Chas. Davison — British Earthquakes, 1891. 303 



area ; while at St. Clether, which is near the eastern boundary, the 

 intensities of the two shocks were approximately equal. 



I conclude, then, the shocks were really distinct, the focus of the 

 second being to the south of that of the first ; and it is obvious that, 

 if this were the case, the interval between the two shocks should 

 have been least at the southern end of the disturbed area, an infer- 

 ence which agrees with the estimate already quoted of the apparently 

 short interval at Michaelstow. 



The position indicated on the map as that of the epicentrura 

 probably corresponds closely with the superficial position of the 

 centre of intensity of the whole seismic focus. The distance between 

 the two foci may have been as much as two or three miles, but Dr. 

 Wade's interesting observation at Boscastle of the sound being heard 

 during the whole interval between the shocks seems to show that 

 the foci were not completely isolated. If the shocks were due to 

 fault-slipping,^ we may infer, from the form of the disturbed area 

 and from the facts summarised in the preceding paragraphs, that the 

 direction of the fault must be north and south. Boscastle, again, is 

 the only known place where the sound was heard continuously 

 between the two shocks, and, as mentioned above, the sound-area 

 may have overlapped the disturbed area along its south-east margin. 

 These facts seem to indicate, but not with certainty, (1) that Bos- 

 castle must be near the spot where the normal to the seismic focus 

 meets the surface of the earth, and (2) that the upper margin of the 

 focus from which the sound-vibrations proceeded lay to the east or 

 south-east of the epicentrum ; i.e. that the fault must hade to the 

 westward. Lastly, if this conclusion be correct, the line in which 

 the fault intersects the surface must pass to the east of the epicentrum, 

 and at a distance from it probably not much greater than one or two 

 miles.^ 



Summing up on the supposition that the earthquake was fault- 

 formed : the first shock was probably caused by a slip within a 

 small area about a mile north of the point marked as the epicentrum ; 

 but the slip continued southward for about two miles, though, so 

 slight in extent that only earthquake-sounds were produced, an 

 interval of perhaps five or more seconds being necessary for the 

 slipping over this distance to take place ; at the southern end the 

 slip being again great enough within a small area to produce a 

 shock of approximately the same intensity as the first. 



It is interesting to notice the relation between this earthquake and 

 that felt in East Cornwall on October 7, 1889. The boundary of 

 the disturbed area of the latter is indicated on the map by a dotted 

 line. Its epicentrum lay about 2f miles S.W. of Altarnon, and the 



^ There is no fault marked in the Survey Map of the district with which the 

 earthquake can be connected. 



* The fact that no shock was noticed at Otterham is in favour of the position 

 above assigned to the fault ; for, if the earthquake were due to fault-slipping, the 

 earth-waves iu the rock-masses on either side of the fault would start in opposite 

 phases of vibration and might possibly interfere to a very great extent along the line 

 of fault. (See a paper " On the Existence of Undisturbed Spots in Earthquake- 

 shaken Areas," Geol. Mag. Dec. III. Vol. III. 1886, p. 157). 



