- © 2004 by the Seismological Society of America
For more than 100 years, scientists have been searching for a temporal relation between earthquakes and the Earth's tides (Schuster, 1897; Emter, 1997). Such a relation is plausible because the tides wax and wane with predominantly diurnal (12-hour) and fortnightly (14-day) periods. This plausibility is strengthened by the fact that the stressing rates in the Earth's crust resulting from the tides are far greater than from other known loads, such as tectonic plate motions (Vidale et al., 1998).
Studies from the last 10 years have focused on diurnal tides. The highest-resolution studies find at best a few percent variation in the rate of earthquake occurrence due to diurnal tide stressing (Tsuruoka et al., 1995; Vidale et al., 1998; Ohtake et al., 2001), except at the places with the largest tides (Cochran et al., 2004). Some studies suggest the tidal triggering of earthquakes varies with tectonic region; the highest correlations have been found in ocean ridge environments (Tolstoy and Vernon, 2002). The lack of a strong correlation of earthquakes with the diurnal tides suggests a delay may occur between achievement of high tidal stresses and the triggering of an earthquake. If the delay is of the order of a few days, than we should be able to capture the correlation within a window spanning several days about the times of highest tidal stress. This method has been pioneered by James Berkland (http://www.syzygyjob.com/) and termed “syzygy.” Syzygy refers to the new and full moon, and primarily depends on the Sun-Earth-Moon angle (Figure 1). The syzygy hypothesis was tested by analyzing more than 2,000 earthquakes in the San Francisco Bay area (McNutt and Heaton, 1981). This showed that predictions based on the Sun-Earth-Moon angle are not sufficiently powerful to make societally useful earthquake risk …