GNGTS 2013 - Atti del 32° Convegno Nazionale

Campi Flegrei caldera unrest: a possible scenario for the next eruption S. Carlino 1 , A. Troiano 1 , M.G. Di Giuseppe 1 , R. Somma 1 , C. Kilburn 2 , A. Tramelli 1 , C. Troise 1 , G. De Natale 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Italy 2 Dept of Earth Sciences, Aon Benfield UCL Hazard Centre, UK The eruption of calderas is among the most intense and variable volcanic phenomena in the world, spanning from quite lava dome emplacement to large eruptions (VEI>5). Caldera dynamic has been widely studied by many authors to understand the mechanisms of erup- tion, resurgence and subsidence using either numerical than analogue methods (Lipman, 1997; Acocella, 2007 and reference therein). Magma withdrawal and related under-pressurization of magma chamber is the most common mechanism to explain the caldera collapse. Subsequent to large ignimbrite eruptions, the activity within calderas is frequently characterized by recurrent period of uplift and subsidence. This behavior is generally explained in terms of magma intru- sion and/or disturbance of geothermal fluids in the shallow crust, which in turn are both source of ground deformations and seismicity (Troiano et al. , 2010). A major goal is, therefore, to de- termine the relative contribution of each process, since the potential for eruptions significantly enhanced if magma movements represents the primary component. An important case study is represented by the Campi Flegrei caldera, CFc, (west to Naples city) where the volcanic risk is very high due to 350,000 people living inside the caldera and neighboring. This volcanic area is characterized, since Holocene, by regional tectonic extension as a consequence of Tyrrhenian basin spreading. Geophysical, geological and archeological data have shown an area of back- ground subsidence across the caldera, which arose at least since Roman time, with average rate of 1.7 cmy -1 (Woo and Kilburn, 2010). This process in periodically spaced out by uplift phases which, during historic time, can be recognized before the last eruption (1538 A.D., Monte Nuo- vo) with more than 10 m of uplift, during 1970-72 with maximum uplift of 1.7m, and during 1982-84 with 1.8 m of uplift (the deformation pattern of the two latest episodes is the same and centered in Pozzuoli harbor). The last episode of unrest indicated the possibility of an im- minent eruption, forcing the authorities to evacuate Pozzuoli town, which was also damaged by the continuous seismic activity and ground deformation; however, the unrest essentially ended in December 1984, without any eruption occurring (De Natale et al. , 2001). The un- rest observed at CFc can be ex- plained both in terms of magma and/or fluids migration, but the controversy relating to what is the source (magma, hydrother- mal, or hybrid) is still a matter of debate. The non-uniqueness of the physical models adopted in the literature is provided by the different interpretations of the available geophysical data (Carlino and Somma, 2010). For instance, the inversion of both gravity and deformation data from the 1982-1984 unrest fits well with sources of different natures and shapes: magmat- ic or hydrothermal spherical sources in a viscoelastic or plas- Fig. 1 – Adapted Leak Off Test for in situ permeabulity assessment. The ateps 1-2-3 correspond to stop (P1-P3-P5) and start (P2-P4-P6) of the pump. The drop of pressure (P1-P2-P3-P4-P5-P6) corresponds to the fluid migration through the wall rock.The pressure drop rate was utilized to assess the in-situ permeability. During the test, was also evaluated the minimum principal stress with standard test. 243 GNGTS 2013 S essione 1.3