Mechanical memory in explosive collapse caldera systems: how loading rate and cyclic deformation may condition the transition to caldera collapse

Abstract Explosive collapse calderas that form late in the evolution of large central volcanic complexes represent the end point of a mechanical history spanning reservoir construction over 10 5 –10 6 years, repeated unrest over months to decades, eruptive withdrawal over hours to days, and roof failure over minutes to days. Because researchers commonly investigate these intervals separately, it remains unclear how repeated loading conditions prepare the crust for collapse. Moreover, quantities with identical dimensions need not describe the same deforming component or constitutive regime. Characteristic bulk strain-rate estimates are approximately 10⁻ 14 –10⁻ 12 s⁻ 1 during reservoir assembly, 10⁻ 11 –10⁻ 9 s⁻ 1 during ordinary unrest, 10⁻ 7 –10⁻ 5 s⁻ 1 during Plinian withdrawal, and 10⁻ 6 –10⁻ 3 s⁻ 1 during rapid roof-scale collapse increments. Local fault-core rates may be orders of magnitude higher and are not directly comparable with kilometre-scale deformation. In this contribution, we propose the mechanical-memory hypothesis: the response of a caldera system to a new loading episode depends on state changes retained from earlier cycles, including residual stress and strain, damage connectivity, permeability, elastic moduli, thermal structure, and fault state. Memory is not synonymous with progressive weakening; it emerges from competition between accumulation processes (damage growth, structural linkage, and persistent stress heterogeneity) and recovery processes (viscous relaxation, fracture healing, sealing, and stress redistribution). Loading rate matters because it controls which processes can operate during and between cycles. The hypothesis predicts that proximity to withdrawal-driven collapse should be expressed more reliably by persistent localization, cross-signal coupling, and incomplete recovery than by acceleration of any single deformation signal. Restricted to collapse preceded by magma withdrawal and reservoir decompression, this framework defines measurable proxies and testable priorities for monitoring, field analysis, experiments, and coupled reservoir–conduit–fault models.