10.— ON THE OBSERVATION OF EARTHQUAKES AND 

 OTHER EARTH MOVEMENTS. 



By John Milne, F.R.S. 



At the present time the greatest seismic activity in the African 

 Continent is to be found in the high lands of Algeria and in the 

 vicinity of the Great Lakes, and Abyssinia. If from the former of 

 these districts we travel Eastwards in the direction of Tunis, or 

 Westwards through Morocco and turn Southwards down the Western 

 side of the Continent, the activity rapidly decreases. A similar 

 decrement is met with if from the central region we go Northwards 

 down the Nile Valley or Southwards towards Cape Colony. Not 

 only is the frequency small, but the shocks themselves are 

 insignificant. They are local in character, and probably represent 

 slight adjustments on lines of existing faults. For ten years at least 

 Africa has not produced a single world-shaking earthquake, the infer- 

 ence from which is that during this period no large fault has been 

 created. If, however, we regard the African Continent as a mass 

 which extends Eastwards to the floor of the Indian Ocean, we find 

 that the sites of several megaseismic efforts have been traced to this 

 submerged frontier. From the changes which have taken place in 

 soundings after a large earthquake which has originated beneath a 

 sea or ocean, but more particularly from measurements made on land, 

 when it has been found that valleys have been contracted, and the 

 lengths of trigonometrical lines have been altered and from other 

 observations, the conclusion arrived at is that a world-shaking earth- 

 quake originates from the faulting, shattering, and the displacement 

 of an area only to be measured by many thousands of square miles. 

 If we had a knowledge of the depth to which the faulting extends, 

 a cubic measurement might be made of the magnitude of these molar 

 displacements. Inasmuch as they transmit sufficient energy through 

 the crust of our earth to create movements which may be recorded at 

 the antipodes of their origin, it seems probable that the displacement 

 downwards extends to a considerable depth. If we regard the crust 

 of our world as a layer of materials which conveys elastic vibrations 

 at about the same rate as they are conveyed by the rocks we see, 

 then seismological investigations indicate that this covering is less than 

 30 miles in thickness. In relation to the area which is displaced, 

 fracturing to this depth, or, at least, to a large fraction of the same, 

 might be compared to the formation of the tiny cracks we sometimes 

 see in the varnish covering an ordinary globe. The origins of the 

 displacements along the line of the Mozambique synclinal have 

 apparently been too far from the African coast for the resultant 

 vibrations to be felt, but they have been recorded in very distant 

 countries. At the present time in Africa the British Association type 

 of seismograph, which is not adapted to record local earthquakes 

 which can be felt, but only to record unfelt teleseismic motion, is only 

 to be found at Cape Town and Cairo. Forty other similar instruments 

 are installed in Europe, Asia, America, and Australia. Capt. H. E. 

 Lyons, R.E., of the Egyptian Survey Department, now proposes to 

 establish observing stations at Khartoum and Lake Victoria. The 



