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

- 2 CHAPTER XXI EARTHQUAKES AND PRINCIPLES OF ANTI- EARTHQUAKE CONSTRUCTION Tue NATURE oF EARTHQUAKE ActioN—The earth's crust is in constant tremor. The earth travels along its orbit 3000 miles a minute, and places near the Equator revolve around its axis 1000 miles an hour. As the crust is irregular in structure it is always quivering like a badly mounted Aywheel. The surface is affected by continual variations in temperature, in the weight of the atmosphere, in the distribution of snow, rain, and tidal water, by blows on hill- sides from the wind, the hammering of surf on the coast, and the slip of material down hills and oceanic slopes. The crust is violently jerked when blocks sink owing to the loss of support, or slide over one another under the lateral pres- sure of the contracting crust. All these agencies produce waves of distortion, which range from tremors perceptible only by the most sensitive instruments, to shocks which devastate a province and slay a hundred thousand people. The Greeks knew that an earthquake spreads radially from its place of origin, which Aristotle called the centrum. The term is convenient though the centrum may be a large area and not a point. Above the centrum is the epicentrum or epifocal area, where the vibration emerges up the seismic vertical. Around the epifocal area the angle of emergence decreases outward, and is recorded by cracks in masonry or plaster, which tend to be normal to the earthquake path. Lines representing the angles of emergence converge to the centrum. The epicentrum may be determined from the times at which an earthquake was felt at different. localities. Homo- sersts are lines joining places shaken at the same time. Isoseists are lines through places where the shock was of equal severity. The meigoseismic area is that of the greatest

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