On 18.6-Year and Approximately Nine-Year Variations in Seismicity and Crustal Deformation in Japan and Its Interpretation Based on Decadal Variations in Tremors

Abstract:

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 (8.85 and 18.61 years) has been studied by relatively few researchers and clear correlations have not been obtained. In this study, by investigating historical large earthquakes that occurred over 1,000 years in Japan, it is shown that a stronger periodicity of 8.85 years and a weaker periodicity of 18.61 years appear in the seismicity along the Japan Trench and the Nankai Trough, respectively. The past strain and tilt observations conducted during the 1950s through the 1970s indicate that, nationwide, gradual compression repeated every 8–10 years in the direction of relative plate motion. These compression periods are in accordance with the periods of higher seismic activity in the area along the Japan Trench. This agreement between the phase of the increasing seismicity and the compression periods seen in the long-term cyclic crustal deformation suggests that the periodicity is not merely a coincidence and that the transient crustal movement may be relevant with the tides. Considering the seismiity in the Nankai region, periodicity of 18.61 years should be superimposed on the crustal deformation with periods of 8-10 years. However, the approximately twenty-year observation was too short to extract it. According to a recently proposed model based on the combination of the nonlinear fault rheology and oscillating tidal stress, the above transient crustal deformation with periods of approximately 9 years may be caused by long-term variations in the occurrence rate of tremors. The initiation of the past slow slip events in the Tokai area also agreed with the phase of the 9-year mode except for the largest event from the year 2000.