Vertical Ground Displacement at Campi Flegrei (Italy) in the ﬁfth Century: Rapid Subsidence Driven by Pore Pressure Drop, Geophys

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Geophysical Research Letters

RESEARCH LETTER Vertical ground displacement at Campi Flegrei 10.1002/2013GL059083 (Italy) in the fifth century: Rapid subsidence Key Points: driven by pore pressure drop • An integrated approach constrains fast subsidence at Campi Flegrei in Micol Todesco1, Antonio Costa1, Alberto Comastri1, Florence Colleoni2, Giorgio Spada3, fi the fth century 1 • Fast subsidence can be caused by and Francesca Quareni compaction due to decompression of 1 fi 2 hydrothermal system Istituto Nazionale di Geo sica e Vulcanologia, Sezione di Bologna, Bologna, Italy, Centro Euro-Mediterraneo sui • A sequence of unrest events preceded Cambiamenti Climatici, Bologna, Italy, 3Dipartimento di Scienze di Base e Fondamenti, Università di Urbino “Carlo Bo”, last eruption with no secular subsidence Urbino, Italy

Supporting Information: • ReadMe Abstract Campi Flegrei (Italy) caldera has experienced episodes of ground deformation throughout its • Figure S1 geological history, alternating between uplift and subsidence phases. Although uplift periods are typically • Table S1 more alarming, here we focus on subsidence, looking for its driving mechanisms and its role in the caldera • Appendix S1 • Appendix S2 evolution. Historical and archaeological records constrain ground deformation over the last two millennia. Here we revise such records and combine them with published radiometric dating and with the simulation of Correspondence to: sea level change. The resulting analysis highlights for the first time a rapid subsidence during the fifth M. Todesco, century. We show that rate and magnitude of this subsidence are consistent with the compaction of porous [email protected] material caused by a pressure drop of ~ 1 MPa within the hydrothermal system. We interpret this event as the decompression of the hydrothermal system following an unrecognized episode of unrest, during Roman Citation: times. These findings redefine the pattern of ground deformation and bear important implications for Todesco, M., A. Costa, A. Comastri, F. Colleoni, G. Spada, and F. Quareni volcanic hazard assessment. (2014), Vertical ground displacement at Campi Flegrei (Italy) in the fifth century: Rapid subsidence driven by pore pressure drop, Geophys. Res. Lett., 41,1471–1478, 1. Introduction doi:10.1002/2013GL059083. Calderas are volcanic structures that undergo significant ground deformation, with uplift and subsidence that

Received 18 DEC 2013 in case precede eruptive phases. Uplift is usually ascribed to magma ascent or to perturbation of the Accepted 6 FEB 2014 hydrothermal system [Casertano et al., 1976; Gaeta et al., 1998; Hurwitz et al., 2007; Fournier and Chardot, Accepted article online 7 FEB 2014 2012]. Subsidence can be caused by various mechanisms characterized by different timescales, such as Published online 6 MAR 2014 eruptions or magma migration [Dzurisin et al., 1994; Kaneko et al., 2005], regional extension [Dzurisin et al., 1994], elastic rebound, compaction of caldera filling deposits [Yokoyama and Nazzaro, 2002], magma cooling and contraction [Kwoun et al., 2006], or the poroelastic deformation associated with hydrothermal fluid circulation [Castagnolo et al., 2001; Waite and Smith, 2002; Todesco et al., 2004; Hurwitz et al., 2007; Rinaldi et al., 2010]. Campi Flegrei (Figure 1) is an ideal location to study these phenomena, as the presence of Roman ruins along the shore provides an unusual benchmark, which recorded the ground displacement with respect to sea level during the last two millennia. The marble pillars of the ancient Roman market in Pozzuoli, known as Serapeum, have fascinated generations of scientists, from Lyell [1830] on, who contributed to unfold the history of the caldera. Reconstruction of ground deformation is important to understand the mechanisms driving the unrest episodes of this active volcanic system, which recently experienced periods of uplift accompanied by seismicity. Here we consider all available geological, geochronological, and archaeological data to draw a unique and comprehensive picture of vertical ground displacement at Campi Flegrei. A brief reconstruction of the history of Serapeum is provided in Appendix S1 in the supporting information, together with the information we used to reconstruct its vertical movements (Table S1). Our goal is to assess the timescale associated with ground deformation and to provide temporal constraints on the vertical displacement of the Serapeum pillars. The uncertainties associated with our estimate are discussed in the supporting information (Appendix S2). The history of vertical ground displacement reveals that a rapid subsidence (of the order of hundreds of mm/yr) took place during the fifth century A.D. We show that subsidence is likely due to compaction of shallow, porous rocks, driven by abundant discharge of volcanic fluids. We speculate that such deflation phase may have been preceded by a volcanic unrest that pressurized the hydrothermal system.

