Geophysical and hydrogeological experiments from a shallow hydrothermal system at Solfatara Volcano, Campi Flegrei, Italy: Response to caldera unrest

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jb004383, 2007 Geophysical and hydrogeological experiments from a shallow hydrothermal system at Solfatara Volcano, Campi Flegrei, Italy: Response to caldera unrest Pier Paolo G. Bruno, 1 Giovanni P. Ricciardi, 1 Zaccaria Petrillo, 1 Vincenzo Di Fiore, 2 Antonio Troiano, 1 and Giovanni Chiodini 1 Received 8 March 2006; revised 10 December 2006; accepted 26 February 2007; published 5 June [1] Integration of high-resolution geophysical and hydrogeological investigations at Solfatara Volcano, Campi Flegrei, Italy, allowed us to (1) image the shallow and intermediate subsurface to the crater, (2) elucidate patterns in the shallow subsurface degassing, and (3) refine and upgrade volcano-monitoring strategies for this dynamic area. Our results show that the subsurface to the crater can be divided roughly into two zones: a dry, outcropping layer overlying a horizon saturated by hydrothermal fluids. Within this saturated zone, intersections of dominant NW- and ENE-striking structural lineaments act as preferential escape conduits for the fluids which generate high microseismic noise amplitudes in the southeastern part of the crater. Hydrogeological data suggest an uprising of the isotherms below Solfatara crater, and a marked increment of fluid degassing, over the last 40 years. Sudden variations of both seismic noise level and noise cycling are positively correlated with early stages of ground inflation during the AD 2000 uplift. We believe therefore that monitoring of seismic noise can be used for upgrading early warning strategies in this sector of the Campi Flegrei volcanic system. Citation: Bruno, P. P. G., G. P. Ricciardi, Z. Petrillo, V. Di Fiore, A. Troiano, and G. Chiodini (2007), Geophysical and hydrogeological experiments from a shallow hydrothermal system at Solfatara Volcano, Campi Flegrei, Italy: Response to caldera unrest, J. Geophys. Res., 112,, doi: /2006jb Introduction [2] Volcanoes can dissipate large amounts of heat via fumaroles and diffuse degassing of hydrothermal-volcanic fluids through soils. This process has been recently investigated in detail at Solfatara, a volcano located within the active Campi Flegrei caldera of southern Italy (Figure 1a). Solfatara releases about 1500 t/day of volcanic-hydrothermal CO 2 as a result of diffuse degassing through soil (Figures 1b and 1c). During this process, about 3350 t/day of steam condenses, generating hydrothermal fluid circulation [Chiodini et al., 2001]. The heat released by condensation ( J/day) represents the largest part of the total heat dissipated by the hydrothermal degassing at Solfatara ( J/day [Chiodini et al., 2001]). This value, which is only a fraction of the total convective hydrothermal flux at the whole Campi Flegrei caldera, is still much higher than other known energy released within the caldera during the current period of quiescence of Campi Flegrei, as a result of thermal conduction, earthquakes, 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Naples, Italy. 2 Consiglio Nazionale delle Ricerche, Istituto Per l Ambiente Marino e Costiero, Naples, Italy. Copyright 2007 by the American Geophysical Union /07/2006JB004383$09.00 and ground deformation. Hydrothermal fluids at Solfatara are believed to induce and/or enhance rock deformation [Oliveri del Castillo and Montagna, 1984; Bonafede, 1991; De Natale et al., 1991; Gaeta et al., 1998] by increasing the pore pressure (and hence modifying the effective stress) and rock temperature (inducing thermal expansion). Chiodini et al. [2003] suggested that at Campi Flegrei (as well as at other similar calderas, such as Long Valley, Yellowstone, and Rabaul), periods of intense magmatic degassing affect both chemical changes of fumaroles and ground deformation during unrest crises. That is, when degassing exceeds the physical capabilities of the host rock and the ability of the hydrothermal system to remove the fluids, the resulting overpressure can produce ground deformation [Oliveri del Castillo and Montagna, 1984]. Therefore it is important to delineate the physical properties of the shallow hydrothermal system. We will show that the fluid and heat transport associated with hydrothermal circulation at Solfatara generate clear geophysical anomalies and high levels of seismic noise. We will also show how periods of intense magmatic degassing, which can potentially trigger unrest phases, affect levels and patterns of microseismic noise. These changes can be used to improve monitoring techniques and hazard assessment at Campi Flegrei. [3] Our surveys therefore aim to (1) image the shallow and intermediate subsurface (<500 m) of the crater, identifying discontinuities, such as fractures and faults, that may 1of17

2 Figure 1. (a) DTM map of Campi Flegrei Caldera with sketches of the main tectonic and volcanic features [Orsi et al., 1996; Bruno et al., 2003]. (b) Structural map of the Solfatara area with azimuthal distribution of cracks in selected sites plotted on rose diagrams [Bianco et al., 2004]. Yellow triangles: position of the three-component seismic stations (ACC and SOF). (c) Aerial photograph of Solfatara with distribution of CO 2 flux [Chiodini et al., 2001] and the position of geophysical profiles. Abbreviations: ER1, ER2, ER3, and ER4: dipole-dipole geoelectrical profiles (blue); TDEM: transient time-domain electromagnetic profile (green); SR1 and SR2: P wave seismic tomography profiles (black); MASW1 and MASW2: seismic Rayleigh wave profiles (red); MT: magnetotelluric profile (dashed); red triangles: MT and controlled-source audiomagnetotelluric (CSAMT) coincident soundings; black triangles: CSAMT soundings; red square: dipolar antenna. act as pathways for gases; (2) estimate depth, morphology, and the physical parameters of the water-saturated subsurface zone; and (3) evaluate any relationship between hydrothermal fluids and the groundwater system in this part of Campi Flegrei. To our knowledge, this is the first attempt at detailed imaging of the subsurface (beneath the crater) features of a shallow degassing system within an active volcanic area by integration of geophysical, geochemical, and hydrogeological surveys. 2. Geology and Hydrothermal Features [4] Solfatara volcano is a 0.6-km-wide tuff cone, situated 180 m above sea level to the NE of Pozzuoli, within the densely populated Campi Flegrei Caldera (Figure 1a). The 2of17

