- Email: [email protected]
Coseismic geochemical variations in some gas emissions of Umbria region (Central Italy)
Phys. Chem. Earth (A). Vol. 25. No. 3. no. 289-293, 2000 0 2000 Eiskvier Science Ltd All rights reserved 1464- 1895/00/$ - see front matter
Pergamon
PII: S 1464- 1895(00)00046-6
Coseismic Geochemical (Central Italy) J. Heinickel,
F. Italiano’,
Variations
V. Lapenna3,
in some Gas Emissions
G. Martinelli4
of Umbria
Region
and P. M. Nuccio5
‘Saechsiche Akademie der Wissenschanften zu Leipzig, Freiberg, Germany 21st. Geochimica dei Fluidi, CNR, Palermo, Italy 31st. Metodologie Avanzate di Analisi Ambientale de1 CNR, Potenza, Italy “Serv. Geologic0 e Cartografico, Regione Emilia-Romagna, Italy ‘Dip. CFTA, Universiti di Palermo, Italy Received
1 June
1999; accepted
29 July 1999
Abstract. Coseismic geochemical variations have been detected in some gaseous vents and natural springs during the last seismic crisis occurred in Umbria region (Central Apennines). that started on September 26”, 1997 with several moderate earthquakes (up to Ml 5.8). The results of chemical analyses performed on both gas and water samples taken at a weekly rate, combined with analyses on continuous gas flow rate nearby San Faustino site, suggest that the variations can be interpreted as possible consequence of the crustal permeability changes induced by earthquake shaking.
\. MARCHE
’ ._ _A,D,RlATIC SEA ;
0 2000 Elsevier Science Ltd. All rights reserved.
Introduction LATIUM
In recent years many research groups focused their attention to the analysis of anomalous patterns in geochemical parameters possibly related to earthquakes (e.g. Thomas, 1988; Tsungai and Wakita, 1995; Di Be110 et al., 1998). In this framework a geochemical survey based on continuous and spot measurements has been carried out in a seismic active area of Central Italy (Fig. 1). The Umbria Apennine is affected by frequent seismicity associated with an extensional strain field. In this area earthquakes generally occur in the sedimentary cover at shallow depth (Deschamps et al., 1984; Haessler et al., 1988). At least 22 (Ml>.5) local earthquakes occurred between 1279 and 1984 (Boschi et al., 1998). Focal mechanisms of mainshocks occurred in 1979, 1984 and 1997, also confirmed by further stress indicators (Montone et al., 1997), highlighted active extension processes in the crust characterized by NE-SW and E-W directions (Frepoli and Amato, 1997; Amato et al., 1997). Travertine rocks occur in the vicinity of CO> gas emissions of Umbria Apennine, evidencing deep fluid ascent toward surface (Barbier and Fanelli, 1976). COz. emissions are widely Correspondence to: F. Italian0 Istituto di Geochimica dei Fluidi - CNR, Palermo E.mail:
i
Fig.1 - Location of the sampling sites in Umbria Region (Central Appenines, Italy). The September-November epicentres location is also shown by the grey squares
distributed in Umbria region and can be related to the slightly anomalous heat flow pattern possibly responsible for thermometamorphic processes at depth (Cataldi et al., 1995; Chiodini et al., 19951, while intense local faulting (e.g. Cello et al.. 1997) can be considered responsible for CO: uprising. Crossing the data of seismic hazard evaluation, thermal water data-base and historical information on local deep fluid behaviour in concomitance with past seismic events (Martinelli and Albarello, 1997) allowed to .previously select the gas emissions located at Montecastello di Vibio and Massa Martana and a thermal spring located at Triponzo for a geochemical monitoring. The Montecastello di Vibio vent releases CO2 dominated gas at a high flow rate which appears not to be influenced by seasonal variations. The Massa Martana CO? dominated gas emission occurs close to the San Faustino
(Italy)
[email protected] 289
290
J. Heinicke
et al.: Coseismic
Geochemical
.,/
Variations
199.5). Furthermore we applied the run theory (Le Boutillier and Waylen, 1988) to identify anomalous patterns in geochemical time series. More details about the mathematical background of these methodologies are given elsewhere (e.g. Cuomo et al.. 1996; Di Bello et al. 1998: Di Belle et al., 1999). Finally, the relationships among results concerning the detected temporal variations in chemical composition at Montecastello di Vibio site (gas samples) and Triponzo site (water samples), the analysed flow rate time-series (Massa Martana-San Faustino site), and the seismic activity of the area, were evaluated all together. 3.