(a) (b) Monte Nuovo

Napoli

Monte Serapeum Nuovo Solfatara Serapeum * Pozzuoli Pozzuoli ITALY Baia

Campi Flegrei Miseno

Bacoli

Miseno

Figure 1. (a) The Campi Flegrei caldera, with the position of Serapeum, the Solfatara crater, and Monte Nuovo. (b) Submerged Roman ruins in the Pozzuoli Bay. Contour lines every 5 m (modiﬁed after Dvorak and Mastrolorenzo [1991]).

2. Unrest at Campi Flegrei The Campi Flegrei caldera was formed by two major events: the Campanian Ignimbrite eruption, 39 ka ago [Fedele et al., 2011; Costa et al., 2012], and the Neapolitan Yellow Tuff eruption, 15 ka ago [Deino et al., 2004]. Later activity mainly consisted of smaller, intracaldera explosive events (predominantly phreatomagmatic) that were clustered in short periods separated by quiet intervals [Di Vito et al., 1999; Fedele et al., 2011]. The presence of raised marine terraces and sediments alternating with beach deposits and subaerial volcanic products indicate repeated and widespread episodes of uplift and subsidence [Di Vito et al., 1999; Bellucci et al., 2006; Isaia et al., 2009]. Roman ruins along the coast contain tracks of marine organisms, providing evidence for periods of partial submersion. The Serapeum pillars, in particular, hold biological borings caused by the activity of marine organism (Lithophaga lithophaga, among others) [Morhange et al., 1999, 2006]. The upper limit of these perforations (presently 7 m above sea level) indicates the maximum submersion and corresponds to the biological sea level during the lifetime of the marine colony. Radiometric dating of the fossil remains collected at this elevation identified three separate age groups: A.D. 400–530, A.D. 700–900, and finally at the end of fifteenth century [Morhange et al., 1999, 2006]. These ages record the death of the marine fauna, which in this case relates to the emersion of the colony (i.e., ground uplift), since no evidence supports other causes of death (such as changes in water salinity). The onset of Lithophaga on a new substratum requires at least a few years, and the shells grow to their maximum size in about 70–80 years [Morhange et al., 1999]. Given the size of the colony, the pillars must have reached their maximum submersion several decades before the onset of each uplift phase. Last emersion culminated in the A.D. 1538 Monte Nuovo eruption, ending more than 3000 years of eruptive quiescence. The appearance of new

stretches of land preceding this eruption is documented in historical chronicles that conﬁrm a large, widespread ground deformation [Parascandola, 1947; Di Vito et al., 1987; Guidoboni and Ciuccarelli, 2011]. Geodetic measurements of ground elevation are available since 1905 and indicate episodes of ground uplift in 1953, 1969, and 1982, totalling approximately 4 m of vertical displacement [Del Gaudio et al., 2010]. The pattern of uplift and subsidence is symmetrical [Dvorak and Berrino, 1991], with maximum displacement at the caldera center. A subsidence phase between 1985 and 2005 was periodically interrupted by minor (few centimeters), short-lasting episodes of uplifts. Since 2006, the caldera has been inﬂating again, at a slow but steady rate [D’Auria et al., 2011].

2.1. Other Evidence of Vertical Ground Displacement Constrain on past vertical ground displacement derived from the fossil fauna found in the Averno lake (Figure 1b). The lake was connected to the sea by a canal, built in 37 B.C., and abandoned 10–20 years later. The presence of marine species within the fossil record of the lake testifies to increasingly frequent marine transgression between the years 50 and 400 A.D. [Welter-Schultes and Richling, 2000]. In particular, a substantial increase in the proportion of marine species was observed after A.D. 350 and was interpreted in terms of ground subsidence. The marine contribution declined between A.D. 500 and 700. Further information comes from the other archaeological remains on the western side of the bay. The town of Baia became an appreciated location for Roman notables since the beginning of the first century B.C. Evidence of restoration and written accounts by Symmachus (A.D. 340–402) suggests that the area was still above the sea level at the beginning of the fourth century. This is supported by similar evidence found at other locations like Punta Epitaffio, and Misenum, only abandoned at the end of the fourth century [Maniscalco, 1998]. Here tracks of marine organisms on ancient buildings, as well as the presence of marine sediments within the ruins, suggest that subsidence of the coastline took place between the fourth and fifth century. The presence of graves dating to the Late Antiquity (250–750 A.D.) within the submerged Roman ruins in Baia suggests that these were again above sea level in the sixth century. In Misenum, the town abandonment was followed by new constructions in the sixth to seventh century, which were later buried under beach and marine deposits, suggesting a new sinking phase between seventh and eighth century [Maniscalco, 1998].