3 area has been blanketed by two large eruptions: the Campanian Ignimbrite (39 ka [De Vivo et al., 2001]) and the Neapolitan Yellow Tuff (14 ka [Deino et al., 2004, and references therein]). Over the last 14 ka, volcanic activity has occurred mainly within the caldera, commonly focused along its rim and regional faults [De Astis et al., 2004; Bruno, 2004; Bruno et al., 2003; D Antonio et al., 1999]. The erupted products, which range in composition from K- basalts to trachyte and phonolite, were mainly generated during explosive eruptions [De Vivo et al., 2001]. [5] Uplift of the Campi Flegrei caldera floor, followed by renewed volcanic activity, occurred between 10 and 5 ka b.p. [Cinque et al., 1985]. Sea level measurements, made on the ruins of a Roman market built near the seashore town of Pozzuoli, indicate a slow subsidence of the area since Roman times. This subsidence was first interrupted by a 7-m uplift, which took place west of Pozzuoli between 1530 and 1538 AD, ending with the Monte Nuovo eruption in 1538 AD [Di Vito et al., 1987]. After the 1538 eruption, the subsidence continued until Then, rapid uplift started again, totaling about 170 cm during and 182 cm during [Berrino et al., 1984]. [6] In contrast to the sinking phases, uplift episodes are accompanied by seismic activity. Earthquakes occur mostly along the coast around Pozzuoli, at Solfatara, and within the bay. Seismic activity does not extend outside the margin of the caldera and abruptly terminates at 3.5- to 4-km depth, suggesting a sharp transition from brittle to ductile behavior. The maximum recorded magnitude (4.0) was measured in 1984 [Orsi et al., 1999]. Earthquakes are likely associated with the upward migration of a pressure front triggered by an excess of fluid pressure from a magmatic intrusion, and with the brittle readjustment of the inflated system occurring along some lubricated structures [Bianco et al., 2004]. [7] Geothermal wells drilled during the 1980s enabled detailed investigation of the distribution of the hydrothermal alteration minerals at Campi Flegrei [Chelini and Sbrana, 1987]. The shallower (<300 m) hydrothermal alteration zones, composed of argillitic and illite-chlorite mineral assemblages, form at temperatures <250 C. These assemblages induce a reduction in the rock permeability and represent the caprock for the hydrothermal reservoir. At greater depths ( m), a Ca-Al silicate alteration zone, composed of abundant neogenic hydrothermal minerals, increases instead rock permeability. At the deepest level, a thermometamorphic alteration zone ( m) likely represents a deeper portion of the hydrothermal reservoir. [8] Stratigraphic and geochronological data show that Solfatara formed between 3.8 and 4.1 ka [Di Vito et al., 1999]. Phreatic eruptions also occurred during the 12th century [Rosi and Santacroce, 1984]. The volcanic products consist of breccia covered by accretionary lapilli-rich, dunebedded ash, and lapilli beds [Rosi and Sbrana, 1987]. These deposits cover about 0.8 km 2 and are extremely altered by hydrothermal activity. Solfatara volcano is also affected by intense, diffuse degassing (Figure 1). The distribution of the hydrothermal alteration in the subsoil of Solfatara is unknown because of a lack of deep wells. However, we hypothesize that hydrothermal zones indicative of higher temperatures are present at relatively shallow depths because of the presence of a marked thermal anomaly caused by the expulsion of hot gases. Mud and soil samples show that interaction between low ( ) ph, high-temperature waters, and the altered trachitic rocks of Solfatara forms alunite and opal-bearing assemblages, while an association of alunite-opal-10 Å halloysite-pyrite forms from waters with ph 4.5. [9] The subrectilinear NE and SW rims of Solfatara are cut by two normal faults that strike NW-SE (Figure 1b [Bianco et al., 2004]). Outside the crater, two NW-SE striking faults cut the eastern part of the tuff cone. These faults move in response to a normal stress field, characterized by a subvertical s1 and a NE-SW striking subhorizontal s3. Two main crack systems striking NW-SE and NNE-SSW/NE-SW are also present at Solfatara (Figure 1b [Bianco et al., 2004]). Most of these fractures are subvertical with dips > 60 and are filled by hydrothermal minerals. The NW-SE crack system is related to the main fault segments affecting the tuff cone. Carbon dioxide degassing paths are also oriented along the same direction (Figures 1b and 1c [Chiodini et al., 2001]). New NE-SW cracks also formed on Solfatara crater during the inflation episode of [Rosi and Sbrana, 1987; Acocella et al., 1999]. 3. Data Acquisition and Processing 3.1. Electromagnetic and Electrical Data [10] Resistivity is very sensitive to fluid saturation and temperature as well as fracture properties of the rock matrix and chemical reactions between the pore electrolytes and the minerals [Llera et al., 1990]. Since these parameters are modified by processes generated by volcanism, geoelectrical methods are the main exploration tools used in volcanic and hydrothermal areas. We used different techniques with different resolution to explore to varying depths. To investigate the subsurface structure down to several hundreds of meters in depth, we performed three wideband magnetotelluric (MT) soundings (acquired in two frequency bands of 2048 and 64 Hz) and six controlled-source audiomagnetotelluric soundings in the frequency band Hz along a 500-m N-S profile inside the Solfatara crater (Figure 1). We also examined in higher resolution the subsurface of the crater down to 80 m by using both dipole-dipole DC electric profiling and transient electromagnetic techniques. [11] The MT soundings were acquired with two complete synchronous Metronix stations (each one equipped with three magnetic and two electric channels) to allow for multivariate data analysis. The use of the Robust Multivariate Error-in-Variables method (RMEV [Egbert, 1997]) to estimate the tensor impedance allows for a more detailed analysis of coherent electromagnetic noise, mainly originating from railroads and power lines in this part of Campi Flegrei. Examination of MT data between 0.1 and 10 s showed a good signal-to-noise ratio, whereas severe nearfield effects were present at higher periods. We therefore limited our MT data inversion to 10 s. [12] Controlled-source audiomagnetotelluric (CSAMT) soundings were acquired to improve our understanding on the shallower structure and to evaluate possible static shift problems of the resistivity curves estimated by MT. CSAMT data were acquired using a STRATAGEM EH-4 system source, which uses a 50-Hz notch filter to limit nearfield effects due to electrical networks, and two orthogonal 3of17

