An approximately 9-yr-period variation in seismicity and crustal deformation near the Japan Trench and a consideration of its origin

It is well known that the statistical probability of earthquake occurrence changes over the course of a day due to periodic variations in the tidal stress acting on faults. However, periodicity on a decadal scale has been studied by relatively few researchers. It has been reported that an approximately 10-yr periodicity is observed globally for the seismicity of M-8-class large earthquakes. However, the mechanism underlying this periodicity has not yet been revealed. In this study, the decadal-scale periodicity of earthquakes along the Japan Trench is investigated. A new finding is presented that in northeast Japan, the probability of the occurrence of historical earthquakes with an M >= 6 that have occurred during the past 1000 yr has increased approximately every 9 yr. Periodicity becomes even more apparent for large earthquakes with an M > 7.5 and approximately half the recorded events intensively occurred within two successive years on a cycle of approximately 9 yr. This implies the presence of a periodic stress disturbance at an appreciably regular interval. The past strain and tilt observations conducted in Japan during the 1950s through the 1970s indicate that, nationwide, gradual compression repeated every 8-10 yr in the direction of relative plate motion. These compression periods are in accordance with the periods of higher seismic activity discussed above. As a first step in investigating the origin of earthquake periodicity, periods associated with lunar motion are considered. It is shown that long-term motion primarily governed by the period of the lunar perigee is synchronized with the cyclic variation in seismicity and crustal deformation described above. Decadal changes in tidal stress, as calculated using an ordinary theory of solid Earth tides, are too small to cause periodic variations in seismicity. Therefore, the conditions by which tidal stress is sufficiently amplified to trigger an earthquake are investigated. The results show that, if one assumes that a tidal force acts on a spherically asymmetric block-like upper mantle beneath the Pacific Plate, the computed phase and amplitude can explain the observations. Otherwise, it is difficult to consider direct tidal force alone as the main source of periodic variations in seismicity. Other possibilities should be considered, such as unknown interactions between the plate boundaries and the ocean/atmosphere with a period of approximately 9 yr or a resonance between the period of the tidal force and a recurrence period of slow slip events in the transition zone on the plate boundary. Apart from understanding the origin, the important fact confirmed in this study is that in some areas, the occurrence of large earthquakes, if considered as a group, appears to be strongly governed by a periodic stress disturbance rather than by completely random processes. Elucidating the wide-range approximately 9-yr mode helps us narrow a range in occurrence time in a probabilistic midterm prediction of large interplate earthquakes.