The Cause and Character of Earthquakes1

Abstract

THE stress-difference required to produce fracture in average hard rocks, as they are met with at the surface, is round about 1,000,000 grammes per centi metre square, and, allowing for the greater strength at depth, which is indicated by the experiments of Prof. Adams and the computations of Prof. Barrell, we may put the breaking strength of the earth's crust at about double this quantity, so that to reach this point in one year from starting, the strain would have to increase at the rate of about 1400 grammes per centi metre square in each quarter of a day. According to the late Sir George Darwin the stress-differences set up by the moon in the latitude of Italy would amount to about 20 grammes per centimetre square in an incompressible Earth, and in a compressible Earth with an incompressible crust, a condition much more akin to what we have reason to suppose is the reality, the stress-differences would be many times this figure; but even the lower amount is nearly 112 per cent, of the growth required to reach breaking point in one year; it would be close on 15 per cent, if the period is increased to ten years, and, with anything approach ing this proportion, a periodicity would result which could not have escaped detection before now.