Journal of Volcanology and Geothermal Research 280 (2014) 104–110

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Journal of Volcanology and Geothermal Research

j ourna l homepage: www.e lsev ie r .com/ locate / jvo lgeores

Possible coupling of Campi Flegrei and Vesuvius as revealed by InSARtime series, correlation analysis and time dependent modeling

T.R. Walter a,⁎, M. Shirzaei a,b, A. Manconi c, G. Solaro d, A. Pepe d, M. Manzo d, E. Sansosti d

a Dept. Physics of the Earth, Deutsches GeoForschungsZentrum (GFZ), Telegrafenberg, D-14473 Potsdam, Germanyb School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USAc Istituto di Ricerca per la Protezione Idrogeologica (IRPI), Consiglio Nazionale delle Ricerche (CNR), Strada delle Cacce 73, 10135 Torino, Italyd Istituto per il Rilevamento Elettromagnetico dell' Ambiente (IREA), Consiglio Nazionale delle Ricerche (CNR), Via Diocleziano 328, 80124 Napoli, Italy

⁎ Corresponding author. Tel.: +49 331 2881253; fax: +E-mail address: twalter@gfz-potsdam.de (T.R. Walter)

http://dx.doi.org/10.1016/j.jvolgeores.2014.05.0060377-0273/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 November 2013Accepted 2 May 2014Available online 11 May 2014

Keywords:Volcano interactionEruption couplingInSAR monitoringCampi FlegreiStress transfer

Volcanoes are often considered as isolated systems, however, evidences increase that adjacent volcanoes are di-rectly coupled ormay be closely related to remote triggers. At the Italian volcanoes Campi Flegrei andVesuvius, aswell as adjacent volcano-tectonic systems, all located in the Campania Volcanic Province with ~2 million inhab-itants, a new analysis of satellite radar data reveals allied deformation activity. Here we show that during the 16-year records from 1992 to 2008, identified episodes of deformation have occurred in correlation. Albeit differ-ences in the quantity of deformation, the sign, frequency and rate of pressure changes at reservoirs beneathCampi Flegrei and Vesuvius can be very similar, allowing to infer that pressure changes originating from a mag-matic or tectonic source external to the shallow volcanomagma plumbing systems is a likely cause. Such a fluid-mechanical coupling sheds light on the earlier episodes of correlated eruptions and deformations occurring dur-ing the historical roman times.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Volcanoes located close to each other may conjointly become activeand are hence considered especially hazardous, as documented for vol-canoes in Iceland (Sturkell et al., 2010), Alaska (Hildreth and Fierstein,2000), Kamchatka (Eichelberger and Izbekov, 2000), and elsewhere(Miklius and Cervelli, 2003). The reasons of a feedback of adjacent vol-canoes are only poorly understood andmay be locally different, includ-ing the triggering by large tectonic earthquakes, faulting and associatedstress changeswithin the crust (Nostro et al., 1998) and the competitionof volcanoes for common reservoirs (Eichelberger and Izbekov, 2000) orby pore pressure diffusion (Gonnermann et al., 2012). These observedexamples and causes are basedmainly on singular, possibly sporadic oc-currences of conjoint activities. As this work shows, however, volcanoesthat are dormant may display an associated activity over the course ofdecades to millennia.

The volcanoes Vesuvius, Campi Flegrei, and Ischia, together withabundant faults, are located in the direct vicinity of Naplesmetropolitanarea, a densely populated area (pop. N 8000/km2). These volcanoesare since long known to be subject to deformation episodes, whichhave been barely investigated conjointly, however. Campi Flegrei andVesuvius, in specific, have been identified as the sources of some ofthe most explosive (Pingue et al., 2000) and far reaching eruptions in

49 331 2881204..