4 dipolar antennas energizing in the frequency band Hz. Near-field effects are present in the first two CSAMT sites in the vicinity of the source (Figure 1c); here data were acquired using only the natural magnetotelluric source. The other four soundings recorded electromagnetic data in the plane wave approximation [Goldstein and Strangway, 1975], allowing us to estimate the resistivity MT impedance tensor. Resistivity and phase curves, estimated for the six CSAMT soundings in the complete acquisition band ( Hz), showed a good signal-to-noise ratio and a smooth pattern. Resistivity curves suggest the CSAMT are consistent with the DC electric and transient electromagnetic data. The lower band (64 Hz) relative to the three MT stations (sites 1, 3, and 6 MT, coincident with CSAMT sites; Figure 1c) shows good resistivity and phase down to 0.1 Hz. As the frequency decreases (band Hz), resistivity and phase patterns are consistent with near-field effects rendering the MT tensor not definable [Quian and Pedersen, 1991]. We substituted the noise-contaminated band of MT data with the noise-free band estimated by the CSAMT. The CSAMT and magnetotelluric data were then processed with a two-dimensional inverse code [Rodi and Mackie, 2001] on transverse electric (TE) and transverse magnetic (TM) modes separately. As discussed by Siripunvaraporn et al. [2005], three-dimensional structures are better constrained on twodimensional sections by the inversion of the TM mode only. However, inversion of TM and TE modes achieved very similar results. A joint inversion of both TE and TM modes allowed us to reconstruct the electric image along the northsouth profile. The final RMS value was about 6. This error also takes into account the approximation of a threedimensional structure with a two-dimensional model. [13] Four electric profiles were acquired using the dipoledipole configuration and an array of 64 electrodes. As a source, we used the IRIS Syscal R2 system, with a maximum voltage of 800 V and a maximum output current of 2 A for the DC electrical measurements. The electrode spacing was 7 m for profile ER4 and 4 m for the other three profiles. Digital stacking of repeated waveforms in the field provided an increase of signal-to-noise ratio. The raw data were processed using an Occam s nonlinear inversion algorithm that yields a minimum roughness solution consistent with the data type and accuracy. This algorithm, based on a finite element forward solver, is described by LaBrecque et al. [1999]. This method searches the smoothest resistivity profile related to experimental measurements. Taking into account the rough volcanic environment in which the data were acquired, together with the approximation of a threedimensional structure along a two-dimensional profile, the average RMS-associated uncertainty on inversion of the DC electric profiles of 9 was considered satisfactory. [14] The procedure utilized for subsoil exploration using transient electromagnetic techniques (time domain electromagnetic method or TDEM) is based on the propagation of an induced electromagnetic field; a steady current is forced to flow in a loop for several milliseconds to allow the turn-on transient in the ground to dissipate. An ultrafast TDEM system was employed to evaluate the transient decay of the secondary magnetic field in a coincident loop configuration with a radius of 25 m. The time window ranged from 4 to 239 ms with 24 channels of signal integration. Time domain electromagnetic data lack spatial resolution at near surface. However, we recovered it using data from electrical profile ER4 which is coincident to TDEM (Figure 1). TDEM apparent resistivity data were transformed to an effective subsurface resistivity by means of one-dimensional inversion analysis using conventional least squares approximation [e.g., Meju, 1998]. A two-dimensional section was obtained by spatial interpolation of 10 TDEM soundings Active Seismic Data [15] Two P wave seismic refraction surveys and two Rayleigh wave measurements, made using the multichannel analysis of surface waves or MASW method [Park et al., 1999], were conducted along two NE and NW heading profiles with the aim of locating P wave velocity (Vp) anomalies related to degassing structures and estimating the distribution of S wave velocity (Vs) in the shallow subsurface of the crater (Figure 1). Another goal was to provide an estimate of Vp/Vs variation with depth. All seismic data were recorded using a 48-channel 21 bit engineering seismograph and 40-Hz (P wave) and 4.5-Hz (Rayleygh wave) vertical geophones. [16] Two seismic refraction profiles were acquired using 200- to 300-g blasting explosive charges inside 1-m-deep shot holes. Geophone spacing was 5 m for profile SR1 and 8 m for profile SR2. The source move out was twice the geophone spacing. First arrival traveltimes were handpicked, checked for consistency (using reverse, split, and offset profile configurations [Ackermann et al., 1986]), and interpreted using the generalized reciprocal method [Palmer, 1980]. This method allows for detection of vertical and lateral seismic velocity changes and for resolution of irregularities of the refracting surfaces. The generalized reciprocal method is also suitable for identifying thin layers where velocity inversion occurs. The optimum shotgeophone distance, defined on the basis of velocity analysis function and time-depth function, allows the interpreter to generate the migrated subsurface seismic section. Refraction data results were refined using a high-efficiency tomographic iterative reconstruction technique that updates the velocity of a single cell by using all the rays passing through that cell (simultaneous iterative reconstruction technique, SIRT [Menke, 1989]). The SIRT method is a local optimization method. That is, it requires a good a priori model (for example, a model interpreted using the generalized reciprocal method), since the solution usually is near this model. In other words, this approach gives a weighted spatial averages [Menke, 1989] of the true model to achieve a reliable model of the subsoil. The calculation of theoretical raypaths and associated traveltimes is solved by the finite difference solutions of the eikonal equations of Vidale [1988]. This method solves the stability problems in the presence of significant velocity heterogeneity due to discontinuous gradient of the wavefront. The final RMS error was 1.8 ms for seismic profile SR1 and 2.2 ms for SR2. [17] Multichannel analysis of Rayleigh wave dispersion (MASW; Figure 1) was carried out using explosives, a sledgehammer, and noise (stacks of 10 windows of 32 s each). The dispersion curve was picked in the frequencyslowness domain with the aim of overcoming (1) the frequency-wave number domain requirement of a large number of traces and (2) the undesirable effect of spatial padding and spectral leakage in the power spectrum. The 4of17

