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Geological Society, April ro.-"Notice of some of the Secondary Effects of the Earthquake of the soth January, 1869, iu Cachar.“Communicated by Dr. Oldhsm, of Calcutta, with remarks by Mr. Robert Mallet, C.E., F. R.S. This earthquake was a severe one, being strongly felt in Calcutta, distant from the meizoseismic area about 200 miles, and far into the plain of Bengal. The effects were examined on the spot a few weeks after the shock by Dr. Oldhatn, who anticipates being able to fix the position and depth of the centre of impulse by following the same methods as those first employed by Mr. Mallet with respect to the great Neapolitan earthquake of 1857. These results have not yet been received; but Dr. Oldham has forwarded an extremely interesting letter on the circumstances of production of very large earth-fissures, and of the welling up of water from these, derived from the water-bearing ooze-bed, upon which reposed the deep-clay beds in which the fissures were formed. Di-. Oldham rightly views all these fissures, which were all nearly parallel to and not far distant from the steep river banks, as "secondary effects,“and not clue to fractures produced by the direct passage of the wave of shock. He also shows that the welling up or overflowing of the water in the fissures was a secondary effect also, and negatives the notion entertair ed on the spot of mud-volcanoes, &c., having originated at those fissures. The chief aim of Mr. Mallet's remarks was to point out the importance to geologists of rightly comprehending the dynamics of production of these phenomena, and to show that the older notions of geologists as to earthquake-fissures are untenable. He explained clearly, aided by diagrams, the train of forces by which the elastic wave of shock, on passing out of the deep-clay beds where these have a free side forming the steep river banks, dislodges certain portions and throws them off towards that free side and that this is but a case of the general law in accordance with which such elastic waves behave towards more or less incoherent deposits reposing on inclined or on level beds, under various conditions. Mr. Mallet also explained the dynamic conditions under which the water from water-bearing beds, such as that of ooze beneath the Cachar clay beds, becomes elevated in the fissures formed, and gave approximate expressions for the minimum height to which the Water CSfl rise In relation to the velocity of the elastic wave particle. The paper concluded with some explanatory remarks upon the continual noises, like the irregular fire of distant artillery, heard long after the shock had passed, and when the country had become perfectly quiescent. The noble collection of photographs which were made by Dr. Oldham, and forwarded to Mr. Mallet, illustrative of the physical features of the huge earth-fissures and other effects of this earthquake, were exhibited to the Fellows present, and are well worthyof attentive study. Sir Henry James inquired whether there was any trace of fissuring in the lower beds beneath the slimy ooze. Mr. Scott wished to ascertain the author's opinion as to the possibility of predicting earthquakes on meteorological grounds, as had been done by M. Boulard, several of whose prophecies were said to have been fulfilled. Mr. D. Forbes gave some details of the earthquake of Mendoza, a town situated on a vast alluvial plain at the foot of the Andes, in which the phenomena remarkably coincided with those detailed by Dr. Oldham. In that case he found that the rumours as to fire and smoke having been emitted from fissures were entirely without foundation, the presumed smoke having been nothing but dust. The earthquake was felt over a distance of 1,200 miles; and wherever the firm rock came to the surface there was no trace of fissure, though portions of the rock were overthrown. But in the plain, consisting of 30 or 40 feet of alluvial soil, the whole ground was in places fissured, and in some districts the surface completely furrowed, and even the turf turned over. He had witnessed numerous earthquakes, and in some cases had been in deep mines during their occurrence, when the sound only could be heard, and he could testify to their effects being confined to the surface. The direction of the fissures was invariably at right angles to the line of shock. In South America au the earth. quakes could be traced to volcanic centres. The President inquired as to the distinction to be drawn between the primary and secondary effects of earthquakes, and whether the author thought that no fissures were attributable to the direct action of earthquakes. As to the cause of the sound, like that of a cart carrying iron bars or of an artillery waggon, he wished for further information. Mr. Mallet, in reply, explained that fissures only take place where masses were comparatively free in one disection. They might extend to enormous depths, though they often closed in rapidly. With regard to the power of predicting earthquakes, he disbelieved in it wholly, and considered that any fulfilment of such prophecies must be due to accident; earthquakes are so numerous, that the chances of such fulfil. ments are great. The blow or impulse originating earthquakes could not be attributed solely to one cause. It arose often fi-om deep subterranean volcanic action; but it also especially in the case of long-continued tremors, like those of Comrie or Pignerol arose from the breaking up or the grinding over each other of rocky beds at a great depth, through the tangential pressures produced in the earth's crust by secular cooling. The arrested impulse of the fall of the Rosberg in Switzerland produced a sensible earthquake. Fissures in hard rock could not be produced diredily by the shock, because the velocity of impulse in such rock greatly exceeded that of the elastic wave particle. The earth's crust was at present not in a state of tension, but of compression, through secular cooling.
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Societies and Academies . Nature 5, 513–516 (1872). https://doi.org/10.1038/005513a0
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DOI: https://doi.org/10.1038/005513a0