Vol50,4,2007 513 ANNALS OF GEOPHYSICS, VOL. 50, N. 4, August 2007 Key words geochemical monitoring – seismicity – tectonic lines 1. Introduction The thermal springs of Porretta are located in the Northern Apennines over an area crossed by several tectonic lineaments (fig. 1). The thermal waters have been locally utilized for medical purposes since the Etruscan age. A large scientif- ic literature on the therapeutic characteristics of Porretta thermal waters has been written during the past five centuries and many observational data were recorded. In particular Capponi (1608) noted that «after five local seismic events water flow-rate increased up to four times more than usual and their temperature was too hot to touch with the hands». Post seismic thermal and flow rate anomalies were also detected on April 20, 1765, April 11, 1787, June 10, 1904, December 28, 1908, June 5, 1910 and July 24, 1911 togeth- er with anomalies in methane gas bubbling and in local methane dry gas emissions (Facci et al., 1995). Demetrio Lorenzini, a local chemist who utilized cold groundwaters and thermal waters for chemical preparations, observed water flow rate and temperature anomalies in concomitance with a local seismic event in 1872 (Lorenzini, 1898). Lorenzini monitored water level fluctua- tions in a drilled well close to the thermal springs in the period 1873-1882 to study possible seis- mic precursors in cooperation with M.S. De Rossi (De Rossi, 1875). He observed that in concomitance with local seismic events a water level decrease in a local well accompanied a flow rate increase of the thermal springs. A detailed statistical processing of the available data recorded by Lorenzini was carried out by Albarello et al. (1991) revealing The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring Nicola Ciancabilla (1), Manuela Ditta (2), Francesco Italiano (2) and Giovanni Martinelli (3) (1) Agenzia Regionale Prevenzione e Ambiente (ARPA) dell’Emilia Romagna, Sezione di Bologna, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Italy (3) Agenzia Regionale Prevenzione e Ambiente (ARPA) dell’Emilia Romagna, Sezione di Reggio Emilia, Italy Abstract The thermal springs of Porretta are located on a seismically active area of the Northern Apennines. In 19th Century a chemist identified anomalous behaviour of the thermal waters in concomitance with local seismic events. Recent studies assessed the geochemical features of the circulating fluids (e.g., waters carry a dissolved CH4-dominated gas phase with a radiogenic signature of the helium isotopic ratio) and observed anomalous hydrologic and geochemi- cal signals possibly related to crustal strain phenomena due to local seismic events. Long-term geochemical moni- toring was carried out from 2001 to 2006 with the aim of detecting the behaviour of the circulating fluids possibly coinciding with seismic activity. The collected data reveal a sensitivity of the thermal waters to the activity of the main fault crossing the village of Porretta and identify a «seismogenic» structure crossing the village. Mailing address: Dr. Manuela Ditta, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Via Ugo La Malfa 153, Palermo, Italy; e-mail: m.ditta@pa.ingv.it 514 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli Fig. 1. Location of the investigated area on the Northern Apennines and the recognized tectonic structures (af- ter biblio). The earthquakes that hit the area from 1980 to 2002 (Castello et al., 2005) and the recent 2003-2005 events are also plotted. It is easy to observe an high density of events on the tectonic line crossing Porretta Terme. 515 The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring possible precursory beaviours of water level fluctuations. After a local Mw = 5.3 seismic event in 2003 (Seismic Bulletin, 2007) whose epicen- tre was located close to the village of Monghi- doro about 25 km to the East of Porretta Terme, significant anomalies in temperature, flow rate and bubbling gas emissions were recorded both by visual observations and direct measurements. To better understand possible relations among local seismicity and hydrogeochemical patterns of Porretta thermal springs a geochemical prospection and a continuous monitoring pro- gram were carried out. This paper deals with the main results obtained from the collected geo- chemical data focussing on the relationship be- tween fluids and tectonic structures. 