The fortnightly tide and the tidal triggering of earthquakes

Abstract

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