54 



KNOWLEDGE. 



February. 1911. 



The sour.J w hich forms so essential a part of an 

 earthqual'c is -a low, ru'nbliv^g or grating noise. 

 Near the centre it is henr.i by all observers, but as 

 the distance increases it becomes inaudible to a 

 larger and larger perc_ itr;;;- of persons. The reason 

 is that the numbe- r_ brations occurring everN- 

 second is very ne;!r lIi which forms the lower limit 

 of audibility, or a:~.;t sixteen to thirt\--t\\o a 

 second: and. uit'p ; slight decrease in strength, 

 these vibrations il to affect the ear. Moreo\'er, 

 this limit varit i different persons, so that some, 

 and not ot'i : are able to hear the sound. Thus, 

 in one :_. and even in the same house, one 



observe: will tleclare that the shock was unaccom- 

 panied by sound, while another will record a noise 

 louder than thunder or than main- traction engines 

 passing together. 



Some obser\-ers tiiui it liifficult to describe the 

 ^ound. remarking that it resembles no noise with 

 which they are acijuainted. The majorit\" compare 

 it to some well-known t\"pes. Those most frequenth- 

 referred to are the rapid passing of heav\- waggons, 

 traction-engines, steam-rollers, or railwa\' trains, on a 

 hard road, over a bridge, or through a tunnel : the 

 deep roll of thunder, and generally distant thunder ; 

 and the rising of a strong wind or a chimne\' on hre. 

 Less frequentl}' in strong earthquakes, but more 

 often in slight ones, we hax'e comparisons to sounds 

 of short duration, such as the tipping of a load of 

 stones, the fall of a heav\- muffled weight, the firing 

 of a distant gun. nr a blast in a quarr\-. 



By most obserxers, by two out of ex'ery three. 

 the sound is heard just before the shock begins. 

 A smaller proportion, about two out of e\'er\- five. 

 hear the sound after the shock is over. In man\" 

 foreign countries the sound is heard before the shock 

 only, and this has led to the impression that the 

 waves which form the sound tra\el more rapidh' 

 than those w hich form the shock. The true explana- 

 tion of the discrejiancy is that the British, as a race, 

 are more capable of hearing low sounds than most 

 other people. Tliey possess a lower limit of audi- 

 bilit}', which, in earthquake-countries. ma\- be of 

 service in enabling them to escape into the open air 

 with the first rumble of the on-coming earthquake. 

 That the two series of waves travel with ajjpro.xi- 

 mately, if not quite, the same velocity is evident from 

 the fact that, in strong earthquakes, the percentage 

 of observers who hear the sound before the shock, 

 and also of those who hear it after the shock, is 

 almost exactly the same at all distances from the 

 origin. If there were any appreciable difference in 

 velocity, the sound and shock would soon become 

 separated from one another. 



Simple and Twin EARTHguAKES. 



The great majorit}- of British earthquakes belong 

 to the first of the three classes mentioned at the 

 beginning of this paper. Of the two hundred and 

 fifty earthquakes, two hundred and thirt\-nine 

 were simple, and ele\en twin. The latter include, 

 however, most of the strong shocks felt in this 



country. Of these, nine in number, two were 

 simple earthquakes, the average area disturbed by 

 them being twent\--nine thousand square miles. The 

 remaining seven were twins, and disturbed, on an 

 average, an area of forty-seven thousand square miles. 

 The latter include also the four strongest earthquakes 

 of the period considered, namely the Pembroke earth- 

 quakes of 1892 and 189j. the Hereford earthquake 

 of 1896, and the Swansea earthquake of 1906. The 

 average disturbed area of these four earthquakes is 

 sixty-eight thousand square miles, or three-quarters 

 of the total area of Great Britain. 



.-\nother point in which simple and twin earth- 

 quakes differ is in the number of minor shocks which 

 attend them. The total number of such shocks 

 during the twenty-one \'ears is se\ent\-one. all but 

 five of which preceded or followed the nine strong 

 earthquakes and the Inverness earthquake oi 1890. 

 This earthquake, which belongs to the class of simple 

 earthquakes, just falls short of the test required to 

 place it among the strong earthquakes. The area 

 included within the isoseismal of intensity four, is 

 four thousand three hundred and fort\' square miles, 

 and the area disturbed by it about seven thousand 

 five hundred square miles. Of the sixt\--six minor 

 shocks which attended these ten earthquakes, fifteen 

 were fore-shocks and fift\'-one after-shocks : and, of 

 the after-shocks. thirt\"-three followed the three 

 simple earthquakes, and eighteen the seven twin 

 earthquakes. In other words, the average number 

 of after-shocks of a simple earthquake is eleven, and 

 of a twin eartlKpiake between two and three. 



.MiNOK Shucks. 



I5esides the slight shocks which are intimately 

 connected with the strong earthtjuakes, there have 

 been two series of shocks, for the most part of slight 

 intensity and confined to certain limited districts. 

 Both of these districts are in Scotland, and are 

 responsible for the large number of slight shocks 

 felt in that country. The first series lasted for about 

 twelve years, from 1888 to 1899, and were confined 

 to Glen Garrw a vallev in the west of Inverness- 

 shire. In this interval fift\' slight shocks ware 

 recorded, but owing to the mountainous character of 

 the district little more is known about them than 

 their time of occurrence and their nature at one or 

 two places. The shock was invariably a slight 

 tremor of very brief duration, and accompanied by a 

 sound compared most frequently to a carriage 

 passing, but occasionally to thunder. 



The series of earthquakes felt chiefiy in the district 

 lying to the north of the Firth of Forth and south of 

 the Ochil Hills is more interesting, on account of the 

 larger number of observers, though here again the 

 presence of a range of hills to the north prevents us 

 from determining the disturbed areas of all but the 

 strongest shocks. This series began with four shocks 

 in the year 1900, and then ceased, with one excep- 

 tion in 190.3. until 1905. when both the number and 

 intensit\' increased. , In this year ten were recorded, 

 one of them being felt over an area of a thousand 



