Simultaneous inversion of deformation and gravity changes in a horizontally layered half-space: Evidences for magma intrusion during the 1982–1984 unrest at Campi Flegrei caldera (Italy)

A very large uplift (about 1.8 m) occurred in the period 1982–1984 at Campi Flegrei caldera, Italy, without
culminating in an eruption. A still-standing controversy accompanies the interpretation of deformation and
gravity changes recorded during the unrest, which were interpreted to result from the sub-surface magmatic
reservoir by some authors and from the hydrothermal system or hybrid sources by others. Here for the first
time we take into account crustal layering while inverting leveling, EDM, and gravity data using uniformlypressurized
sources, namely small vertical spheroids and finite horizontal penny-shaped sources. The weight
of EDM data in the misfit function is chosen from a trade-off curve in order to balance the compromise
between fitting the leveling and the EDM data well. Models using a homogeneous medium cannot give a
good simultaneous fit to leveling and EDM deformation data of the 1982–1984 unrest, whereas incorporating
a layered structure (determined from seismically derived estimates of the P wave speed for the crust, and not
adjusted to improve the fit in any of the inversions) allows a significantly better fit. Also, layering affects the
sub-surface mass redistribution effects on gravity changes, and we show that the retrieved intrusion density
is in full agreement with densities for highly evolved magmas expected at the Campi Flegrei caldera for
depths of 3 to 4 km, ruling out hydrothermal fluids as the primary cause of the 1982–1984 unrest. The source
of the 1982–1984 CF unrest was probably a shallow (about 3-km deep) penny-shaped magma intrusion fed
by a deeper magma chamber; source overpressure was few MPa.

AGIP, 1987. Modello geotermico del sistema flegreo (Sintesi). Technical Report,
SERGMESG, San Donato, Italy.
Amoruso, A., Crescentini, L., 2007. Inversion of leveling data: how important is error
treatment? Geophys. J. Int. 171, 1352–1362. doi:10.1111/j.1365-246X.2007.03585.x.
Amoruso, A., Crescentini, L., Scarpa, R., 2002. Source parameters of the 1908 Messina
Straits, Italy, earthquake from geodetic and seismic data. J. Geophys. Res. 107.
doi:10.1029/2001JB000434.
Amoruso, A., Crescentini, L., Fidani, C., 2004. Effects of crustal layering on source
parameter inversion from coseismic geodetic data. Geophys. J. Int. 159, 353–364.
doi:10.1111/j.1365-246X.2004.02389.
Amoruso, A., Crescentini, L., Scarpa, R., 2005. Faulting geometry for the complex 1980
Campania–Lucania earthquake from levelling data. Geophys. J. Int. 162, 156–168.
doi:10.1111/j.1365-246X.2005.02652.x.
Amoruso, A., Crescentini, L., Linde, A.T., Sacks, I.S., Scarpa, R., Romano, P., 2007. A
horizontal crack in a layered structure satisfies deformation for the 2004–2006
uplift of Campi Flegrei. Geophys. Res. Lett. 34, L22313. doi:10.1029/2007GL031644.
Barberi, F., Hill, D.P., Innocenti, F., Luongo, G., Treuil, M. (Eds.), 1984. The 1982–1984
Bradyseismic crisis at Phlegrean Fields (Italy). Bull. Volcanol., vol. 47, pp. 173–411.
Battaglia, M., Segall, P., 2004. The interpretation of gravity changes and crustal
deformation in active volcanic areas. Pure Appl. Geophys. 161, 1453–1467.
Battaglia, M., Troise, C., Obrizzo, F., Pingue, F., De Natale, G., 2006. Evidence for fluid
migration as the source of deformation at Campi Flegrei caldera (Italy). Geophys.
Res. Lett. 33, L01307. doi:10.1029/2005GL024904.
Berrino, G., 1994. Gravity changes induced by height–mass variations at the Campi
Flegrei caldera. J. Volcanol. Geotherm. Res. 61, 293–309.
Crescentini, L., Amoruso, A., 2007. Effects of crustal layering on the inversion of
deformation and gravity data in volcanic areas: an application to the Campi Flegrei
caldera, Italy. Geophys. Res. Lett. 34, L09303. doi:10.1029/2007GL029919.
Davis, P.M., 1986. Surface deformation due to inflation of an arbitrarily oriented triaxial
ellipsoidal cavity in an elastic half-space, with reference to Kilauea Volcano, Hawaii.