Fig. 2 - Data from continuous monitoring at S.Faustino site: (a) gas flow rate time series measured at San Faustino site during the period September October 1997; (b) environmental temperature values measured dunng the same period close fo the working station. The output signals measured in
millivolt are directly related to gas flow rate and temperature respectively. sparkling water exploited for spring source, a naturally bottling. Even in this site the flow rate is characterised by a
constancy in time and apparent no sensitivity to seasonal variations. At those sites vadose waters can occasionally show bubbling activity accompanied by foam generation. of Triponzo (sulphurous water at The thermal spring T=29”C) has been considered for a weekly sampling of the waters, that have shown significant changes discharged mainly of the water level and the flow rate during seismic crisis.
in some Gas Emissions
Results
and Discussion
3. I. Duily jlow rate variations We analysed a gas flow rate time series measured at S. Faustino station during the period September - November 1997 (Figs. 2 and 3). The first graph was obtained using a sampling interval of 40 min, while in the second one hourly values are given. All the flow rate measurements were constantly compared to meteo-climatic parameters. We analyse the temperature values measured by means of San Faustino station and the climatic data-base (temperature, barometric pressure and rain) of Servizio Idrografico Italiano. By a preliminary inspection the recorded flow-rate data show a daily cyclic component related to ambient temperature and an irregular behaviour at higher frequencies. In the first set of data there is a period of 5 days with data missing due to technical troubles problems related to intense rainy period. Finally it is quite evident that there is an anomalous pattern in the flow rate gas emission during the last part of November, this variation is not preceded by strong temporal fluctuations of meteo-climatic parameters.
2. Methods
The adopted monitoring strategy was based on both continuous (in situ automatic recording) and discontinuous (weekly sampling) data acquisition. In approaching the problem of possible correlation between geochemical signals and seismic sequences, it is necessary to discriminate anomalous patterns from background noise applying objective methods (e.g. Wyss, 1991; Evans, 1997). In this paper advanced statistical methods (Cuomo et al., 1996) were adopted to analyse flow-rate data collected by a continuous monitoring approach at Massa Martana (S. Faustino) gas vent. In particular we use the spectral analysis (Jenkins and Watts, 1968) to pick out cyclic components possible induced by meteo-climatic variations and to detect scaling laws in power spectra (Feder, 1988; Turcotte,
Fig. 3 - Data as in fig. I: (a) time series measured at San Faustino site during the period October-November 1997; (b) environmental temperature values outside and inside the station, measured during the same period.
I. Heinicke
et trl.: Coseismic
Geochemical
Fig. 4 - Power spectra of the first 5 I2 dimples of gas flow rate time seT,es recorded at San Faustlno station during the period (October 8 - November 30, 1997) as shown In Flg.3. In the graph a qpikc corresponding to a daily component and a wahng lau are evidenced In order to substantiate this observational evidence we apply some statistical methodologies mentioned in the previous section. First of all. the time series are normalised (c.g. Lero mean and unit variance). In a successive step the power spectra of many sub-samples of experimental data have been estimated using the well-known FFT algorithm (Fig.4). In most of the power spectra we detect a spike corresponding to daily cycle. possibly related to climatic effects, and a very interesting power law. Today it is widely accepted that the power law index can give information about the physics underlying the processes that product the observed signals (Turcotte, 199.5). In particular. we analyse the relationship between the scaling exponent in power spectra and the Hurst exponent (H). The estimate of the Hurst exponent is a robust statistical tool that has a wide variety of interesting applications (Feder. 