Continuous GPS measurements at about 500 sites in Japan reveal a time periodic component of crustal strain accumulation within the island arc. The magnitude of this variability is about 20--30% of the steady annual component of deformation driven by coupling across the subduction zone. The amplitude of temporal variability is strongly organized in space, and its correlation with the secular velocity field suggests a geodynamic origin for the phenomenon. The observed periodicity may help to explain the seasonality of occurrence of the major earthquakes (Magnitude >7.5) associated with the Japanase subduction zone. ¿ 2001 American Geophysical Union