Europe (Orsi et al., 2004). In more recent times, Vesuvius violentlyerupted in 1631 and remained almost continuously active until 1944,with intervening periods of repose that on average endured less than5.5 yrs (Carta et al., 1981). In contrast, historical volcanic eruptions atCampi Flegrei are more rare, last 1538 (Di Vito et al., 1999).

In the following we first describe the methods and observations re-trieved from radar interferometry. Correlation of the displacement timeseries is then analyzed through statistical tests, as well as with a time-dependent modeling approach, showing stability of the geometry andposition of the sources. This is followed by a more general discussionon the correlated behavior of the sources with general implications.

2. Methods

2.1. InSAR

For this work we have used 165 SAR data acquired by the ERS-1/2satellites and extended the time series by 62 SAR data acquired by theENVISAT satellite, in ascending and descending mode. This datasetwas interferometrically processed using the Small BAseline Subset(SBAS) InSAR technique (Berardino et al., 2002a; Lanari et al., 2007).Deformation time series for ~30,000 pixels with a spatial resolution of100 × 100 m were accordingly generated by applying the SingularValue Decomposition (SVD) method to the set of multi-looked andunwrapped interferograms (Pepe and Lanari, 2006).

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In addition, atmospheric artifacts in the deformation time-serieswere detected and filtered out because of their different space–time be-havior with respect to the deformation signal (Ferretti et al., 2001;Berardino et al., 2002a). Each of the computed coherent pixels wasthen, used for the following deformation analysis.

For the study areas of Campi Flegrei and Vesuvius, the SBAS tech-nique was tested and validated by independent data (Casu et al.,2006). Comparison between the SBAS results and leveling andGPSmea-surements showed that the InSAR approach provides measurementswith an accuracy of 0.1 cm/year for the mean deformation velocityand of 0.5 cm for the displacement time series measurement (Casuet al., 2006; Lanari et al., 2007). Nevertheless, as in any InSAR study,even after atmospheric filtering residual artifacts cannot be entirelyexcluded. Because this work focuses on the correlation between defor-mation occurring at high altitude (on Vesuvius summit, 1281 m a.s.l.)with a signal at low altitude (Campi Flegrei, few tens of meters a.s.l.),and both are consistent in time and space, we can reasonably assumethat the atmospheric effect is negligible in the following analysis.

2.2. Statistics

Statistical evaluationswere performed to test the type and degree ofcorrelated deformation signals. This was performed in two ways. Firstwe tested the linear relationship between Campi Flegrei and Vesuviusonly. Cross correlation of the two univariate time series was computedutilizing the MATLAB function “corrcoef”. The correlation coefficient,say R, has values within the range −1 ≤ R ≤ 1, indicating the degreeof linear dependence between the variables (increasing linear relation-ship is positive, decreasing linear relationship is negative). The closerthe coefficient is to either -1 or +1, the stronger the correlation be-tween the variables. Variables that are independent tend to R = 0.

Furthermore, we are interested in analyzing the frequency depen-dent cross correlation, since observed short-term variances may occurwithout any apparent temporal proximity. To identify such a shortterm deformation signal we used a recently proposed approach(Shirzaei et al., 2013). We first decomposed the signal using multi-resolution analysis (Mallat, 1989) based on Morlet wavelet (Torrenceand Compo, 1998). Following the time–frequency decomposition ofthe deformation data at each pixel, we estimated the linear cross corre-lation coefficient between all the pixels and those located at CampiFlegrei maximum deforming area, limiting the signals to the scaledomain 1 ± 0.5 yr. The uncertainty for these coefficients has been esti-mated using the weighted total least square approach.

