Path-Dependent Landslide Initiation Through Response-Generated Memory Under Mainshock–Aftershock Loading

Earthquake sequences can affect slope stability before recovery from an earlier event is complete, yet reduced-order models commonly treat successive earthquakes as independent inputs or prescribe cumulative damage through event-based increments. We introduce a bounded, response-generated memory state into a delayed two-block landslide model. The state reduces incremental friction, evolves exclusively through computed dissipative response, and heals continuously between earthquakes. Its critical value is derived independently from the characteristic roots of the delayed mechanical subsystem. Paired aftershock-only and mainshock–aftershock experiments use corrected accelerograms from the 2011 Redcliffs sequence as recorded inputs rather than calibration data. Across 1,255 admissible deterministic comparisons, a subcritical mainshock reduced the aftershock activation threshold in every parameter cell; 1,235 threshold intervals were strictly separated, while 20 converged to the aftershock-only limit under strong healing. Threshold reduction was almost entirely determined by retained memory (Spearman rank coefficient ρS = 0.999683). Under stochastic forcing, all 33 primary parameter cells showed the same direction of reduction, and 32 paired 95% bootstrap confidence intervals excluded zero. Independent 4,096-realization ensembles yielded reductions of 18.0–26.2% under white-noise and Ornstein–Uhlenbeck forcing. Resetting memory reproduced the aftershock-only response exactly, whereas removing displacement delay eliminated delayed activation. These results identify a causal mechanism by which a non-activating earthquake can transiently lower the activation threshold of a subsequent event, while distinguishing dimensionless model activation from physical landslide failure.