The fortnightly tide and the tidal triggering of earthquakes

In this paper we test the southern California network earthquake catalog and the -wide earthquake catalog for a fortnightly tidal periodicity and find none. In that test the hypothesis of tidal triggering of earthquakes, it is usually to resolve the tidal shear stress onto the plane of the fault and in the of the slip vector to determine if the stress is compatible with the fault (Heaton, 1975, 1982). This analysis requires accurate focal mechanisms and of the focal plane on which slip occurred for as many earthquakes as to improve the statistical sampling of the data set. However, if we consider fortnightly tide, which is a simple amplitude modulation, focal mechanism is not required. Our analysis assumes that we have a large data base of with varying mechanisms. The alignment of the slip vector with the shear stress will vary from earthquake to earthquake. However, if tidal of earthquakes does occur, more earthquakes should be triggered out of total population when the tidal stress is large than when it is small. If the semi-diurnal tide is normalized to an amplitude of 1.0, the fortnightly tide have an amplitude of 0.17 on this scale and a period of about 2 weeks (Munk MacDonald, 1960). Therefore, the effect on the occurrence of earthquakes by tidal shear stress will be modulated with a 2 week period. Considering the tide allows us to utilize entire catalogs of data to test the tidal triggering without knowledge of the focal mechanisms.