Holclen — Earthquake- Intensity in San Francisco. 429 



= destructive effect = the maximum acceleration due to the 

 impulse. 



It would be logical to express I in fractions of the accelera- 

 tion due to gravity, i. e., 9810 mm per I s . As these fractions are 

 usually small, it is convenient to give the values of I in terms 

 of millimeters per I s . 



The observations of Ewing, Milne and Sekiya on Japanese 

 earthquakes give for each shock a and T, from which V and I 

 can be computed. Very frequently a description of the effects 

 of the shock on buildings, etc., is given by them, which descrip- 

 tion is often sufficiently minute to justify the characterization 

 of the shock by one of the degrees of the Rossi-Forel scale. 



I have carefully examined all the writings of the three gen- 

 tlemen named, accessible to me, and after rejecting all doubt- 

 ful cases, I have found twenty-one shocks ranging in intensity 

 from I to IX, in which the a and T were determined by in- 

 struments and in which I could assigmthe Rossi-Forel intensity 

 with confidence. The following table is the result : 



Equivalents of the degrees of intensity of Earthquake shocks on 

 the Rossi-Forel scale, in terms of the acceleration due to the 

 velocity of the shock itself* 





T V 2 2na 













~ ~a~~T> 





Kossi-Forel 







Scale. 



Intensity. 



Diff. 



I 



corresponds to 20 mm per l a 







II 



" 40 " 



(20) 



III 



" 60 " 



(20) 



IV 



" 80 " 



(20) 



V 



" 110 " 



(30) 



VI 



" 150 " 



(40) 



VII 



" 300 " 



(150) 



VIII 



500 " 



(200) 



IX 



1200 " 



(700) 



So far as I know, this is the best determination possible from 

 the meager data now available. 



The observations at Berkeley and Mt. Hamilton are espe- 

 cially directed toward obtaining better values of these rela- 

 tions. A few years of observations will determine them, at 

 least for the lighter shocks (I-VI). 



* It is interesting to observe the influence of long period in diminishing the 

 destructive effect of a shock of given amplitude. Thus a shock of intensity VIII 



has 1= -— = 500 mm per I s by observation. If T = 0-1 8 , a, — 0-l mm , while if 



T = I s , a = 13 mm , and so for other cases. 



