Re: GREEN ROADS 1538
Harriet Wehrenberg
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Defining and understanding the shallow transfer of magma at volcanoes is crucial to forecast eruptions, possibly the ultimate goal of volcanology. This is particularly challenging at felsic calderas experiencing unrest, which typically includes significant changes in seismicity, deformation and degassing rates. In fact, caldera unrest is particularly frequent, affects wide areas and its evidence is often complicated by the presence of a hydrothermal system: as a result, forecasting any eruption and vent-opening sites within an existing caldera is very difficult1.
Despite the restless activity of Campi Flegrei, the recent unrest episodes did not culminate in eruption, so that any possibility to define the pre-eruptive shallow transfer of magma (that is, from the magma reservoir to the surface) at Campi Flegrei remains elusive. Indeed, this definition is a crucial step in order to identify and understand pre-eruptive processes, and thus to make any forecast. To fill this gap, we focused on the last eruption of 1538, reconstructing its pre-eruptive deformation pattern. For this, we exploited the unique historical, archaeological, geological and long-term geodetic record of the caldera to carefully determine the height variations (and related errors) of 20 selected sites along its coastline (Fig. 1 and Supplementary Table S1). The details of this complex and multidisciplinary approach are provided in the Methods and in Supplementary Information sections.
The integrated analysis of geomorphological, sedimentological, paleontological, archaeological and historical data allowed a detailed and quantitative reconstruction of the evolution of the ground displacements predating the Mt. Nuovo eruption along the coastline of the Pozzuoli Bay (Fig. 1). A representative example of the multidisciplinary procedure adopted for such a detailed description of the historical elevation changes for the Capo Miseno area is included in the supplementary material.
The nature of the data (inferred from historical and archaeological records) makes it difficult to precisely infer the amount and extent of horizontal deformation that accompanied the vertical deformation. However, records of ground tilt offer an additional constraint on the deformation field. To this aim, tilt changes between 1536 and 1538 have also been reconstructed (Supplementary Information and Supplementary Fig. S4).
We did not attempt to model any contribution to the deformation field from the Campi Flegrei hydrothermal system or from structural discontinuities associated with the caldera. The nature, extent, and permeability of the pre-1538 hydrothermal system are highly uncertain, so any attempt to model the effect of magma accumulation on it would have introduced a set of largely unconstrained variables. Therefore, any modelling considering the role of the hydrothermal system would have just introduced a higher set of non-constrained variables. Similar considerations also hold in considering any pre-existing discontinuity in the modelling: our general knowledge and data on the subsurface of Campi Flegrei are still too limited to include a reliable and univocal analysis taking into account for pre-existing fractures.
Another limit of our analysis is that elastic deformation models have very similar near-field vertical deformation for a range of source geometries25. Resolution of the geometry of a source would require the inversion of 3D deformation data24 that cannot be inferred from the existing historical and archaeological records.
As a result of these two processes, the laterally propagating sills increase their dip, forming inclined sheets and then subvertical dikes feeding the eccentric vents. These stress variations may also explain the clustering of vents on the NE portion of the caldera, where there is a stronger topographic gradient36.