2. Study sites and geological background The thermal manifestations located in the Porretta area are caracterized by spontaneous spillage of about ten springs inside the town (fig. 1). Among those, five springs named Bove, Marte, Donzelle, Leone and Sale, are located to the southwest where the old hotel of the Thermal baths is located (fig. 2). The water temperature ranges from 34°C to 37°C; they are salty waters enriched in bromium and iodine ions and charac- terized by a methane-dominated dissolved gas phase. The other springs named Puzzola 1, Puzzola 2, Porretta Nuova, Porretta Vecchia and Majoc- chi, are spread in proximity of the Reno River 1 km from the thermal baths. They are also salso- bromoiodine waters with bubbling and dissolved CH4-dominated gases, with lower temperature (20-26°C) and salinity compared to the above mentioned springs (Ciancabilla et al., 2004). Water and gas samples were collected at Por- retta Nuova, Puzzola 1, Bove and Sale springs to investigate the Porretta thermal system; the se- lected springs are considered the most represen- tative of the whole thermal system. Fig. 2. Sampling sites at Porretta thermal area. The thermal waters are close to the old and new thermal plants respectively located on the Rio Maggiore and Reno rivers. 516 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli 2.1. Geology and tectonic setting The Northern Apennines are a NE-verging thrust and fold belt characterized by a crustal extension of about 2.5 mm/yr and by moderate and large normal faulting seismic events (Hun- stad et al., 2003; Chiaraluce et al., 2004). The extension along NW-trending faults is balanced by compressive trends recognized in the nearby Po Valley area (Montone et al., 2004). Tectonic processes are accomodated by complex mecha- nisms on pre-existing structures. While the ex- tension along the Apenninic belt is related to the Adriatic lithosphere retreat, compressive trends are probably related to subordinate processes (Piccinini et al., 2006). No signifi- cant historical earthquakes are reported in the Porretta Terme area but evidence for wide- spread low-slip Quaternary faulting along the entire length of the Reno River were recently obtained by detailed strata terraces analysis (Bierma et al., 2005). Local seismicity is char- acterized by relatively frequent low magnitude events consistent with a low-slip Quaternary faulting. The village of Porretta Terme is located at the confluence of the Rio Maggiore and the Reno rivers, over a land that is mostly made up of a for- mation of so-called scaly clays (Palombini shales), close to a large formation of marly-are- naceus and calcarenitic turbidite. The geologic setting in the high Reno valley area, where the thermal sources of Porretta are located, is charac- terized by two main rock formations: the Tuscan units, characterized by litic arenaceus silt forma- tions (Porretta, Macigno and Cervarola forma- tions) and the Ligurian units, made by clays and strongly fossilized shales with rocky mass char- acterized by different nature and size. The geo- logic structure of the over said units is to be tied up to the contraction (middle Cretaceous) and to the closing (middle Eocene) of the Piemonte-Lig- urian ocean (located between the African and the European plate). The compressive stress pushed and crossed over (Oligo-Miocene) the Creta- ceous Eocene formations (Ligurian facies) on the Triassic-Oligocene formations (Tuscan facies), then everything was pushed as a wave through East shaping folds and overlaps (Duchi et al., 2005). From a geologic point of view the Porret- Fig. 3. Schematic section showing the recharge area/thermal springs relative positions and the main structures governing the groundwater circulation through the different lithotypes. 517 The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring ta area consists of an anticlinal fold with the nu- cleus made by Oligo-Miocene «Macigno» sedi- ment. The northern side of the fold is vertical and the southern side goes down toward SW through a series of fold and faults with anti Appennine di- rection and other one with sub-vertical immer- sion and orthogonal to the first (Bencini and Duchi, 1980). The turbidite formations (Tuscan Units) represent the recharge basin for the inves- tigated springs (Cianciabilla et al., 2004 and ref- erences therein). In particular the water discharge in Porretta baths crosses the Porretta Formation (a sand-silt grain stone entirely similar to the rock in feeding area) and rises to the surface through a deep regional tectonic dislocation (fig. 3). The Porretta Formation acts as a natural draining wedge in a low permeability setting (sandstone unities). It connects the warm overpressure wa- ters accumulated in depth to the external environ- ment with its stratification joints and fractures. 