Theoretical Constraints on Tidal Triggering of Slow Earthquakes

Tidal stress is a periodic stress acting globally on the Earth, driven primarily bythe gravitational forcing of the Moon and the Sun. Understanding how tidalstress can trigger seismic events is essential for constraining tectonicenvironments that are sensitive to small, periodic stress perturbations.Here we investigate tidal triggering on stable sliding, velocity-weakening (VW)rate-and-state frictional (RSF) faults using a spring-block framework. We firstapply idealized step-like and boxcar normal stress perturbations todemonstrate a resonance-like amplification of slip rate when the perturbationduration approaches the intrinsic RSF time scale. Building on this observation,we perform non-dimensional analyses and numerical simulations withsinusoidal tidal-like perturbations to identify the key parameters controllingtidal triggering and their admissible ranges. We further characterize thetriggered events through observable quantities, including radiation efficiencyand tidal phase. Our results show that resonance effects allow tidal stress to trigger bothregular periodic and complex temporal slip events on otherwise stable slidingVW faults. The triggering behavior is primarily controlled by two non-dimensional parameters: the normalized perturbation period and thenormalized perturbation amplitude. Increasing the normalized period shiftsevent timing from peak tidal stress toward the maximum stress rate, whereasincreasing the normalized amplitude promotes a transition from slow to fastslip events. The parameter space permitting tidally triggered slip eventssuggests that the RSF parameter,$a\sigma$, which characterizes theinstantaneous frictional strength of an interface, should not exceed tens tohundreds of kilopascals, and that the characteristic slip distance for frictionalweakening is likely on the order of micrometers.