184 REPORT — 1897. 



traced to earthquakes originating at great distances, it is, for the present 

 at least, assumed that the cause of the former is similar to the cause of 

 the latter. If this is the case, the only place towards which we can turn 

 to find the origin of the former appears to be beneath our oceans, and 

 when they are of a magnitude approaching that of June .3 their origins 

 must have been very far from land, otherwise a sensible shaking would 

 have beeii observed upon the nearest shores. 



If we take the three classes of records to which we have referred in 

 conjunction, the conclusion to which they point is not simply that the 

 submarine evidences of seismicity are more nvmierous than those on land, 

 but also that they are very much more intense. 



The Character of Submarine Seismic Districts. — If we compare 

 together the characters of the districts where earthquakes of submarine 

 origin are frequent with those where they are practically unknown, the 

 differences are striking. In the former the land, as shown on the seaboard, 

 usually consists of strata which are geologically new ; it exhibits evidences 

 of recent elevation, some of which can be traced to historical times, whilst 

 its average slope from the mountains in the interior down beneath the 

 ocean is, over a considerable distance, relatively very steep.' The unit of 

 distance over which such slopes have been measured is taken at 2°, or 

 120 geographical miles. The following are a few examples of such slopes : — 



West Coast, South America, near Aconcagua . 1 in 20-2 ] 



Tlie Kurils ifrom Urap 1 in 22-1 | Sei.sraic 



Japan, west coast of Nippon 

 Sandwich Islands nortliwards 

 Australia generally 

 Scotland from Ben Nevis 

 South Norway 

 South America, eastwards . 



1 in 30-4 r districts. 



1 in 23-5 ) 



1 in 91 S 



1 in 158 (Non-seismic 



1 in 73 I districts. 



1 in 943 I 



The conclusion derived from this is, that if we find slopes of con- 

 siderable length extending downwards beneath the ocean steeper than 1 in 

 35, at such places submarine earthquakes, with their accompanying land- 

 slips, may be expected. On the summit of these slopes, whether they 

 terminate in a plateau or as a range of mountains, volcanic action is 

 frequent, whilst the earthquakes originate on the lower portions of the 

 face and base of these declivities. 



The Cause of Seismic Strain, Deformation, Thrust, and Crush. — "We 

 assume that the contours referred to in the last section are mainly the 

 result of rock-movement, and that seismic strain, due to a tendency to 

 further adjustment, is greatest where earthquake origins are most frequent. 

 The home of the volcano is evidently the place where the rocks have been 

 most deformed, whilst that of the earthquake is at the base of steep sub- 

 oceanic slopes where most deformation is in progress. The nature of the 

 forces in operation producing this deformation is twofold. First, there 

 is the horizontal thrust, so strongly emphasised by Lapworth, which may 

 or may laot tend to increase the height of the mountain ranges bounding 

 its line of action ; and, secondly, a factor dependent on gravity, which, 

 acting on the side of subaerial and marine denudations, tends to lower 

 them. Earthquakes are for the most part spasmodic accelerations in 

 processes with these characters. 



' See ' Note upon the Geographical Distribution of Volcanoes,' by J. Milne, Geol. 

 Mat)., April 1880. Also address to the Geological Section of the British Association, 

 in 1892, by Professor C. Lapworth, LL.D., F.R.S. 



