USEFUL ANALOGY

Auckland Star, Volume LXXIII, Issue 194, 18 August 1942, Page 4

USEFUL ANALOGY

EARTHQUAKE CAUSES

LIKE BENDING OF A STICK

(Ey L. Bastings, D.Sc., Fellow of the Institute of Physics, London; former Demonstrator in Electricity, Cavendish Laboratory, Cambridge, etc.). ' rTbis is the first of a series of topical scientific articles by o. w ® r £ ers gas? permit.] What is the cause of earthquakes? Is there any connection between earthquakes and volcanoes? Does the weather or the sun control the time or place of occurrence of an eartn duake? Can earthquakes ever be predicted?, These and many other similar questions are being asked repeatedly by the man in the street Many of these questions are answered dogmatically, and wi assurance by those who delight to air their little knowledge. But -w hat has science to say in answer to these questions? The ultimate cause of earthquakes is as yet unknown. It is quite clear that some force of long duration is acting constantly in many parts of the earth, building up strains in its interior, and that the con s^i stresses eventually reach a stage at which the rocks near theearths surface are no longer able to resist the strain, and a break occurs. A useful analogy is provided by the bending of a stick held in the hands. The stick gives gradually under the strain until suddenly a limit is reached, and the stick breaks. Such a break occurring within the eartn often takes the form of a crack or fault, which may stretch for ten or a hundred miles in a uniform direction. Effects on Surface When this break reaches up to the surface, its effects become evident, roads and fences are displaced, and buildings and bridges are shattered. But often the original movement is so deep down in the earth that the fault does not appear on the surface at all, and its occurrence can be inferred on ly from instrumental evidence. But while it is clear that many earthquakes originate through sudden movements on old or new faults of this kind, the ultimate cause of the strain which finds relief in the break is not yet clearly understood. There is in general no connection between earthquakes and volcanic activity. And it is not true to say that volcanic countries are more subject than other lands to severe earthquakes. There is one type of earthquake sometimes associated with volcanic activity; but such earthquakes are never felt more than a few miles from their origin.

- The instrumental evidence also shows that these volcanic earthquakes originate very near the earth's surface, just as volcanic action also does. But most earthquakes originate at depths reckoned in tens or even hundreds of miles. Such deep-seated disturbances can evidently have no close connection with such superficial phenomena as volcanoes. No 'Quake-Proof Buildings When a fault extends up to the surface and intersects a building, there is no reasonable possibility of designing a structure sufficiently robust to survive such violent treatment. But usually the fault itself is not the chief cause of damage. When the break occurs, deep beneath the surface, the whole neighbourhood is thrown into violent tremors and these tremors spread out from the origin in all directions in a series of wavelike movements.

Two varieties T)f these waves are well-recognised. They advance at different rates, and so arrive at a distant point at appreciably different times. By measuring such a timeinterval, we can ascertain our distance from the origin of the earthquake.

Most people who have experienced such an earthquake as shook Wellington province last month have noticed the preliminary tremor followed a few seconds later by the main shock. These two incidents represent the arrival of the two chief kinds of wave-movements; and the fact that, in the Wairarapa shock the second succeeded the first after about 10 seconds, indicates that the origin was some 60 miles from \\ ellington.

When the distance to the origin fiom two or three well-separated places has thus been discovered it is not a difficult matter to locate the origin of the disturbance with some degree of certainty. As a rough working rule, the distance to the origin may be taken as six miles for each second of interval between the arrival of these two qu°ake mentS usually felt in an earth-

(To be continued)