362 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 31 



quakes on structures, and the design of structures to resist or reduce 

 destruction by earthquakes. Tliere is particular interest in regard 

 to the large structures which modern civilization makes necessary, 

 such as office buildings, factories, bridges, high dams, and other 

 structures. 



For a number of years engineers have been gathering information 

 regarding earthquake effects on structures and to some extent are 

 basing design of structures on the observed facts. At Stanford Uni- 

 versity they are operating a large shaking table on which types of 

 structure and models of buildings are tested under conditions simu- 

 lating earthquakes. With all this, they feel the need of more exact 

 knowledge in regard to motions of strong earthquakes, a demand 

 which the seismologists have not yet met. 



The reason for this is a practical one. Seismologists, like engi- 

 neers, have to be practical. Severe earthquakes are of relatively 

 infrequent occurrence at any given place, even where great damage 

 has occurred in the past. Instruments suited to record strong mo- 

 tions will not record weak ones. Accordingly, in order that we 

 may have material with which to work, the instruments must be 

 able to record earthquake waves coming from a considerable dis- 

 tance, but instruments able to do this will not record strong motion. 

 The valuable information which the seismologist has been able to 

 obtain about the interior of the earth has been accompanied by the 

 introduction of many perplexing problems. It is, therefore, not 

 surprising that much energy has gone into perfecting instruments 

 and methods for recording distant earthquakes. In fact, the prin- 

 cipal problems in this field are well on the way to solution, though 

 we may continue to expect new instruments and methods from time 

 to time. 



The seismologist in developing these instruments has acquired 

 much information which is useful for the design of the more rugged 

 and relatively insensitive instruments which must be used to record 

 strong motion. His knowledge of wave transmission will also be 

 essential to the correlation of the information regarding strong 

 motions when available. It is for the engineer to state exactly 

 what information he needs and to make use of it after it has been 

 obtained. The seismologist by his previous accomplishment has 

 justified the confidence of the engineer in his ability to solve the 

 problems. 



This accomplishment has included both instrumental development 

 and the development of theories to explain the complexities of the 

 records in terms of transmission through the earth and, consequently, 

 reasonable deductions about the physical conditions of the other- 