2.3. Modeling

We simulate the observed displacement fields at Campi Flegrei andVesuvius volcanoes based on an earlier published inversion technique(Shirzaei and Walter, 2010). Because of the time dependent displace-ment observations, we apply this modeling approach capable to inte-grate the full capacity of the data time series (Shirzaei et al., 2013).The aim is to track the behavior of the deformation sources over thetime. Previous studies (Segall and Matthews, 1997; Larson et al., 2001;Fukuda et al., 2004) implemented a Kalman filtering approach fortime dependent modeling of the geodetic data. Here, we consider timedependent modeling technique using a combination of Genetic algo-rithm (GA) (Shirzaei and Walter, 2009) and Kalman filtering (KF)(Shirzaei and Walter, 2010). The inversion filtering is repeated untilthe rootmean squared error (RMSE) approaches the range of the obser-vation error (0.5 cm). Analytical solutions of a spherical pressurizedsource (McTigue, 1987) applied at both volcanoes were assumed. Weuse a Poisson ratio of 0.25 and a rigidity of 7 GPa. Changing rigidity af-fects the source strength, and introducing material heterogeneitiesmay affect both the source strength and dimension. Here we assumematerial properties typical in the region (Manconi et al., 2010).

3. Results

3.1. Deformation at Campi Flegrei and Vesuvius

The deformation behavior at both volcanoes (Fig. 1) illustrates aprominent long-term subsidence of the volcano centers. In addition,the data suggest periods of uplift in 2001, and again in 2006. While atCampi Flegrei these mini-uplifts exceed 5 cm, at Vesuvius these barelyreach 2 cm, both of which are clearly above the detection limit. Themini-uplift deformation area is rather large at Campi Flegrei, and affectsthe ~10 km wide inner caldera basin. At Vesuvius, in turn, only theupper region with a ~2 km diameter is affected by weak mini-uplifts.

At Vesuvius and Campi Flegrei, some of the mini-uplift periods seenby our InSAR products have been associated with a clearly identifiedseismicity increase and gravity changes (Berrino et al., 2006), and alsoto geochemical changes in the form of diffuse carbon dioxide degassingpeaks (Granieri et al., 2009). Therefore, independent geophysical andgeochemical data support the observed trend changes at the adjacentvolcanoes. Far from the volcanic centers, the deformation is generallyminor. Exceptions are found at a girdle surrounding the N and NE mar-gin of the Vesuvius cone at ~10 km distance, and at sites of alleged bur-ied faults (Fig. 1, see also Supplementary material). Although theamplitude of observed deformation zones is highly variable, the tempo-ral behavior seems to be similar.

3.2. Correlation of deformation activity

To investigate the degree and type of the mutual relation betweenthe deformation signals at Pozzuoli harbor and Vesuvius summit, crosscorrelation of the two univariate time series was computed. To thisend, we selected deformation time series in areas on the top of Vesuviusand at the Pozzuoli harbor, we normalized the time series to their corre-sponding maximum deformation values, thus ending up with a signalvarying between −1 and 1. Finally, we generated the bivariate plotsshown in Fig. 2, for ascending and descending data respectively. Theso obtained bivariate plots were used to determine the correlationvalue, with an upper and lower bound in the 95% confidence region.The computed correlation value for the descending data is 0.88 (upperbound 0.84, lower bound 0.92), and the one for the ascending data is0.82 (upper bound 0.75, lower bound 0.87). The degree of linear depen-dence between the variables is high if the coefficient is either close to−1 or to +1. Hence, both ascending and descending datasets show acorrelation value that is statistically significant and supports the hy-pothesis that the deformation behavior at the volcanic systems ofCampi Flegrei and Vesuvius appears synchronous.

Furthermore, we are interested in analyzing the spatial scale of crosscorrelation, possibly also occurring at other sites (e.g. Ischia Island andburied faults). As explained in the method section, following thetime–frequency decomposition of the deformation time series at eachpixel (Shirzaei et al., 2013), we estimated the linear cross correlation co-efficient between all pixels and those located at Pozzuoli harbor byusing only the portion of the signal within the scale domain 1 ±0.5 yr. This analysis confirms, once more, that the deformation signalsare strongly spatio-temporally correlated (Fig. 2b). Results do notshow a significant topographic correlation. Instead, the correlationmaps show that the volcanic centers (Campi Flegrei, Vesuvius, IschiaIsland) correlate, as well as parts of the buried fault girdle surroundingVesuvius. This result is found independently in ascending and descend-ing data.