3. Sampling and analytical procedures Samples of thermal waters, dissolved and bubbling gases were collected to be analyzed for their chemical and isotopic composition. Water temperature, pH value and electrical conductivity were measured directly in the field. The water samples were analyzed for major and minor ele- ments by ion chromatography (Dio-nex DX-120 equipped by Dionex CS12A and AS14A columns, reproducibility within ±2%). The HCO3 amount was measured by volumetric titration with 0.1 N HCl. Mass spectrometric analyses were carried out by a Finnigan DeltaPlus XP and by an Analitical Precision 2003 mass spectrome- ters respectively for D/H and for the δ18O isotopic ratios. D/H isotopic analyses of water were per- formed using the Kendall and Coplen (1995) technique (reaction with Zn at 450°C), while 18O/16O analysis were carried out by the CO2-wa- Table II. Analytical results for dissolved and bubbling gases. Dissolved gas data in cc/STP per litre; bubbling gas data in vol% for O2, N2, CH4 and CO2, in ppm for He and H2. Dissolved gas Date He H2 O2 N2 CO CH4 CO2 R/Ra He/Ne sample Puzzola1 02/03/2005 < < 0.05 1.7 8.68E−05 3.52E+01 10.7 n.a. n.a. Bove 02/03/2005 < < 0.07 1.2 1.53E−04 4.94E+01 29.6 n.a. n.a. Porretta Nuova 02/03/2005 < 2.0E−03 0.05 2.1 3.74E−05 3.15E+01 6.2 n.a. n.a. Sale 02/03/2005 < < 0.04 2.4 7.49E−05 2.80E+01 22.9 n.a. n.a. Bubbling gas Date He H2 O2 N2 CO CH4 CO2 R/Ra He/Ne sample Porretta Nuova 02/03/2005 25 < < 11.1 < 88.0 0.9 0.04 8.28 Sale 02/03/2005 < 605 < 1.2 < 94.5 4.2 n.a. n.a. Table I. Chemical composition of the sampled thermal waters. Concentrations in meq/l; TDS in mg/l. EC= = electrical conductivity in mS/cm at 20°C. The isotopic composition of deuterium and oxygen are reported in δ unit per mil versus V-SMOW standard. Sample Date T°C pH EC Li Na K Mg Ca F Cl Br SO4 HCO3 TDS d18O dD Puzzola 102/03/05 21.1 7.54 3,85 0.224 36.66 0.89 0.56 1.68 0.15 29.34 0.012 0.54 10.9 1987 -7.33 -50 Porretta 02/03/05 29.1 7.69 2,66 0.150 26.78 0.61 0.28 0.37 0.16 17.88 0.008 0.48 10.5 1310 -6.88 -52 Nuova Sale 02/03/05 35.3 7.25 8,81 0.499 79.67 1.91 0.73 1.23 0.23 70.32 0.000 0.11 16.9 4442 -6.80 -51 Bove 02/03/05 35.5 7.28 8,65 0.480 84.73 1.95 0.89 1.70 0.21 70.04 0.030 0.11 17.0 4561 -6.88 -52 518 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli ter equilibration technique (Epstein and Mayeda, 1953). The results are reported in ‰ per mil units versus the V-SMOW standard. The standard devi- ations of measurements are approximately ±1% for D/H and ±0.2% for the 18O/16O. The chemical composition of the dissolved and bubbling gases was determined by gas- chromatography by a Perkin Elmer Autosystem XL gas chromatograph equipped with a 4m Carboxen 1000 column, using Ar carrier and a double detector (HWD and FID Detectors). The detection limits are 2 ppmv for He and H2, 1 pp- mv for CH4 and CO2 and 500 ppm vol. for N2 and O2. The analytical error is ±5% for He, and ±3% for the other gaseous species. The dis- solved gases were extracted following the method proposed by Capasso and Inguaggiato (1998). Helium was purified in an ultra high vacuum line for helium isotope analyses fol- lowing standard procedures (Italiano et al., 2004). The He to Ne ratio was measured by an in-line quadrupole mass spectrometer (QMS, VG Quartz). The isotopic analyses of the puri- fied helium fraction were performed by a mod- ified static vacuum mass spectrometer (VG5400TFT, VG Isotopes) that allows for the concomitant detection of 3He and 4He-ion beams, reducing the 3He/4He measurement er- ror to very low values. Typical uncertainties in the range of low-3He (radiogenic) samples are below ±5%. The analytical results are listed in tables I-II. 3.1. Continuous monitoring Following tales and visual observations re- ported by local inhabitants, we found out that the thermal springs of Porretta Terme have under- gone changes coinciding with seismic