How the Sun and Moon Can Nudge Faults: New Model Explains Tidal Triggering of Slow Quakes

# Summary

# What the model does

The researchers modeled a single patch of a fault as a block connected to a spring and governed by rate-and-state friction. Rate-and-state friction captures how frictional resistance depends on slip velocity and the evolving contact state at the fault interface. The model applies periodic tidal perturbations representative of lunisolar forcing and tracks how the patch responds.

Two parameters of the tidal forcing matter: amplitude (how strong the stress) and period (how long the stress lasts). Small amplitudes tend to leave a fault sliding slowly and quietly. If the amplitude surpasses a threshold and the period falls in the right range, the model produces amplified slip events. When the tidal period matches the patch's natural response timescale, resonance boosts even modest tidal stresses into slow earthquakes.

# Why resonance matters

# Types of timing patterns

The study tested whether seismic events align with the tidal stress maximum, the maximum stressing rate (when tidal stress is changing fastest), or appear without a clear tidal phase. Results depend on the fault's frictional properties and the details of the tidal cycle. Some patches respond at stress maxima, others nearer the maximum stressing rate, and some show more complex, less predictable timing.

# Real-world connections

Observed correlations between tidal cycles and tremor or low-frequency earthquake (LFE) activity in regions such as southwest Japan and the Cascadia subduction zone match patterns the model can reproduce. The modeling is best suited to simulating repeated, local LFEs — single-patch events — rather than broad tremor episodes that involve many interacting patches.

# Practical implications

Because tidal forcing is well known and predictable, matching observed seismic timing to modeled responses could help infer fault properties like frictional strength and how far a patch must slip before weakening. That reverse-engineering could improve interpretation of slow earthquake records and contribute to forecasting efforts focused on stress accumulation and release along subduction zones.

# Final point

The work provides a concrete physical mechanism linking tiny tidal stresses to measurable fault slip through resonance between forcing period and the fault patch response time. It narrows the gap between observed tidal modulation of seismicity and a testable physical model.