Seasonality of great earthquake occurrence at the northwestern margin of the Philippine Sea plate

A significant seasonality is found in the occurrence time of past great earthquakes (M ≥ 7.9) in the northwestern margin of the Philippine Sea plate. Among the thirteen earthquakes cataloged, five took place in December, and all the earthquakes are included in the seven months from August to February. The probability that such a skewed distribution occurs by chance is as small as 2.0%. This seasonal concentration of earthquakes suggests that a small stress increase may trigger an earthquake when the future focal zone is at a critical condition to release a large rupture. As a candidate factor of the earthquake triggering, we tested a possibility of stress change caused by annual variation of the atmospheric pressure. However, the incremental stress is no larger than 30 Pa, and it hardly accounts for the seasonality of the great earthquakes.