activity. Variations in water level, gas bubbling, turbidity etc. are the most common reported features. To better understand the possible effects of seismic- ity on the Porretta Terme thermal waters, a mul- tiparameter device (Green-span CTD 350 Sen- sor) was installed at the Puzzola1 spring (in 2001, in 2004 and in 2006 respectively) and one more probe was installed in the Bove thermal spring (T≅35°C) to monitor warmer water probably of deeper origin (from July 2004 to March 2005). The probes provided conductivi- ty, temperature and pressure data with an accu- racy of ± 0.2% FS, ± 0.1°C, 0,02% FS, respec- tively (FS = full scale). The aim of the continu- ous monitoring activity was also to detect the possible man-induced noise to discriminate nat- ural-induced signals from artifacts. 4. Results and discussion 4.1. Chemical composition of the thermal waters All the sampled waters show a Cl-alkaly and a Na-dominated chemical signature, how- ever a remarkable difference in salinity, 1300 and 4600mg/l and conductivity, 2600 µS and 8600 µS, marked the two groups of thermal springs from the new and old thermal plants re- spectively (fig. 2). The triangular diagrams in fig. 4 display the alkali enrichment of the water springs feeding the Porretta Terme system. The lower Cl and Na contents of Porretta nuova and Puzzola 1 samples (fig. 5) are interpreted as a dilution process by parent waters of which the Sale and Bove springs represent the high salin- ity components. It may be the result of a mixing process between shallow meteoric waters and groundwaters which rise up through the main lithological contact or through tectonic align- ments. The δ18O values reveal a meteoric origin of thermal springs (Minissale et al., 2000), in- filtrating through local arenaceous rocks of «Macigno» and «Porretta» formations and tem- peratures consistent with a source of the main aquifer at a depth of about 1 km. Figure 5 shows the clear Na and Cl enrich- ment that induces Na/Cl ratios well above the stoichiometric ratio of sea-water. A groundwa- ters halite interaction process cannot explain a Na abundance as high as 1950 mg/l. Since the springs are also enriched in other alkaline ele- ments (fig. 6), an interaction with clay layers where large cationic Na-Mg exchange occurs can be considered (Shoeller, 1955). According to Nanni and Zuppi (1986), salty waters are the ex- pression of a mixing process between waters of meteoric origin and trapped brines inside the Neogenic marine sediments underlyng the lig- 519 The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring urean allochthonous flysh. The brines would be squeezed and pushed toward the surface through the overlapping surfaces of the Ligurian units. CH4 gas phase, always present in the springs, further facilitates the brines to rise and favour ionic exchange processes between clays and salty waters. As a result (fig. 7) the waters catch Na and release Mg and subordinately Ca. Although Bencini and Duchi (1980) found an interaction of the Porretta springs with soluble evaporitic deposits (see Ca/SO42- ratio in fig. 8), the Puzzola1, Sale and Bove springs show a large increase in Ca concentration not balanced by the SO42- ions, which do not support the interpretation of Bencini and Duchi (1980). Furthermore, the waters are undersaturated with respect to the min- erals anhydrite, gypsum, and halite (table III), confirming that the waters do not emerge from evaporitic rocks. The Dolomite saturation index approaches zero showing the occurrence of equi- Fig. 4a,b. Ternary plot a) Ca-Mg-Alkali and b) HCO3-Cl-SO4. The triangular diagrams display the composition of four main springs of Porretta Terme plant. All the samples are enriched in alkali element with high Cl content. Fig. 5. meq/l Na versus meq/l Cl diagram. The figure distinguishes two water groups: Puzzola 1 and Porretta Nuova, marked by lower salinity and Bove and Sale springs characterized by higher concentrations. The Porret- ta Nuova and Puzzola1 samples represent the more diluted terms of a hypothetical mixing between a meteoric and a groundwater terms (dashed line) in the Porretta thermal system. All the samples are Na-enriched with re- spect to the stochiometric Na/Cl ratio and the Na/Cl ratio of seawater (black and dash-dot line respectively). Fig. 6. meq/l K versus meq/l Cl and meq/l Li versus meq/l Cl concentration at the Porretta terme springs show- ing the enrichment in alkaline elements with respect to sea water. 