Hogben. — Earthquake-origins in the South-west Pacific. 139 



Art. XIII. — Earthquake-origins in the South-west Pacific in 1910. 



Bv George Hogben, M.A., F.G.S. 



[Read before the Wellington Philosophical Society, 4th October, 1911.] 



The most interesting problems in connection with seismology at the 

 present time are those relating to the paths of earthquake-waves through 

 the earth. The paths of the so-called long waves, which show the maximum 

 amplitude, lie, it is generally agreed, along arcs approximately parallel to the 

 earth's surface, at no great depth below the surface. Their mean velocity 

 of propagation is in almost all cases very near to 3-3 kilometres per second, 

 or 200 kilometres (or 125 miles) per minute. The velocity of the prelimi- 

 nary tremors is much higher — often four times as great, or even more. 

 These waves, being the first to be recorded, must travel by the brachisto- 

 chronic path from the origin to the places of observation, and, whether 

 this path be approximately rectilinear or not, the high speed of the waves 

 shows that they must be transmitted through a medium or media of much 

 greater elasticity than that possessed by the surface rocks. The deter- 

 mination of the actual path of these preliminary tremors is therefore the 

 point upon which attention is being just now especially directed. The 

 problem is mainly a geometrical problem, and obviously the first step is 

 the determination of the positions of the epicentra of the earthquakes 

 discussed. These epicentra are likely to be most correctly ascertained 

 when the data used are those from observatories so near the origin that 

 it may be reasonably presumed (a presumption to be tested by the agree- 

 ment of the results) that the medium through which the waves travel is 

 homogeneous, or nearly so, and yet not so near the origin that the ordinary 

 errors of observation can substantially affect the results. If the paths of 

 the preliminary tremors can be ascertained in such a way that we can 

 formulate a general law, then we shall be able to draw, with a reasonable 

 degree of certainty, inferences as to the constitution of the earth's interior 

 — as to the density, elasticity, and thickness of the successive shells of 

 which the earth is made up. 



#It therefore becomes the duty of the seismological observers in any 

 region of the world to ascertain as nearly as may be the positions of the 

 origins or of the epicentra of the principal earthquakes occurring in that 

 region. Accordingly I have devoted myself during the last twenty years 

 to the determination of earthquake-origins within the New Zealand region, 

 and incidentally, at the request of the Seismological Committee of the 

 Australasian Association, to rinding the origins of some other Australasian 

 earthquakes. It will be of more service to the solution of the problems 

 in hand, however, if this work is extended to a wider region, and accord- 

 ingly the results of systematic inquiry into the earthquake-origins of the 

 whole south-west Pacific are now placed before you. Those in the present 

 paper relate to the year 1910. 



The records used are those received from the Milne seismograph stations, 

 which are published twice a year by the British Association Seismological 

 Committee, edited by Dr. John Milne, F.R.S. ; also records received from 

 the Directors of the observatories at Apia, Batavia, Manila, and River- 

 view, Sydney (the instruments at all the last-named observatories are of 

 the Wiechert type). For these I am indebted to the courtesy of the respective 

 Governments of Germany, Holland, and the United States, and to the kind 

 offices of the Rev. Father Pigot, Director of the Riverview Observatory. 