5 analysis chosen adds a spectral power-ratio calculation to McMechan and Yedlin s [1981] technique; in this way, it was possible to obtain a spectral normalization of the records. Furthermore, to enhance the spectra, a stack in frequency-slowness domain gathered several traces in a unique frequency-slowness group, following the rules of near and far offset, as explained in the study by Park et al. [1999]. To carry out the solution of the eigenvalue problem and to perform the inversion procedure, we utilized the code described by Lai and Rix [1998], based on the Occam s inversion algorithm [Constable et al., 1987]. The starting model was obtained (autonomously from seismic refraction data) using shear-wave velocities, density, and Poisson ratio values measured on different rock samples from Solfatara [Nunziata et al., 1999]. In addition, several values of shearwave velocities were tested, choosing the one having the smallest initial RMS deviation between the picked and the calculated dispersion curve Microseismic Noise [18] It is well known that in many geothermal systems, fluids at depth create a background of seismic noise [Kieffer, 1984]. The analysis of seismic noise at Solfatara involves the separation of hydrothermal noise from that of a different origin (for example, meteorological, cultural, etc.). A threecomponent recording station (labeled as SOF: Figure 1c) was deployed for more than a year in the SE corner of Solfatara, near the most active fumaroles. Study of SOF data shows that noise amplitude varies in a 24-hour period; it is characterized by high levels during the day and low levels between 2:00 AM and 5:30 AM. To limit the effect of cultural noise, spatial measurements of seismic noise at Solfatara were conducted during the night. The device consisted of an array of four vertical 1-Hz geophones set up with a cross geometry. To separate hydrothermal from random noise, we applied the technique described by White [1973] to the signals recorded by the array, progressively increasing the spacing between geophones from 1 to 10 m. In this way, it was possible to obtain the maximum coherence on signals with wavelengths greater than the reciprocal distance between the sensors. The processing showed that the maximum coherence of nonrandom (hydrothermal) noise is located in the 10- to 15-Hz frequency band, which was used for spatial seismic noise comparison within Solfatara crater. A 10-m array spacing was used to sample the spatial distribution of noise. The four signals of the array were sampled and continuously stacked for 450 s Gravity Data [19] Oliveri del Castillo et al. [1968] acquired highresolution data in the crater before the uplift episodes of and of We used these data to compare with newly acquired data to ascertain whether or not the shallow structure of the volcano has remained mostly unchanged over the last 40 years, or indeed, to see if the two more recent uplift episodes have influenced both shallow degassing and groundwater pathways. The grid of stations consisted of 50 observation points spaced 50 m from each other. Measurements were made using a Worden gravimeter with a sensitivity of mgal. The observed gravity values were corrected by Oliveri del Castillo et al. [1968] to a local datum of 97 m above sea level, using a density value of 1.40 g/cm 3 for surrounding masses. We applied a simple boundary analysis technique which consists of the study of the horizontal derivative of the gravity [Cordell, 1979; Cordell and Grauch, 1985], with the aim of enhancing the boundaries of anomalous mass sources Hydrogeological Data [20] The release of large amounts of hydrothermal volcanic fluids at Solfatara does affect the pattern of underground water circulation and temperature. To quantify these changes, we measured the water table level and temperature in about 30 boreholes at Solfatara and nearby. The water table was determined using an open electric circuit that closes when the water level is reached. Temperatures were measured using a digital thermometer with an accuracy of ±0.1 C. The data were acquired over a period of 3 days during the month of July Daily variations of both temperature and water level were averaged making five measurements every 5 hours. Continuous monitoring of two boreholes outside Solfatara showed that water table oscillations did not exceed 2 m and groundwater temperatures remained almost constant (±2 C) during the 3-day time span. At Solfatara, two measurements of the water level and temperature were done at the OAK well and at the mud pools, where the saturated zone outcrops at the surface (Figure 1c). 4. Results 4.1. Electromagnetic and Electrical Data [21] The sensitivity of electrical resistivity to a wide range of variables (of both host rock and pore fluids) makes it difficult to predict quantitatively the electrical response of a hydrothermal area. However, experimental data and modeling [Detwiler and Robefls, 2003] show that electrical resistivity is an indicator of saturation in geothermal systems. Integrated analysis of the electromagnetic and DC electrical data illustrates that the subsurface of the shallow crater can be approximately divided in two electrical zones: an outcropping, resistive layer A that sits above a conductive (1 15 W m) body B (Figures 2 4). Layer A is about m thick in the northwestern side of the crater, progressively becoming thinner toward the central part of the crater until it disappears below the mud pools area (Figure 3). We compared the results of our resistivity profiles with the CO 2 flux and temperature patterns at the surface [Chiodini et al., 2001, 2005]. It can be seen that, in general, soil CO 2 degassing and temperatures are higher where layer A is thinner, or absent. Electrical resistivity in zone A is compatible with a nonsaturated argillitic alteration zone affected by CO 2 degassing. The lowresistivity zone B corresponds to the hydrothermal aquifer recharged by natural condensates. Magnetotelluric data show that this saturated area extends down to a depth of m below the crater floor (see Figure 2). Below 400 m, the resistivity increases. We interpret this fact as indicative of either a change in geology or, more likely (considering the increasing temperature gradient), an increase in the gas and steam fraction with respect to water. Roberts et al. [2002] found that in fractured samples, the partial replacement of conducting brine with insulating water vapor is followed by a gradual increase in resistivity. 5of17