Our model highlights the importance of considering pre-eruptive lateral magma propagation from the centre of Campi Flegrei caldera. Many calderas worldwide show different vent patterns. Outside the caldera, these include radial fissures along the flank of the caldera edifice and circumferential fissures along the outer caldera rim; these distal patterns of circumferential and radial dikes have been recently explained as due to the unloading due to the caldera depression, the depth to the magma reservoir and the density of the magma36 [references therein]. Within the caldera, vent patterns may have a wide variability, including scattered vents in a central or eccentric position and/or parallel fissures along regional structures [e.g.41]. Within this variability, some intra-caldera vent patterns appear more frequent. For example, most of the geodetically monitored (since the late 1980s) restless and erupting calderas show vents opening at the periphery of the area most uplifted during the unrest, often induced by sill-like magmatic sources. Examples include mafic and felsic calderas, such as Fernandina, Cerro Azul and Sierra Negra (Galapagos), Rabaul (Papua New Guinea), Aso and Usu (Japan) and Okmok (Aleutians)42 and references therein]. These examples suggest that the features found at Campi Flegrei may be relatively common at calderas worldwide, inasmuch as they have been documented at several well-monitored sites. Still, the longer-term geological record shows that some other calderas exhibit centred eruptive vents (e.g.1), suggesting that the proposed mechanism of lateral magma transfer does not apply in all cases. These deviations from our proposed conceptual model may be explained by several factors, such as the presence of a stronger regional extension, lack of a strong topographic caldera depression (both hindering lateral magma propagation), or the shape and size of the magmatic system.
The reconstruction of the surface deformation (uplift and tilt) preceding the 1538 eruption is based on precise information on height variations at twenty sites along the Campi Flegrei coastline (Fig. 1) derived from archaeological, historical, geomorphological and stratigraphic evidence of sea-level persistence. Many archaeological remains of known age (Roman constructions and other later monuments and artefacts, such as harbour structures, roads, thermal baths, fishponds, churches and farms) permit the definition of the position of the coastline at the time of their construction. Moreover, paleontological, sedimentological and geochronological analyses of exposed and drilled sediment sequences from these twenty sites allowed the definition of their sedimentation environment, age and vertical motion. Vertical movements have been also inferred from the displacement of wave-cut notches and other erosional features, and by comparing the present depth of submerged structures and features (Fig. 1) with historical images and descriptions. Additional information has been obtained from chronicles and papers on the urban development of the town of Pozzuoli, and by comparing historical illustrations printed before and after the Monte Nuovo eruption. We also took into account for the thickness of the Monte Nuovo eruption deposits and the present height of their depositional surface, deduced from lithological, sedimentological and paleontological analysis of 200 drill cores and logs (Supplementary Table S2 summarizes methods and related data, as well as the references used in the reconstruction of the ground deformation for each site and for selected periods). Finally, we considered information on sea-level variation during the past 2,000 years17.
Parameters of the caldera best-fitting deformation sources have been inferred by inverting uplift and tilt by using the dMODELS software package44. The software implements a number of analytical solutions for possible sources (sphere, spheroid, sill-like and opening crack/dike) in an elastic, homogenous, flat half-space. Although actual volcanic sources are not embedded cavities of simple shape, we assume that these models may reproduce the stress field created by the actual magma intrusion or hydrothermal fluid injection. The dMODELS software employs a nonlinear inversion algorithm to determine the best-fit parameters for the deformation source by searching the minimum the cost function (chi square per degrees of freedom):
where N is the number of data points, P the number of model parameters, are the experimental data, the modeling results, and the data uncertainties. The non-linear inversion algorithm is a combination of local optimization (interior-point method45) and random search. This approach is more efficient for hyper-parameter optimization than trials on a grid46. See also Fig. 4, Supplementary Fig. S4, Supplementary Tables S3 and S4.
We tested four source geometries: a spherical source47, a prolate spheroid48, a horizontal penny-shaped source49 and a dike50 all in an elastic, homogeneous, isotropic half-space. The details of the models obtained by inversion of the data are listed in Supplementary Tables S3 and S4.
M.A.D.V. and V.A. coordinated the work and wrote the manuscript. C.D.G., G.P.R., C.R. and R.S. provided the historical and archaeological uplift data; M.A.D.V., G.A., D.B., A.C. and S.d.V. provided the geological uplift data; F.T. carried out the 14C age determinations; M.B. inverted the deformation data; V.A. elaborated the proposed model. All authors contributed ideas and input to the research and writing of the paper. This work is dedicated to the memory of our collegue and acknowledged master Paolo Gasparini, former director of Osservatorio Vesuviano.
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