Published online by Cambridge University Press: 01 May 2009
SOME recent studies on the periodicity of earthquakes have thrown light on their origin and especially on that of their attendant after-shocks. The periods that are of most service for this purpose are the solar periods of one year and one day, the lunar periods of 29·6 and 14·8 days, and the period of 42 minutes due to the throbbing of the earth in consequence of the crustal displacement that gives rise to the earthquake. As a rule, the maximum epoch of the annual period falls either in midwinter or in midsummer, that of the diurnal period about midnight or noon, that of the period of 29·6 days close to the times of either new and full moon, that of the period of 14·8 days at or about the times of new and full moon or of first and last quarter, and that of the 42 minute period either with the return movements after their passage to the antipodes and back or half-way between these returns.
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