5 6 Fig. 9. N2-CO2-CH4 diagram showing a CH4-domi- nated gas phase for both dissolved and bubbling gases. 520 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli librium with dolomitic rocks for water circulating at relatively shallow levels. During their uprising towards the surface, they mix with waters of deep- er origin (brines) hosted in Pliocene sediments thus reaching their final Na-Cl connotation. 4.2. Chemical and isotopic features of dissolved and venting gas The chemical composition of the gas phase of both bubbling and dissolved gases is dis- played in table II. The dissolved gases exhibit a CH4 amount up to five orders of magnitude higher than that of air saturated water. The N2- CO2-CH4 diagram (fig. 9) displays the chemical composition of the bubbling gases collected at Porretta Nuova and Sale springs, besides the chemical composition of the dissolved gases Fig. 8. Ca versus SO4 contents for the collected water samples showing a clear Ca enrichment.Only Porretta Nuova falls on the stoichiometric Ca/SO4 ratio line. Table III. Saturation indexes (SI) calculated for the main mineralogical phases in equilibrium with the thermal waters. It is evident how aragonite and cal- cite are close to the saturation showing the attain- ment of the equilibrium for the less saline waters (Puzzola 1 and Porretta Nuova, new thermal plant), while more concentrated waters (Sale and Bove, old thermal plant) are equilibrated with calcite and dolomite. SI calculated by the WATEQP software (Appelo, 1988) and expressed as the logarithm of the ion activity ratio pertinent to a mineralogical phase, and the equilibrium constant of its solubility product. Phase Puzzola 1 Porretta Nuova Sale Bove Anhydrite −3 −3.6 −3.98 −3.86 Aragonite 0.03 −0.34 −0.13 0.04 Calcite 0.17 −0.2 0.01 0.18 Dolomite −0.04 −0.34 0.04 0.32 Fluorite −0.6 −1.24 −0.7 −0.64 Gypsum −2.77 −3.4 −3.82 −3.69 Halite −4.7 −5.05 −4.07 −4.05 Fig. 7. meq/l Na versus meq/l Mg diagram. The values for the sampled thermal springs show a high Na content suggesting an interaction with Na-rich clay minerals. 521 The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring Fig. 10. Temporal variations of conductivity and water level at the Puzzola spring recorded during November 2001-January 2002.The vertical black line marks the Md = 3.1 seismic event that occurred on December 6, 2001. Fig. 11. Temporal variations in water temperature (21-23°C) and water level (0.5-3 m) in the Puzzola spring from August 2003 to February 2005. The earthquakes that occurred within a 20 km-radius area centered on Por- retta Terme town are shown as vertical grey lines. The rainfalls are reported as daily mean value for the entire monitoring period (0-16 cm). The rain-gauge is located close to the Porretta thermal center. 522 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli collected at all the sampling sites. The graph exhibits their very similar composition. The He concentration measured in the free gas collect- ed at Porretta nuova spring is distinctly higher than the atmosphere. On the other hand the high He/Ne ratio highlights a negligible atmospheric contribution. The isotopic He composition as low as 0.04 R/Ra, reveals a clear radiogenic sig- nature. This information rules out any release of mantle-derived components over the study area. According to carbon and hydrogen isotopic da- ta of gases collected over the Emiliano Appen- nine (Duchi et al., 2005), the CH4 gas phase would represent the extreme stadium of the thermal degradation of organic substance trapped in the sediments. 4.3. Long-term automatic monitoring During the 2001-2004 observation period many variations in the water level and conductiv- ity were recorded (figs. 10-11). The data were an- alyzed besides the shocks occurred over a 20 km- radius area (table IV). A slight anomaly in con- ductivity and water level at the «Puzzola 1» 06/12/2001 44.091 11.076 3.1 08/06/2002 44.274 10.618 4 09/06/2002 44.386 10.692 2.5 11/06/2002 44.395 10.693 2.6 13/06/2002 44.391 10.692 2.7 18/06/2002 44.396 10.683 4.2 05/07/2002 44.396 10.702 3.1 08/07/2002 44.386 10.711 2.7 09/07/2002 44.397 10.696 3.3 13/08/2002 44.396 10.689 2.5 06/09/2002 44.063 11.286 2.5 11/10/2002 44.397 10.688 2.7 18/11/2002 44.221 10.778 2.5 10/02/2003 44.137 10.812 2.5 30/03/2003 44.256 10.793 