3.3. Modeling of the pressurized sources

External loading causes dynamic, static and quasi-static stresschanges (Hill et al., 2002), i.e. a force acting on a volcano reservoirwall (Pinel and Jaupart, 2003). Conjecturing this entity is similar in am-plitude for the adjacent volcanoes, the force acts per unit area and can

Fig. 1. The Bay of Naples and the two volcanoes under study. a. Overview of the cumulative deformation retrieved by exploiting SAR data acquired by radar satellites from 1992 to 2008and structural information after (Bianco et al., 1998). Green dots show radar pixels with little deformation, whereas red dots represent pixels with displacement values equal or largerthan 10 cm. Note also the deforming girdle to the North of Vesuvius. The map shows the descending orbit data. b, c. Close up views of the two volcanic centers of Campi Flegrei andVesuvius. d, e. Time series for pixels located in the Campi Flegrei caldera (Pozzuoli harbor area) and at Vesuvius (summit cone area). The long-term subsidence at both volcanoes isinterrupted by short-term uplift periods, as in 2001 and 2006, suggesting a possibly correlated activity. Time series is shown for ascending and descending data. (For interpretation ofthe references to color in this figure legend, the reader is referred to the web version of this article.)

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be determined bymodeling the observed deformation (McTigue, 1987).To estimate relevant source parameters, such as location, radius andpressure, weminimize themisfit between the results of the observationand the model (Shirzaei and Walter, 2010). The correlated volcano de-formation data can be well explained by simulating spherical pressuresources located underneath the volcanic centers. Consistent withvolcano-tectonic earthquake locations (De Natale et al., 2006), the pres-sure sources are found to be slightly closer to the surface beneath Vesu-vius (at ~2 km depth) than beneath Campi Flegrei (~3 km) (Fig. 3). Themean and standard deviation residuals of the model simulations aregenerally less than 0.5 cm, hence, for the InSAR case,within the accuracyof the observation method. The mean annual pressure change in both

sources decreased by about 0.6 MPa from 1992 onwards. Since late2005, the pressure has been increasing by almost 2 MPa per year,which is associated with the highest and most prolonged inflationphase within the observation period. Since the deformation at the sur-face is cubic wise inversely proportional to the source dimension(McTigue, 1987), the loading of the edifices allows to estimate the rela-tive volumes of the underlying reservoirs, being about 10 times largerunder Campi Flegrei; this has important implications for the fluid andmagma storage dimensions, and, therefore, for the volcanic hazard.The Ischia signal and also the deformation girdle surrounding Vesuviusvolcano, however, aremore plausibly explained by other geologic inter-pretations (Lanari et al., 2002; Berardino et al., 2002b; Borgia et al.,

Fig. 2. a. Bivariate plots showing displacements at Campi Flegrei vs. displacements at the top of Vesuvius (each normalized to the corresponding maximum displacement value) for eachepoch of the time series (see color bar). The plotted displacements are obtained by averaging values in small areas around the Pozzuoli harbor (Campi Flegrei) and the top of Vesuvius,respectively, as indicated bywhite circles in Fig. 1. The rather good alignment of the bivariate plot points shows the degree and the order of themutual relation between the two deformingvolcanoes. The computed cross-correlation coefficient is 0.88 and 0.82 for descending and ascending results, respectively. b. Statistical linear cross correlation with respect to a pixel nearPozzuoli harbor (white cross). The cross correlation coefficient is computed by using a wavelet decomposition and retaining only the portion of the signal within the scale domain 1 ±0.5 yr since deformation data show that the duration of the mini-uplifts is approximately 1 year. Pixels that have a similar behavior to the Pozzuoli harbor are shown in red for thetwo independent descending and ascending datasets, respectively. c. Pixel uncertainty for the linear cross correlation coefficients shown in (b). As seen, the centers of both volcanoesare displaying a correlated deformation activity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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2005; De Natale et al., 2006; Manzo et al., 2006), not further modeledhere, but discussed in the following section.

