402 STUDIES FOR STUDENTS: 
tremors, V, that of the second preliminary phase,’ and B that of the 
main or large waves in the seismograph; and if A be the distance in 
megameters (1 megameter equals 1,000 kilometers), then 
1+A=V,-V, 
and 
34=B-—\N,. 
Benndorf has proven by many determinations the correctness of 
these formulas,? which are known as “‘Laska’s Rules,” and which 
may be stated in simple form as follows: 
1. The duration of the first preliminary tremors in minutes, less 
one, is the distance of the seat oj disturbance in megameters. 
2. The duration of all preliminary vibrations in minutes, less one, 
is thrice the distance of the seat oj disturbance in megameters. 
An illustration of the precision in the measurement of distance 
with unfelt quakes is given by Alfani, the director of the Ximeniana 
station at Florence, for the Indian earthquake of April, 1905,3 the 
error in determining the distance being only 32 kilometers if the 
geographic center of the affected district be regarded as the starting- 
point of the shocks. 
Omori, on the assumption that the velocity of the waves which 
produce the first preliminary tremors (over an arcual path) is 13.7 
kilometers per second, and that the velocity of the second preliminary 
tremors is 7.2 kilometers per second, has deduced a formula for 
finding the time of occurrence of an earthquake from the observa- 
tions at a distant station.4 
His formula is 
bo = be ae TeLOS 
where /, is the time of occurrence of the earthquake, ¢,the commence- 
« Laska finds that earthquakes less than 500 kilometers distant produce no second 
preliminary phase in the seismograph. 
2 H. Benndorf, “Ueber die Art der Fortpflanzung der Erdbebenwellen im Erdin- 
nern,” Mitth. d. Erdbeben-Kom. d. k. Acad. d. Wiss. 2. Wien, N. F., No. 29 (1905), 
p- 19. 
3 P. G. Alfani, “Tl terremoto d’India del 4 Aprile, 1905, e le registrazioni sismiche 
all’ osservatorio Ximeniana di Firenze,” Rivista geografica Italiana, Anno XII (1905), 
fasc. V, pp. 1-6. 
4 F. Omori, ‘““On the Estimation of the Time of Occurrence at the Origin of a 
Distant Earthquake from the Duration of the First Preliminary Tremors Observed at 
Any Place,” Bull. E. I. C., Vol. I, No. 1 (1907), pp. 1-4. 
