Dr. C Davison — British Earthquakes. 301 



observers, and was compared to passing waggons, etc., in 32 per cent, 

 of the records, to thunder in 53 per cent., to wind in 5, to loads 

 of stones falling in 5, and to miscellaneous sounds in 5 per cent. 



The chief interest of this earthquake consists in its relation to the 

 Carnarvon earthquakes of June 19th, 1903. The isoseismal 7 of the 

 principal earthquake (which occurred at 10.4 a.m.) is an elongated 

 ellipse, 33i miles long and 15 miles wide, with its centre 4 miles 

 west of Penygroes, and its longer axis directed N. 40° E. The earth- 

 quake was probably caused by a slip along a submarine continuation 

 of the Aber-Dinlle fault, part of the course of which is shown in 

 Fig. 2. Shortly afterwards, on June 19th at 10.9 a.m. and 11.8 a.m., 

 and June 21st at 8.6 a.m., three after-shocks occurred with their foci 

 near the north-east margin of the principal focus. The boundary of 

 the disturbed area of the second of these shocks is represented by the 

 broken line in Fig. 2. It is 20 miles long, 13 miles wide, and 

 contains 219 square miles, its centre being 8 miles north-east of that 

 of the principal earthquake and 2f miles south-west of that of the 

 earthquake of 1906, and the direction of its longer axis IT. 47°E. 

 This line may also be taken to represent approximately the boundaries 

 of the other two after-shocks referred to above. 



It is clear that the earthquake of 1906 must be regarded as an 

 after-shock of the earthquake of 1903, its focus being situated within 

 or slightly beyond the region displaced in 1903. 



6. Berly Earthquake : August 27 th, 1906. — Time of occurrence, 

 5.56 a.m.; intensity, 5; centre of isoseismal 5, in lat. 53° 0-8' IS"., 

 long. 1° 42"3'W. ; number of records 131, from 79 places, and 85 

 negative records from 79 places. (.Fig. 3.) 



Time of Occurrence. — The number of records of the time of occurrence 

 is 72, and the mean of 13 records, which are considered by their 

 observers as accurate to the nearest minute, is 5.56 a.m. 



Isoseismal Lines and Disturled Area. — The continuous curves on the 

 map (Fig. 3) represent isoseismals of intensities 5 and 4, and the 

 broken line the isoseismal 7 of the Derby earthquake of July 3rd, 

 1904. The isoseismal 5 is elliptical in form, 17 miles long, 11 miles 

 wide, and 147 square miles in area. The longer axis is directed about 

 IS". 25° E. The centre of the curve lies 1 mile south-east of Ashbourne 

 and is less than a mile from the centre of the isoseismal 7 of the 

 earthquake of 1 904. The isoseismal 4 (which is not so accurately drawn) 

 is 48 miles long, 36 miles wide, and contains about 1,360 square 

 miles. The distance from the isoseismal 5 both to the north-west and 

 south-east is 13 miles. The shock was also felt at eight places outside 

 the isoseismal 4 — at Sheffield, 3 miles to the north ; AVoUaton, 

 ITottingham, Sherwood, Euddington, and Calverton, which lie east 

 of the isoseismal at distances of 1^, 4, 4, 5, and 5^ miles respectively ; 

 and at Thrumpton and Kingston-upon-Soar, which are respectively 

 2h and 3 miles to the south-east. The whole disturbed area must 

 therefore contain about 2,100 square miles. 



Nature of the Shock. — To the great majority of the observers the 

 shock appeared to consist of a single series of vibrations, with a mean 

 duration of 2-7 seconds. According to 11 observers, however, the 

 shock consisted of two distinct parts, separated by an average interval 



