430 Holden — Earthquake-Intensity in San Francisco. 



Absolute intensity of Earthquake action at San Francisco. 



417 shocks of all intensities have been recorded at San Fran- 

 cisco in the years 1808-1888. Of these, 200 were described so 

 definitely that their intensities could be assigned on the Rossi- 

 Forel scale with tolerable certainty. This work has been 

 done with great care and is summarized in the following table : 



No. of shocks actually recorded at San Francisco (1808-1888) for which (he in- 

 tensity is known. 



Intensity on Eossi-Forel Scale. Number of Shocks. 

 I 8 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



IX 



. 4 

 .55 

 .50 

 .58 

 .12 

 . 4 

 . 7 

 . 2 



Total, 



.200 



Beside the 200 shocks of known intensity, there are 217 

 shocks printed in my catalogue. No doubt a great number of 

 the lighter shocks (I, II, III,) are not recorded at all. 



Earthquake action is so irregular and lawless, that it is not 

 possible to make any estimate however rough of the number 

 of these lighter shocks. Experience has amply proved that 

 the average intensity of San Francisco shocks is not above IV 

 on the Rossi-Forel scale. The vast majority of our shocks 

 are II and III and the average is certainly below IY. I shall, 

 therefore, assume this fact as a basis for computation. 



The 200 shocks of known intensity are evaluated and 

 summed up in the following table : 















Units of Acceleration 



8 shocks 



of 



intensity I 



correspond 



to 8x 



20 



= 



160 



4 " 





II 



u 



" 4x 



40 



= 



160 



55 





III 



u 



" 55 x 



60 



= 



3300 



50 " 





IV 



u 



" 50 x 



80 



= 



4000 



58 " 





" V 



(i 



" 58 x 



110 



= 



6380 



12 " 





VI 



tc 



" 12 x 



150 



= 



1800 



4 " 





VII 



" 



" 4x 



300 



= 



1200 



7 " 





. " VIII 



1i 



" 7x 



500 



= 



.3500 



2 " 





IX 



a 



" 2x 



1200 



= 



2400 



200 recorded shocks of known intensities correspond to 22900 units. 



The average recorded shock corresponds to I = 114 units or 

 approximately to Y on the scale. This simply proves that all, 

 or nearly all, the shocks of intensity Y and more severe have 

 been recorded and that the lighter shocks have been neglected. 



As has been said 417 shocks in all have been noted (of which 

 only 200 are accurately described). I assume the 217 shocks of 



