High tides, which typically occur twice a day, are caused when ocean water is moved by the gravitational pull of the moon. But twice a month, during a full or new moon, tides are especially high because the moon, earth and sun all line up together. (These twice-monthly tides are known as "spring" tides.)
Big quakes can occur when this additional weight of tidal water strains geological faults, according to the study. Though this theory is not new, this is the first study to display a firm, statistical link.