4. Discussion

Apparent time synchronous subsidence and mini-uplift episodeshave been observed at the volcanoes Campi Flegrei and Vesuvius byusing InSAR time series from 1992 to 2008. These episodes could bewell simulated using pressurized source models, allowing time consis-tent constraints on the location, depth and size of the reservoirs. Inaddition to these two volcanic/hydrothermal sources, we observed de-formation episodes with similar frequency but different scales at re-gions associated with tectonic faults and on Ischia Island.

To explore the spatial dependency of the deformation episodes withrespect to preexisting faults known from geologic and structural map-ping, Figs. 1 and 2 show the InSAR data together with the structuralmap summarized by (Bianco et al., 1998). Some of these faults are

covered by sedimentary deposits of Pleistocene to Holocene age. Ourcross correlation maps show that the locations of correlated deforma-tion are foremost found at or in the immediate vicinity of tectonic faults(Fig. 2b). This is found even if these faults are buried and not expressedby steep topography (see also supplementary material). Interestingly,similar localizations display lower density materials (Cella et al.,2007), possibly indicating unconsolidated and/or fluid bearing rockmasses.

How can the correlated deformation at the Neapolitan volcanoesand the buried faults be explained? We cannot provide a definite an-swer yet, but we can think of a variety of possible scenarios that includeinternal and external, near field and far field geologic interactions. Onepossibility is that deformation at one volcano, for instance at CampiFlegrei, with its large hydrothermal reservoir, imposes a stress changeat theneighboring volcanoes and fault systems.However,we also testedthe distance–source relationship, computed static stress transfer andexplored possible delays typical for lateral poroelastic transients, all of

Fig. 3. a. The deformation data at the two volcanoes was simulated by assuming the exis-tence of pressurized sources beneath Campi Flegrei (green symbols) and Vesuvius (bluesymbols). The reservoir under Campi Flegrei is found deeper and larger than that beneathVesuvius. b. Although the depth and radius may vary over time, a similar pressure evolu-tion at both volcanoes effectively reproduces the observed deformation data as a functionof time, suggesting that loading might occur from an extrinsic trigger. The results areshown for descending data (the ascending ones are reported in the Supplementaryinformation). (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

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which were below significant values, arguing against such an interpre-tation. Another possibility is that the migration of fluids, possibly fromlarger depths, is causing correlated activity changes at sites susceptibleto small changes, such as volcanoes and faults. A similar concept was re-cently proposed also for the correlated activity at Kilauea and MaunaLoa volcanoes (Gonnermann et al., 2012; Poland et al., 2012). Fluid–rock interactions have already been suggested by stress field analysis(Bianco et al., 1998). Also, measurements of diffuse carbon dioxidedegassing have revealed a main control of the regional stress field onthe location and pattern of gas migration (Aiuppa et al., 2004) andmay allow further testing on this hypothesis. This is supported by thefact that all sites identified as showing correlated deformation activityare zones of high porosity and fluid flow (see also Segall, 2010), beingprone for the detection of the changes of fluid emissions (Granieriet al., 2009). A common pool of magma situated beneath all theCampanian volcanoes was furthermore recently suggested based onpetrological studies (Pappalardo and Mastrolorenzo, 2012). Anotherpossibility is that tectonic loading of the region affects the Campi Flegreiand Vesuvius sites, whichwould possibly show a sign in other geophys-ical measurements.