1988). This exponent is a useful measure of the fractal dimension of the time series (D=2-H) and there is an interesting relation between the rate of decay of power spectrum and the H value: (a=2H+l). This relation becomes (a=H+ l/2) when non-stationary effects are filtered out. The knowledge of this exponent allows us to well describe the time fluctuations of experimental data: for H=1/2 the past and future increments are completely independent: for O.S
Variations
in some Gas Emissions
291
related to seismic activity (i.e. Di Bell0 el al.. 199X). This is one of the more controversial aspects of short-term earthquake prediction. A suitable tool to solve this problem is the crossing theory o theory of runs. In the run theory abnormal events arc treated as rare or unlikely events. Assuming that the experimental time series is a stationary and Gaussian process. the number of crossings below/above some truncation level becomes a Poissonian process. Considering the number of observations to be large enough to permit a good estimation of the empirical curves of probabilities of abnormal events, it is possible to fit the curves theoretical probability coming from the observations. Unfortunately, in many applications the size of the records is large enough to assess the structure of the empirical time series of interest, but not enough to have good statistics about extreme events. In this case the probability curves of extreme events must be obtained from models fitting to observational data (Jenkins and Watts, 1968; LeBouitillicr and Waylen. 1988). Previous studies about this problem (e.g. Cuomo et al., 1996 and reference therein) allow us to define optimal to detect abnormal values modelling time criteria fluctuations of experimental values. For the sake of brevity we do not report all mathematical background underlying this theory, but WC simply describe the adopted technique to detect significant anomalous pattern in geochemical time series. The applied procedure has been subdivided in the following steps: a) a time interval (in our case 2 months) is selected; b) the mean value and the standard deviation are estimated; c) the time series are normalised; d) values above/below ?20 arc detected; c) only anomalous patterns characterised by a very low occurrence probability (
Fig. 5 - Temporal variation of HcICO? and CH.JCQ ratios at site during the period September Montecastello di Vibio sampling November 1997, the bars indicate the earthquake occurred in the investiytcd
iwa
292
J. Heinicke
et al.: Coseismic
Geochemical
3.2. Chemical analyses of gas and water samples The chemical composition of the sampled thermal waters was determined by high-pressure liquid chromatography (Dionex 2001HPLC), while a gas chromatograph (Perkin Elmer 8500) was used for chemical gas analysis. The analytical results of samples taken at a weekly rate from Montecastello di Vibio and Triponzo (figs. 5 and 6) display temporal variations in most of the components of both the gas and the liquid phase. The CO;! dominated gas emission of Vibio contains also variable amount of N2 (3~1 I%), C% (0.2.5+0.27%) and He (IO+15 ppm by vol.).
OAS
OAO
Variations
in some Gas Emissions
Figure 5 displays temporal variation of He/CO* and CH~/COI ratios. Both of the ratios increased in coincidence with seismic energy release. The observed variations are interpreted as a result of mixing process between two CO?rich sources having different amounts of He and CH4 and located at different depths, as a consequence of crustal permeability variations that may influence the mixing proportions between the gas sources Gas output variations were recorded on the field (Montecastello di Vibio site) by visual observations together with the gas pressure variations detected at San Faustino site. The output variations were also in coincidence with gas-chemical upsetting as shown in Fig. 5.
