Tidal stresses from the Moon and Sun, tiny gravitational forces that flex Earth’s crust, can trigger slow earthquakes through a resonance effect, according to recent modeling published in JGR Solid Earth (Zhou et al., 2026).

Slow earthquakes (or slow- slip events) release energy over hours to months rather than seconds. They are usually imperceptible at the surface but occur on some plate-boundary faults and can precede or relate to larger conventional quakes.

Solid- Earth tides (distinct from ocean tides) produce stresses of only a few kilopascals, comparable to a gentle hand press, yet observations show they can modulate or trigger these slow events on certain faults (for example, in southwest Japan and the Cascadia subduction zone, where activity often peaks at ~12- and 24- hour tidal intervals).

How the mechanism works

Researchers used a simplified spring- block model of a single fault patch combined with rate- and- state friction laws (friction that depends on slip speed and the evolving “state” of the contact surface).

Key findings include:

• Faults that are already sliding stably can be nudged out of that state by weak periodic tidal forcing.
• When the period of the tidal stress matches the fault’s natural response timescale (the time needed for friction and other interface properties to adjust), resonance amplifies the effect, analogous to pushing a swing at the right rhythm or rubbing a wet finger around a wine-glass rim. Even small amplitudes can then produce accelerated slip.
• Outcomes depend on both amplitude and period:
  • Low- amplitude forcing may leave the fault sliding quietly.
  • Above a threshold amplitude, and with a matching period, the fault can produce slow slip events (or, under some conditions, faster events).
• Triggering can align with peak tidal stress, the peak rate of stress change, or more complex/chaotic patterns, depending on the specific fault properties and the phase of the tidal cycle. sciencealert.com

The model reproduces observed tidal modulation of slow earthquakes and low- frequency earthquakes. Because tidal forcing is predictable, matching observed seismic patterns to tidal cycles can help constrain fault properties (such as frictional strength and critical slip distance).

Caveats and context

The simulation treats an isolated fault patch and is therefore best suited to repeated local low- frequency events rather than large- scale tremor involving many patches.

Real sensitivity also depends on local geology (fluid pressure, rock type, etc.). Tidal stresses are far too weak to trigger large “fast” earthquakes on their own in most cases; any influence on major quakes remains subtle and is not useful for deterministic prediction.

Overall, the work supplies a physical framework explaining why such minute, continuous tidal stresses can still matter for slow slip, and it may ultimately improve understanding of stress buildup and release on subduction zones that host megathrust earthquakes. The Phys.org piece by Paul Arnold reports on this same body of research.

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The spring- block model (also called the slider- block or spring- slider model) is a simple but powerful mechanical idealization used in earthquake science, especially for studying rate- and- state friction and the conditions under which faults stick, slip, or produce slow earthquakes.

Basic setup

Imagine a block (representing a patch of a fault) sitting on a rough surface (the opposing side of the fault).

• The block is attached to a spring.
• The other end of the spring is pulled at a constant slow speed (this represents the long- term tectonic loading or plate motion).
• Friction acts between the block and the surface, resisting motion.

As the far end of the spring is pulled, the spring stretches and stores elastic energy. When the force in the spring exceeds the frictional resistance, the block slips forward. After slipping, the spring relaxes, friction may heal, and the cycle can repeat.This captures the essential competition between:

• Elastic loading (the spring), and
• Frictional resistance on the fault surface.

Rate- and- state friction (the key ingredient in the paper)

In modern versions of the model (including the one used by Zhou et al. 2026), friction is not a fixed number. It depends on:

• Slip velocity (the “rate” part): how fast the surfaces are sliding relative to each other.
• State variable $\theta$: a measure of the “quality” of the contact (how well the microscopic asperities are interlocking or how much time has passed since the last slip). The state evolves with time and slip according to an evolution law (commonly the aging or slip law).

Two important frictional parameters appear:

• (a): controls the direct (instantaneous) velocity dependence of friction.
• (b): controls how much friction evolves (weakens or strengthens) with the state variable.