3. Reconstruction of Late Antique Subsidence Our reconstruction of vertical ground displacement at Campi Flegrei is based on the assessment of the position of Serapeum pillars with respect to sea level. A brief summary of the available archeological evidence on the building of Serapeum is provided in the supporting information (Appendix S1). To estimate its elevation through time, we combined this information with an estimate of the sea level variations along this stretch of coast. In the Mediterranean region abundant archaeological markers allow a detailed reconstruction of ancient coastlines [Auriemma and Solinas, 2009]. In the NW Mediterranean Sea, the sea level rise since Roman times was evaluated to be ~50 cm [Pirazzoli, 1976]. Estimates carried out for central Tyrrhenian Sea range from 1 to 1.4 m [Lambeck et al., 2004]. This corresponds to rates of relative sea level rise ranging from 0.25 to 0.7 mm/yr, consistent with predicted, present-day isostatic rates [Lambeck et al., 2011]. These sea level changes primarily result from glacial isostatic adjustment (GIA), due to the melting of the polar ice sheets since the Last Glacial Maximum ~21,000 years ago. We address the process with a model that solves the sea level equation [Farrell and Clark, 1976; Spada and Stocchi, 2007]: two numerical simulations were carried out considering different ice melting scenarios and different viscosity profiles for the Earth’s mantle. The first is ICE-5G (VM2) model (here model P) [Peltier 2004], while the second was developed at the Research School of Earth Science of the Australian National University by K. Lambeck and coworkers [Lambeck et al., 1998, and references therein] (model L). The obtained sea level changes (Figure 2) bound the upper and lower values proposed in the literature for the last 2000 years. Model L implies larger isostatic disequilibrium hence greater relative sea level variations. Our results are largely unaffected by the choice of the GIA model: in the following, we consider the model P, but the subsidence rate computed with model L does not differ significantly. Both curves are shown in Figure 2 for completeness, but the rate for model L is not used. In reconstructing the vertical ground displacement we take as a reference level the upper limit of perforation on Serapeum pillars (Figure S1). At the time of the market construction this level was ~13 m above present- day sea level (Table S1). The construction of a second floor suggests that subsidence had already begun at the

14 time of the Severian restorations 12 (during the third century, Appendix S1). 10 We assume that the 2 m difference in 8 ﬂoor elevations reﬂects the magnitude 6 of the actual ground displacement. The 4 columns were still out of the water at 2 the time of last restoration (A.D. 394). 0

Elevation above Assuming that the second ﬂoor was at −2 present sea level (m) Arecheological data (Parascandola, 1947) the sea level of the time, the complete −4 Ground level (Del Gaudio et al., 2010) Time of lithodom emersion column submersion required at least −6 Sea level (Model P) Sea level (Model L) 7 m of subsidence (Appendix S2). −8 0 200 400 600 800 1000 1200 1400 1600 1800 2000 To infer the timing of such subsidence, we rely on the radiometric dating of fossil remains from Serapeum pillars 0 200 400 600 800 1000 1200 1400 1600 1800 2000 ﬁ Year (Figure 2, bottom). The rst group of radiometric dates ranges in time from Figure 2. Reconstructed vertical motion of the Roman pillars of Serapeum. A.D. 378 to 540, with large errors, Greendots:elevationofthereferenceline (i.e., the upper limit of the perfo- spanning from ±114 to ±138 [Morhange rated sector, Figure S1), above present sea level. The elevations are based on sea level change corresponding to model P (blue, solid line) although both et al., 1999, 2006]. Their average value, sea level lines are shown for completeness. Black dots: ground elevation and its associated corrected standard measurements [after Del Gaudio et al., 2010]. Red arrows: times of pillar deviation is A.D. 450 ± 70. Ages older emersion, according to (bottom) radiometric dating of fossil marine organ- than A.D. 394 must be discarded, as we isms. Shaded areas: time intervals associated with subsidence (blue) or uplift know that the pillars were above the (yellow) based on other evidence (see text for discussion). water at the time of last restoration. The average value (A.D. 450) is also questionable, as it implies very fast rates of subsidence (see discussion in Appendix S2). For this reason the younger age (540 ± 125) obtained by Morhange et al. [1999] is considered here as a reference for the estimation of subsidence rate. This age is also in better agreement with archaeological evidence suggesting that uplift did not begin before the sixth century. Taking this age as the beginning of the uplift and considering a time lapse of a century for the growth of the Lithophaga colony, the complete columns submersion occurs in A.D. 440, i.e., in 46years, corresponding to an average subsidence rate of the order of 160mm/yr. The estimate above is affected by large uncertainties, being based on indirect evidence and a few assumptions. An assessment of the resulting error is carried out in Appendix S2, where each source of uncertainty is carefully examined to constrain its reasonable bounds. The resulting admissible range of subsidence rates varies between 50 and 500 mm/yr, with our best guess being of the order of 200 mm/yr. Further radiometric ages established for fossil remains on the pillars indicate more periods of subsidence and uplift [Morhange et al., 1999, 2006], culminating in the A.D. 1538 Monte Nuovo eruption. Unfortunately, pillar elevation during the Middle Ages is not constrained by historical or geological data, and rates of ground deformation cannot be estimated for that period.