1
1
I A%-91
I at-91
I Du-91
330 -
Jm-91
At@1
hi.97
Fig. 6 - Temporal variations
t!u-97
.hm97
As-91
at-97
k-97
of the main dissolved ions in thermal waters from Triponzo spring. Concentration
A general, probably earthquake-induced, increase of the regional gas output may also be responsible for the observed pH decrease which accompanied changes in water chemistry at Triponzo spring (Fig. 6). The pH decrease can in fact be due to an increased CO* interaction with groundwater that leads of improving CO;? dissolution. At Triponzo thermal spring, the concentration of Mg, Na and K measured at the beginning of the seismic crisis (September-October 1997) were above those measured in a preseismic sample (1997, July), while Ca content was
units in mcq./l
below the previous, pre-seismic, level when pH value was also lower by about 0.3 pH units (fig. 6). On the contrary, the concentration of the main anions (Cl‘ and S04“) did not show significant variations until the beginning of the seismic crisis. During the occurrence of the most intense seismic swarms, all the dissolved ions underwent significant modifications (from - 20% up to +70%; see fig. 6) of their pre-seismic concentration. At the beginning of October, together with the occurrence of high magnitude (Ml> 5; Amato et al., 1997) shocks, the concentration of
.I. Heinicke
Ca, K and SOJ increased
ef al.: Coseismic
Geochemical
by a sudden decrease. while Cl, Na. Mg and HCOi displayed an opposite behaviour. Triponzo thermal water is the result of a mixing between a SOJ-rich water (thermal component) and a carbonatic water. The observed variations are thus interpreted as a result of changed mixing ratios between the two components, that. even in this case. is interpreted as due to modifications of crustal permeability at depth. followed
4. Conclusions The monitored gas emissions have shown intense geochemical upsetting during the occurrence of seismic events, evtdenced by both the continuous flow rate monitoring and the analytical results of the water and gas samples collected at a weekly rate during the September Wh - November 301h observation period. The observed geochemical variations suggest changes in crustal permeability probably due to crustal deformations associated to subductive process which characterise the Apenninic chain. These variations appear to be linked to a more general seismogenic process involving crustal deformation than to single events. The crustal permeability that we suppose to be the main parameter related to the observed variations, underwent modifications that allowed mixing of gas reservoir and water tables with different chemical equilibrium. Similar behaviour was already observed for other seismic areas around the world, as reported, for example, by King ( 1986) and Thomas (1988). The analysis of gas flow rate measurements obtained by means of San Faustino station allows us to consider the time series as a sum of a deterministic component related to climatic fluctuations and a stochastic component that is not characterised by purely random fluctuations. The presence of scaling laws in the power spectra highlights an antipersistent time dynamics that is generated by a physical process ruled by a very large number of degree of freedom. Finally the anomalous pattern observed during the last part of November could be considered as possible coseismic geochemical fluctuations. In the meanwhile, since some events of the sequence were preceded or strong accompanied by significant geochemical anomalies, we deduce that geochemical and geophysical monitoring based on both continuous and discontinuous data set collection, has to be performed to better interpret the physical and chemical variations of the natural system. This knowledge may give, in our opinion, the strongest contribution to the still wide and not enough investigated problem of earthquake prediction. Acknowledgments The authors wish to thank Mr S. Francofonte their support during water samples analyses.
and Mrs. G. Volpicelli
in some Gas Emissions
293
della sequenza di Colfionto In Boschi E and Cocco M Eds.“Studi prelimman sulls sequenza \tsmrca dell‘ Appennmo Umbra-Marchtgiano del Setterllbre-Otttrhrt‘ 1997” 1.N G I-9
Barbter. E. and M F~ncllt. ( 1976) Mam iI-acture\ of Italy from Erts satelhte images and correlation with thermal springs, volcanoes and earthquakes In. Aokr. H and bzuka. S Eds
Bak, P.. Tang. C and Wresenfeld, Phys Rev. A. 3X. 364-374.
K.
( 1988). Self-organized
criticality.
Boschi, E., Guidobom E , Ferraro. G.. Valensise. G. (199X). 1 terremoti ,;ud ortentale dal 98 a.C. al dell’ Appennino Umbro-Marchigiano.Area 1984 1.N.G -S.G A .267 pp Bologna.
Box, G.E.P and Jenkins Day. S. Franctsco
G M
(1976).
Time Serves Analysis.
Holden-
Cataldt, R.. Mongelli, F.. Squurct. P..Taffi. L.. Ztto. G., Calore, C., (199.5). Geothermzal rankrng of Italian tcrrttory Geothermics. 24, 1IS -129
Cello, G., MazLoli. S.. Tondi. E.. Turco, E.. (1997). Active tectonics in the central Apennines and possible tmplications for seismic hazard analysis in peninsular Italy. Tectonophysics, 272.43-68
Chiodini , G.. Frondini, F.. Ponztani, F., (1995). Deep structures carbon dioxide degassing in Central Italy. Geothermtcs, 24, 81-94.
and
Cuomo, V . Di Bello. G., Lapcnna. V., Macchiato. M., and Serio, C. (1996). Parametric ttme seties analysis of extreme events in electrical earthquake precursors, Tectonophysics, 262f I-4). 159.172.
