Theoretical Constraints on Tidal Triggering of Slow Earthquakes

Tidal stress is a periodic stress acting globally on the Earth, driven primarily by
the gravitational forcing of the Moon and the Sun. Understanding how tidal
stress can trigger seismic events is essential for constraining tectonic
environments 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 first
apply idealized step-like and boxcar normal stress perturbations to
demonstrate a resonance-like amplification of slip rate when the perturbation
duration approaches the intrinsic RSF time scale. Building on this observation,
we perform non-dimensional analyses and numerical simulations with
sinusoidal tidal-like perturbations to identify the key parameters controlling
tidal triggering and their admissible ranges. We further characterize the
triggered events through observable quantities, including radiation efficiency
and tidal phase. Our results show that resonance effects allow tidal stress to trigger both
regular periodic and complex temporal slip events on otherwise stable sliding
VW faults. The triggering behavior is primarily controlled by two non-
dimensional parameters: the normalized perturbation period and the
normalized perturbation amplitude. Increasing the normalized period shifts
event timing from peak tidal stress toward the maximum stress rate, whereas
increasing the normalized amplitude promotes a transition from slow to fast
slip events. The parameter space permitting tidally triggered slip events
suggests that the RSF parameter,$a\sigma$, which characterizes the
instantaneous frictional strength of an interface, should not exceed tens to
hundreds of kilopascals, and that the characteristic slip distance for frictional
weakening is likely on the order of micrometers.