THE TIME AND ORIGIN OF THE SHOCK, 11 
TaBLE 1.— Times, Velocities, and Errors for 
Various Positions of the Focus. 








Yo. Zoe bh. eat A? 
20 0 31.0 Pfaff 5.0 
20 20 29.8 7.2 10.2 
40 0 30.1 8.3 4.9 
40 20 28.9 8.7 5.9 
50 20 29.2 8.8 5.3 
eee ee 

If now we assume a velocity of 7.2 km./sec., and repeat the calculations determining 
only tp, we get the following results for various positions of the centrum: 
TaBLE 2.— Times and Errors for Various Positions of the Focus. 







Yo- Zo to. A?, Yo. Zo. bh. Az: 
20 0 30.5 9.3 50 0 27.8 7.6 
20 20 29.8 10.2 50 20 27.4 hed 
30 0 29.8 8.9 60 0 26.7 8.3 
30 20 29.2 7.2 60 20 26.2 7.6 
40 0 28.9 6.7 60 40 25.1 6.7 
40 10 28.8 6.9 100 0 21.9 16.6 
40 20 28.4 6.6 100 20 21.6 16.0 









There is not a very great difference in the sums of the squares of the errors for values of 
Yo varying between 20 km. and 60 km., but they increase at both of these extreme dis- 
tances (errors of a few tenths are insignificant as they are partially due to approxima- 
tions in the calculations); and the times of course become earlier as the origin is placed 
further northwest. We may therefore adopt for the approximate position of the centrum 
of the violent shock, yy = 40 km.+ 20 km., 2) = 20 km. + 20 km., and for the time of 
occurrence, fy = 5° 12™ 28° + 2 seconds. The individual errors of the times of obser- 
vation become at San Rafael, Mare Island, Berkeley, and Mount Hamilton, respectively, 
+ 0.6 second, + 0.4, — 2.1, + 1.3. If in these calculations we had used a velocity of 6 
km./sec. or 8 km./sec., our results would not have been altered beyond the uncertainty 
indicated. We may therefore write for 
ty = 13" 12™ 28° + 2 seconds G. M. T., 
X = 122° 48’ W. + 5’, 
gd = 38° 03’ N. + 4’, 
2) = 20 km. +20 km. 
The violent shock 
The point lies between Olema and the southern end of Tomales Bay. This position of the 
point of beginning of the violent shock receives some confirmation by the fact that the 
violence of the shock was probably as great in this neighborhood as anywhere, and that 
the displacements along this part of the fault-line were the greatest recorded. 
THE DEPTH OF THE FOCUS. 
There are two ways of determining the depth of the focus: by observations of the times 
of arrival of the shock at various stations and by a consideration of the distribution of the 
damage and other effects produced by the disturbance over the surface of the earth. The 
two methods do not determine identically the same point. The time method gives the 
location and depth of the point where the shock started, whereas the method depending 
upon the distribution of intensity gives the depth of the whole of the fault-plane. 
