Dr. G. Davison — The Stafford Earthquakes of 1916. 30$ 



surface were curved and increasing in inclination to the horizon as it 

 approached the surface of the earth, the position above given would 

 be the line in which a tangent-plane at the depth of the focus to the 

 fault-surface meets the surface of the earth, and this ought to lie 

 some distance to the south of the actual fault-line. 



This apparent displacement of the presumed fault-line is specially 

 characteristic of deep-seated earthquakes. In British earthquakes, 

 the only test that we possess of the greater or less depth of the 

 seismic foci is the less or greater rate of decline in the intensity of the 

 shock at the surface. A rough test of this rate of decline is the area 

 enclosed within the isoseismal 4 for shocks of different intensity at 

 the epicentre. For earthquakes of intensity 7, this average is 

 22,000 square miles in England and Wales, and less than 1,000 

 square miles in Scotland ; for those of intensity 5, the corresponding- 

 figures are 780 and 450. Thus, the rate of decline of intensity 

 outwards from the epicentre is much more rapid in Scotland than in 

 England; and this implies that the average depth of focus is much 

 less in Scotland than in England and Wales. Now, Scottish earth- 

 quakes which cannot be correlated with known faults are very rare. 

 In English and Welsh earthquakes it is quite an exception to be able 

 to associate an earthquake Avith a known fault; and, in the few 

 cases in which this is possible, the area within the isoseismal 4 is 

 always small. It is therefore not surprising if we cannot point to 

 any definite fault in the case of the Stafford earthquake. 1 Should 

 we not rather look on the results of the earthquake investigation as 

 adding to our knowledge of the structure of the crust at a depth 

 which is far beyond the range of the field-geologist's methods ? 



Returning to the Stafford earthquake, the positions of the two 

 epicentres cannot be determined with accuracy, but, from the 

 form of the isoseismal 7 and the course of the synkinetic band 

 (which must pass between the epicentres), it follows that the 

 eastern epicentre must lie about two miles north-east of Stafford and 

 the western epicentre about twelve miles north-west of Eccleshall. 

 The distance between the epicentres would thus be about 8 or 

 9 miles, which differs little from the average (10 or 11 miles) 

 for British twin -earthquakes. 



For very many years, possibly for centuries, there has been no 

 perceptible movement along the earthquake fault. Then, suddenly, 

 two earthquakes occurred so closely together that the later could 

 not possibly be a consequence of the earlier, for it took place before 

 the earth-waves had time to traverse the interfocal region. The 

 only movement that could produce such practically simultaneous 

 displacements is one of rotation. We may conceive the earthquake- 

 fault as cutting transversely a crust-fold (Fig. 3) the crest C and 

 trough T of which are separated by a distance of about nine miles. 

 If a small step took place in the growth of the fold, that is, if the 



1 Close to Eccleshall there is a small post-Triassic fault (Geol. Siirv. map, 

 sheet 22, S.W.), which is parallel to the longer axis of the isoseismal 7, and 

 hades to the north. If this fault were continued to the east it would occupy 

 the position assigned to the fault-line by the seismic evidence. 