Cuomo, V , Lapenna. V., Macchrato, M.. Seno. C., and Telesca, L., (1998). Linear and nonlinear dynamics in geoelectric time series: implications with earthquake prediction. Trctonophysics, 287, 279-298.
Deschamps, earthquake 36.
A., Iannaccone, G., Scarpa. R.. (1984). The Umbrian (Italy) of 19 September 1979. Annales Geophysicae, 2. 29-
DI Bello, G., Heinicke, J.. Koch, U.. Lapenna, V., Macchiato, M., Martinelli, G.. and S. Piscitelli (1998). Geophysical and geochemical parameters jointly monitored in a seismic area of Southern Apennines (Italy). Physics and Chemtstry of Earth Vol 23. N.9IO, 909-914.
Evans. R , (Ed.). (1997). Spectal Issue on “Assessment earthquake predtction”. Geophys. J. Int., Vol. I31
of schemes
for
Feder, 1.. ( 1988). Fractals. Pleun Press. New York.
Frepoli. .A. and A. Amato, c1997) Contemporaneous extension and compression in the Northern Apennines from earthquake fault-plane solutions. Geophys. J. Int.. 129. 368.388
for Haesslrr. H.. Gaulon, R.. Rivera. L , Console. R., Fropneux, M.. Gasparini, C.. Mattel. L.. Patau, G , Siciliano. M.. Cistemas. A., (1988) The Prrugia (Italy) earthquake of 29 April 1984: a microearthquake survey. Bull Seism.Soc Am.,78.1948-1964.
References Amnto. A., Azzara, R., Basili. A., Chiarabba, C., Di Bona M., Margheriti, L, Mnzza, S.. Mele, E., Bittarelli, G., Chinraluce, L.. Piccinini, D.. F.. Deschamps. A.. Gaffet, S , (1997) Studio
Variations
Cimini. G.B.. Cocco. M., F.. Selvnggi, G.. Boschi. Ripepe. M.. Courboulex, sismolopico prcliminare
Jenkins, GM., and Watts, D.G.. (196X). Spectral applications. Holden-Day, Okland-CA. 520~~.
analysis
an
its
Pergamon
PII: S 1464- 1895(00)00046-6
Coseismic Geochemical (Central Italy) J. Heinickel,
F. Italiano’,
Variations
V. Lapenna3,
in some Gas Emissions
G. Martinelli4
of Umbria
Region
and P. M. Nuccio5
‘Saechsiche Akademie der Wissenschanften zu Leipzig, Freiberg, Germany 21st. Geochimica dei Fluidi, CNR, Palermo, Italy 31st. Metodologie Avanzate di Analisi Ambientale de1 CNR, Potenza, Italy “Serv. Geologic0 e Cartografico, Regione Emilia-Romagna, Italy ‘Dip. CFTA, Universiti di Palermo, Italy Received
1 June
1999; accepted
29 July 1999
Abstract. Coseismic geochemical variations have been detected in some gaseous vents and natural springs during the last seismic crisis occurred in Umbria region (Central Apennines). that started on September 26”, 1997 with several moderate earthquakes (up to Ml 5.8). The results of chemical analyses performed on both gas and water samples taken at a weekly rate, combined with analyses on continuous gas flow rate nearby San Faustino site, suggest that the variations can be interpreted as possible consequence of the crustal permeability changes induced by earthquake shaking.
\. MARCHE
’ ._ _A,D,RlATIC SEA ;
0 2000 Elsevier Science Ltd. All rights reserved.
