L52 The Earth Beneath
An earthquake comes like a thief in the night, without warning. It was necessary, therefore, to inventinstruments that neither slumbered nor slept. Some devices were quite simple. one, for instance,consisted of rods of various lengths and thicknesses which would stand up on end like ninepins.when a shock came it shook the rigid table upon which these stood. If it were gentle, only the moreunstable rods fell. If it were severe, they all fell. Thus the rods by falling, and by the direction inwhich they fell, recorded for the slumbering scientist the strength of a shock that was too weak towaken him and the direction from which it came.
But instruments far more delicate than that were needed if any really serious advance was to bemade. The ideal to be aimed at was to devise an instrument that could record with a pen on paper themovements, of the ground or of the table, as the quake passed by. While I write my pen moves, butthe paper keeps still. With practice, no doubt, I could in time learn to write by holding the still whilethe paper moved. That sounds a silly suggestion, but that was precisely the idea adopted in some ofthe early instruments (seismometers) for recording earthquake waves. But when table, penholderand paper are all moving how is it possible to write legibly? The key to a solution of that problemlay in an everyday observation. Why does a person standing in a bus or train tend to fall when asudden start is made? It is because his feet move on, but his head stays still. A simple experimentwill help us a little further. Tie a heavy weight at the end of a long piece of string. With the hand heldhigh in the air hold the strings so that the weight nearly touches the ground. Now move the hand toand fro and around but not up and down. It will be found that the weight moves but slightly or not atall. Imagine a pen attached to the weight in such a way that its point rests upon a piece of paper onthe floor. Imagine an earthquake shock shaking the floor, the paper, you and your hand. In the midstof all this movement the weight and the pen would be still. But as the paper moved from side to sideunder the pen point its movement would be recorded in ink upon its surface. It was upon thisprinciple that the first instruments were made, but the paper was wrapped round a drum whichrotated slowly. As long as all was still the pen drew a straight line, but while the drum was beingshaken the line that the pen was drawing wriggled from side to side. The apparatus thus described,however, records only the horizontal component of the wave movement, which is, in fact, muchmore complicated. If we could actually see the path described by a particle, such as a sand grain inthe rock, it would be more like that of a bluebottle buzzing round the room; it would be up and down,to and fro and from side to side. Instruments have been devised and can he so placed that all threeelements can be recorded in different graphs.
When the instrument is situated at more than 700 miles from the earthquake centre, the graphicrecord shows three waves arriving one after the other at short intervals. The first records the arrivalof longitudinal vibrations. The second marks the arrival of transverse vibrations which travel moreslowly and arrive several minutes after the first. These two have travelled through the earth. It wasfrom the study of these that so much was learnt about the interior of the earth. The third, or mainwave, is the slowest and has travelled round the earth through the surface rocks.