The horizontal force on a continent, at the land/ocean boundary, is calculated according to the concept of reflected tide waves, in which there is a time rate of change (reversal) of the linear momentum brought in by the tide to the continent. From the text books, the Stokes drift for surface gravity waves propagating in arbitrary constant mean depth, which is directly related to the linear momentum of the wave, is adapted to the tide wave by using the approximation that the wavelength of the tide is very much larger than the vertical length of the water column. This horizontal force is a maximum at full and new moon when the tide amplitude is greatest. Evidence is cited for the correlation of earthquakes at full moons. The tendency of the reflected tide force on the continents is to cause compression of the solid material in the horizontal direction and it is suggested that this possibly could trigger earthquakes occasionally.
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