Spatio‐temporal variation of the tidal triggering effect on earthquake occurrence associated with the 1982 South Tonga earthquake of Mw 7.5

We observed a clear tidal triggering effect on earthquakes closely related to the occurrence of the 1982 South Tonga earthquake (Mw 7.5) by precisely measuring the correlation between the earth tide and earthquake occurrence using 385 shallow earthquakes of reverse‐fault type in the Tonga‐Kermadec subduction zone. The result of statistical analysis indicates that the tidal effect, having been predominant for the normal stress on the fault plane, concentrated in and near the future focal area for several years preceding the South Tonga earthquake. The high correlation disappeared after the earthquake. The frequency distribution of phase angles for the pre‐event period exhibited a peak at the phase angle where the tidal normal stress is at its maximum to accelerate the fault slip, which indicates that the observed high correlation is not a statistical chance but physically reasonable.