182 REPORT— 1897. 



the sea are more numerous and more intense than those originating on 

 land, the inference is that bradyseismic activity and phenomena which 

 accompany earthquakes, like landslides, are also more pronounced beneath 

 the sea than they are on land. 



Bradyseismical movements include movements of upheaval or depres- 

 sion, by which rocks are bent, folded, faulted, ordisplaced, by thrust, together 

 with those which are the result of overloading, and may be exhibited as basal 

 crush. One set of movements involve the idea of elastic and seismic 

 strain, whilst the others a gravitational effect. 



2. Sedimentation and erosion. — Submarine landslides which in part 

 are due to earthquakes. 



The effects of overloading, submarine springs and currents. 



.3. Changes evidenced by cable interriqjtions and soundings. 



4. Conclusions. 



1. Bradyseismic Action, 



Earthquakes the Origin of ivhich are Submarine. — The earthquakes 

 which have a submarine origin may be divided into three groups : — 



1. Those which have been felt and recorded on land, and which, 

 therefore, may be assumed, in the generality of cases, to have originated 

 on a coast-line or within a few hundred miles off in the ocean. 



2. Those which have been recorded on shipboard out at sea, either as 

 tremors or as severe movements. Many of these disturbances are 

 probably volcanic. 



3. Those which have not been felt on land, but have been distinctly 

 recorded there. In this group we find many of the earthquakes which 

 shake the world. 



As illustrative of the frequency of the first group, I will quote from 

 observations made in Japan. • Between 1881 and 1883 in North Japan 

 the writer found that, out of 419 shocks, no less than 218 of them had 

 originated beneath the ocean. There had been 137 which had originated 

 on or near the seaboard, and therefore some of these had been of sub- 

 oceanic origin, whilst only 64 had originated inland. A large number of 

 these earthquakes came from the deep water off the mouth of the Tonegawa, 

 the largest river in Japan, which, as it approaches the sea, crosses the 

 alluvial plain of Musashi. 



Between 1885 and 1892 no less than 8,331 earthquakes were recorded 

 in Japan — that is, on the average during this period of eight years there 

 were about one thousand shocks per year.'^ A glance at the map showing 

 the distribution of origins of these disturbances shows that nearly all of 

 them have originated along the eastern seaboard, and have been frequent 

 near the alluvial plains. Between January 1885 and December 1888, 

 when seismic activity was in a normal state — that is to say, when there 

 were no long series of after-shocks — 2,018 earthquakes were recorded, of 

 which at least 1,034, or 50 per cent., originated beneath the sea. In 

 Japan, therefore, along a coast-line of 1,140 miles, there has recently 

 been at least about 250 submarine shocks per year. In some years there 

 have been 500. 



From a seismic map of the world, I should estimate that round the 



' ' On 387 Earthquakes observed during Two Years in North Japan,' by John 

 Milne, Trans. Seis. Sec, vol. vii. pt. ii. 

 * Trans. Seis. Soc, vol. xx. 



