The Cause of Earthquakes

New Zealand Tablet, 27 September 1906, Page 15

The Cause of Earthquakes

An interesting paper on earthquakes, by the Rev. H. V. Gill, S.J., was read at a recent meeting of the Royal Dublin Society, in which he gives a brief acpount of some simple experiments with rotating bo-dies, and an application of the principles illustrated by, them to explain a -possible connection) between seismic disturbances at different localities. A perfectly balanced teetotum or top will spin about the axis of its spindle with great regularity. If a weight be attached to one side, 'it . will spin with a ' wobble.' By placing an equal weight exactly opposite the first, regular spinning may be restored. The same result may be obtained by placing several equal weights at equal distances round the circumference. A hollow teetotum was made three-eighths inch deep, and one and one-fourth inch in diameter. This was made with great care, and could be covered by .a plate which screwed on. A number of steel ' ballbearing ' balls were also obtained. 1. The teetotum v»as set spinning, and sppn with perfect evenness. While it was spinning, a "Ball onefourth inch diameter was dropped in. At once the teetotum began to wobble, and the ball could be beard running round the inner edge. A second ball of the same size was then dropped in ; a slight movement of the balls was heard which ceased at once, the wobble disappeared, and once more the teetotum spun perfectly evenly, just as before it had been disturbed. The balls were found to have placed themselves at the opposite ends of a common diameter. The same result was obtained when three or more balls of equal size were dropped in. In all cases the balls took up positions equally distant round the circumference. 2. A small weight was attached to the circumference of the teetotum. It spun with a wobble. After several trials with balls of different sizes, one was found which produced even spinning, taking up a position opposite the weight. 3. The teetotum was half filled with water and covered. It spun quite regularly. By carefully arranging the amount of water in, the teetotum and the weight of a ball whigh was also placed inside it, it was possible to obtain perfectly even spinning. All the above results depended upon well-known principles, and are introduced to illustrate the matter under consideration. • By spinning the teetotum on plates of 'smoked glass, very interesting curves may be, obtained. Without entering into the discussion concerning The Exact Shape of the Earth, or the precise state— whether solid or liquid— of its interior, we may consider it as a sphere rotating

with great velocity, containing, at least at certain localities, masses of matter which are capable of undergoing displacement. We are, besides, led to believe that volcanic disturbances" and earthquakes are ' merely ■ external indications of much greater disturbances beneath the surface. of. the earth. , >- - Suppose- at a place near the equator there is, owing to whatever cause, an upward or downward displacement of matter. This ,would correspond to the addition or subst'raction of a weight at a point on the circumference of the ' teetotum. The earth ' would spin with a wobble. The effect of this would be to produce a stress on other points situated on a narrow belt round the earth's circumference. The" places of weakest crust would be the first to yield to this stress. If this resulted in a displacement of 'matter at a point diametrically opposite to the original r disturbance, it would correspond to the addition' of a ball to a weighted teetotum. If, however, the first place to yield were some point .not diamerically op- ■ posite, the presence of these two disturbances would, as illustrated by the teetotum experiments, produce a displacement at some thiid point. This would correspond to a teetotum containing three balls. A Sudden Displacement would be manifested as an earthquake, while a more gradual displacement would produce a , volcanic eruption. In the ideal case the three localities would -be expected to be equally distant. It is, however, evident that if the displacements were not of equal intensities, the distances between the places need not be the same, though, in general, wp should expect them to be separated by considerable intervals. That there is a connection between the frequency of earthquakes and displacements of the poles was called attention to twenty years ago by Professor Milne, and has been studied by him in detail, It has been calculated that every great earthquake between 1895 and 1902 caused an average displacement of the pole. The displacement of matter in connection with earthquakes is also a fact recognised by writers on this subject. The theory here suggested is that one such displacement, whether it be gradual as in the case of an eruption, or more sudden as in the case of an earthquake, would be expected to give rise to corresponding displacements at other places more or less symmetrically situated with reference to the earth's axis of revolution, producing earthquakes or eruptions -as the case might be. The following principle on which this view is founded is illustrated by the teetotum exneriments :— A rotating body, containing matter capable of shifting its position, tends to keep itself in a state of regular revolution about its axis, owing to the way in which the movable matter automatically disposes itself with reference to the axis of the body. There were certain circumstances connected with the disturbances which characterised the middle of the month of April, 1906, which seemed strangely to fit in with The Views Here Proposed. In Ihe first place the three disturbances took place within the space of ten days': Vesuvius very active on April 8 ; great earthquakes in Formosa, April 14; earthquake at San Francisco, April 18. Secondly, these Ihree " places are all situated on a narrow belt parallel to the equator. Thirdly, the distances between the places are comparable. Fourthly, as far as can be iudged, the disturbances were of comparable intensities. All these facts seem to justify the suggestion that these disturbances may have been connected in the way pointed out in this paper. The following sequence of causes and effects might, in view of the theory here suggested, be tentatively proposed : Vesuvius has been more or less active for some considerable, time past. A displacement of material in its neighbourhood would have given rise, owing to causes illustrated by the teetotum experiments, to a stress round the earth's circumference. The places of weaker crust would have given way ; Formosa would have been the - first to yield : hence ,the earthquakes there of March 17 arid April 14. The- final effect of these two causes— i.e., Formosa and Vesuvius— would have been 'the earthquake at San Francisco. The latter place 'is where we should ha.ye expected the connected earthquake to occur according to the- views . just set forth, especially as that place is located' in a region of weaker crust. It is, hardly necessary, to add that, in phenomena of so ' complicated a nature, and depending on many causes, the effect of the forces referred to in this paper would, in many casos, be expected to be partially or totally neutralised by other influences.