The difference $a – b$ (or the ratio $a/b$) determines the overall behavior:

• If $a – b < 0$ (velocity- weakening), the fault can become unstable.
• If $a – b > 0$ (velocity- strengthening), sliding tends to be stable.

Critical stiffness $k_c$

The spring has a stiffness (k) (force per unit stretch). There is a critical value $k_c$ that depends on the frictional parameters, normal stress, and the characteristic slip distance $d_c$:

$$k_c \propto \frac{(b – a)\sigma}{d_c}$$

• When $k < k_c$ the system is unstable → classic stick-slip (regular earthquakes).
• When $k > k_c$ the system is stable → continuous stable sliding (creep).
• When (k) is only slightly larger than $k_c$k_c (the regime studied in the paper), the fault is stable but marginally so. Small external perturbations can push it into large slip-velocity amplifications via resonance.

How the paper uses the model

Zhou et al. (2026) focus exactly on this near-critical, velocity- weakening, stably sliding regime. They add periodic (tidal- like) stress perturbations, either changes in normal stress or shear stress, and show that:

• When the period of the tidal forcing is close to the natural frictional response time of the fault, resonance occurs.
• Even tiny perturbations (a few kPa) can then produce bursts of accelerated slip (slow earthquakes or even faster events).

The model is deliberately simplified (a single block = a single fault patch), which makes the mathematics tractable and allows clear identification of the controlling dimensionless parameters (normalized period and amplitude).

Real faults are more complex (many interacting patches, fluids, etc.), but the spring- block model isolates the essential frictional dynamics that can explain why some faults are so sensitive to the tiny stresses from the Moon and Sun.

In short: the spring- block model turns the complicated physics of a tectonic fault into a manageable mechanical system whose behavior is governed by the competition between spring stiffness and rate- and-state friction, exactly the tool needed to understand tidal triggering of slow earthquakes.

Earth’s crust really does experience continuous, tiny stress changes caused by the gravitational pull of the Moon and the Sun. These are called tidal stresses (or solid- Earth tides).

Why they exist

Just as the Moon and Sun raise ocean tides by pulling on the water, they also slightly deform the solid rock of the Earth itself. The solid Earth is elastic, so it bulges by a few tens of centimeters. That deformation produces stresses throughout the crust and upper mantle.

How large are they?

• Typical amplitudes are only a few kilopascals (kPa), often 1-10 kPa, sometimes up to ~100 kPa when both solid- Earth tides and ocean loading are combined.
• For comparison: atmospheric pressure is about 100 kPa, and the pressure under a gentle hand press is a few kPa. Lithostatic stresses at depth are measured in megapascals (MPa) to hundreds of MPa, so tidal stresses are extremely small relative to the background forces acting on faults.

Why they still matter

Even though the stresses are tiny, they are:

• Continuous and perfectly periodic (mainly ~12- hour and ~24- hour cycles, plus longer fortnightly and monthly variations).
• Applied globally and predictably.

On most faults these stresses are negligible. But on certain faults that are already close to failure and have the right frictional properties (especially the near-critically stable, velocity- weakening patches studied in the spring- block model), the periodic tidal stresses can be amplified through resonance.

When the tidal period matches the fault’s natural response timescale, even a few-kilopascal “nudge” can trigger measurable slip, the slow earthquakes (tremor, low- frequency earthquakes, slow- slip events) that are observed to correlate with the tides in places such as Cascadia and southwest Japan.

In short: the gravitational forces of the Moon and Sun really do continuously stress Earth’s crust by tiny amounts, and under the right conditions those tiny stresses can trigger slow earthquakes.

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Theoretical Constraints on Tidal Triggering of Slow Earthquakes

Even very small tidal stresses (a few kilopascals, comparable to the pressure of a gentle hand press) caused by the Moon and Sun can trigger slow earthquakes on certain faults through a resonance mechanism. When the period of the tidal stress matches the fault’s natural response timescale, the effect is amplified, much like pushing a swing at the right rhythm.

