Note on the influence of the Moon on earthquakes
Harry Fielding Reid · Transactions American Geophysical Union · 1926
Since the time of Perrey much statistical study has been devoted to seeking a relation between the frequency of earthquakes and the position of the moon. Different investigators have arrived at contradictory conclusions; so that the results of all this work have been disappointing. I think we shall see the reason for this if we look at the problem from a dynamical standpoint. The earth is tidally distorted by the moon and stresses must be set up which will add to those growing independently in the earth and influence the time of occurrence of a particular earthquake, and a preliminary estimate indicates that this influence may be important. Yet the influence depends not only upon the position of the moon, but also upon the direction of the strike of the fault where the fracture is about to take place with respect to the azimuth of the moon and on the dip of the fault. Of two faults similarly situated with respect to the moon, but with the downthrow on opposite sides of the fault, one will be advanced and the other retarded. In any large catalogue of earthquakes, faults will be involved striking in various directions, with different dips and with downthrown sides both towards and away from the moon. We conclude, therefore, that although the moon may have a definite effect on any particular fault, the hour‐angle for its greatest effect will differ much for different faults, and no hour‐angle will be especially predominant when a long list of earthquakes are studied statistically. This is only a preliminary note on a subject still under investigation.