Laboratory measurements of damage evolution in dynamic volcanic environments: From slow to rapid strain events

The mechanical response and failure of volcanic rock depends on its physical characteristics (e.g. porosity) and the magnitude of stressing events, leading to varying degrees of damage over time. Although stressing events impart permanent strain on material, and in some cases trigger landslides, the degree of damage in rocks with varying physical properties and under different stress rates is rarely systematically measured. Here, we aim to characterize damage evolution of dacitic rocks ranging from 10-35% porosity from Unzen volcano in Japan, which is prone to collapse. In addition to standard uniaxial compression and tension tests, damage was determined during slow (time-dependent brittle creep) and fast (load oscillation earthquake simulation) experiments. Brittle creep experiments were conducted in 24 hour stepped-load cycles, increasing from 40 to 70% of their expected failure load. Dynamic earthquake loading was simulated by moving the top piston by a corresponding displacement to induce a desired stress fluctuation, simulating longitudinal stress (i.e. compression and dilation) of multiple seismic waves. Damage is measured using strain rate data during testing. We find that rocks with varying physical properties experience varying degrees of damage during stressing events. Low porosity samples tend to suffer more damage than high porosity samples, suggesting more porous samples may buffer damage more effectively. During dynamic stressing events, strain tends to be more prominent during initial events. Identifying how material heterogeneity influences damage accumulation can help to define damage in rock masses after stressing events.