Figure 3. The evolution of deformation regimes during elastic-brittle behaviour.

(a) As the applied differential stress increases from zero, the deformation of natural rock (black curve) deviates increasingly from elastic behaviour (sloping dashed line), evolving from the elastic (white shading), through quasi-elastic (yellow) to inelastic (magenta) regimes of behaviour. The start of the inelastic regime (blue circle) coincides with deformation under a constant maintained stress. (b) The deviation from elastic behaviour is caused by faulting. The total deformation caused by fault movements is represented by the cumulative number of VT events. The number of VT events increases exponentially with deformation in the quasi-elastic regime, but linearly with deformation in the inelastic regime. (c) The evolution from quasi-elastic to inelastic deformation was observed (blue curve and line) during the 23-year precursory unrest before the 1994 eruption at Rabaul caldera, Papua New Guinea²⁴. The quasi-elastic trend is given by ΣN=4,120 exp (h/λ_ch), where λ_ch=0.53 m (r²=0.98). The black dashed lines show the episode of rapid uplift during 1983–1985.