Earthquakes and other earth movements

by John Milne

At Florence, before a period of earthquakes there is an increase in the amplitude and frequency of vertical movements. These vertical movements do not appear to coincide with the barometrical disturbances, but they appear to be connected with the seismic disturbances.

They are usually accompanied with noises in the telephone, but as the microphone is so constructed as to be more sensitive to vertical motion than to horizontal motion, this is to be expected. This vertical motion would appear to be a local action, inasmuch as the accompanying motions of an earthquake which originates at a distance are horizontal.

Storms of microseismical motions appear to travel from point to point.

Sometimes a local earthquake is not noticed in the tromometer, whilst one which occurred at a distance, although it may be small, is distinctly observed. To explain this, Bertelli suggests the existence of nodes. Similar conclusions were arrived at by Rossi when experimenting on different portions of the sides of Vesuvius. Galli noticed an augmentation in microseismic activity when the sun and moon are near the meridian. Grablovitz found from Bertelli’s observations a maximum two or three days before the syzygies, and a minimum three days after these periods. He also found that the principal large disturbances occurred in the middle of periods separating the quadrature from the syzygies, the apogee from perigee, and the lunistigi period from the nodes, whilst the smallest disturbances happened in the middle of periods opposed to these.

P. C. Melzi says that the curves of microseismical motions, earthquakes, lunar and solar motions, show a concordance with each other.

With the microphone Rossi hears sounds which he describes as roarings, explosions, occurring isolated or in volleys, metallic and bell-like sounds, ticking, &c., which, he says, revealed natural telluric phenomena. Sometimes these have been intolerably loud. At Vesuvius the vertical shocks corresponded with a sound like volleys of musketry, whilst the undulatory shocks gave the roaring. Some of these sounds could be imitated artificially by rubbing together the conducting wires in the same manner in which the rocks must rub against each other in an earthquake. Other sounds were imitated by placing the microphone on a vessel of boiling water, or by putting it on a marble slab and scratching and tapping the under side of it.

These, then, are some of the more important results which have been arrived at by the study of microseismic motions. One point which seems worthy of attention is that they appear to be more law-abiding than their more violent relations, the earthquakes, and as phenomena in which natural laws are to be traced they are certainly deserving our attention. As to whether they will ever become the means of forewarning ourselves against earthquakes is yet problematical. Their systematic study, however, will enable us to trace the progress of a microseismic storm from point to point, and it is not impossible that we may yet be enabled to foretell where the storm may reach its climax as an earthquake. These, I believe, are the views of Professor di Rossi, who is at the present time engaged in the establishment of a system of microseismic observations throughout Italy.

Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer was in a state of agitation, and similar disturbances were observed at Livorno, Florence, and Bologna.

Since February 1883 I have observed a tromometer in Japan, and such results as have been obtained accord with results obtained in Italy. The increase in microseismical activity with a fall of the barometer is very marked. Other peculiarities in the behaviour of the instrument will be referred to under ‘Earth Pulsations.’

Cause of microseismic movements.—As to the cause of tromometric movements, we have a field for speculation. Possibly they may be due to slight vibratory motions produced in the soil by the bending and crackling of rocks produced by their rise upon the relief of atmospheric pressure. If this were so we should expect similar movements to be produced at the time of an increase of pressure. Rossi suggests that they may be the result of an increased escape of vapour from the molten materials beneath the crust of the earth, consequent upon a relief of pressure. The similarity of some of the sounds which are heard with the microphone to those produced by boiling water are suggestive of this, and Rossi quotes instances when underground noises like those which we should expect to hear from a boiling fluid have been heard before earthquakes without the aid of microphones. One instance was that of Viduari, a prisoner in Lima, who, two days before the shock of 1824, repeatedly predicted the same in consequence of the noises he heard.