Fig. 4.Historical data suggest independent eruption occurrence at Campi Flegrei (CF) andVesuvius (VS). Eruptions at Vesuvius are dominant during the periods of subsidence(Pingue et al., 2000; Troise et al., 2007), while no eruption at Vesuvius has occurred duringuplift periods. Eruptions are shown only if date is known and the occurrence was con-firmed, and when the volcano explosivity index (VEI) was larger than three (Siebert andSimkin, 2002-).

4.1. Limitations

The herein described dataset is an exceptionally large InSAR time se-ries, with hundreds of interferograms from different viewing directions,characterized by an inferred precision of about 0.5 cm (Casu et al.,2006). Nevertheless, our study and interpretation could be affected byseveral errors in data, processing or modeling. The fact that the concur-rent uplift and subsidence episodes at Campi Flegrei and Vesuvius occursimilarly at low altitude and at high altitude gives us confidence that thecorrelation is real and not related to unfiltered noise due to atmosphereheterogeneities. Moreover, the fact that the deformation episodes aretime and space consistent, i.e. they remain located at the same placeand show clear trends, argues against any environmental and non sta-tionary artifact. Finally, also independent gas flux data suggests a syn-chronous activity change at both volcanoes (Chiodini et al., 2012),

supporting our observations and interpretation. We note, however,that the correlated activity along the buried faults remains to be furthertested and explained, as well as a regional influence needs to be ex-plored in additional independent data. Moreover, although our sourcemodel results, possibly representing a fluid-bearing rock formation,are stable and agree with many earlier published results (Lundgrenet al., 2001; Lanari et al., 2004; Trasatti et al., 2008; Zollo et al., 2008;Manconi et al., 2010), they assume a very simplified structure, linearelasticity, and did not explore more complex source geometries suchas recent studies suggest (D'Auria et al., 2012). A new uplift episodethat has occurred since late 2009 affected the Campi Flegrei caldera,but so far no detectable changes are reported frommonitoring Vesuviusvolcano (Samsonov et al., 2014). Therefore future investigations willneed to address the particular modalities of concurring deformationepisodes.

4.2. Coupling in the past and present?

Let us assume that the interpretation of coupled deformationsources is correct in the decades of InSAR observations. Which evi-dences for a coupling can be derived in a longer time span? Magmaticandhydrothermal reservoirs, such as the one thought to have contribut-ed to the cataclysmic AD 79 and 1631 Vesuvius eruptions, have beentectonically controlled and bounded by tectonic faults (Bianco et al.,1998, 1999). Concerning ground motions, only the large scale, near-continuous subsidence of the region was hitherto considered to be tec-tonic in origin (Bianco et al., 1998, 1999), whereas localized small upliftperiods have been explained by shallow and local volcanic rechargingalone (Lundgren et al., 2001; Trasatti et al., 2008).

Particularly, as earlymonitoring records, e.g. as in 1982, suggest thatchanges at Vesuvius and Campi Flegrei might correlate in time but notso much in amplitude (De Natale et al., 2006), the local source condi-tions appear as the dominant actor. Differentiation history analysissuggest that a long-lived common pool of magma situated beneaththe Campanian volcanoes was and is present (Pappalardo andMastrolorenzo, 2012). We briefly review the volcano eruption data atCampi Flegrei and Vesuvius, as documented since Roman times: a com-pilation of the data was provided by Lee Simkin of the Smithsonian In-stitution (Siebert and Simkin, 2002-). The catalog contains the dates oferuptions at Campi Flegrei (catalog ID 0101-01) and Vesuvius (catalogID 0101-02) in the past 2500 years (see Fig. 4, and, for further details,the supplementary information). Comparison of eruptions at Vesuviusto the deformation history at Campi Flegrei (Troise et al., 2007) impliesa coupling: episodes of subsidence at Campi Flegrei concurred with pe-riods of more frequent eruptions at Vesuvius. The currently inferredsynchronicity of the deformation data at the volcanic systems appearsto be in opposition to the historical records available since the Romantimes, however, may allow speculations on the modalities of interac-tions as described below.

