Popular Science Monthly/Volume 27/July 1885/Earthquake Phenomena
AT the present time the earth seems to be in a state of great seismological action. Different parts of the world have recently been disturbed by earthquakes which have caused wide-spread destruction. Those in Spain, which began December 24th, and have lasted, with slight interruption, down to the time of writing, have been among the most destructive of recent earthquakes. Over two thousand people have been killed, many more wounded, and thousands of houses destroyed. Such a state of affairs can not help arousing an interest in this phenomenon.
The earth is constantly quivering, some point on the surface being the seat of a slight quake nearly every moment of the day. By far the larger number of these are of little intensity, being felt only by delicate instruments, and the majority of cases come from volcanic regions. So few facts are known, that we can neither draw deductions nor even determine the causes. It is reported, however, that earth-quakes more commonly occur at night, and that they are more abundant in winter than in summer.
The only settled facts about earthquakes are, that they are the result of some shock imparted to the rocks at a considerable distance beneath the surface, and that this shock reaches the surface in a series of concentric rings, all points on the circumference of each ring receiving the shock at the same moment, even though they may be hundreds of miles apart. In other words, all points at equal distances from the center of the earthquake receive the shock at the same moment. Although this is theoretically the case, according to well-known physical laws, still, in practice, the facts are somewhat different; for the shock is retarded or accelerated according as the rock opposes or favors the passage of the wave. The seventy of the shock in a given place is dependent upon a variety of causes. These are: 1. The strength of the original shock; 2. The distance from the earthquake center; and, 3. The kind of rock on which one is standing, loose gravels greatly diminishing: the force of the shock. The destructiveness of earthquakes depends rather upon the suddenness of application than the amount of motion. In that at Rio Bomba, it is reported for a fact that a man was hurled across a stream a distance of one hundred feet, and landed on an elevation fifty feet higher than his original position. It is an undoubted fact that objects are frequently thrown great distances. In the Mississippi Valley, during the earthquakes of 1811 to 1814, the tops of trees were twisted and entangled, and strong log cabins were thrown to the ground. Rivers are sometimes checked in their flow, and, in past geological ages, some have been completely turned from their course by earthquakes.
At least four theories have been seriously advocated by scientists to explain these phenomena. The first, which is now abandoned, is based upon the supposition that the earth's interior is in a fluid condition. This being the case, the combined action of the sun and moon upon this molten mass beneath the surface causes it to surge and swing in tides of liquid fire. It is the shock from this that we feel on the surface. (I dismiss this theory, as one having no value whatever).
Another theory is, that earthquakes are due to volcanic action. The passage of the immense quantities of gas which escape from volcanoes must necessarily cause shock after shock. This gas, under pressure of thousands of pounds, is contained in a subterranean boiler, from which it is continually trying to escape. The moment the pressure becomes sufficient, the walls in some part give way, and the transmission of this shock reaches us as an earthquake. When the pent-up gases have broken through successive strata, always coming nearer to the surface, they finally reach the uppermost stratum, and this, which has been weakened by previous eruptions, gives way before the pressure, and lets the steam out as a volcanic eruption. It is thus, then, that the continual succession of earthquakes in volcanic regions is produced. When, however, we study the country about many earthquakes, and find no evidence of volcanoes, we are forced to abandon this theory for those regions, and look further. This is the case with all New England earthquakes, and with those in Spain.
Let us look at the next theory. We all know that water is continually soaking into the ground, and is dissolving out all matter which it can. Limestone is one of these substances, and it is to this property that we owe the great caverns of Kentucky. Water not only works on or near the surface, but it even penetrates thousands of feet into the earth, and at this depth possesses even greater power of solution. Indeed, we have every reason to believe that deep down in the earth's crust vast caverns have been excavated by this erosion of water, and that in some places, especially limestone regions, these are numerous. Granting, then, that there are such great holes in the ground, and supposing that one of them should suddenly collapse, we can readily imagine a shock which would cause wide-spread devastation. But this theory also will apply merely to certain limited localities, and we have not yet found a general theory—one for all regions.