Introduction LATIUM
In recent years many research groups focused their attention to the analysis of anomalous patterns in geochemical parameters possibly related to earthquakes (e.g. Thomas, 1988; Tsungai and Wakita, 1995; Di Be110 et al., 1998). In this framework a geochemical survey based on continuous and spot measurements has been carried out in a seismic active area of Central Italy (Fig. 1). The Umbria Apennine is affected by frequent seismicity associated with an extensional strain field. In this area earthquakes generally occur in the sedimentary cover at shallow depth (Deschamps et al., 1984; Haessler et al., 1988). At least 22 (Ml>.5) local earthquakes occurred between 1279 and 1984 (Boschi et al., 1998). Focal mechanisms of mainshocks occurred in 1979, 1984 and 1997, also confirmed by further stress indicators (Montone et al., 1997), highlighted active extension processes in the crust characterized by NE-SW and E-W directions (Frepoli and Amato, 1997; Amato et al., 1997). Travertine rocks occur in the vicinity of CO> gas emissions of Umbria Apennine, evidencing deep fluid ascent toward surface (Barbier and Fanelli, 1976). COz. emissions are widely Correspondence to: F. Italian0 Istituto di Geochimica dei Fluidi - CNR, Palermo E.mail:
i
Fig.1 - Location of the sampling sites in Umbria Region (Central Appenines, Italy). The September-November epicentres location is also shown by the grey squares
distributed in Umbria region and can be related to the slightly anomalous heat flow pattern possibly responsible for thermometamorphic processes at depth (Cataldi et al., 1995; Chiodini et al., 19951, while intense local faulting (e.g. Cello et al.. 1997) can be considered responsible for CO: uprising. Crossing the data of seismic hazard evaluation, thermal water data-base and historical information on local deep fluid behaviour in concomitance with past seismic events (Martinelli and Albarello, 1997) allowed to .previously select the gas emissions located at Montecastello di Vibio and Massa Martana and a thermal spring located at Triponzo for a geochemical monitoring. The Montecastello di Vibio vent releases CO2 dominated gas at a high flow rate which appears not to be influenced by seasonal variations. The Massa Martana CO? dominated gas emission occurs close to the San Faustino
(Italy)
[email protected] 289
290
J. Heinicke
et al.: Coseismic
Geochemical
.,/
Variations
199.5). Furthermore we applied the run theory (Le Boutillier and Waylen, 1988) to identify anomalous patterns in geochemical time series. More details about the mathematical background of these methodologies are given elsewhere (e.g. Cuomo et al.. 1996; Di Bello et al. 1998: Di Belle et al., 1999). Finally, the relationships among results concerning the detected temporal variations in chemical composition at Montecastello di Vibio site (gas samples) and Triponzo site (water samples), the analysed flow rate time-series (Massa Martana-San Faustino site), and the seismic activity of the area, were evaluated all together. 3.
Fig. 2 - Data from continuous monitoring at S.Faustino site: (a) gas flow rate time series measured at San Faustino site during the period September October 1997; (b) environmental temperature values measured dunng the same period close fo the working station. The output signals measured in
millivolt are directly related to gas flow rate and temperature respectively. sparkling water exploited for spring source, a naturally bottling. Even in this site the flow rate is characterised by a
constancy in time and apparent no sensitivity to seasonal variations. At those sites vadose waters can occasionally show bubbling activity accompanied by foam generation. of Triponzo (sulphurous water at The thermal spring T=29”C) has been considered for a weekly sampling of the waters, that have shown significant changes discharged mainly of the water level and the flow rate during seismic crisis.
in some Gas Emissions
Results
and Discussion
3. I. Duily jlow rate variations We analysed a gas flow rate time series measured at S. Faustino station during the period September - November 1997 (Figs. 2 and 3). The first graph was obtained using a sampling interval of 40 min, while in the second one hourly values are given. All the flow rate measurements were constantly compared to meteo-climatic parameters. We analyse the temperature values measured by means of San Faustino station and the climatic data-base (temperature, barometric pressure and rain) of Servizio Idrografico Italiano. By a preliminary inspection the recorded flow-rate data show a daily cyclic component related to ambient temperature and an irregular behaviour at higher frequencies. In the first set of data there is a period of 5 days with data missing due to technical troubles problems related to intense rainy period. Finally it is quite evident that there is an anomalous pattern in the flow rate gas emission during the last part of November, this variation is not preceded by strong temporal fluctuations of meteo-climatic parameters.