4. The Mechanism of Subsidence Ground deformation in active calderas is commonly interpreted in terms of pressure or volume changes of a source of deformation at depth, typically a magma chamber [Waite and Smith, 2002; Farrell et al., 2010; Shelly et al., 2013], a hydrothermal system, or both [Casertano et al., 1976; Gaeta et al., 1998; Amoruso et al., 2014]. Volume or pressure changes at the source act on the surrounding rocks causing a deformation, whose magnitude and temporal evolution depend on the rheological properties of the subsurface rock. In this framework, subsidence is caused by a pressure drop or a volume contraction of the source, possibly due to magma cooling and crystallization or to the migration of magma or hydrothermal ﬂuids. The timescale of deformation and the magnitude of the resulting surface displacement may help in discriminating among possible driving mechanisms. Typical subsidence rates of calderas range from a few to a few tens of millimeters per year [Newhall and Dzurisin, 1988; Hurwitz et al., 2007]. Vertical ground displacement at Campi Flegrei was ﬁrst described as a long-term, secular subsidence with an estimated, constant rate spanning from

Table 1. Results From Fitting Equation (4) With Campi Flegrei Subsidence Data for the Period 1985–1988a

Parameter (Dimension) h0 = 500 m h0 = 600 m h0 = 1000 m

ϕ0 () 0.48 0.41 0.32 Pc (MPa) 0.67 0.64 0.44 A (MPa db) 488,785 488,785 488,785 b () 0.82 0.82 0.82

aFor comparison, values proposed by Zoback [2008] for uncemented sedimentary rocks are for A = 5410 MPa db, b = 0.164, and ϕ0 = 0.27.

15 to 20 mm/yr and periodically interrupted by uplift or eruption [Dvorak and Mastrolorenzo, 1991; Bellucci et al., 2006]. Our reconstruction does not reveal such a monotonic, long-term trend and rather highlights a sequence of deformation episodes, with a rapid subsidence between A.D. 400 and 500, when meters of displacement took place within decades. Hydrothermal fluid flow can cause pressure changes within the considered timescale and is in good agreement with the available data on the caldera. The onset of subsidence was explained assuming a decompression of the large hydrothermal system that feeds the Solfatara emissions [Bonafede, 1991; Gaeta et al., 1998, 2003; Gottsmann et al., 2006; D’Auria et al., 2011; Amoruso et al., 2014]. The subsidence rate reached a maximum value of ~150 mm/yr between January 1985 and October 1986 [Del Gaudio et al., 2010], which is the same order of magnitude of the subsidence rate we have estimated for the fifth century. A decompression of the hydrothermal system can be accompanied by compaction: when the pore space is saturated with fluids, pore pressure mitigates the effects of lithostatic load. If the pore pressure drops, the confining pressure acting on rock grains correspondingly increases, eventually leading to compaction as shown in the theory of soil consolidation [Biot, 1941]. This mechanism is known to generate ground subsidence during reservoir exploitation in geothermal power plants [Zoback,2008;Allis et al., 2009]. To evaluate whether a pore pressure drop can account for subsidence at Campi Flegrei, we performed some simple calculations. We assume that the subsidence is entirely due to the loss of pore volume, without deformation of the solid grains, and we consider that for a given cross section, such volume loss only occurs along the vertical direction. Under these assumptions, the observed subsidence can be expressed in terms of porosity loss as ϕ ϕ Δh ¼ h 0 (1) 0 1 ϕ

where h0 is the initial thickness of the compacting layer, ϕ0 is the initial porosity, and ϕ is the porosity after compaction. In the case of Campi Flegrei, the compacting layer may range from thousands of meters (thickness of the caldera ﬁll deposits) to a few hundred meters (the single tuff layer within the caldera), while typical porosity values range between 0.3 and 0.5 [Vanorio et al., 2002]. For any combination of these values, the porosity loss necessary to achieve 10 m of

h (m) subsidence is within a few Δ percent. Rock compaction can therefore account for the magnitude of the observed deformation. The compaction rate depends on the hydraulic and elastic properties of the porous material and on the magnitude of the pore pressure change, as Figure 3. Observed ground subsidence (crosses) from January 1985 to June 1988, described by the general theory of after the last large uplift episode (1982–1984) and before the onset of the so-called consolidation [Biot, 1941]. “mini” uplifts (in 1988). The different lines show the ﬁts obtained with equation (4) for different choices of the initial layer thickness. The corresponding parameters For the sake of simplicity and are listed in Table 1. because limited information are

available on the system, here we infer the timescale for the compactionbasedonanempirical relation established in reservoir φ = 0.5 geomechanics [Zoback, 2008]. Laboratory experiments showed h (m)