Let us travel for a moment in a mountainous region, and we will see thousands of feet of rock folded, twisted, and bent in every conceivable manner. Now, it is a well-proved fact that these rocks were originally deposited in a horizontal layer, and that they have since undergone contortion. It can be conceived, then, that such rocks must be in a constant state of strain ready to relieve themselves at any favorable moment. Let us suppose that we have a long board held down on each end by a bank of earth. Remove a part of one bank, and a point will be reached when the board will spring up with considerable force, striking anything above it a severe blow. This is the condition of our mountain-rocks. Material is constantly being eroded from some and piled upon others. The time finally comes when they snap or spring, and, striking the rocks above, send out an earthquake shock. Quarrymen frequently feel a slight shock, and immediately after hear a report, showing that the strain in some rock has been relieved. A most curious instance is reported where a very long column of granite rock was being split from a quarry by the use of wedges. At last the mass was split from the bed, and instantly it expanded itself with such suddenness as to produce a very perceptible jar; and when the rock was returned to the place where it originally lay it no longer fitted, and the drill-holes did not coincide as before. Here, then, we have an earthquake on a small scale started by artificial means. Probably many of the slight shocks result from similar causes, or from the action of frost, or by contraction and expansion caused by the daily change in temperature.
These, then, are the theories for earthquakes, but neither of the three can be called general. We can usually tell when volcanic action is the cause, and are thus limited in our choice to two. In some cases the cavern theory may apply, but these are rare, and can usually be determined, so that in any region not volcanic the third theory is the most probable. This accounts for the fact that earthquakes are much more frequent and violent in mountain-regions of recent origin. The Appalachian, which are the oldest mountains in America, are, therefore, very free from earthquakes, because the rocks have, as a rule, long since relieved themselves; while, on the other hand, the comparatively new Rockies are the seat of more frequent tremblings.
There is a theory for the cause of earthquakes, which is rather neglected by scientific men, but which, I think, will explain many of those phenomena not otherwise accounted for.
At the mouth of the Ohio, and at Newburyport, Massachusetts, both of which have in times past been the seat of very severe earthquake shocks, the layers of rocks are not badly bent. And, further, I do not see how the theory of bent rocks can explain the frequent repetition of shocks which we have recently seen in Spain, in Newburyport in 1727, and near the mouth of the Ohio in 1811. These regions had been comparatively quiet for years, and suddenly a severe shock was felt, followed by a series of successive shocks, which, in the case of Spain, have not yet ceased, although the original shock occurred December 24, 1884. The theory, which I have spoken of, and which would explain this, is that there are great quantities of gas accumulated at certain points beneath the surface under great pressure, and that this gas, in its attempt to free itself, bursts open the rocks, causing shock after shock, until it has finally relieved itself sufficiently to remain quiet. This, one will see, is similar to the volcanic theory, except that there is no necessity for the presence of a volcano. What this gas is, does not matter; it may be accumulations of steam, or it may be evolved from petroleum, or it may be carbonic-dioxide gas evolved by acids working on calcareous rocks. In the Newburyport earthquake, which was the most severe ever recorded in New England, large quantities of gas escaped to the surface; and, on the Ohio, gas also escaped. This shock, which was at first wide-spread, finally narrowed itself down to a very limited marshy area and died out.
To sum up, then, it may be said: 1. That in volcanic regions earthquakes are a part of an eruption, premonitory warnings; 2. That in a limestone country the falling in of the walls of caverns may account for some; 3. That in regions of recent mountain-making the sudden release of tension causes many; 4. That the pressure of pen tup gases on the surrounding rocks, which are finally burst, may be the cause of a large number, more especially those which are followed by a long-repeated series of shocks; and, finally, that in any one of these regions either or all of the other causes may (with the exception of volcanic in non-volcanic regions) enter into the production of earthquakes.