286 Proceedings of the Royal Society of Edinburgh. [Sess. 
The centres of the isoseismal lines of all these earthquakes lie on the north 
side of the fault, along a band which is roughly parallel to the fault-line. 
In the great majority of the shocks, it is impossible to define the 
boundary of the disturbed area towards the north. The shocks were not 
strong enough to be felt beyond the Ochil Hills. In these cases, it may be 
inferred that the epicentres were as a rule not far from the fault-line. In 
the earthquakes of the first and second classes, however, the isoseismal 
lines or the boundary of the disturbed area can usually be drawn, and it 
is a significant fact that, in every one of these earthquakes, by far the 
larger part of the disturbed area lies on the north side of the fault-line. 
For instance, in the earthquake of May 3, 1912, the portion of the area 
within the isoseismal 7 on the north side was 10*3 times as great as that 
on the south side. For the isoseismals 6, 5, and 4, the corresponding figures 
were 4*4, 3*7, and 2*8. Now, in the Inverness earthquake of September 18, 
1901, the areas within the isoseismals 8, 7, 6, 5, and 4 on the south-east 
side of the fault were respectively 1*9, 1*6, 2T, 1*9, and 20 times the areas 
on the north-west side. From the unusual expansion of the disturbed 
areas of the stronger Ochil earthquakes on the north side of the fault, it 
may be inferred that the epicentral areas were of considerable magnitude 
in a horizontal direction at right angles to the fault-line. 
Another important feature of the stronger earthquakes is their great 
intensity considering the smallness of the disturbed areas. Taking the 
three principal earthquakes, that of 1905 was of intensity 6 and disturbed 
an area of about 1000 square miles; those of 1908 and 1912 were of 
intensity 7 and disturbed areas of about 1000 and 605 square miles. Now, 
in other British earthquakes of intensity 7 since the year 1889, the 
disturbed area ranges from 12,000 to 63,600 square miles, the average being 
31,000 square miles. Thus, the seismic foci of the stronger Ochil earth- 
quakes must have been at a very slight depth below the surface. 
From these two conclusions, it may be inferred that the originating 
fault is inclined at a very small angle to the horizon. 
The portion of the fault affected by the recent movements extends from 
a mile or two east of Tillicoultry to a short distance west of Bridge of 
Allan — that is, over a length of about nine miles. 
In order to study the distribution of the epicentres along the fault, it 
will be convenient to divide the portion affected into four regions. That 
in the neighbourhood of Dunblane, Bridge of Allan, and Airthrey may be 
called the western region ; that between Airthrey and Menstrie the west- 
central region. The region between Alva and Tillicoultry may be called 
the eastern region; that from Menstrie to Alva the east-central region. 