Fig. 5. Conceptualmodel for the observed correlated deformation activity at the Neapolitanvolcanoes. Geodetic data inversion suggests shallow reservoirs beneath Campi Flegrei(A) and beneath Vesuvius (B). Temporarily pressure changes at these reservoirs may beassociated with some crustal fault (C) reactivation. The reason for such a triggering maybe extrinsic, such as a deeper magmatic recharging and or regional tectonic extensional re-gime, allowing the apparent communications of the magmatic and tectonic structures.

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4.3. Implications

As confirmed by historical eruption records from other volcanoes,the lack of synchronous eruptions does notmean that they are indepen-dent systems. The largest eruption of the 20th century, i.e., the explosive1912 eruption of Novarupta, caused the pressure in the Katmai magmachamber to suddenly drop and the ground to subside into a caldera(Eichelberger and Izbekov, 2000). One may speculate if an eruption atone of the Neapolitan volcanoes may change the fluid pressure at theother system(s), or to which degree these adjacent volcanoes are fedby the same source (Fig. 5). Moreover, the recognition of a coupled ac-tivity of the two volcanoes might also be used for future monitoringstrategies and possible assessment of future volcanic unrest.

As periods of uplift at Campi Flegrei are conjointly occurring withless-eruptive periods at Vesuvius, a similar mode of coupling mayexist, acting both positively and negatively. Alternatively, an apparentinteraction of adjacent volcanoes may be triggered by their volcano-tectonic environment, such as near field and far field earthquakes(Manga and Brodsky, 2006). This explains correlated volcanic activityat many regions around the World, similarly as geyser and mud volca-nism (Manga and Brodsky, 2006). In the case of the coupled occur-rences, we did not recognize any systematic link to earthquakes;however, this requires a deeper study to warrant any conclusion.

In addition, an interesting question is to which degree the reneweduplift period from2009 onwards (not included in our study) is conjoint-ly occurring at Campi Flegrei and Vesuvius. Fluid pressure within mag-matic and hydrothermal reservoirs might further increase, as recentlysuggested (Chiodini et al., 2012), potentially reaching a fracture criteri-on of this host rock (Fialko and Rubin, 1997) and initiating a new unrestepisode at one of the dangerous volcanoes. We speculate that an erup-tion at one of the Neapolitan volcanoes (e.g. Vesuvius) could also de-crease the magma pressure beneath the other volcano and couldcause subsidence (e.g. Campi Flegrei). This hypothesis needs to be test-ed and explored in future experimental and numerical modellingstudies.

5. Conclusions

Using an InSAR time series from 1992 to 2008 we were able to mapthe deformation field at the Neapolitan Volcanoes. We observe that de-formation changes appear in chorus at Campi Flegrei and Vesuvius.Using a cross correlation approach we confirm this synchronous signal,and also detect similar ones at Ischia Island and along (buried) faults. Noconcluding explanation can be provided for this synchronous activity.However, using simple pressure sourcemodels, we can simulate the de-formation behavior at Campi Flegrei and Vesuvius, clearly depicting

two reservoirs that are isolated in space but follow the same pressurechanges. We also analyzed historical data and found, indeed, that thelong-term records of uplift and subsidence at Campi Flegrei appear tobe correlated with eruptions at Vesuvius. Periods of pronounced subsi-dence at Campi Flegrei are associatedwith periods of major and numer-ous eruptions at Vesuvius. Periods of uplift at Campi Flegrei, in turn, aresynchronized with non-eruptive periods at Vesuvius. This may indicatethat both volcanoes are magmatically or tectonically linked.

Acknowledgments

Constructive reviews byMirella Piña-Gauthier and Fabio Luca Bonaliare greatly appreciated. The work benefited from discussions withG. Chiodini on an earlier version of this paper. This work is financiallysupported by the Helmholtz Alliance EDA, and contributes to theINSARAP project, a European Space Agency grant.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2014.05.006.

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