Δ that uncemented rock samples φ = 0.3 subjected to increments of confining pressure, after a first instantaneous deformation, keep straining at constant pressure [Chang et al., 1997]. This creeping behavior is related to the motion of particles adjusting to the new fi Figure 4. Temporal evolution of ground subsidence resulting from the compac- con ning pressure. The tion of a 600 m thick layer, for different values of confining pressure (dashed lines) corresponding evolution of porosity and initial porosities (dotted lines). Where not otherwise specified, the initial can be described as [Zoback, 2008] porosity of 0.4 and the confining pressure is 2 MPa. The star shows the duration and magnitude of the fifth century subsidence with the related uncertainties. Pc The choice of best guess and errors are fully discussed in Appendix S2. ϕðÞ¼P ; t ϕ tb (2) c i A

where ϕi is the porosity after the instantaneous compaction, and the second term describes the time- dependent evolution, where Pc is the conﬁning pressure (MPa) (i.e., the applied pressure change), t is time (days), A and b are empirical parameters which express the rheological properties of the compacting layer: A is a scalar associated with the magnitude of creep compaction and b is related to the apparent viscosity of the system (smaller b for greater viscosities). Note that relationship (2) is valid for small porosity variations, i.e., d ϕi ϕ(t)=Δϕ <<1. The instantaneous term can be expressed as ϕi = ϕ0(Pc/P*) ,whereP* is a reference pressure (i.e., P* = 1 MPa) and d is an empirical parameter, related to the rock compliance (smaller d for stiffer rocks) [Zoback, 2008]. Experiments carried out with uncemented sand provided small d values, implying a

negligible instantaneous compaction, with ϕi close to ϕ0 [Zoback, 2008]. Geodetic measurements at Campi Flegrei after January 1985 conﬁrm a time-dependent response to the depressurization of system, without instantaneous compaction [Del Gaudio et al., 2010]. In the following, we assume that vertical displacement is entirely driven by creep compaction with a negligible instantaneous component, i.e.,

ϕi ¼ ϕ0 (3)

Combining equations (1), (2), and (3), we obtain a relation describing the temporal evolution of caldera subsidence: P A1tb Δh ¼ h c (4) 0 1 b 1 ϕ0 þ PcA t

The thickness of the compacting layer and its initial porosity can be constrained by geological information, and we can estimate the magnitude of pressure changes in our system. On the contrary, values for the empirical parameters A and b in equation (4) are not available for Campi Flegrei. To constrain their values, we used equation (4) to ﬁt the subsidence data for the period 1985–1988 (Figure 3). In this time span, the data show a clear trend, and the available measurements are just enough to estimate all the equation parameters (A,

b, Pc,andϕ0) assuming different thicknesses of the compacting layer, h0. Keeping in mind the limitations due to the paucity of data, we obtained a satisfactory ﬁt with the empirical relation. The values of initial porosity and pressure inferred for different thicknesses of the compacting layer are in good agreement with available data and suggest that the 1985–88 subsidence was driven by the compaction of a shallow and porous layer. The retrieved values of the empirical parameters A and b were then assigned to equation (4) to quantify the amount of subsidence due to different pore pressure drops. Figure 4 shows the temporal evolution of subsidence calculated for different initial thickness of the compacting layer, (i.e., 500, 600, and 1000 m) and displays the effect of different values of initial porosity (from 0.3 to 0.5). Figure 4 also shows the magnitude and timing of the ﬁfth century subsidence, with associated error bars. The large uncertainties that affect our estimate

are discussed in greater detail in Appendix S2, together with our choice of the best guess. A comparison between such a best guess and the curves plotted in Figure 4 suggest that a pressure drop in the range of 1–2 MPa could have caused a complete column submersion. These values are consistent with the estimated pressure decline that caused more than 10 m of subsidence in Wairakei, New Zealand [Allis et al., 2009]. Numerical simulations show that pore pressure changes of this order of magnitude easily result from changes in the feeding rate of magmatic gases at the base of the hydrothermal system or may arise from changes in surface degassing due to permeability transients [Todesco et al., 2003, 2004; Rinaldi et al., 2010]. The subsidence during the fifth century is consistent with the time-dependent compaction due to deflation of the hydrothermal system. While pore pressure changes of the order of a few megapascal easily characterize hydrothermal circulation, the occurrence of a pressure drop usually premises a previous phase of pore pressure buildup. We speculate that an unrest phase during Roman times could have provided the excess of magmatic fluids then discharged during Late Antiquity. Our reconstruction depicts a lively history of vertical ground displacement, in which the long repose time before eruptive activity appears to be fragmented into several, connected episodes of uplift and subsidence. In particular, the Monte Nuovo eruption, in 1538, was preceded by more than a thousand years of ground deformation, with repeated transitions between unrest and quiet phases. Our reconstruction provides a new perspective on the history of volcanic activity at Campi Flegrei.

