Tidal triggering of earthquakes posits that the gravitationally driven deformation of the Earth—principally arising from the relative geometry of the Sun, Moon and Earth (syzygy)—can modulate stress on faults that are already close to failure and thereby advance or delay seismic slip. Tidal effects act through two primary pathways: direct elastic deformation of the crust (solid‑Earth or body tides) that alters the regional strain field, and indirect hydrostatic loading and unloading associated with ocean tides that change the overburden pressure on coastal and near‑shore faults. Because ocean unloading reduces the normal (clamping) stress on a fault, it can diminish frictional resistance and promote slip on faults favorably oriented to the tidal stress perturbation.
Empirical investigations over more than a century have produced mixed outcomes. Correlations between tidal cycles and seismicity tend to be more consistent in volcanic settings and at mid‑ocean spreading centers, where tidal stress changes combine with high pore pressures and weakened rock; by contrast, many continental earthquake studies have failed to identify a universal tidal triggering signal. Methodological shortcomings contribute to this heterogeneity: neglecting tidal phase, site‑specific fault geometry (notably fault dip and sense of slip), and rigorous statistical treatment can mask genuine associations or produce spurious positives. For example, time series of ocean tide amplitude (one such record was collected near the Golden Gate in 1970 and used in early tidal–seismic correlation efforts) illustrate attempts to match tidal forcing to seismic windows, but interpretation depends critically on analysis choices.
More robust findings have emerged when fault type and tidal phase are explicitly considered. Systematic work has not supported a generalized increase in earthquakes during epochs of maximum tidal range (spring tides), but has detected a small yet statistically significant rise in seismicity near low tide in some coastal regions. The sensitivity of faults to tidal unloading is strongly dependent on fault kinematics: thrust (reverse) faults, which are clamped by overburden, are preferentially promoted to slip by unloading, whereas strike‑slip faults typically show little or no response to ocean tidal loading.
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Tidal forcing produces its clearest and most reproducible signal in non‑volcanic tremor (NVT): the small stress oscillations associated with tides correlate well with variations in tremor occurrence, demonstrating that very modest stress perturbations can modulate fault slip in weakly coupled regimes. Volcanology likewise exploits the predictable character of Earth tides both to test and calibrate sensitive deformation instruments and, in some cases, to identify tidal modulation or triggering of volcanic events. Overall, tidal triggering is a subtle, context‑dependent phenomenon whose detectability hinges on local fault properties, pore‑pressure conditions, tidal phase, and the rigor of statistical methods.