2.5 06/04/2003 44.291 10.981 2.7 06/04/2003 44.266 10.987 3 07/04/2003 44.306 10.991 2.7 11/04/2003 44.264 10.982 2.6 11/04/2003 44.268 10.970 2.7 21/04/2003 44.324 10.701 2.5 24/04/2003 44.268 10.965 2.7 01/06/2003 44.344 10.708 2.6 11/07/2003 44.376 10.606 2.7 04/08/2003 43.978 10.991 2.5 18/08/2003 44.362 10.749 2.9 18/08/2003 44.364 10.717 2.5 18/08/2003 44.379 10.723 2.8 15/09/2003 43.936 11.299 2.5 Table IV. Md ≥ 1.8 seismic events within a 20 km-radius area centered on Porretta Terme town during the 2001- 2006 observation period. Date Latitude Longitude Magnitude Date Latitude Longitude Magnitude 06/11/2003 44.389 10.628 2.9 11/12/2003 44.326 11.055 3.2 12/12/2003 44.288 11.019 2.8 13/12/2003 44.297 11.038 2.8 04/01/2004 44.273 10.946 2.9 05/01/2004 44.169 10.783 2.5 23/02/2004 44.396 10.789 2.5 16/03/2004 44.248 10.959 2.6 04/05/2004 44.295 10.957 2.6 10/06/2004 44.021 11.002 2.1 22/07/2004 44.258 11.076 1.9 23/07/2004 44.155 10.820 2.6 30/07/2004 44.221 10.620 2.6 22/08/2004 44.349 10.738 2.5 10/09/2004 44.277 10.738 2.5 09/10/2004 44.352 10.828 2.7 26/12/2004 44.046 11.109 2.6 11/01/2005 44.092 10.823 1.8 01/03/2005 44.272 11.080 2.2 29/01/2006 44.324 11.409 2.0 02/02/2006 44.239 11.470 2.0 05/02/2006 44.301 11.491 2 03/04/2006 44.155 11.393 2 05/06/2006 44.259 11.275 2 17/08/2006 44.282 11.550 2 21/06/2006 44.319 11.451 2 14/06/2006 44.272 11.449 2 21/11/2006 44.200 11.580 2.0 523 The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring spring followed the seismic shock occurred on December 6th, 2001 lasting also as post-seismic effect (fig. 10), when the water level increased by a factor of two. After two months, in January 2002, the continuous monitoring stopped, howev- er the physico-chemical parameters were still above the pre-seismic values. The trend shown by the electric conductivity might highlight that higher values than those previously measured lasted for a long time span after the seismic shock as a consequence of a slow deformation process as already observed along the Apennines Chain (Italiano et al., 2004). The increases in the water level observed in 2003 at the «Puzzola 1» site, (when the continuous monitoring had started again) were accompanied by sharp temperature drops probably due to changes in the mixing pro- portions between a shallow cold water compo- nent and a deeper thermal one. Figure 11 displays that the above-mentioned modifications have oc- curred coinciding to other natural phenomena: seismicity and rainfalls. Both of them are well Fig. 12. Temporal variations in conductivity at the Bove spring during June 2004-February 2005 observative interval. The earthquakes occurred within a 20km-radius area centered on Porretta Terme town are shown as ver- tical black lines. Only the July 2004 shocks caused temporary modifications in the electrical conductivity. The recorded spikes are a consequence of the «anthropic disturbances» due to chlorine addition to the water. 524 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli known to be able to induce modifications on the groundwaters, however, the absence of carstic systems in the area does not allow fast water cir- culation that, on the other hand, can be enhanced by the opening of microfractures due to seismic shocks. As a matter of fact, the collected data make it hard to discriminate which, between seis- micity and rainfall, is responsible for the ob- served modifications. A possible model can be as follows: a long-term deformation process is ac- tive along that sector of the Apennines Chain; sometimes ruptures occur generating shocks of generally low magnitude; the intrusion of rain water along the fault lines acts as a lubricant and allows small movements of the fault planes under stress accumulation. The concomitant occurrence of low-energy seimic events, rainfalls and modi- fications to the mixing proportions of different water bodies is the consequence of a complex. These observations stress the role of fault movement on the mixing ratios changes at depth that can modify, for example, the electrical con- ductivity without changes in the water tempera- ture. Coinciding with the shocks in June and Ju- ly 2004 respectively, we observed a slight water level increase not associated with any tempera- ture variation. The same shocks induced modifi- cations at the Bove site (fig. 12) where the fol- lowing macroscopic observations fit