Acknowledgments References The work was partially supported by the Allis, R., C. Bromley, and S. Currie (2009), Update on subsidence at the Wairakei-Tauhara geothermal system, New Zealand, Geothermics, 38, DPC-INGV project V2—“Precursori.” We 169–180. wish to thank Shaul Hurwitz and an Amoruso, A., L. Crescentini, and I. Sabbetta (2014), Paired deformation sources of the Campi Flegrei caldera (Italy) required by recent anonymous reviewer for the useful (1980–2010) deformation history, J. Geophys. Res. Solid Earth, doi:10.1002/2013JB010392, in press. comments that greatly improved the Auriemma, R., and E. Solinas (2009), Archaeological remains as sea level change markers: A review, Quat. Int., 206, 134–146, doi:10.1016/j. original manuscript. quaint.2008.11.012. Bellucci,F.,J.Woo,C.Kilburn,andG.Rolandi(2006),Grounddeformation at Campi Flegrei, Italy: Implications for hazard assessment, in The Editor thanks Shaul Hurwitz and Mechanisms of Activity and Unrest at Large Calderas, Special Publications, vol. 269, edited by C. Troise, G. De Natale, and C. Kilburn, Joachim Gottsmann for their assistance pp. 141–157, Geol. Soc., London. in evaluating this paper. Biot, M. A. (1941), General theory of three-dimensional consolidation, J. Appl. Phys., 12(2), 155–164. Bonafede, M. (1991), Hot fluid migration: An efficient source of ground deformation: Application to the 1982–1985 crisis at Campi Flegrei- Italy, J. Volcanol. Geotherm. Res., 48(1–2), 187–198. Casertano, L., A. Olivieri del Castillo, and M. T. Quagliariello (1976), Hydrodynamics and geodynamics in the Phlegraean fields area of Italy, Nature, 264(5582), 161–164, doi:10.1038/264161a0. Castagnolo, D., F. S. Gaeta, G. De Natale, F. Peluso, G. Mastrolorenzo, C. Troise, F. Pingue, and D. G. Mita (2001), Campi Flegrei unrest episodes and possible evolution towards critical phenomena, J. Volcanol. Geotherm. Res., 109(1–3), 13–40, doi:10.1016/S0377-0273(00) 00302-4. Chang, C., D. Moos, and M. D. Zoback (1997), Anelasticity and dispersion in dry unconsolidated sands, Int. J. Rock Mech. Min. Sci., 34(3–4), 48.e1–48.e12, doi:10.1016/S1365-1609(97)00204-9. Costa, A., A. Folch, G. Macedonio, B. Giaccio, R. Isaia, and V. C. Smith (2012), Quantifying volcanic ash dispersal and impact of the Campanian Ignimbrite super-eruption, Geophys. Res. Lett., 39, L10310, doi:10.1029/2012GL051605. D’Auria, L., F. Giudicepietro, I. Aquino, G. Borriello, C. Del Gaudio, D. Lo Bascio, M. Martini, G. P. Ricciardi, P. Ricciolino, and C. Ricco (2011), Repeated fluid-transfer episodes as a mechanism for the recent dynamics of Campi Flegrei caldera (1989–2010), J. Geophys. Res., 116, B04313, doi:10.1029/2010JB007837. Deino, A. L., G. Orsi, S. de Vita, and M. Piochi (2004), The age of the Neapolitan Yellow Tuff caldera-forming eruption (Campi Flegrei caldera– Italy) assessed by 40 Ar/39 Ar dating method, J. Volcanol. Geotherm. Res., 133(1), 157–170. Del Gaudio, C., I. Aquino, G. P. Ricciardi, C. Ricco, and R. Scandone (2010), Unrest episodes at Campi Flegrei: A reconstruction of vertical ground movements during 1905–2009, J. Volcanol. Geotherm. Res., 195(1), 48–56, doi:10.1016/j.jvolgeores.2010.05.014. Di Vito, M., L. Lirer, G. Mastrolorenzo, and G. Rolandi (1987), The 1538 Monte Nuovo eruption (Campi Flegrei, Italy), Bull. Volcanol., 49(4), 608–615, doi:10.1007/BF01079966. Di Vito, M. A., R. Isaia, G. Orsi, J. Southon, S. De Vita, M. D’Antonio, L. Pappalardo, and M. Piochi (1999), Volcanism and deformation since 12,000 years at the Campi Flegrei caldera (Italy), J. Volcanol. Geotherm. Res., 91(2), 221–246. Dvorak, J. J., and G. Berrino (1991), Recent ground movement and seismic activity in Campi Flegrei, southern Italy: Episodic growth of a resurgent dome, J. Geophys. Res., 96(B2), 2309–2323, doi:10.1029/90JB02225. Dvorak, J. J., and G. Mastrolorenzo (1991), The mechanisms of recent vertical crustal movements in Campi Flegrei caldera, southern Italy, Spec. Pap. Geol. Soc. Am., 263, 49. Dzurisin, D., K. M. Yamashita, and J. W. Kleinman (1994), Mechanisms of crustal uplift and subsidence at the Yellowstone caldera, Wyoming, Bull. Volcanol., 56(4), 261–270, doi:10.1007/BF00302079. Farrell, W. E., and J. A. Clark (1976), On postglacial sea level, Geophys. J. R. Astron. Soc., 46(3), 647–667, doi:10.1111/j.1365-246X.1976.tb01252.x. Farrell, J., R. B. Smith, T. Taira, W.-L. Chang, and C. M. Puskas (2010), Dynamics and rapid migration of the energetic 2008–2009 Yellow- stone Lake earthquake swarm, Geophys. Res. Lett., 37, L19305, doi:10.1029/2010GL044605. Fedele, L., D. D. Insinga, A. T. Calvert, V. Morra, A. Perrotta, and C. Scarpati (2011), 40Ar/39Ar dating of tuff vents in the Campi Flegrei caldera (southern Italy): Toward a new chronostratigraphic reconstruction of the Holocene volcanic activity, Bull. Volcanol., 73(9), 1323–1336, doi:10.1007/s00445-011-0478-8.

