CAUSE AND EFFECT.

Star (Christchurch), Issue 9430, 2 January 1909, Page 4

CAUSE AND EFFECT.

A CHAT ABOUT EARTHQUAKES. SOME INTERESTING OBSERVATTONS. Avoiding all technicalities, and j speaking as far as possible from the popular standpoint Mr G. Hogben, In-spector-General of Schools, who has made a close study of earthquakes and their- causes, bad some interesting remarks to make on this subject when questioned by a " New Zealand Times " reporter in Wellington. "People," said Mr Hogben, "connect earthquakes with volcanic action, but there is only a small class of earthquake caused by such action, in the same way that an explosion in a mine would cause a shaking of the earth. These, however, are generally of a local '. character, and do not cover a wide area. For instance, the earthquakes at Rotorua at the time of the eruption in 1 1886, and those at Wnakatane lately, were of that desoritpion, and were not felt far away. These were due to the explosion of steam and other eases, but they were not earthquakes oF the -ind_ that make big disturbances ; they are entirely different. PRIMARY CAUSES. "It may be said that there are two primary causes of earthquakes. One is the unequal shrinkage of layers of the earth's crust. The consequence of that is that rocks hearer to the surface are really too large to occupy the space they would naturally fall into as the inner part of the crust shrinks. Therefore, the outer crust must crumble, and become contorted in various ways in order to repack itself; it has got to repack itself according to the space available. That repacking generally takes place about certain lines or planes, ueualjy aclled axis. This move--ment of repacking leads to the movement of the rocks along its axis or parallel to them, forming various kinos of faults. "This fault movement that takes place is generally very small, but now and then there is a little instability of position, and there will be sudden movements. This gives rise to vibrations of different kinds which go over the whole of the earth's crust, if the movement is at all considerable. These vibrations we call earthquakes, which may produce secondary effects, throwing down rocks that may open chasms, but in their train bringing down buildings, etc. Nearly all these results are secondary. Thus what people know as earthquakes are nearly all secondary results. Of course, sometimes it hapens that a place is on ah actual mas 3 that is moved,- That is not secondary. .. For instance in 1855 in Wellington, the actually moving mass reached right up to tie surface, and raised the land surface around Wellington higher. That was not secondary, but primary. THE LAND AND SEA. "As to the other cause: That is goring on wherever there is land or sea, to a certain extent, but it is not always sufficient to cause earthquakes. In the natural course of things, the land surface is denuded by rainfall, snow, glaciers, etc., and part of i£ is always being worn away and carried by the rivers into the sea. This is constantly going on. The rocks below the land and the rocks bejow tEe sea are all being pressed; upon by what is above them. If you take the pressure off the land and put it on to the sea-bed you remove the pressure on the rocks underlying the land and increase the pressure on the rocks underlying the ocean bed. That will make the land rise and the ocean bed sink; The result is distortion, and ultimate dislocation near the junction, of the sea and land, frequently followed by ibe fomation , of a monoclinal fault. The movement of that fault at times when it is sudden would give