2. Methods
The adopted monitoring strategy was based on both continuous (in situ automatic recording) and discontinuous (weekly sampling) data acquisition. In approaching the problem of possible correlation between geochemical signals and seismic sequences, it is necessary to discriminate anomalous patterns from background noise applying objective methods (e.g. Wyss, 1991; Evans, 1997). In this paper advanced statistical methods (Cuomo et al., 1996) were adopted to analyse flow-rate data collected by a continuous monitoring approach at Massa Martana (S. Faustino) gas vent. In particular we use the spectral analysis (Jenkins and Watts, 1968) to pick out cyclic components possible induced by meteo-climatic variations and to detect scaling laws in power spectra (Feder, 1988; Turcotte,
Fig. 3 - Data as in fig. I: (a) time series measured at San Faustino site during the period October-November 1997; (b) environmental temperature values outside and inside the station, measured during the same period.
I. Heinicke
et trl.: Coseismic
Geochemical
Fig. 4 - Power spectra of the first 5 I2 dimples of gas flow rate time seT,es recorded at San Faustlno station during the period (October 8 - November 30, 1997) as shown In Flg.3. In the graph a qpikc corresponding to a daily component and a wahng lau are evidenced In order to substantiate this observational evidence we apply some statistical methodologies mentioned in the previous section. First of all. the time series are normalised (c.g. Lero mean and unit variance). In a successive step the power spectra of many sub-samples of experimental data have been estimated using the well-known FFT algorithm (Fig.4). In most of the power spectra we detect a spike corresponding to daily cycle. possibly related to climatic effects, and a very interesting power law. Today it is widely accepted that the power law index can give information about the physics underlying the processes that product the observed signals (Turcotte, 199.5). In particular. we analyse the relationship between the scaling exponent in power spectra and the Hurst exponent (H). The estimate of the Hurst exponent is a robust statistical tool that has a wide variety of interesting applications (Feder. 1988). This exponent is a useful measure of the fractal dimension of the time series (D=2-H) and there is an interesting relation between the rate of decay of power spectrum and the H value: (a=2H+l). This relation becomes (a=H+ l/2) when non-stationary effects are filtered out. The knowledge of this exponent allows us to well describe the time fluctuations of experimental data: for H=1/2 the past and future increments are completely independent: for O.S
Variations
in some Gas Emissions
291
related to seismic activity (i.e. Di Bell0 el al.. 199X). This is one of the more controversial aspects of short-term earthquake prediction. A suitable tool to solve this problem is the crossing theory o theory of runs. In the run theory abnormal events arc treated as rare or unlikely events. Assuming that the experimental time series is a stationary and Gaussian process. the number of crossings below/above some truncation level becomes a Poissonian process. Considering the number of observations to be large enough to permit a good estimation of the empirical curves of probabilities of abnormal events, it is possible to fit the curves theoretical probability coming from the observations. Unfortunately, in many applications the size of the records is large enough to assess the structure of the empirical time series of interest, but not enough to have good statistics about extreme events. In this case the probability curves of extreme events must be obtained from models fitting to observational data (Jenkins and Watts, 1968; LeBouitillicr and Waylen. 1988). Previous studies about this problem (e.g. Cuomo et al., 1996 and reference therein) allow us to define optimal to detect abnormal values modelling time criteria fluctuations of experimental values. For the sake of brevity we do not report all mathematical background underlying this theory, but WC simply describe the adopted technique to detect significant anomalous pattern in geochemical time series. The applied procedure has been subdivided in the following steps: a) a time interval (in our case 2 months) is selected; b) the mean value and the standard deviation are estimated; c) the time series are normalised; d) values above/below ?20 arc detected; c) only anomalous patterns characterised by a very low occurrence probability (
Fig. 5 - Temporal variation of HcICO? and CH.JCQ ratios at site during the period September Montecastello di Vibio sampling November 1997, the bars indicate the earthquake occurred in the investiytcd
iwa
292
J. Heinicke
et al.: Coseismic
Geochemical
3.2. Chemical analyses of gas and water samples The chemical composition of the sampled thermal waters was determined by high-pressure liquid chromatography (Dionex 2001HPLC), while a gas chromatograph (Perkin Elmer 8500) was used for chemical gas analysis. The analytical results of samples taken at a weekly rate from Montecastello di Vibio and Triponzo (figs. 5 and 6) display temporal variations in most of the components of both the gas and the liquid phase. The CO;! dominated gas emission of Vibio contains also variable amount of N2 (3~1 I%), C% (0.2.5+0.27%) and He (IO+15 ppm by vol.).