6 Figure 2. Resistivity section obtained by the twodimensional inversion of CSAMT-MT data. Red square, CSAMT antennas; red triangle, CSAMT-MT station; black triangle, CSAMT station (see location in Figure 1). Color scale is log(resistivity) in W m. Labels A, B, and E refer to anomalous features discussed in the text. CO 2 flux [Chiodini et al., 2001] and temperature [Chiodini et al., 2005] patterns are plotted for comparison along the profile strike. In particular, in fractured tuff samples, vaporization/ condensation can result in resistivity changes that are more than an order-of-magnitude greater than those measured in intact samples. [22] Within conductive zone B, we found three anomalous higher resistivity areas, including (1) a subcircular body C, imaged at the southeastern end of electrical profile ER4 (Figure 3), and at the northwestern end of profile ER2, beneath the main fumaroles (Figure 4); (2) an elongated body D discovered along profiles ER4 and TDEM (Figure 3); and finally (3) an elongated body E which splits hydrothermal aquifer B in two parts on the magnetotelluric profile (Figure 2). Body E is also visible as a subcircular, resistive area (E) along electrical profiles ER1 and ER2 (Figure 4) and most probably represents the electrical image of a NW-SE striking fault cutting Solfatara crater. Most of these resistivity anomalies can be related to faults and/or fractures affected by the expulsion of gas-rich hydrothermal fluid. Gas and steam pressure drives the liquid phases outside of the fractures, ensuring a relative increase in resistivity for the fractures with respect to the neighboring saturated zones of rock. Old or inactive degassing pathways are also imaged as higher resistivity and higher-density zones if the fractures are filled by resistive alteration minerals such as alunite and opal instead of water. The distinction between active and inactive degassing pathways is discussed in section Active Seismic Data [23] Because of the geological conditions at the site and logistics that limited the maximum source-receiver offset as well as data quality at far offsets, the seismic surveys achieved a limited depth penetration. Thus only the very shallow parts of the crater (i.e., the top 30 m) were imaged. Interpretation of seismic refraction profiles (SR: Figure 5) reveals a first layer with a P wave velocity of m/s, sited above a substratum with higher P wave velocity ( m/s). The width of the first layer varies from 8 to 20 m, comparable in thickness to layer A determined by the resistivity survey. Underneath layer A, the P wave velocity increases considerably (more gradually on seismic profile SR1), and its lateral distribution appears heterogeneous. A high-velocity anomaly of 2600 m/s is recognized on SR1 (Figure 5a: location m). Two other highvelocity zones are also present on seismic profile SR2 (about 2400 m/s; Figure 5b). The anomaly to the southwestern end of profile SR2, located at the margin of the model, might not represent a real geological feature because of uncertainty associated with low seismic ray coverage. The second high-velocity anomaly on profile SR2 ( E in Figure 5b) matches with the top of the high-resistivity body E, found along electrical profiles ER1 and ER2. CO 2 flux and soil temperature are higher along P wave anomaly E (Figure 5b). [24] Inversion of the Rayleigh wave dispersion curves (Figure 6) allowed us to estimate, about in the middle of P wave profiles, the variation of shear wave velocity to a depth of 32 m. Using the P wave velocities of Figure 6, horizontally averaged along the overlap area common to both Rayleigh wave and P wave profiles (Figure 1), we were able to estimate the Vp/Vs value. The calculated Vp/Vs values are equal to about 2 between 0- and 16-m depth (corresponding to layer A ); thereafter, the ratio increases within layer B, reaching a maximum value of 4.5 at 28-m depth. Average Vp/Vs values for layer A are compatible with dry unconsolidated soils/rocks and also with the presence of gas. Small amounts of gas significantly reduce the compressional velocities, while Poisson s ratios for gas-saturated rocks are lower than those for fluidsaturated rocks. Recent experiments [Zimmer et al., 2002] show that Vp/Vs values around 4, similar to those found below 16- to 20-m depth, are instead caused by a very high degree of water saturation in the soils/rocks Microseismic Noise [25] The seismic signal at Solfatara is dominated by continuous noise rather than discrete events. The temporal variation of seismic noise amplitude, recorded at seismic station SOF, follows a quasi 24-hour period (Figure 7). The high noise amplitude recorded during the day cannot be due to human activity, since the spectral pattern of seismic station ACC (Figure 1b), which is closer than station SOF to the main sources of cultural noise, is characterized by a lower amplitude, and it does not show any daily periodicity (Figure 7a). Furthermore, it is worth noting that the variation of H 2 O/CO 2 is consistent with noise cycling (Figure 7b). We therefore conclude that at Solfatara, the main source of the seismic noise is not cultural, but hydrothermal. 6of17

