1915-16.] The Ochil Earthquakes of the Years 1900-1914. 285 
Length of Focus . — Taking the difference between the longer and shorter 
axes of inner isoseismal lines as a measure of the length of the seismic 
focus, in earthquakes of the first class, the average length of the focus 
is 5 miles, and in those of the second class 4 miles. For the third class, 
the corresponding figures cannot be ascertained. It is, however, clear from 
the nature of the shock, the nature of the sound, and the time-relations 
of the sound and shock, that the focus in earthquakes of the third class 
was usually of very small dimensions. 
IV. Origin of the Earthquakes. 
The seismic evidence is sufficient to determine the approximate direction 
and the hade of the fault, and to trace roughly the position of the fault- 
line : — (i) The direction of the longer axes of the isoseismal lines is 
E. 13° N. in the earthquakes of September 17 and 22, 1900; E. 27° N. in 
that of July 23, 1905; E. 29° N. in that of September 21, 1905 ; E. 18° N. 
in those of October 8, December 28 and 30, 1906 ; E. 25° N. in the two 
earthquakes of October 20, 1908 ; and E. 7-1° N. in the earthquake of 
May 3, 1912. The average of these ten directions is E. 19° N. (ii) The 
direction of the hade of the fault is indicated by the relative positions 
of the isoseismal lines of the earthquakes of September 21, 1905, and 
May 3, 1912. As these lines are farther apart to the north than to the 
south, the originating fault must hade to the north, (iii) The fault-line 
must therefore run approximately in the direction above indicated and 
pass through a point a short distance to the south of the centres of, the 
inner isoseismal lines of these earthquakes. From the unusual intensity 
of some of the shocks at Alva, Menstrie, Airthrey, and Bridge of Allan, 
and from the fact that so many of the shocks are felt only at one or 
a few of these places, it may be inferred that the fault-line traverses the 
district in their immediate neighbourhood. 
On the maps of the earthquakes is shown the course of the great Ochil 
Fault in the epicentral districts. Its mean direction there is E. 13° N. ; its 
hade is unknown from geological evidence, and the fault-line passes 
through or near the Hillsfoot villages. As it satisfies two of the elements 
required by the seismic evidence, it may be concluded that the Ochil 
earthquakes are due to movements along this great fault. 
Assuming this conclusion to be correct, there can be no doubt, from the 
evidence provided by the principal earthquakes of 1905 and 1912, that the 
fault hades to the north. This inference is supported, if it needs further 
support, by the evidence of the principal earthquake of 1908 and of all the 
earthquakes of the second class and of several of those of the third class. 
