Earthquakes, their Cause and Effect

Radio Record, Volume II, Issue 41, 26 April 1929, Unnumbered Page

Earthquakes, their Cause and Effect

HE recent severe local earthquake at Arthur’s Pass prompted the request by 3YA to Mr. H. T. Skey, Director of the Magnetic Observatory, Christchurch, to deliver a talk on Earthquakes, their cause and effects. As earth tremors and volcanic action are of general interest to New Zealanders, this paper is reproduced for the benefit of permanent record ‘and the interest of those who did not hear it.

HAT is an earthquake; also why? Well, most of us have felt one: things move, there is some noise, and we are glad it is over, for us; and next morning we look at the papers to see whereabouts it did most damage, and we naturally conclude that near there the trap went off, in the upper rocks of the earth’s crust, and that there a large amount of "potential," or stored energy, suddenly became freed, or "kinetic," and had to spread out and distribute itself throughout the earth. Energy, we know, is the capacity of doing work, and so when the energy stored at a point becomes freed it immediately sets about doing work. How, then, does-the energy become fixed, and how does it become freed? MODERN science teaches us that the earth is almost entirely solid, with the exception of the sea on its surface. On the land, elevations are formed with mountains and valleys. The mountains have weight and must be supported by their stiffness and. the stiffness of their surroundings. At the bottom of a large mountain such as Mount Rolleston the stress in the rock is tremendous, and it. is not until a depth of about sixty miles below the surface is reached that the stress is uniform: this is called the depth of compensation, or the depth of the isostatic layer, because it is found that the. total mass of rock above every large block of it is practically the same. Hence, under mountains the density of the material is less than under low plains or large valleys, and as the mountains become lower throtigh denudation they

must tend to rise, but to a less extent than they are denuded: hence denudation is a great cause of'stress changes in the rock in mountainous country, and when in any part the stress exceeds the strength of the rocks that bear it and are elastically strained by it, even as the wound-up spring of a clock is strained, then the breaking point is reached, and an earthquake occurs. Very often the release of strain is accompanied by a marked dislocation of the strata, forming what is known as a fault, and in some cases the relative levels of the rocks on either side of a fault or crack may be altered by many feet, and the fault may extend to a length of over a hundred miles. In the great Californian earthquake of 1906 the fault movement occurred along the northern half of an already existing fault line, the San Andreas fault, which had been traced to a length of over six hundred miles. IN the case of the recent earthquake near Arthur’s Pass, and its aftershocks, the actual fault movement could not be identified on the surface, and can only be located precisely by the aid of field seismographs. The actual damage sustained at Arthur’s. Pass was confined to chimneys and fireplaces: one large concrete building sustained some cracks; the railway ballasting settled in parts and had to be made up; cracks appeared in filled-in roadways, and there were numerous rockfalls where the material had been insecure, ‘along the valley of the Bealey river. There was no structural damage to the Arthur's Pass-Otira tunnel. In the Riviera earthquake of 1887 the shock was very weak. or not

felt at all in the tunnels of the Nice to Genoa Railway, and none of the tunnels was damaged in the slightest degree, which is very reassuring. T Arthur’s Pass the settlement is built largely on filled-in or made ground and in this an earthquake wave would have a much larger amplitude of movement than in the solid rock. It is this fact that contributed largely to the damage that did occur. At Otira, almost no structural damage was caused, and it is quite possible that the settlement was shaded from the earthquake waves, as. frequently happens, even in railway cuttings of fair depth. N the largest earthquake shock at Arthur’s Pass the energy was set free at 10hrs. 50min. 30secs. p.m. on March 9 last, while at the Observator- here (Christchurch), nothing happened till 10hrs. 50mi.n 42secs., and we know that the earthquake effect took about 12 seconds to travel from the origin to Christchurch, travelling through the earth just as a sound travels through a substance; and like the sound it travelled as a wave through the earth, in fact the first effect to reach us from an earthquake origin is just sound waves, mostly of extremely low pitch and mostly too low to be heard by us (except by the way objects are moved against one another), but near the origin higher pitched waves are heard as a roaring or rushing sound. But in an earthquake besides the sound waves produced at the origin, there are other kinds of waves produced in the earth. Chief -Continued on page 2.

Earthquakes

(Continued from cover.) among these is the "transverse" wave, which also travels out from the origin to carry away the energy that is freed. In this wave the movements of the earth particles in the advancing wave are at right angles to the direction in which the energy is travelling, while in the sound wave, or the "longitudinal" wave, as it is called, the relative movements of the particles in the advancing wave are to and fro, but in the same ‘direction as the energy is travelling, so that it is a wave of compression, whereas the action «in: the transverse wave is a shearing -action. Now, in all ordinary matter the resistance offered by the matter to a shearing action is less than the resistance offered to a compression, and thus it comes about that in the earth the transverse wave travels at a lower velocity than the sound wave does, in fact, at little more than half the velocity. At any observatory the times of arrival of these two waves of energy is observed on an earthquake recorder or seismograph, and from the time-difference, or the length of time elapsing between their respective arrivals, a very close estimate may be made of the distance of the observatory from the earthquake origin.