7 Figure 3. NW-SE striking electrical (ER3 and ER4) and TDEM profiles with CO 2 flux [Chiodini et al., 2001] and temperature [Chiodini et al., 2005] patterns plotted along the profile strike. Electrical anomaly D dipping to the NW, with r of W m is consistently found along profiles ER4 and TDEM. Figure 4. NE-SW oriented electrical profiles ER1 and ER2, with CO 2 flux [Chiodini et al., 2001] and temperature [Chiodini et al., 2005] patterns plotted along the profile strike. Both profiles show in their central part a subcircular, 100-m-wide 40-m-high anomalous body E, located at 30-m depth within the conductive substratum B. Body E is characterized by a resistivity anomaly (20 70 W m). The rapid change of resistivity along body E on ER1 and ER2 implies that chemical and physical properties are changing quite rapidly in the direction perpendicular to the two lines. 7of17

8 Figure 5. Seismic tomograms SR1 and SR2 with CO 2 flux [Chiodini et al., 2001] and temperature [Chiodini et al., 2005] patterns plotted along the profile strike. Continuous lines show depth and morphology of the refracting interface evaluated with the generalized reciprocal method [Palmer, 1980]. Peripheral and deeper parts of the tomographic models, with low ray density, are not shown. Highvelocity body E matches with the top of body E shown on MT, ER1, and ER2. Figure 6. Vs depth profile (1) and Vp/Vs value (2) relative to multichannel analysis of Rayleigh wave dispersion along MASW1 and MASW2 soundings (see Figure 1 for position). Average depth of seismic interface A-B (16 m) is drawn for both graphs for comparison. On line MASW1, the estimated Vs curve shows a rapid increase ( m/s) in the first m; thereafter, Vs remains almost constant down to 32 m. On line MASW2, a similar trend is seen, although Vs continues to increase to a depth of 24 m, reaching a maximum value of about 540 m/s at 32 m. 8of17

9 Figure 7. (a) Amplitude of the seismic noise recorded by the three-component 1-Hz seismic stations SOF and ACC (see Figure 1 for location) during the period March Gain parameters are equal for both stations. Each point is the average of the amplitude of the 10- to 15-Hz frequency window, of the vertical component signal on a 5-min window time. (b) Comparison between seismic noise amplitude pattern recorded on 21 March 2000 and H 2 O/CO 2 variation (same day) between 8:00 and 19:00. Both signals were transformed in the nondimensional z-score space [DeLaughter et al., 2005]. [26] Daily fluctuations of seismic noise at Solfatara are likely witness of the intrinsic instability of the hydrothermal system. Noise cyclical variation is also common in other hydrothermal systems. For example, Vandemeulebrouck et al. [2005] compared analogue experiments to two volcanic lake areas in New Zealand and found that there is an agreement between seismic noise, thermal activity, and water level cycling, which is due to gravitational rather than the purely convective instability nature of these hydrothermal systems, without any change in the underlying magmatic system. During volcano monitoring at Campi Flegrei, as well as in other similar calderas, it is crucial to be able to discriminate intrinsic hydrothermal instabilities from ones of magmatic origin. For example, the level of seismic noise at SOF abruptly increased on 22 March 2000 (Figure 7a) at the beginning of a minor uplift episode at Campi Flegrei [Civetta, 2001]. Moreover, the noise cycling changed, with superimposition of other cycles on the typical preuplift 24-hour sequence (Figure 7a). This AD 2000 uplift caused an increase in the flux of emissions at fumaroles corresponding to an increase of seismic noise amplitude of magmatic rather than intrinsic hydrothermal origin. [27] The spatial distribution of noise (Figure 8) has its maxima along the SE and NE borders of the volcano, especially where the most active fumaroles are located. Spatial noise distribution in the crater is correlated with CO 2 degassing (Figure 1). Seismic noise decreases in the mud pools area, where degassing is less energetic and it is 9of17

