1902.] R. D. Oldham — Tidal Periodicity in Earthquakes of Assam. 139 



On Tidal Periodicity in the Earthquakes of Assam. — By R. D. Oldham, 

 Superintendent, Geological Survey of India. 



(Communicated by permission of the Director of the Geological Survey of 

 India). [Received July 21st, Read August 6th, 1902.] 



I. — Introductory. 



Evei* since earthquakes were first studied there have been repeated 

 and persistent attempts to trace the action of the sun, the moon, and 

 the planets in producing them, or at the least in influencing their rela- 

 tive frequency. Mallet, from the discussion of his great earthquake 

 catalogue l , found that there was a marked periodicity, which caused 

 earthquakes to have a maximum frequency towards the end of each 

 century, with a minor, but nearly as great, maximum a little before the 

 middle ; and, more l'ecently, Dr. A. Cancani has remarked a similar pecu- 

 liarity in the earthquakes of Italy. 2 Periods of this length, however, 

 have no direct and obvious connection with the movements of the 

 heavenly bodies, and more interest attaches to variations of shorter 

 periods. Perrey, and following him Mallet, 8 believed that they had de- 

 tected such variations, and that the frequency of earthquakes showed 

 a relation to the distance of the sun and the moon from the earth, and to 

 their relative positions in the heavens, at the syzygies and quadratures. 

 As a result of this careful investigation it had been generally accepted 

 that earthquakes were more frequent during winter than iu summer 

 and during the night than during the day. 



In 1889 the subject was again attacked by M. P. deMontessus de 

 Ballore,* who started by preparing a catalogue of 45.000 earthquakes. 

 From this he proceeded to discuss the diurnal periodicity, and found 

 that though each individual list and record showed a distinct periodicity, 

 there was no agreement among them and that the larger the number 

 of shocks taken the more uniform became the resulting distribution of 

 earthquakes throughout the day and night. In a subsequent paper b 

 he applied the same treatment to the seasonal periodicity with a similar 

 result and came to the conclusion that there was no real variation in 

 the frequency of earthquakes, which he regarded as a purely geolog- 

 ical phenomenon, unaffected by either astronomical or meteorological 

 influences. 



About the same time Dr. Davison began his investigation of 



1 Rep. Brit. Ass., xxviiii, (1858). 



2 Boll. Soc. Sismol. Ital. vii, 205-209 (1901). 

 8 Brit. Ass. Rep., xxviii, (1858). 



* Archives des Sciences Physiques et Naturelles, 3. Ser., xxii, 409, (1889). 

 6 Archives des Sciences Physiques et Naturelles, 3. Sev., xxv, 50-1, (1891). 



