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Darwin Correspondence Project

From Robert Thomson   1 May 1878

Beaufort, South Carolina.

Wed. 1st. May 1878

Dear Sir:

Having read only recently, in your “Naturalists Tour Around the World”, with much interest the account of the great earthquake in Chile of Feby. 1835, I beg leave to submit to your attention a conjecture of my own as a possible explanation of certain phenomena connected with that and similar events. I mean an explanation more particularly of the great sea wave accompanying that and other earthquakes, to which you refer as a subject of much obscurity at that time, and of which I am not aware that any satisfactory explanation has ever since been given.1 Allow me to say however that I have no special knowledge of the subjects and have not access to any more detailed account of the earthquake of 1835, than that which is given in your narrative.

1. The theory I have to offer is based upon the idea of what I consider strictly undulatory or wave motion: or in other words, it is the application of the principle of wave motion in water to the case of the undulations of the earth crust, or earthquakes. It postulates the following proposition, that wherever there is an elevation of the earth surface there must be a corresponding adjacent depression. Now applying this principle to the special case of 1835, I venture to say that when the coast of Chile was raised above its former level, at the same time by way of compensation, the earth crust adjoining the region of elevation was depressed below its former level to a corresponding extent. So I come to the conclusion that the bottom of the Sea adjoining the coast of Chile was depressed below its former plane; that the level of the superincumbent sea was correspondingly lowered; and that as a natural consequence the great sea waves came rolling down from the higher plane of the undisturbed ocean, to become converted, upon reaching shallow water, into tremendous breakers.

2. I am considering the earthquake of 1835 as one grand undulation, which I suppose moved from a westerly direction to the coast of Chile, and was finally arrested with its crest elevated in the line of the coast or of the mountains, and thus became permanent, with attendant depressions in the sea bottom and also perhaps in the land on the east of the line of permanent elevation. I thus eliminate from consideration the attendant minor oscillations which may be compared to the secondary waves or ripples upon some great sea swell.

diagram

3. To account for the gentle swelling of the water along the shore, just before the great sea waves came in, I should say that it occurred when the crest of the grand earth undulation was approaching the shore, and quietly raised the superincumbent water, to be correspondingly lowered when the earth wave had passed on to the dry land, and what was the crest became the consequent hollow. I conceive that such a gentle elevation of the water on the shore might occur without a subsequent subsidence below the former level; and in that case there would be no attendant sea waves, as in the earthquake at Chiloe to which you refer as having happened a few years before that of 1835.2 Perhaps the grand earth wave in such a case would be considered as arrested before reaching and elevating the land. Was there no permanent elevation of the land at Chiloe?

4. It appears to me that such a theory is in general harmony with the facts given in your narrative, and especially with your suggestion of the place of origin of the sea waves. I need hardly say that we should expect several rather than one great sea wave to be formed, and that immediately behind the first, two or more minor waves would follow and move onward to restore the natural level of the sea. I have perhaps generalized with more freedom in this matter from the fact that I am not hampered with much detailed information concerning the subject. And I am not conversant with the progress of knowledge in reference to earthquakes since the year 1835, nor have I ready access to the observations of recent explorers and writers.

If the idea I have ventured to bring forward has sufficient interest, although very unwilling to trespass upon your time and attention, I should like to know how far it seems compatible with your own observations, and how far tenable in the present state of knowledge. I remain with great respect | yours very truly | Robt. Thomson

Professor Chas. Darwin | England

Footnotes

In his Journal of researches, pp. 377–9, CD discussed earthquake waves (often known now as tsunami or seismic sea waves) and hypothesised that they were the results of disturbances in the equilibrium of an undulation. CD had arrived at Concepción, Chile, a couple of weeks after the earthquake that happened on 20 February 1835, and recorded feeling the tremors and earth movement on that day at Valdivia, over two hundred miles to the south (ibid., pp. 368–9).

Bibliography

Journal of researches: Journal of researches into the geology and natural history of the various countries visited by HMS Beagle, under the command of Captain FitzRoy, RN, from 1832 to 1836. By Charles Darwin. London: Henry Colburn. 1839.

Summary

On earthquakes, and the generation of massive sea-waves that accompany them.

Letter details

Letter no.
DCP-LETT-11492
From
Robert Thomson
To
Charles Robert Darwin
Sent from
Beaufort, S.C.
Source of text
DAR 178: 118
Physical description
3pp

Please cite as

Darwin Correspondence Project, “Letter no. 11492,” accessed on 22 September 2021, https://www.darwinproject.ac.uk/letter/DCP-LETT-11492.xml

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