Figure 5. Accumulation of stress at Campi Flegrei since 1950.

The numbers and numbered circles show reference positions in the time series in Fig. 2. (a) The trend (solid blue curve) shows an overall accumulation of stress and increase in the proportion of inelastic deformation, interrupted by partial stress relaxation (3–4) and recovery (4–5) during 1972–1982. Ground oscillation since 1984 is consistent with a drop (6–7) to lower pore pressure (blue dashed curve) and subsequent recovery (7–8). Deformation occurs in the elastic (white shading) and quasi-elastic (yellow) regimes, and approaches the inelastic (magenta) regime. (b) The variation of total number of VT events with combined corrected uplift during the rapid uplifts of 1950–1952, 1969–1972 and 1982–1983 (1i+2i+3i in Fig. 2) shows a short transition from elastic to quasi-elastic behaviour. In the quasi-elastic regime, the VT event number increases as ΣN=295 exp (h/λ_ch) and λ_ch=1 m (r²=0.99). A return to the main VT-uplift trend (blue solid curve) may coincide with the emergence of inelastic crustal deformation. (c) The variation of total number of VT events with uncorrected uplift shows almost the same trend as for corrected uplift, with ΣN=408 exp (h/λ_ch) and λ_ch=1 m (r²=0.98).