with the recorded data: i) an anomalous gas bubbling ac- tivity was observed inside the Bove spring from the beginning of June 2004; ii) the electrical conductivity probe recorded a variation that in- creased the background value from less than 8300 to about 9000 µS. Such a variation, which lasted more than a month, started about one month before the seismic sequence of July 22- 30th, when the electrical conductivity reached its maximum value. Similar behavior was never observed even when earthquakes with equiva- lent magnitudes occurred around Porretta. The modifications recorded during the 2006 moni- toring period highlighted how deformation can be considered responsible for geochemical modifications in the absence of seismic activity. Although the variation in the water level (from 60 to more than 80 cm) started together with the low-energy shock which occurred on 21st No- vember 2006 (fig. 13), it is hard to accept that a low-energy shock which occurred 20 km away from the observation site induced similar modi- fications to the groundwater circulation. Howev- er, following the above mentioned model, the deformative effect may be responsible for the observed phenomena as the low energy shock was a rupture episode that occurred at a distance but was generated by the same Apennines defor- mative activity. 5. Conclusions The thermal springs of Porretta represent an interesting study site to better understand the relation between fluids and seismogenic struc- tures. In this sense the results we obtained are of general interest even though the study area is small and of local interest. The discharged thermal waters are fed by meteoric waters which reached saturation with carbonatic rocks, mixed to brines coming from Pliocene sediments. A methane-dominated gas phase is dissolved at all the thermal springs that sometimes become gas-oversaturated. Fig. 13. Temporal variations of the water level at the Bove spring during October-December 2006 observ- ative interval. One earthquake occurred within a 20 km-radius area, however because of the low energy is not considered as responsible for the observed modifications (see text). 525 The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring The temporal variations of the geochemical features of both gas and thermal waters were not investigated (e.g., by a monthly sampling) but long-term continuous monitoring investigated water temperature, electrical conductivity and water level even though with much datamissing (Greenspan probes from 2001 to 2006). The in- terpretation of the geochemical variations high- lights the importance of the fluid monitoring in a seismogenetic structure. Some of the recorded modifications occurred in concomitance with both earthquakes located on the tectonic linea- ment crossing the town of Porretta, and rainfalls. The model we propose interprets the modifica- tions of the water composition as a consequence of local stress accumulation, where the infiltrat- ing rain water allows local fault-planes under stress to slide generating low-magnitude earth- quakes. The obtained results are in full agree- ment with previous observations showing how geochemical modification to the circulating flu- ids can be induced by crustal deformation be- sides crustal ruptures (earthquakes). As a conse- quence, the seismogenic behaviour of a tectonic line can be recognized from the modifications occurring to the uprising fluids due to local stress accumulation. Acknowledgements The authors thank H. Woith and F. Yang whose criticisms and suggestions greatly im- proved the paper. The research work was sup- ported by the Grant IMONT-2002 «Monitorag- gio chimico-fisico e valutazione delle possibili correlazioni con eventi sismici in alcune sor- genti idrotermali dell’Appennino Tosco Emil- iano Romagnolo: approcci innovativi per la va- lutazione delle caratteristiche di permeabilità degli acquiferi» and by the project «Gestione idrogeologica e monitoraggio ambientale delle acque termominarali Porrettane». REFERENCES ALBARELLO, D., G. FERRARI, G. MARTINELLI and M. MUC- CIARELLI (1991): Well-level variation as a possible seis- mic precursor: a statistical assessment from Italian his- torical data, Tectonophysics, 193, 385-395. APPELO, C.A.J. (1988): WATEQP Program, Institut voor Aardwetenschappen, Vrije Universiteit, Amsterdam. BENCINI, A. and V. DUCHI (1980): Studio geochimico su