132 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
TaBLE 16.— Transmission Intervals (in minutes) for Three Earthquakes. 
First PRELIMINARY TREMORS. SECOND PRELIMINARY TREMORS. 



DISTANCE. 
Indian. Calabrian. California. Average. Indian. Calabrian. California. Average. 
° min. min. min. min. min. min. min. min. 
10 2.6 2.244 2.4 2.41 4.6 4.06 3.85 4.17 
20 4.5 4,26 } 4.3 4.35 8.4 8.05 7.6 8.02 
30 6.2 Laity 6.1 6.14 ik! 11.26 10.9 LETS 
40 v6 frit Phar (255 13.9 13.80 13.8 13.83 
50 9.0 8.51 9.0 8.84 16.2 16.24 16.3 16.25 
60 10.6 9.67 10.2 Aacsiehe 18.9 18.68 18.6 18.73 
70 2 10.78 P15 20.7 21.12 20.6 20.81 
80 13.9 11.89 I P4ao 24.7 DEH f 22.20 Be A 
90 i Kay 4 13.00 13.25 ZT 26.00 24.0 
100 1723 14.11 14.2 30.4 28.56 25.6 







In table 16 have been collected the transmission intervals in minutes for the first and 
second preliminary tremors of the Indian earthquake of April 4, 1905, the Calabrian 
earthquake of September 8, 1905, and the California earthquake of April 18, 1906. The 
data for the Indian earthquake are taken from plates 111 and rv of Professor Omori’s report, 
that of the Calabrian earthquake from table 2 of Professor Rizzo’s first memoir,’ and that 
of the California earthquake from table 11, page 120 of this report. A very close 
agreement exists up to 50° for the first preliminary tremors and up to 70° for the second 
preliminary tremors, with the exception of the interval at 20°, and we may accept the 
averages given as representing to a very fair degree of accuracy the time intervals 
necessary to travel the corresponding distances. The four intervals marked are from 
Professor Rizzo’s second memoir” and refer to the Calabrian earthquake of October 
23, 1907; they are a little shorter, and I think a little more accurate, than the 
corresponding intervals for the earlier earthquake. 
DETERMINATION OF THE DISTANCE OF THE ORIGIN OF AN EARTHQUAKE. 
Professor Milne in 1898* showed that the distance of an earthquake from the recording 
station could be determined by the interval of time between the beginning of the disturb- 
ance and the arrival of the large waves, and he drew a preliminary curve to represent this 
relation. In 1902* he gave more accurate results based on more abundant data. Pro- 
fessor Omori has followed up this subject and has drawn curves and given equations to 
determine the distance of the origin from the duration of the first preliminary tremors and 
from the interval between the first preliminary tremors and the long waves.’ His rela- 
tions are linear, one equation being given for near origins and a second for distant origins. 
In fig. 31 curves are drawn which are taken directly from the hodographs in plate 2 and 
show the interval elapsing between the first and second preliminary tremors, between the 
first preliminary tremors and regular waves, and between the second preliminary tremors 
and regular waves. ‘These three curves are of course not independent; any one of them 
could be deduced directly from the other two. By means of them a typical seismogram 
will give two independent determinations of the distance of an earthquake origin. These 


* Nuovo Contributo allo Studio della Propagazione dei Movementi Sismici, Acad. R. d. Scienze di 
Torino, 1907-1908, vol. urx, p. 415. 
? Sulla Velocita di Propagazione della Onde Sismiche, Acad. R. d. Scienze di Torino, 1905-1906, vol. 
LXVII. 
3 Report Seis. Com. B. A. A. S., 1898. 
* Same, 1902. 
5 Pub. Earthquake Investigation Commission in Foreign Languages, Nos. 5 and 13, Bull. Imperial 
Earthquake Investigation Commission, vol. 1, pp. 144-147. Also Report on the Great Indian Earthquake 
of 1905. Publications, etc., No. 24, pp. 179-186. 