10 Figure 8. Spatial distribution of the amplitude of seismic noise (*10 4 cm/s; 10- to 15-Hz frequency window) at Solfatara crater. Symbols: (1), mesh of measuring positions; (2) caldera borders; (3) regional faults; (4), proposed E-E fault; (5), location of the anomalies discovered by the geophysical surveys. Notice that Bouguer anomaly 3 coincides with resistivity anomaly C. Label 3 was omitted. close to background levels in the NW sector of Solfatara. There is also a correlation between high noise amplitude and the anomalous areas imaged by the two-dimensional geophysical profiles (Figure 8) Gravity Data [28] The Bouguer anomaly data (Figure 9a) acquired by Oliveri del Castillo et al. [1968] display a gravity minimum sited just south of the boiling mud pools (point 2 in Figure 9a) and other two minima on the western and eastern edges of the crater rim (points 1 and 3, respectively). By contrast, a positive Bouguer anomaly occurs on the northern rim of the crater (point 4 in Figure 9a). Bouguer anomalies 2, 3, and 4 of these data correlate with the maxima of the present seismic noise distribution in the crater. Bouguer anomaly 1, however, does not correlate with either CO 2 discharge or with anomalous seismic noise zones. A possible interpretation is that the uplifts of and have modified, in part, the shallow degassing system, deactivating some of the pre-1969 paths, such as the area of Bouguer anomaly 1. [29] The horizontal derivative map of Bouguer anomalies (Figure 9b) presents its maxima clearly aligned along NW and ENE directions, coincident with the strike of regional tectonic lineaments bounding and intersecting Solfatara crater. These faults, probably filled by hydrothermal alteration minerals and/or by gas and liquid phases, form a measurable gravity anomaly. There is no correlation between the hypothesized NW-SE striking fault (imaged by two-dimensional methods and labeled as E on Figure 9b) and the gravity data Hydrogeological Data [30] Water table levels exhibit a difference of m between Solfatara and the surrounding areas (Figure 10). In the crater, the water table outcrops at the boiling mud pools (97 m above sea level), and it is found at 90 m above sea level in the OAK well (Figure 1). Water level drops at m above sea level just outside the western boundary of the caldera and at m above sea level less than 300 m away from the caldera rims. Precipitation affecting Solfatara produces an average amount of 730 t/day of water [Ricciardi and Siniscalchi, 2006] which is largely insufficient to explain the upwelling of the saturated zone. [31] The upwelling water table below Solfatara correlates with a peak of about 98 C in groundwater temperature (Figures 10b and 10c). Water temperatures inside and outside Solfatara differ by about 60 C. Two other temperature peaks of 65 C and 85 C, also accompanied by rising water tables, are found to the south and northwest of Solfatara (Gerolomini and Agnano: Figure 10c) where fumaroles and high CO 2 degassing have been described [i.e., Chiodini et al., 2001]. Temperature measurements made at OAK well during the period [De Luca, 1880] and today show that (1) the water temperature has increased from 57 C to98 C, implying an uprising of the isotherms below Solfatara crater; and (2) the saturated zone has risen, on average, from 12 m to the present-day 7 m below ground 10 of 17

11 Figure 9. (a) High-resolution Bouguer anomaly map of Solfatara crater, acquired by Oliveri del Castillo et al. [1968], draped over the local topography. (b) Horizontal derivative of the Bouguer anomaly data. Symbols: see Figure 8. surface, suggesting an increment of hydrothermal fluid condensation and therefore a marked increment of fluid degassing. 5. Discussion 5.1. Combined Interpretation of Geophysical Data [32] At Solfatara, geophysical data reveal that the very shallow subsoil beneath the crater is made up of two zones: a discontinuous and more resistive layer (A) with a Vp/Vs value of 2 and thicknesses between 0 and 70 m, overlying a very low-resistivity layer (B) that in its shallow part has a Vp/Vs value >4. Zone A corresponds to a dry argillitic alteration zone affected by CO 2 degassing. The lowresistivity zone B corresponds to the hydrothermal aquifer recharged by natural condensates [Todesco et al., 2003]. The magnetotelluric data (given in Figure 2) show that zone B extends down to at least m below ground surface. Resistivity and seismic velocities (Figures 2 6) are also consistent with zones A and B having been affected by lateral heterogeneities that can be explained in terms of the interaction between structural patterns and degassing dynamics within shallow hydrothermal circulation cells. Faults and fractures in the crater most likely act as preferential degassing pathways, as shown by the NW elongated shape of the CO 2 degassing structure in Figure 1. The pattern of CO 2 flux, soil temperature, and seismic noise can help to discriminate between active and inactive paths. Active degassing areas are characterized by high seismic noise, high CO 2 fluxes, and high soil temperatures. However, comparison between the different maps is difficult to assess by simple visual evaluation. Therefore in order to explore 11 of 17

12 Figure 10. (a) Three-dimensional topographic map of Solfatara area. (b) Three-dimensional map of the water table level. (c) Color map of the water table temperature (in C). Dots show the measuring points. Two direct measurements of the water table level and temperature were made within Solfatara crater in the OAK well (Figure 1c) and at the boiling mud pools, where the saturated zone outcrops at surface. The trend of the water table below the crater was reconstructed using resistivity information. any correlation between the different parameters, we have stacked them in the nondimensional space of the z-score [DeLaughter et al., 2005]. The z-score associated with the ith observation of a random variable x i is given by: z i ¼ ðx i mþ=s where m is the mean and s is the standard deviation of all observations x 1...x n. A stack of noncorrelating parts of the maps would produce a diminishing value of the modulus. By contrast, stacked correlated zones would produce an increase of the modulus. It is assumed that there is an in-phase correlation among the parameters; that is, high values of CO 2 flux correspond to high value of Bouguer anomalies etc. [33] In order to be able to compare with previous maps, we used the distribution of CO 2 flux and temperatures measured at surface of Solfatara during AD 2000 [Chiodini et al., 2001, 2005]. The maps scaled in the z-score space (Figure 11) illustrate, as might be expected, that temperature and CO 2 flux patterns are very similar, with a correlation coefficient of The stacking of selected maps (Figure 12) enables us to see that high noise levels are positively correlated with CO 2 degassing and soil temperature along the NE and SE rims, where the most vigorous fumaroles are located. These areas are also characterized by anomalous gravity values on the AD 1968 Bouguer anomaly map, confirming that some of these zones (i.e., anomalies 3 and 4 ) were already active before the inflation episodes of and In Figure 13, the fault patterns affecting Solfatara have been superimposed on the stack of three maps: seismic noise, Bouguer anomaly, and CO 2 flux. From this figure, we can see that degassing is concentrated mostly at the intersection of faults and frac- 12 of 17

13 Figure 11. CO 2 flux, Bouguer anomaly, soil temperature, seismic noise, and horizontal derivative of Bouguer anomaly, all rescaled using the z-score technique [DeLaughter et al., 2005]. Labels 1 to 4 and C, D, E, and F show the position of the anomalies discovered by the geophysical surveys. Notice that Bouguer anomaly 3 coincides with resistivity anomaly C. Label 3 was omitted. 13 of 17

