312 



DISCOVERY 



Earthquakes 



By J. J. Shaw 



Ilowiranj Sccn-tanj. Scisinulnriical CommilU-r "/ (lie lirilish Assuclaliun 



The investigation of earthquake phenomena, or 

 Seismology, constitutes a most interesting study. It 

 is really remarkable that, whilst other sciences have 

 attracted so many workers, seismology has been 

 almost entirely neglected. Perhaps this is because 

 experimenters have not yet realised its fascinations, 

 or maybe because earthquakes are not of everyday 

 occurrence. 



It is not necessary to live in an earthquake country 

 in order to engage in their study ; since it is possible 

 to construct instruments, called seismographs, for the 

 purpose of recording in diagram form — the seismogram 

 — the various wave motions which are propagated 

 uniformly throughout the Earth's mass. 



By this means the exact time of their occurrence 

 can be computed, also the precise area where they 

 occur can be located, even if they are in the bed of 

 an ocean ; in fact, the greater number of shocks are 

 located at the bottom of the sea. 



An earthquake is by far the greatest demonstration 

 of energy of which we have knowledge. It is capable 

 of setting the whole Earth pulsating from pole to pole. 

 The great earthquake in China in 1920 shook moun- 

 tain ranges down until the valleys were filled up. 

 The recent catastrophe in Japan furnishes us with an 

 example of its destructive force. It is not uncommon 

 for hundreds of thousands of lives to be lost in a few 

 seconds. 



Several attempts have been made to calculate the 

 amount of energy liberated ; in the case of a severe 

 shock it is estimated to be of the order of 

 50,000,000,000,000 (fifty billion) kilowatts. 



This energy radiates outwards in every direction : 

 not only does it traverse the whole surface of the 

 Earth, but it permeates also every cubic yard within. 

 It is propagated in the form of waves of various types, 

 each type having a characteristic speed. 



Determining the Distance 



In the same way that the distance of a gun may be 

 determined from the flash and the sound, so also may 

 an earthquake's distance, and the time at which it 

 occurred, be computed. 



There are three principal types of wave available 

 for observation. The first to arrive at an observing 

 station comes through the interior of the Earth in the 

 form of a concussion, and is known as a compressional 

 wave. The speed of this wave varies from 7 to 13 

 kilometres per second, depending upon the depth to 



which the chord, along which it travels, penetrates. 

 This phase is denoted by P (primary). (See Fig. i.) 



The second type of wave travels via the same 

 chordal route, but at a slower speed than the primary 

 waves, viz. from 4 to 7 kilometres per second. These 

 are transverse waves, so called because the vibrations, 

 as in light waves, are at right angles to the path along 

 which they are travelling. These waves are not only 

 slower in speed, but the pulsations are also fewer per 

 second, and have greater power. These characteristics 

 give a wider and more open diagram, enabling them 

 to be distinguished from the preceding primary waves. 

 They are designated by the letter S (secondary). 



-SHOWING THE PATHS OF THE PRIMARY, SECONDARY, 

 AND LARGE WAVES. 



Waves of the third type are similar to those waves 

 which travel across a pool — they pass outwards from 

 the epicentre, as the point on the surface above the 

 origin is called, in ever-increasing circles, at 3 J kilo- 

 metres per second, along the Earth's crust. They are 

 responsible for the largest part of the record and are 

 known as the large waves, or maximum. 



The evidence that these waves travel along the 

 surface lies in the fact that, no matter how far they 

 are propagated — and they sometimes pass more than 

 once round the Earth — they travel at a constant 

 speed relative to the distance measured on the sur- 

 face ; whereas the ratio of the length of a chord to 

 the surface distance is constantly changing. 



Since it is possible to recognise these different types 

 of wave in the seismogram, it is obvious that these 

 are the necessary data for estimating the distance 

 they have travelled. 



