The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Engineering geology

Collection:: Economics Books

EARTHQUAKES 251 (Rep. Brit. Assoc., 1917, pp. 13-14; 1922, Phil. Trans. A., vol. 222, pp. 45-6). Prof. H. H. Turner, from the time of arrival of an earthquake shock at the opposite side of the sarth, assigns the centrum of most earthquakes to a depth of about 145 miles, and some to 310 miles; e.g. the Formosa Earthquake of 14th April, 1906, started at a depth of 280 miles (Rep. Brit. Assoc., 1922, p. 255), and that in China on 16th December, 1920, at more than 80 miles (ibid., p. 256). The Assam Earthquake of 12th June, 1897, which had a meizoseismic area of 150,000 square miles, was at first re- ferred to a depth of 5 miles, but is now assigned by R. D. Oldham to the depth of between 100 and 200 miles (G.S. India, Mem. xlvi, pt. 2, 1026, p. 62). Causes—Certain unstable belts in the earth’s crust are especially liable to earthquakes, which are of three kinds— tectonic, volcanic, and those due to variations in the load on the surface. Tectonic earthquakes are due to unequal movements of the material within or below the crust along great faults and thrust-planes, around subsidences, and along folds which are often traversed by cross-faults.” Volcanic earthquakes are due to the uprush of steam during eruptions which keep the adjacent ground in constant tremor, while single explosions may shake the whole world. Volcanic action often results in local earthquakes by the collapse of cavities left by the ejection of material or the shrinkage of the cooling rocks. Such earthquakes may be of intense violence, but of short range ; those in Ischia from 1881-3, due to subsidence in an extinct volcano, though they culminated in the destruction of the chief town of the island, were barely perceptible in Naples 18 miles away, and were not recorded in Vesuvius Observatory at the distance of 25 miles. Earthquakes have been attributed to landslips, such as the Pamir Earthquake of 1911; but Oldham (Q.%.G.S., xxix, 1923, pp- 243-4) has shown that the landslip at Usoi was not at the epicentre and was a result of the earthquake and not its cause. The slide of material down oceanic slopes often breaks telegraph cables; but the movement may not be recorded by seismographs and so does not cause appreciable vibration in the crust, although the repeated blows of the sutf on the coast of India disturbs the seismo- graph at Calcutta 500 miles away.

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