300 Charles Davison—British Earthquakes. 
Gwithian, Hayle, Helston, Penryn, Redruth, St. Ives, and St. 
Stythians. 
It will be seen from the map that, all over the disturbed area, the 
beginning of the sound preceded that of the shock and the end of 
the sound followed that of the shock, the exceptions occurring 
chiefly at places at some distance from the epicentrum and where 
the slighter vibrations would be less perceptible. 
Origin of the Shocks.—The disturbed area of the first shock is 
approximately circular. This, in the case of so slight a shock, shows 
that its seismic focus was of small dimensions. 
The focus of the second shock must have been of some magnitude. 
This is indicated by its elongated disturbed area, and perhaps also 
by the long duration of the shock. Assuming it to have been fault- 
formed, the direction of the fault in the neighbourhood of the 
epicentrum must be parallel or nearly so to the longer axis of the 
disturbed area, 7.e. must be about east and west. The intensity of 
the shock was greatest at Helston, Nancegollen, Penrose, Porthleven 
and Wendron, all lying to the south of the longer axis of the 
disturbed area. Now, the intensity of a shock should be a maximum 
near the point where the normal to the seismic focus through ‘its 
centre meets the surface. The fault must therefore hade towards 
the south, and consequently the fault-line must lie on the north side 
of the epicentrum. This inference is corroborated by the relation 
which the outer line bounding the sound-area and the area over 
which the shock was very slightly felt bears to the isoseismal line 
of intensity 1V. The distance between these two lines is greater 
on the north side than on the south; and, since the sound-vibrations 
proceed chiefly from the upper and lateral margins of the slip-area 
or seismic focus, it follows that the upper margin must be on the 
north side of the epicentrum; in other words, that the fault hades 
towards the south. Since the centre (C) of the area bounded by 
the outer line is vertically above a point within the upper margin 
of the focus, the line of fault must be a short distance, probably not 
more than a mile or so, to the north of this poimt. The line DD on 
the map represents a probable position of the fault-line, but it may 
be in error by as much as half a mile either to the north or south. - 
Having thus ascertained the position and hade of the fault, at any 
rate approximately, we may now consider the nature of the displace- 
ment or fault-slip which produced the earthquakes. In the first 
place, the line joining the two epicentral points is parallel to the 
fault-line, and therefore the centres of intensity of the two foci 
must have been at about the same depth. The horizontal length 
of the first focus, as we have seen, was probably short. That of 
the second must have been several miles. We have no means 
of determining it accurately ; but, in a case like this, we cannot be 
far wrong in taking it as the difference between the lengths of the 
two axes of the disturbed area. For the isoseismal of intensity IV., 
this difference is five miles; for the outer line it is seven miles. 
Again, the estimates of the duration of the shock and sound at 
Budock and St. Ives, places close to the longer axis of the disturbed 
wi 