Fournier, N., and L. Chardot (2012), Understanding volcano hydrothermal unrest from geodetic observations: Insights from numerical modeling and application to White Island volcano, New Zealand, J. Geophys. Res., 117, B11208, doi:10.1029/2012JB009469. Gaeta, F. S., G. De Natale, F. Peluso, G. Mastrolorenzo, D. Castagnolo, C. Troise, F. Pingue, D. G. Mita, and S. Rossano (1998), Genesis and evolution of unrest episodes at Campi Flegrei caldera: The role of thermal fluid-dynamical processes in the geothermal system, J. Geophys. Res., 103(B9), 20,921–20,933, doi:10.1029/97JB03294. Gaeta, F. S., F. Peluso, I. Arienzo, D. Castagnolo, G. De Natale, G. Milano, C. Albanese, and D. Mita (2003), A physical appraisal of a new aspect of bradyseism: The miniuplifts, J. Geophys. Res., 108(B8), 2363, doi:10.1029/2002JB001913. Gottsmann, J., A. G. Camacho, K. F. Tiampo, and J. Fernández (2006), Spatiotemporal variations in vertical gravity gradients at the Campi Flegrei caldera (Italy): A case for source multiplicity during unrest?, Geophys. J. Int., 167, 1089–1096, doi:10.1111/j.1365-246X.2006.03157.x. Guidoboni, E., and C. Ciuccarelli (2011), The Campi Flegrei caldera: Historical revision and new data on seismic crises, bradyseisms, the Monte Nuovo eruption and ensuing earthquakes (twelfth century 1582 AD), Bull. Volcanol., 73(6), 655–677, doi:10.1007/s00445-010-0430-3. Hurwitz, S., L. B. Christiansen, and P. A. Hsieh (2007), Hydrothermal fluid flow and deformation in large calderas: Inferences from numerical simulations, J. Geophys. Res., 112, B02206, doi:10.1029/2006JB004689. Isaia, R., P. Marianelli, and A. Sbrana (2009), Caldera unrest prior to intense volcanism in Campi Flegrei (Italy) at 4.0 ka B.P.: Implications for caldera dynamics and future eruptive scenarios, Geophys. Res. Lett., 36, L21303, doi:10.1029/2009GL040513. Kaneko, T., A. Yasuda, T. Shimano, S. Nakada, T. Fujii, T. Kanazawa, A. Nishizawa, and Y. Matsumoto (2005), Submarine flank eruption preceding caldera subsidence during the 2000 eruption of Miyakejima Volcano, Japan, Bull. Volcanol., 67(3), 243–253, doi:10.1007/ s00445-004-0407-1. Kwoun, O.-I., Z. Lu, C. Neal, and C. Wicks (2006), Quiescent deformation of the Aniakchak Caldera, Alaska, mapped by InSAR, Geology, 34(1), 5–8, doi:10.1130/G22015.1. Lambeck, K., C. Smither, and P. Johnston (1998), Sea-level change, glacial rebound and mantle viscosity for northern Europe, Geophys. J. Int., 134, 102–144, doi:10.1046/j.1365-246x.1998.00541.x. Lambeck, K., F. Antonioli, A. Purcell, and S. Silenzi (2004), Sea-level change along the Italian coast for the past 10,000 yr, Quat. Sci. Rev., 23(14–15), 1567–1598, doi:10.1016/j.quascirev.2004.02.009. Lambeck, K., F. Antonioli, M. Anzidei, L. Ferranti, G. Leoni, G. Scicchitano, and S. Silenzi (2011), Sea level change along the Italian coast during the Holocene and projections for the future, Quat. Int., 232(1), 250–257. Lyell, C. (1830), Principles of Geology, Being an Attempt to Explain the Former Changes of the Earth’s Surface, by Reference to Causes Now in Operation, J. Murray, London. Maniscalco, F. (1998), Gli edifici