Tidal Forcing of Interplate Earthquakes Along the Tonga-Kermadec Trench

Abstract

We investigated the spatiotemporal variations of the relation between the levels and phase angles of Earth tidal indices and the occurrence of interplate earthquake events along the Tonga-Kermadec trench. As tidal indices, we used volumetric strain (ΔV), shear stress (Δτ), normal stress (Δσ), and the Coulomb failure function (ΔCFF; calculated assuming values of 0.1, 0.4, and 0.7 for the apparent friction coefficient, μ′). We found that seismicity correlated with Δσ rather than Δτ and that earthquakes tended to occur when the tidal force promoted fault slip. Earthquakes tend to occur when the tidal stress levels have large positive values and tend to be suppressed when large negative values. We inferred that μ′ is relatively large on the plate boundary along the Tonga-Kermadec trench, which implies that the response of pore pressure to Δσ is weak. Seismicity at around 20–22°S and 27–29°S was particularly sensitive to Δσ. The b value of the Gutenberg-Richter relation decreased when the difference between the maximum and minimum tidal principal stresses was high, consistent with the results of rock deformation experiments.