J. Geophys. Res. 91, 7429–7438.
De Natale, G., Troise, C., Pingue, F., Mastrolorenzo, G., Pappalardo, L., Boschi, E., 2006. The
Campi Flegrei caldera: unrest mechanisms and hazards. Troise, C., De Natale, G.,
Kilburn, C.R.J. (Eds.), Mechanisms of Activity and Unrest at Large Calderas. Geol. Soc.
London Spec. Publ., vol. 269, pp. 25–45.
Dvorak, J., Berrino, G., 1991. Recent ground movement and seismic activity in Campi
Flegrei, southern Italy, episodic growth of a resurgent dome. J. Geophys. Res. 96,
2309–2323.
Dvorak, J.J., Mastrolorenzo, G., 1991. The mechanisms of recent vertical crustal
movements in Campi Flegrei Caldera, southern Italy. U.S. Geol. Surv. Spec. Pap.
263, 1–47. Fialko, Y., Kazhan, Y., Simons, M., 2001. Deformation due to a pressurized horizontal
circular crack in an elastic half-space, with applications to volcano geodesy.
Geophys. J. Int. 146, 181–190.
Gottsmann, J., Berrino, G., Rymer, H., Williams-Jones, G., 2003. Hazard assessment
during caldera unrest at the Campi Flegrei, Italy: a contribution from gravity–height
gradients. Earth Planet. Sci. Lett. 211, 295–309.
Gottsmann, J., Folch, A., Rymer, H., 2006a. Unrest at Campi Flegrei: a contribution to the
magmatic versus hydrothermal debate from inverse and finite element modeling.
J. Geophys. Res. 111, B07203. doi:10.1029/2005JB003745.
Gottsmann, J., Rymer, H., Berrino, G., 2006b. Unrest at the Campi Flegrei caldera (Italy): a
critical evaluation of source parameters from geodetic data inversion. J. Volcanol.
Geotherm. Res. 150, 132–145. doi:10.1016/j.jvolgeores.2005.07.002.
Ingber, L., 1993. Simulated annealing: practice versus theory. Math. Comput. Model. 18,
29–57.
Judenherc, S., Zollo, A., 2004. The Bay of Naples (Southern Italy): constraints on the
volcanic structures inferred from a dense seismic survey. J. Geophys. Res. 109,
B10312. doi:10.1029/2003JB002876.
Landau, L.D., Lifsits, E.M., 1975. The Classical Theory of Fields, first ed. Pergamon Press,
New York.
Ludwig,W.J., Nafe, J.E., Drake, C.L., 1970. Seismic refraction. Maxwell, A.E. (Ed.), The Sea,
vol. 4. Wiley-Interscience, New York, pp. 53–84. Newhall, G., Dzurisin, D., 1988. Historical Unrest at Large Calderas of the World. U.S.
Geological Survey, Reston, VA. 1108 pp.
Sambridge, M., 1999a. Geophysical inversion with a neighbourhood algorithm — I.
Searching a parameter space. Geophys. J. Int. 138, 479–494.
Sambridge, M., 1999b. Geophysical inversion with a neighbourhood algorithm — II.
Appraising the ensemble. Geophys. J. Int. 138, 727–746.
Trasatti, E., Giunchi, C., Bonafede, M., 2003. Effects of topography and rheological
layering on ground deformation in volcanic regions. J. Volcanol. Geotherm. Res. 122,
89–110.
Trasatti, E., Giunchi, C., Bonafede, M., 2005. Structural and rheological constraints on
source depth and overpressure estimates at the Campi Flegrei caldera, Italy. J.
Volcanol. Geotherm. Res. 144, 105–118.
Walsh, J., Rice, J., 1979. Local changes in gravity resulting from deformation. J. Geophys.
Res. 84, 165–170.
Wang, R., Lorenzo Martín, F., Roth, F., 2006. PSGRN/PSCMP — a new code for calculating
co- and post-seismic deformation, geoid and gravity changes based on the
viscoelastic-gravitational dislocation theory. Comput. Geosci. 32, 527–541.
Williams, C.A., Wadge, G., 2000. An accurate and efficient method for including the
effects of topography in three-dimensional elastic models of ground deformation
with applications to radar interferometry. J. Geophys. Res. 105 (B4), 8103–8120.