THE EAKTHQUAKE PROBLEM HECK 373 



The effects on buildings are of interest. In many cases the 

 destruction is so great or the faihire so varied that they are not 

 instructive. Much can be learned, however, from study of selected 

 details. It is, however, very difficult to deduce the actual earth 

 movements in this way on account of the complexity and variation 

 in the stress applied and our lack of knowledge as to just what 

 part of the activity produced given results. But even then much 

 can be learned, and a large part of existing effort to prevent damage 

 from earthquakes has been obtained from the study of damaged 

 buildings. ]\Iuch work has been done in Japan by the Earthquake 

 Kesearch Institute and other organizations, and buildings are being 

 erected which are expected to resist destruction or even serious 

 damage. The report of the great earthquake of September 1, 1923, 

 contains much valuable information, but is not yet available in 

 English. 



The acceleration of the ground movement is the rate of change 

 in its velocity and it appears to be closely related to the destruc- 

 tiveness of an earthquake. While there are many other complicat- 

 ing factors, engineers in designing structures have generally ac- 

 cepted acceleration as the principal element to consider constants in 

 the formulas which are not satisfactorily known. One of the most 

 important of these is the maximum acceleration; without certain 

 knowledge of this, the engineer is not sure of his factor of safety. 



We have all sorts of statements as to the relation between acceler- 

 ation and damage, and the so-called Mercalli-Cancani scale of earth- 

 quake intensity is a double scale of equivalents. Engineers are be- 

 ginning to adopt 0.1 acceleration of gravity for maximum horizontal 

 and 1/20 g for vertical acceleration. In Italy higher values have 

 been adopted up to 1/6 g. This must be considered in the light of 

 the fact that we have few instrumental determinations of accuracy. 



There has been no lack of effort to make such determinations. A 

 favorite device consists of a series of small columns side by side so 

 arranged that each will fall when a given acceleration has been 

 reached. The information given by such a device is limited to a 

 single observation with the time of occurrence unknown and we can 

 never be certain whether the last column to fall (that responding 

 to the highest acceleration) has responded to the proper maximum 

 or whether a rapid series of impulses of lower acceleration have not 

 through resonance caused the column to fall. 



Other attempts have been made to deduce the maximum accelera- 

 tion from the fall of objects. Here again we do not know w^hether 

 there has been a single impulse or the accumulated effect of several. 

 A typical example of the straits to which seismologists and engineers 

 have been driven is the attempt of Bailey Willis to deduce this in- 



