EARTHQUAKE MOTION TO GREAT DISTANCES. 137 



do not appear to be due to distortional waves of the character contemplated by the 

 theory of wave motion in an infinite solid, Imt rather to be analogous to the elastic 

 surface waves investigated by Lord KAYLEIOH. 



The views of Dr. CANCANI have been opposed by Dr. AOAMENNONE, who, in a 

 series of papers devoted to the investigation of the observed rate of propagation of 

 earthquakes, has shown that there is no indication of the velocities grouping them- 

 selves round the values 5 and 2 '5 kiloms. per second deduced by Dr. CANCANI, but 

 that, on the other hand, the observed values exhibit a great diversity, and not 

 infrequently rise to double of the greater of the two assumed values. 



Dr. CANCANI'S views have, however, been accepted by other seismologists, notably 

 l>y Professor GRABLOWITZ, who has, without any marked success, attempted to apply 

 them to the determination of the place of origin of distant earthquakes observed at 

 Ischia. The idea that the surface undulations due to great earthquakes are of the 

 nature of distortions!, and the so-called "preliminary tremors" of condensational, 

 waves, seems to liave taken root, and in the two most recent papers dealing with 

 the subject by Professor C. G. KNOTT* and Professor J. MILNE, t the former are 

 distinctly treated as distortional waves travelling by a brachistochronic path through 

 the earth. 



When investigating the great Indian earthquake of 1897, I found that the 

 numerous diagrams, for which I was indebted to the generosity of the Directors of 

 the Italian Observatories, showed two distinct phases, or periods of disturbance, 

 preceding the advent of the surface undulations of long period. The first of these 

 was marked by the commencement of the disturbance, the second by a sudden 

 increase, accompanied by a change in the period of the waves ; these features were 

 more or less distinctly exhibited by all the records, and when preparing my report 

 I suggested! that they represented the arrival of the condensational and distortional 

 waves respectively, which had travelled through the earth, while the surface undu- 

 lations of long period had travelled round the surface of the earth ; thus recognising 

 the presence of the three known types of elastic wave motion. 



In this report the suggestion had to remain as such. To have entered on an 

 examination of the records of other earthquakes with a view to its confirmation 

 would have been foreign to the task in hand, besides being rendered impossible by 

 the necessity of completing the report, and it is the object of this paper to show that 

 the records of other great earthquakes confirm the suggestion. 



2. The rate of propagation of earthquake waves is a subject which has for long 

 attracted much attention and been the subject of many studies and experiments. I 

 found, however, that none of these with which I have become acquainted could be 

 directly used, and that it was necessary to go back to the original descriptions, and 



* " The New Seismology " ( Scottish Geog. Mag., 1 vol. 15, 1899, pp. 1-12). 

 t " Earthquake Precursors " ('Nature,' vol. 59, 1899, pp. 414-416). 

 J " Mem. Geol. Sunr. Ind.," vol. 29, 1899. 

 VOL. CXCIV. A. T 