14 Figure 12. Stacked maps obtained using two different sets of data (see labels). For map location, see the upper left chart of Figure 11. tures, and that Solfatara crater can be divided into an inactive sector, located in the NW, and a SE sector that is host to degassing activity. It is worth noting that seismic events accompanying the recent inflation episodes (between 1969 and 2000) show a concentration of seismic activity in both number of events and maximum magnitude in the SW sector of Solfatara [e.g., Orsi et al., 1999, among others]. In Figure 13, the areas of maximum correlation between seismic noise, gravity, and CO 2 are oriented NW and ENE and match with the anomalous zones imaged by our surveys. The highest z-score values are found above zones C, D, and E, and Bouguer anomalies 3 and 4. In particular, the fault E-E (inferred on the basis of magnetotelluric and DC electrical surveys) seems partially inactivated, considering it is affected by hydrothermal circulation only in its SW end where it intersects ENE striking faults Qualitative Modeling of Geothermal Processes [34] Hydrothermal fluids of magmatic origin, as implied by their temperature and the isotopic composition of H 2 O, CO 2, and He [Tedesco et al., 1990; Allard et al., 1991; Panichi and Volpi, 1999], are continuously being discharged at Solfatara crater. A cartoon of the shallow hydrothermal circulation hypothesized for Solfatara volcano is presented in Figure 14. Geothermal fluids, present below the crater floor predominantly in the gas phase, ascend along preexisting structural pathways. Part of the gas phase, including a fraction of steam, feeds the vigorous fumaroles (SOF), located at the intersection of the main faults with the local topography. Other parts of the gas phase are dispersed to air along rock microfractures as diffuse soil degassing. Above the 200 C isotherm, which can be roughly placed at around 300- to 400-m depth on the basis of the magnetotelluric data, the ascending gas is accompanied by large volumes of condensation which contribute to saturate the host rock. Chiodini et al. [2001] evaluated this water supply to be at least 3350 t/day. Condensation of hydrothermal fluids feeds the aquifer below Solfatara crater and likely generates a high hydraulic gradient at surface, causing water to flow from Solfatara toward neighboring areas depending on the permeability properties of the host rock. Considering that there is a 40-m-high water section of exchange between Solfatara and the surrounding shallow aquifer (see Figure 14), which has a cylindrical symmetry, a minimum permeability of 0.05 md is required to allow 3350 tons of water to flow outside the crater each day. 14 of 17

15 Figure 13. Map obtained by stacking the CO 2 flux, seismic noise, and Bouguer anomaly maps of Figure 11, then draped on top of the local topography and tectonic structures at Solfatara. Symbols: see Figure 8. Measured permeability of Campi Flegrei rocks is, on average, higher than this value [Vanorio et al., 2002]. Therefore we believe that the outcropping of the water table below the crater and the resulting high hydraulic table gradient at the edges of Solfatara volcano is due to a reduction of permeability caused by bounding faults filled by hydrothermal alteration minerals. An alternative explanation is that the amount of condensation water feeding the Figure 14. Hydrothermal model of Solfatara caldera along a NW-striking cross section. The top of saturated zone, including its shape, the extent of the gas-saturated zone, and the position of isotherms are hypothetical and have been located according to the findings of our surveys. Isotherms and liquid surfaces are about 200 m deeper than those reported by Chiodini et al. [2003]. The faults shown are also hypothetical and are drawn only for discussion purposes. 15 of 17

16 basal aquifer is larger than the value of 3350 t/day estimated by Chiodini et al. [2003]. 6. Conclusions [35] Integration of geophysical and hydrogeological data provides an opportunity to better understand the main geological features and the shallow hydrothermal activity that occur in the subsurface (0 400 m) of Solfatara. This knowledge is important because hydrothermal fluids can induce or enhance inflation episodes [Oliveri del Castillo and Montagna, 1984; Bonafede, 1991; De Natale et al., 1991; Gaeta et al., 1998]. Geophysical surveys show the presence of several anomalous zones that correlate with the intersection of the main structural lineaments and which act as preferential shallow degassing pathways. High seismic noise levels match with these paths. Furthermore, condensation of hydrothermal fluids causes an upwelling of the saturated zone by about 80 m below Solfatara, when compared to neighboring areas. This upwelling has further increased after Our field observations demonstrate that today, water levels and temperatures have not yet returned to pre-1968 conditions. Comparison between gravity data, recorded before 1969, and our geophysical surveys suggests that the recent inflation episodes at Campi Flegrei have likely modified the shallow hydrothermal system. Future changes in the heat flux from a magma body located underneath Solfatara and/or processes resulting in the formation of argillitic mineral assemblages at relatively shallow depths may influence pressurization events in the hydrothermal system. These will affect measurable physical and chemical changes in the shallow hydrothermal system, together with ground deformation and seismic swarms. Thus time series studies of geophysical and hydrogeological surveys at Solfatara, focused on monitoring changes in the shallow hydrothermal circulation, can be useful aids in evaluating and assessing volcanic hazard at Campi Flegrei. In particular, our data indicate that the seismic noise amplitude, calculated in the 10- to 15-Hz band, positively correlates with fluid degassing activity and is also sensitive to flow variation of the main fumaroles due to overpressurization events. This is highlighted by seismic noise at SOF showing an increase of activity and variation of cycling at the very beginning of ground deformation during the uplift of We therefore suggest integrating the already monitored parameters with real-time monitoring of seismic noise at SOF to improve the early detection of volcanichydrothermal events at Campi Flegrei. [36] Acknowledgments. 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