OAS
OAO
Variations
in some Gas Emissions
Figure 5 displays temporal variation of He/CO* and CH~/COI ratios. Both of the ratios increased in coincidence with seismic energy release. The observed variations are interpreted as a result of mixing process between two CO?rich sources having different amounts of He and CH4 and located at different depths, as a consequence of crustal permeability variations that may influence the mixing proportions between the gas sources Gas output variations were recorded on the field (Montecastello di Vibio site) by visual observations together with the gas pressure variations detected at San Faustino site. The output variations were also in coincidence with gas-chemical upsetting as shown in Fig. 5.
1
1
I A%-91
I at-91
I Du-91
330 -
Jm-91
At@1
hi.97
Fig. 6 - Temporal variations
t!u-97
.hm97
As-91
at-97
k-97
of the main dissolved ions in thermal waters from Triponzo spring. Concentration
A general, probably earthquake-induced, increase of the regional gas output may also be responsible for the observed pH decrease which accompanied changes in water chemistry at Triponzo spring (Fig. 6). The pH decrease can in fact be due to an increased CO* interaction with groundwater that leads of improving CO;? dissolution. At Triponzo thermal spring, the concentration of Mg, Na and K measured at the beginning of the seismic crisis (September-October 1997) were above those measured in a preseismic sample (1997, July), while Ca content was
units in mcq./l
below the previous, pre-seismic, level when pH value was also lower by about 0.3 pH units (fig. 6). On the contrary, the concentration of the main anions (Cl‘ and S04“) did not show significant variations until the beginning of the seismic crisis. During the occurrence of the most intense seismic swarms, all the dissolved ions underwent significant modifications (from - 20% up to +70%; see fig. 6) of their pre-seismic concentration. At the beginning of October, together with the occurrence of high magnitude (Ml> 5; Amato et al., 1997) shocks, the concentration of
.I. Heinicke
Ca, K and SOJ increased
ef al.: Coseismic
Geochemical
by a sudden decrease. while Cl, Na. Mg and HCOi displayed an opposite behaviour. Triponzo thermal water is the result of a mixing between a SOJ-rich water (thermal component) and a carbonatic water. The observed variations are thus interpreted as a result of changed mixing ratios between the two components, that. even in this case. is interpreted as due to modifications of crustal permeability at depth. followed
4. Conclusions The monitored gas emissions have shown intense geochemical upsetting during the occurrence of seismic events, evtdenced by both the continuous flow rate monitoring and the analytical results of the water and gas samples collected at a weekly rate during the September Wh - November 301h observation period. The observed geochemical variations suggest changes in crustal permeability probably due to crustal deformations associated to subductive process which characterise the Apenninic chain. These variations appear to be linked to a more general seismogenic process involving crustal deformation than to single events. The crustal permeability that we suppose to be the main parameter related to the observed variations, underwent modifications that allowed mixing of gas reservoir and water tables with different chemical equilibrium. Similar behaviour was already observed for other seismic areas around the world, as reported, for example, by King ( 1986) and Thomas (1988). The analysis of gas flow rate measurements obtained by means of San Faustino station allows us to consider the time series as a sum of a deterministic component related to climatic fluctuations and a stochastic component that is not characterised by purely random fluctuations. The presence of scaling laws in the power spectra highlights an antipersistent time dynamics that is generated by a physical process ruled by a very large number of degree of freedom. Finally the anomalous pattern observed during the last part of November could be considered as possible coseismic geochemical fluctuations. In the meanwhile, since some events of the sequence were preceded or strong accompanied by significant geochemical anomalies, we deduce that geochemical and geophysical monitoring based on both continuous and discontinuous data set collection, has to be performed to better interpret the physical and chemical variations of the natural system. This knowledge may give, in our opinion, the strongest contribution to the still wide and not enough investigated problem of earthquake prediction. Acknowledgments The authors wish to thank Mr S. Francofonte their support during water samples analyses.
and Mrs. G. Volpicelli
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