sommersi del Palatium dei Severi a Baia: Nuovi dati per la definizione del bradisismo flegreo, in Forma Maris, Forum internazionale di Archeologia Subacquea, edited by P. A. Gianfrotta and F. Maniscalco, Massa Editore, Pozzuoli, Italy. Morhange, C., M. Bourcier, J. Laborel, C. Giallanella, J. Goiran, L. Crimaco, and L. Vecchi (1999), New data on historical relative sea level movements in Pozzuoli, Phlaegrean Fields, southern Italy, Phys. Chem. Earth Part A., 24(4), 349–354. Morhange, C., N. Marriner, J. Laborel, M. Todesco, and C. Oberlin (2006), Rapid sea-level movements and noneruptive crustal deformations in the Phlegrean Fields caldera, Italy, Geology, 34(2), 93–96. Newhall, C. A., and D. Dzurisin (1988), Historical Unrest at Large Calderas of the World, vol. 2, U.S. Geol. Sur. Bull. 1855, 1108 pp. Parascandola, A. (1947), I Fenomeni Bradisismici del Serapeo di Pozzuoli, Stabilimento Tipografico G. Genovese, Naples, Italy. Peltier, W. R. (2004), Global glacial isostasy and the surface of the ice-age Earth: The ICE-5G (VM2) model and GRACE, Annu. Rev. Earth Planet. Sci., 32, 111–149. Pirazzoli, P. A. (1976), Sea level variations in the northwest Mediterranean during Roman times, Science, 194(4264), 519–521. Rinaldi, A., M. Todesco, and M. Bonafede (2010), Hydrothermal instability and ground displacement at the Campi Flegrei caldera, Phys. Earth Planet. Inter., 178(3–4), 155–161. Shelly, D. R., D. P. Hill, F. Massin, J. Farrell, R. B. Smith, and T. Taira (2013), A fluid-driven earthquake swarm on the margin of the Yellowstone caldera, J. Geophys. Res. Solid Earth, 118, 4872–4886, doi:10.1002/jgrb.50362. Spada, G., and P. Stocchi (2007), SELEN: A Fortran 90 program for solving the “sea-level equation”, Comput. Geosci., 33(4), 538–562. Todesco, M., G. Chiodini, and G. Macedonio (2003), Monitoring and modelling hydrothermal fluid emission at La Solfatara (Phlegrean Fields, Italy). An interdisciplinary approach to the study of diffuse degassing, J. Volcanol. Geotherm. Res., 125(1–2), 57–79. Todesco, M., J. Rutqvist, G. Chiodini, K. Pruess, and C. M. Oldenburg (2004), Modeling of recent volcanic episodes at Phlegrean Fields (Italy): Geochemical variations and ground deformation, Geothermics, 33(4), 531–547. Vanorio, T., M. Prasad, and A. Nur (2002), Petrophysical properties of volcanic samples: A contribution to studying ground deformation in Campi Flegrei volcanic area, Italy, EGS General Assembly Conference Abstracts, 27, 3685. Waite, G., and R. Smith (2002), Seismic evidence for fluid migration accompanying subsidence of the Yellowstone caldera, J. Geophys. Res., 107(B9), 2177, doi:10.1029/2001JB000586. Welter-Schultes, F., and I. Richling (2000), Palaeoenvironmental history of the Holocene volcanic crater lake Lago d’Averno (central southern Italy) inferred from aquatic mollusc deposits, J. Quat. Sci., 15(8), 805–812. Yokoyama, I., and A. Nazzaro (2002), Anomalous crustal movements with low seismic efficiency-Campi Flegrei, Italy, and some examples in Japan, Ann. Geophys., 45(6), 709–722. Zoback, M. D. (2008), Reservoir Geomechanics, Cambridge Univ. Press, Cambridge, U. K.