120 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
100 km., respectively ; but the curvature of the lines practically disappears at distances 
from the epicenter equal to twice the depth of the focus. 
Professor Rizzo has made strong inflections in his hodograph of two Calabrian earth- 
quakes.t A straight line would fit the observations of the first preliminary tremors in 
the first earthquake to distances of 2,000 km. rather better than his curves, especially 
for the near stations; and the observations which bend the hodograph of the first pre- 
liminary tremors in the second earthquake are far too inaccurate to justify the curve. 
The observations of the second preliminary tremors in both cases are too few to be deci- 
sive. Moreover, the points of inflections of the curves are at a distance of about 800 km., 
which would correspond to a depth of focus of about 100 km.; whereas Professor Rizzo 
does not think the depth in either case greater than 50 km. 
The curvature of the hodographs near the origin has not been shown in plate 2 because 
the scale is too small. The times given by the curves are measured from the time the 
earthquake occurred, as nearly as this could be determined, and not from the time the 
disturbance reached the surface at the epicenter, as has usually been done. There are 
certain objections to the usual method; the disturbance does not pass from the focus 
directly to the surface and then along the surface to distant points, but it goes directly 
to the distant points, and its time of arrival there, even at such short distances as four 
times the depth of the focus, is not materially affected by this depth, tho the time of 
arrival at the surface is. It is better, therefore, for our base-line to represent the time 
of occurrence of the shock at the focus; and if the scale of the drawing is sufficiently large 
to show the upward curvature of the hodograph, the curve would not pass thru the origin 
but above it, at a distance representing the time necessary for the shock to go from the 
focus to the surface. This will be only afew seconds; perhaps never more than 7 seconds 
for the first preliminary tremors and 10 seconds for the second preliminary tremors, as 
these intervals would correspond to a depth of focus of 50 km. 
In table 11 are shown the velocities of the first preliminary tremors and second pre- 
liminary tremors in kilometers per second, measured along the chord. The velocities are 
not calculated from actual observations at the stations, but from the hodographs. 
TaBLE 11.— Velocities of First and Second Preliminary Tremors in Kilometers per Second along the Chord. 




















DisTANCE. First PRELIMINARY TREMORS.| SECOND PRELIMINARY TREMORS. 
Degrees. Are. Chord. Interval. Wgloctty, Interval. Velocity, chord. 
= km. km. min, min. 
0 0 0 0.0 vee 0.0 4.8 
10 1112 1110 2.4 eke 3.85 4.8 
20 2224 Zotz 4.3 8.6 7.6 4.9 
30 sooo 3297 6.1 9.0 10.9 5.0 
40 4447 4357 lok 9.4 13.8 5.3 
50 5559 5384 9.0 10.0 16.3 +3) 
60 6671 6370 10.2 10.4 18.6 5.7 
70 7783 ; 10.7 20.6 5.9 
80 8894 é . Pp as 6.1 
90 10006 24.0 6.2 
100 11118 ior ae 
110 12230 { aoe coh 
120 13342 { oe ae 
130 14453 ey eine 



1 Sulla Velocité di Propagazione della Onde Sismiche, Acad. R. d. Scienze di Torino, 1905-06, 
vol. Lvu, pp. 309-350; Nuovo Contributo allo Studio della Propagazione dei Movementi Sismici, 
same, 1907-08, vol. Lix, pp. 375-419. 