al- cune acque di Porretta Terme Bologna (Italia), Atti Soc. Tosc. Sci. Nat., Mem., Serie A, 87, 365-374. BIERMA, R., M.C. EPPES and F.J. PAZZAGLIA (2005): Evi- dence for late quaternary faulting in the Reno river val- ley, Northern Apennines mountains, Italy, Geol. Soc. Am., Abstr. Programs, 37 (7), pp. 431. CAPASSO, G. and S. INGUAGGIATO (1998): A simple method for the determination of dissolved gas in natural waters. An application to thermal waters from Vulcano Island, Appl. Geochem., 13 (5), 631-642. CAPPONI, P. (1608): Libro della Medicina delle acque Por- rettane diviso in cinque trattati, Manuscript no. 1260, Biblioteca Comunale dell’Archiginnasio, Bologna. CASTELLO, B., G. SELVAGGI, C. CHIARABBA and A. AMATO (2005): CSI Catalogo della Sismicità Italiana 1981- 2002, versione 1.0. (INGV-CNT, Roma). CHIARALUCE, L., A. AMATO, M. COCCO, C. CHIARABBA, G. SELVAGGI, M. DI BONA, D. PICCINI, A DESCHAMPS, L. MARGHERITI, F. COURBOULEX and M. RIPPE (2004): Complex normal faulting in the Apennines thrust-and- fold belt: the 1997 seismic sequence in Central Italy, Bull. Seismol. Soc. Am., 94, 99-116. CIANCABILLA, N., G.C. BORGIA, R. BRUNI, F. CIANCABILLA, S. PALMIERi and L. VICARI (2004): Le sorgenti dell’Al- ta valle del Reno (Appennino Bolognese): nuovi ele- menti per approfondire la genesi dei movimenti gravi- tativi profondi nei terreni argillitici caoticizzati dell’ap- pennino Tosco-Emiliano, Il Geologo dell’Emilia-Ro- magna, 18, 5-14. DE ROSSI, M.S. (1875): Quadro generale statistico topo- grafico giornaliero dei terremoti avvenuti in Italia nel- l’anno meteorico 1874 col computo di alcuni altri fenomeni, Atti Acc. Pont. Nuovi Lincei, 28, 514-536. DUCHI, V., G. VENTURELLI, I. BOCCASAVIA, F. BONICOLINI, C. FERRARI and D. POLI (2005): Studio geochimico dei fluidi dell’Appennino Tosco-Emiliano-Romagnolo, Boll. Soc. Geol. It., 124, 475-491. EPSTEIN, S. and T. MAYEDA (1953): Variation of 18O content of water from natural sources, Geochim. Cosmochim. Acta, 4, 213-224. FACCI, M., A. GUIDANTI and R. ZAGNONI (1995): Le Terme di Porretta nella Storia e nella Medicina (Nuèter edi- tions), vol. 2, pp. 649. HUNSTAD, I., G. SELVAGGI, N. D’AGOSTINO, P. ENGLAND, P. CLARKE and M. PIEROZZI (2003): Geodetic strain in peninsular Italy between 1875 and 2001, Geophys. Res. Lett., 30 (1181), doi: 10.1029/ 2002GL016447. ITALIANO, F., G. MARTINELLI and A. RIZZO (2004): Seismo- genic-induced variations in the dissolved gases of the thermal waters of the Umbria region (Central Apen- nines, Italy) during and after the 1997-98 seismic swarm, Geochem. Geophys. Geosyst., 5 (11), Q11001, doi: 10.1029/2004GC000720. KENDALL, C. and T.B. COPLEN (1985): Multisample conver- sion of water to hydrogen by zinc for stable isotope de- termination, Anal. Chem., 57, 1437-1440. LORENZINI, D. (1898): Guida dei Bagni della Porretta e Dintorni, Bologna, pp. 167. MINISSALE, A., G. MAGRO, G. MARTINELLI, O. VASELLI and G.F. TASSI (2000): A fluid geochemical transect in the 526 Nicola Ciancabilla, Manuela Ditta, Francesco Italiano and Giovanni Martinelli Northern Apennines (central-northern Italy): fluid gen- esis and migration and tectonic implications, Tectono- physics, 319, 199-222. MONTONE, P., M.T. MARIUCCI, S. PONDRELLI and A. AMATO (2004): An improved stress map for Italy and surround- ing regions (Central Mediterranean), J. Geophys. Res., 109, B10410, doi: 10.1029/ 2003JB002703. NANNI, T. and G.M. ZUPPI (1986): Acque salate e circo- lazione profonda in relazione all’assetto strutturale del fronte adriatico e padano dell’Appennino, Mem. Soc. Geol. It., 35, 979-986. PICCININI, D., C. CHIARABBA, P. AUGLIERA and THE MONGHI- DORO EARTHQUAKE GROUP (2006): Compression along the Northern Apennines: evidence from the Mw 5.3 Monghidoro earthquake, Terra Nova, 18, 89-94. PIOMBO, A., G. MARTINELLI and M. DRAGONI (2005): Post- seismic fluid flow and Coulomb stress changes in a poroelastic medium, Geophys. J. Int., 162, 507-515. SCHOELLER, H. (1955a): Geochemie des Eaux Souterraines, Masson. SCHOELLER, H. (1955b): Le Eaux Souterraines, Masson. SEISMIC BULLETIN (2007): INGV-CNT seismic bulletin (a- vailable on line at: http://www.ingv.it/~roma/reti/rms/ bollettino/). Vol50,4,2007 27 Vol50,4,2007 28 Vol50,4,2007 29 Vol50,4,2007 30 Vol50,4,2007 31 Vol50,4,2007 32 Vol50,4,2007 33 Vol50,4,2007 34 Vol50,4,2007 35 Vol50,4,2007 36 Vol50,4,2007 37 Vol50,4,2007 38 Vol50,4,2007 39 Vol50,4,2007 40