The authors use a classic spring -block model with rate- and- state friction focused on velocity-weakening faults that are stably sliding (stiffness slightly above the critical value).

They first apply idealized step and finite- duration (“boxcar”) normal- stress perturbations to demonstrate resonance- like amplification of slip velocity for specific durations. They then examine realistic harmonic (tidal- like) perturbations and identify the controlling dimensionless parameters through nondimensional analysis and numerical simulations.

Key controlling parameters

Triggering is governed mainly by two normalized quantities:

• Normalized perturbation period ($P_T$)
• Normalized perturbation amplitude ($P_\sigma$)
• Increasing the normalized period shifts the timing of slip events from the peak of tidal stress toward the peak of the stress rate.
• Increasing the normalized amplitude promotes a transition from slow to faster slip events.

Even small perturbations can produce both periodic and temporally complex slip events on otherwise stable sliding faults.

Implications for fault properties

The parameter space that allows triggering implies:

• The instantaneous frictional strength parameter $a\sigma$ is typically only tens to hundreds of kilopascals (much lower than typical lithostatic values).
• The characteristic slip distance for frictional weakening ($d_c$) is likely on the order of micrometers.

Matching observed tidal correlations (e.g., the common 12- and 24- hour peaks of tremor in Cascadia and southwest Japan) with the model predictions can therefore help constrain these frictional properties of the plate interface.

Broader context

Slow earthquakes (including low-frequency earthquakes, tremor, and slow-slip events) are widely observed around the Pacific Rim, often updip or downdip of megathrust seismogenic zones.

Because they are sensitive to tiny stress changes, they provide useful information about stress accumulation and the potential size and extent of future large earthquakes.

The resonance framework offers a physically grounded explanation for why some faults respond strongly to tidal forcing and supplies a practical way to interpret those observations.

Plain- language takeaway: Tiny, continuous gravitational tugs from the Moon and Sun can set off slow, imperceptible earthquakes when their timing matches a fault’s natural “rhythm”. The study maps the conditions under which this happens and shows how those conditions can be used to learn about the mechanical properties of real faults.

Journal information: Journal of Geophysical Research: Solid Earth (2026)

Doi: 10.1029/2026jb033983 (also available as arXiv:2602.06703)

Authors: Yishuo Zhou, Ankit Gupta, Hideo Aochi, Alexandre Schubnel, Satoshi Ide, Pierpaolo Dubernet, Harsha S. Bhat

Abstract

Tidal stress is a globally acting perturbation driven primarily by the gravitational forces of the Moon and the Sun. Understanding how tidal stresses can trigger seismic events is essential for constraining tectonic environments that are sensitive to small stress perturbations. Here, employing a spring–block model with rate-and-state friction, we investigate tidal triggering on velocity-weakening stable sliding faults with stiffness slightly exceeding the critical stiffness. We first apply a step and a boxcar with finite duration normal stress perturbation to demonstrate a resonance-like amplification of slip velocity for specific boxcar durations. Next, we perform nondimensional analyses and numerical simulations with harmonic perturbations to identify the key parameters controlling tidal triggering and their admissible ranges. Triggered slip events are further characterized using physically observable quantities, including radiation efficiency and tidal phase. Our results show that even small stress perturbations can trigger periodic as well as temporally complex slip events on stable sliding faults. The triggering behavior is primarily controlled by the normalized perturbation period and the normalized perturbation amplitude. An increase in the normalized period shifts event timing from the peak of tidal stress toward the peak of stress rate, whereas increasing the normalized amplitude promotes a transition from slow to fast events. This framework helps explain the period-dependent sensitivity and the observed phase preference between tidal stress and maximum slip velocity. Comparison between observed and model-predicted tidal correlation patterns may therefore help constrain the instantaneous frictional strength of the interface, as well as the characteristic slip distance for frictional weakening.