The Mechanism of Tidal Triggering of Earthquakes at Mid-Ocean Ridges

Evidence for the triggering of earthquakes by tides has been largely lacking for the continents but detectable in the oceans where the tides are larger. By far the strongest tidal triggering signals are in volcanic areas of mid-ocean ridges. These areas offer the most promise for the study of this process, but even the most basic mechanism of tidal triggering at the ridges has been elusive. The triggering occurs at low tides, but as the earthquakes are of the normal faulting type, low tides should inhibit rather than encourage faulting. Here, treating the most well documented case, Axial Volcano on the Juan de Fuca ridge, we show that the axial magma chamber inflates or deflates in response to tidal stresses and produces Coulomb stresses on normal faults opposite in sign to those produced by the tidal stresses. If the bulk modulus of the magma chamber is below a critical value, the magma chamber Coulomb stresses will exceed the tidal ones and the phase of tidal triggering will be inverted. The stress dependence of seismicity rate agrees with triggering theory with unprecedented faithfulness, showing that there is no triggering threshold.

💡 Research Summary

Scholz, Tan, and Albino present a comprehensive investigation of tidal triggering of earthquakes at the Axial Volcano on the Juan de Fuca ridge, offering a mechanistic explanation that resolves a long‑standing paradox. While low tides produce tensile vertical stresses that should inhibit slip on steeply dipping normal faults (≈ 67°), observations show a clear increase in normal‑faulting micro‑earthquakes during low tide periods. The authors demonstrate that this apparent contradiction arises from the response of the underlying magma chamber, which acts as a soft inclusion with a bulk modulus (Km) far lower than that of the surrounding crust (Kr).

When the ocean tide falls, the reduction in vertical load causes the relatively compliant magma chamber to inflate. This inflation generates a Coulomb stress change (ΔCFS = Δτ − µΔσ) on the surrounding faults that is opposite in sign to the direct tidal stress. If the magma’s bulk modulus is sufficiently low, the chamber‑induced ΔCFS outweighs the tidal contribution, resulting in a net positive stress on the faults during low tide and thus triggering earthquakes. Conversely, high tides cause chamber deflation, producing a negative ΔCFS that suppresses seismicity. The key control parameter is the ratio Km/Kr; the authors map this relationship in Figure 4, defining red (low‑tide‑triggered) and blue (high‑tide‑triggered) regimes. Using realistic values (Km ≈ 1 GPa, Kr ≈ 55 GPa, effective friction µ′ ≈ 0.4), they obtain χ = ΔCFS/σv ≈ 0.32, placing Axial Volcano firmly in the low‑tide‑triggered domain.

The study then quantifies the seismicity‑stress relationship using two well‑established triggering laws: a rate‑and‑state friction formulation and a stress‑corrosion (subcritical crack growth) model. Both equations fit the observed seismicity rate versus tidal stress curve (Figure 5) with remarkable fidelity, showing no detectable phase lag or hysteresis and confirming that the system operates in the nucleation regime (earthquake nucleation time ≈ 48 h > tidal period). The fit yields an A·σ product of 0.0043 MPa, an order of magnitude smaller than values inferred from deeper continental earthquakes, reflecting the shallow depth (≈ 1.2 km) and low normal stress (≈ 7.2 MPa) of the Axial events. The stress‑corrosion fit implies a stress drop of 0.09–0.17 MPa, consistent with independent estimates for normal‑faulting basaltic earthquakes.

Importantly, the authors demonstrate that seismicity rate varies smoothly to zero as tidal stress vanishes, providing strong evidence that there is no threshold stress for static tidal triggering; stress shadows are simply the negative side of a continuous response function. This contrasts with many dynamic‑triggering studies that invoke fluid‑pressure or permeability changes, which the authors argue are unnecessary for the Axial case because the continuous tidal cycling prevents the development of clogging or other time‑dependent fluid effects.

The paper extends the discussion to other oceanic settings. Similar low‑tide correlations have been reported on the Endeavour segment of the Juan de Fuca ridge and on the East Pacific Rise, where smaller tides and a larger contribution from solid‑Earth tides still produce detectable triggering. The authors caution that in regions where hydrothermal circulation or mixed focal mechanisms dominate, additional processes (e.g., pore‑pressure modulation) may modulate the tidal signal.

In summary, this work provides a physically grounded, quantitatively validated model for tidal triggering at mid‑ocean‑ridge volcanoes. By linking magma chamber compressibility to Coulomb stress changes on adjacent faults, it resolves the paradox of low‑tide normal‑faulting earthquakes, confirms the applicability of classic rate‑state and stress‑corrosion triggering laws without invoking any stress threshold, and offers a framework that can be applied to other volcanic and hydrothermal seafloor environments.