) is usually far larger than 

 the E. and W. component. 



We may then assume as a basis of computation for this shock: 



Number of waves in 10 seconds =z 6 or 7, say 6^/^. 



Period, T, of the representative wave =: 0.5 sec. 



Amplitude of the representative wave (magnified) =: 2.5 mm. 



a = 0.8 mm. 



Velocity of projection r= -tt, z=. 10.0. 

 Intensit}' rz. — zr 12t5. 



This corresponds approximately to V-VI on the Rossi-Forel scale, 

 according to the table in the American Journal of Science, June, 

 1888, p. 429, which was derived from Japanese shocks. 