A THIRD kind of wave is also generated when an earthquake occurs. This is the "Long Wave," and its action is manifested at the surface of the earth, just like an ocean roller manifests itself on the surface of the sea, but with this difference: that the principal oscillatory motion occurs horizontally in the earth’s surface, instead of vertically, as in the ocean roller. Now, these "long waves" on the surface of the earth travel outoe a a

wards from the origin at even a slower speed than the transverse waves travel through the earth, and also help in the same way in obtaining an estimate of the distance of the observing station | from the earthquake origin.

OW for a few figures. It is found that the longitudinal or soundwave takes nearly 23 minutes to go through the earth from one side to the other or along a diameter of the earth; they can go through anything, solid, or even liquid, hard or soft. But it is found that the transverse wave cannot go right through the earth in the same way, that is, through ‘the earth’s centre, because about two-fifths of the way down from the surface to the centre there appears to be a layer of matter that is too plastic or soft to resist a shearing action, so that the transverse wave cannot get through it; what happens to a transverse wave when it somewhat obliquely reaches this layer is that, the wave is broker up; part of the energy of the wave is reflected and carried upwards again. and part of the energy is refracted down through the soft layer as a sound wave which travels on past the centre of the earth again encounters the soft layer, and, in passing through it the second time, is refracted and goes on upward through the harder material, partly as a transverse wave, and it ultimately reaches the surface and can be recorded there by a seismograph. The time it takes to travel is, of course, reduced because it has travelled through the earth’s central core as a longitudinal or sound wave.

HE surface "long waves" from an earthquake origin take, it is found, nearly three hours to go right round the earth and return to their starting point; this they sometimes do repeatedly in the same earthquake. When an earthquake shock has occurred, we have these three sets of waves which go out from the origin, and among them carry away nearly all of the energy set free by the earthquake, and we naturally want to know what hap- — a

pens to them. Foitunately, the earth’s material is not perfectly elastic, and it is not all the same; hence it happens that the waves die down; through friction and heat dissipation by conduction during the wave action in the earth, the whole of the energy of the waves becomes gradually converted into heat, and ultimately is spread throughout the earth; the earth. becomes warmer ever so slightly, but this is hardly an adequate compensation for the loss of fifty thousand lives and immense damage to property, sucb as happened in the great Lisbon earthquake of November 1, 1755, which was severely felt over a great part of Europe. .Three shocks and three sea fiuxes occurred within about seven aninutes of time. The shocks actually were felt over an elliptic area extending from the Canary Islands to Finland, with a width stretching from the north-west of Ireland to the Adriatic sea. ,

Contrast this with New Zealand, where only two or three persons have lost their lives through earthquake action since the arrival of the European on these shores. BESIDES faulting or fracture of the rocks, other causes of earthquakes are earth-slips and voleanic activities. Farth-slips strike a blow by the fall of the great mass of the ‘slip; they are especially liable to happen at sea off the mouths of great rivers, which bring down matter denuded from the land and deposit it in the relatively _ still water of the sea, forming deltas, and often, steep banks are formed under the surface of the sea, which easily break away. OLCANIC action is itself due to pressure movements in the earth’s crust as a rule, and so is only a secondary cause of earthquakes. Yolcanic earthquakes usually have origins quite near the surface of the earth; it is almost certain that the main volcanoes are a kind of insurance against the growth of immense stresses in the earth’s crust, which ultimately would cause earthquakes of extremely disastrous extent.

The late Dr. Omori, a great Japanese seismologist, from an immense number of observations drew a graph ,of the precipitation on the westeri ranges of Japan; he also drew a graph of the frequency of earthquakes in a certain large area in Japan, for the same time; the parallelism of the two graphs was astonishing, and showed definitely that earthquake occurrence is determined very largely by the unequal loading of the earth’s crust and the consequent tendency to isostatic action therein. ROM earthquake data contributed by observations at a great number of places on the world’s surface, Professor Turner, of Oxford, with his assistants, has determined the depths of origin of many large earthquakes of recent years; he finds that some appear to originate at depths up to one twenty-fifth of the earth’s,: radius, | much below the proved depth of compensation in the earth’s crust. One ‘would hardly expect the earth’s substance below the depth of compensation to be fragile or breakable, or that stresses could accumulate in it, as it becomes of importance that, in civilised countries, any earthquakes occurring should have their depths of origin determined as accurately as possible on the spot, by the use of proper selsmographs of modern type.