418 Dr. C. Davison— British Earthquakes. 
1890 and 1901 and Carnarvon in 1903 were followed by 38 after- 
shocks, and the seven strong twin earthquakes by 18 after-shocks. 
DisTRIBUTION IN SPACE. 
The total number of earthquakes which it is possible to associate 
with known lines of fault or folding is 199.1. Of the 199 earthquakes, 
153 were probably connected with faults of the Caledonian system, 
23 with Charnian, 10 with Malvernian, and 13 with Armorican faults. 
In Scotland, with the exception of the Loch Broom earthquake of 
1892, which was connected with a Charnian fault, the remaining 
129 shocks were due to movements along Caledonian faults. In 
England 15 earthquakes were connected with Caledonian faults, 22 
with Charnian, 6 with Malvernian, and 5 with Armorican faults. In 
north-west and central Wales, the earthquakes were connected with 
Caledonian faults, in the south of Wales with Armorican faults. Of 
the nine strong earthquakes, the Inverness earthquake of 1901, the 
Derby earthquakes of 1903 and 1904, the Carnarvon earthquake of 
1903, and the Doncaster earthquake of 1905 were connected with 
Caledonian faults, the Hereford earthquake of 1896 with a Charnian 
fault, and the Pembroke earthquakes of 1892 and 1898 and the 
Swansea earthquake of 1906 with Armorican faults. The last four 
earthquakes were the strongest felt in this country throughout the 
twenty-one years. Twin earthquakes are entirely confined to England 
and the south of Wales. 
OrtciIn oF British EaARTHQUAKES. 
The study of British earthquakes has led to the association of 
a large number of them with known faults, especially in those in 
which there is reason to think that the foci were situated at a small 
depth. In other cases, the investigation of the earthquakes has 
thrown light on the structure of the epicentral districts at depths far 
beyond the reach of methods at the disposal of the field-geologist. 
For instance, the complicated structure of the English Lake District 
is superposed on one of a more simple character, the Carlisle earth- 
quakes of 1901 having originated in a fault not less than 23 miles 
in length, and directed approximately N. 5° E. Similarly, in south” 
Glamorgan, as shown by the Swansea earthquake of 1906, a fault at 
least 223 miles long runs in a direction about E. 5° N. from west of 
Swansea to the neighbourhood of Llwynpia. 
A rough approximation to the length of the seismic focus is given 
by the difference in length between the longer and shorter axes of 
the innermost isoseismal.? In estimating the average length of the 
focus for the different classes of British earthquakes, after-shocks are 
omitted, as in them the length of the focus is governed to some extent 
by that of the principal earthquake. The results are as follows :— 
1 The earthquakes omitted are the 41 shocks felt in Glen Garry from 1889 to 1899, 
2 shocks near Tadcaster in 1890, 5 in Pembrokeshire in 1893, 3 m Annandale in 
1894 and 1896, and 1 near Beddgelert in 1904. 
2 Gerland’s Beitrage zur Geophysik, vol. ix, 1908, p. 224. 
