Dr. C. Davison — T/ie Penzance Earthquake. 489' 



and Zennor, which are respectively \, f, 1, and \\ miles from the 

 curve, and a slight shock being also felt at Clowance, Sennen, and 

 Zennor. 



The shock consisted of a single series of vibrations, whicli 

 gradually increased in intensity and then faded away, the average 

 of 11 estimates of the duration being about 4 seconds. The sound,^ 

 as is usual in slight earthquakes, was heard by all the observers. 

 In 18 per cent, of the descriptions it is compared to passing vehicles, 

 in 22 per cent, to thunder, in 2 to wind, in 3 to the fall of a load 

 of stones, in 2 to the fall of a heavy body, in 52 to explosions or the 

 firing of a heavy gun, and in 2 per cent, to miscellaneous sounds. 

 Thus, in 58 per cent, of the records, the type of comparison 

 employed is one of short duration. The beginning of the sound 

 preceded that of the shock in 53 per cent, of the records, coincided 

 with it in 43, and followed it in 4 per cent. ; while the end of the 

 sound preceded that of the shock in 2 per cent, of the records^ 

 coincided with it in 74, and followed it in 24 per cent. The 

 duration of the sound was greater than that of the shook in 58 

 per cent, of the records, and equal to it in 42 per cent. 



From the above account, it will be seen that the disturbance 

 possesses some resemblance to those which are either wholly or 

 partially artificial in their origin. In particular, the brevity of the 

 sound, as shown by the frequent comparison to explosions of various 

 kinds, is suggestive of heavy gun-firing from a ship about three 

 or four miles south of Marazion. Several correspondents, however, 

 state that no battleships were in the bay on March 3 ; and I am 

 indebted to the Secretary of the Admiralty for the information that 

 " there is no record of any firing having occurred in that locality 

 on the day in question." 



Nor can the disturbance be connected with a fault-slip precipitated 

 by mining operations. For though, as Mr. Clement Eeid kindly 

 informs me, there are old workings under the sea near Marazion, 

 Penzance, and Penlee, none of these is in the required position. 

 And, moreover, the isoseismals are farther apart, and the disturbed 

 area larger, than is usually the case with such an origin. 



It may therefore be concluded, I think, that the observed 

 phenomena were not due to artificial causes, but rather to a slip, 

 three or four miles in length, along a submarine fault about 3^ miles 

 south of Marazion. In several ways — especially in the small 

 disturbed area, the closeness of the isoseismals, and the brevity of 

 the sound — the Penzance earthquake resembles an after - shock 

 of a moderately strong earthquake, and it is probable that the 

 focus was situated at no great depth. The exact direction of the 

 earthquake-fault is somewhat uncertain, owing to the incompleteness 

 of the isoseismals ; but it cannot have deviated widely from that of 

 the lodes in the neighbourhood of Marazion. 



In an interesting paper,^ Mr. Clement Eeid has suggested tbat an 



^ " Ou the probable occurrence of an Eocene outlier off the Cornish Coast": 

 Quart. Journ. Geol. Soc, vol. Ix (1904), pp. 113-117. 



