108 The N.Z. Journal of Science and Technology. [April 
over different epicentral distances by the waves of the various phases of an 
earthquake, or (what comes to the same thing) the velocity of propagation 
of the waves of any particular phase. (The velocity of the waves of any 
type remain constant so long as the transmitting medium is isotropic in 
regard to elasticity, rigidity, and density.) 
Empirical tables drawn up by Zoppritz, from a large number of earth¬ 
quakes recorded at many stations, give the times for P and S waves for 
epicentral distances for each 10 kilometres measured along the arc. These 
tables are at present being critically examined by the Seismological Com¬ 
mittee of the British Association with a view to their correction. (I have 
used the B.A. edition of the tables in this paper.) Dr. Klotz, of the 
Ottawa Observatory, has also issued a very useful set of tables. From the 
P tables we can calculate PR-!, and from the S tables SR 15 and from both 
tables taken together PS—P, S, &c., being taken to denote not only the 
character of the waves, but also the time of transit from the origin. 
Here follow details to show how nearly the observed times (0) of the 
waves recorded on our seismogram agree with the times calculated from 
the table (C). 
Earthquake of 7th September, 1915. — Milne Record taken at 
Christchurch, New Zealand. 
Origin, San Salvador, 14° 5' N., 91° W. Time at the origin, 
1 h. 20 m. 27 s., Greenwich mean civil time (Oh. or 24 h. = midnight). 
Distance of Christchurch from the epicentre = A = 104°-25 — 11,583 km. 
Phase. 
Observed Time 
of Beginning 
(O). 
Calculated 
Time 
(C). 
Residual 
Error 
O-C (secs.) 
Remarks. 
(1) eP . . 
h. m. s. 
1 35 00 ? 
h. m. s. 
1 34 58 
+ 2 
iP .. 
1 35 42 
1 37 58 
+ 44 
Longitudinal waves. 
(2) PR, 
1 40 00 
1 39 09 
+ 51 
Longitudinal waves once re- 
(3) S .. 
1 46 51 
1 47 02 
-11 
fleeted. 
Transverse waves. 
(4) PS .. 
1 49 12 
1 48 22 
+ 50 
Waves abnormally reflected. 
(5) SRj 
1 54 12 
1 53 58 
4 14 
Transverse waves once re- 
(7) Li .. 
2 11 12 
2 11 15 
- 3 
fleeted. 
Surface waves. 
(6) x .. 
2 03 20 
2 03 47 
-27 
?sr 4 .* 
(8) L 2 .. 
2 13 37 
• • 
• . 
Velocity, 3 - 6 km./sec. 
(9) L 3 .. 
2 16 45 
. . 
. . 
Velocity, 34 km./sec. 
(10) l 4 .. 
2 23 30 
• • 
Velocity, 3‘0 km./sec. 
Followed by the 
“ coda ” or “ tail ” of the earthquake. 
Compare Apia : 
iP 
A = 85°*4. 
1 33 24 
1 33 17 
+ 7 
S 
1 44 06 
1 43 50 
+ 16 
• • 
The last two lines are from tffe Wiechert record of the same earthquake 
at Apia, Samoa. The observed and calculated times of P and S agree very 
closely. 
The residual errors of the thirty-six best returns of this earthquake 
published by the B.A.A.S. Committee vary from —64 to +79 seconds. 
Our figures are within these limits. 
* Thus 20°-85 X 5 = 10P-25; 520 sec. X 5 = 2,600 sec.; 1 h. 20 m. 21 s. + 2,600 sec. 
= 2 h. 03 m. 47 s. 
