Tolstoy et al. (2002) have shown that tidal forces influence earthquakes and volcanoes. The purpose of this comment is to add support to their correlation by demonstrating that when the tidal forces are strongest, earthquakes and volcanic eruptions are the most severe.
Three factors determine the magnitude of tidal forces: the distance between the Moon and Earth, the distance between the Sun and Earth, and the alignment of the Sun, Earth, and the Moon. The Sun's influence on tides is only 45% of that of the Moon, but when Earth is closest to the Sun (as it is in December–January), the Sun's effect is at its strongest. This could account for the great earthquakes of central Japan occurring exclusively from August to February since 684 A.D., as reported by Kerr (2001).
Within a day or two of a new moon or a full moon, the tidal forces of the Moon and the Sun act in concert. And if perigee occurs within a day or two of a new moon or a full moon, the tidal effects are further reinforced. As reported by the U.S. Geological Survey, six major earthquakes (magnitude 7.0 or greater) occurred in the first half 2002; four of these were within three days of perigee and a full moon (p < 0.02) (Table 1).
In the second half of 2002, perigee will occur within a few days of a new moon during the periods 7–10 September, 5–8 October, 3–6 November, and 2–5 December. These are the periods when major earthquakes and/or significant volcanic eruptions are most likely to occur.
Note added in proof: Precisely as predicted, a magnitude 7.6 earthquake occurred on 8 September, 2002, at Papua New Guinea. Also, of the five major earthquakes that occurred from September to December 2002, three of the five took place during the predicted dates listed here. The predicted dates comprised a total of 16 days out of the 122-day period from September to December (16/122 = 13%), but 60% of the earthquakes occurred during those dates. See Table 2 added in proof.