Vol. 48, 01, 05ok.qxd

ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005

Key words geochemical monitoring – seismicity –
helium – thermal waters

1. Introduction

It is well-known that earthquakes can provoke
modifications in the natural setting of a seismic

area and may induce modifications in the fluid
phases, such as a decrease/increase in the water
level of wells, changes in the temperature and/or
chemical composition of the groundwaters, and
variations both in the gas discharge flow-rates
and in the chemical and isotopic composition of
the gases (e.g., see King, 1986; Thomas, 1988;
Toutain and Boubron, 1999). At times, precurso-
ry time and duration of the anomalies seem to in-
crease as the magnitude and epicentral distance
grow, but sometimes this is not the case. On the
basis of previous observations carried out in sev-
eral Italian seismic areas, some authors (Favara
et al., 2001; Italiano et al., 2001, 2004; Caracausi

Long-term geochemical monitoring
and extensive/compressive phenomena:

case study of the Umbria Region
(Central Apennines, Italy)

Antonio Caracausi (1), Francesco Italiano (1), Giovanni Martinelli (2), Antonio Paonita (1) and Andrea Rizzo (1)
(1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy

(2) ARPA – Agenzia Regionale Prevenzione e Ambiente dell’Emilia-Romagna,
Sezione Provinciale di Reggio Emilia, Italy

Abstract
Long-term geochemical monitoring performed in the seismic area of the Umbria-Marche region of Italy (i.e. Cen-
tral Apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical
and isotopic variations of both the thermal springs as well as the gas vents. Coincident with the last seismic cri-
sis, which struck the region in 1997-1998, an enhanced CO2 degassing on a regional scale caused a pH-drop in
all the thermal waters as a consequence of CO2 dissolution. Furthermore, much higher 3He/4He isotope ratios
pointed to a slight mantle-derived contribution. Radon activity increased to well above the ± 2σ interval of the ear-
lier seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the
single deep-located shock (March 26, 1998, 51 km deep). The anomalous CO2 discharge was closely related to
the extensional movement of the normal faults responsible for the Mw 5.7, 6.0 and 5.6 main shocks that charac-
terized the earlier seismic phase. In contrast, a clear compressive sign is recognizable in the transient disappear-
ance of the deep-originating components related to the Mw 5.3, 51 km-deep event that occurred on March 26,
1998. Anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic
crisis had terminated. We argue that the observed geochemical anomalies were driven by rock permeability
changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the
detection of anomalies reveals that a seismogenic process is developing. Indeed, comprehensive, long-term geo-
chemical monitoring can provide new tools allowing us to better understand the development of seismogenesis.

Mailing address: Dr. A. Caracausi, Istituto Nazionale di
Geofisica e Vulcanologia, Sezione di Palermo, Via U. La Mal-
fa 153, 90146 Palermo, Italy; e-mail: caracau@pa.ingv.it

Antonio Caracausi, Francesco Italiano, Giovanni Martinelli, Antonio Paonita and Andrea Rizzo

et al., 2002a,b) have suggested that the best way
to approach the broad-ranging problem of fluid
geochemistry and seismogenesis was to collect as
many locally significant geochemical data as pos-
sible and to evaluate it within an interpretative
geochemical model. Thus, the long-term geo-
chemical monitoring of any seismic area enables
us to define the background and detect any anom-
alies, which, besides the knowledge of fluid cir-
culation and interaction, allow us to define the be-
haviour of these anomalies as a seismogenic
process develops. Since the surface chemical/iso-
topic composition is a consequence of deep phys-
ical-chemical conditions and may change due to
re-equilibrations at depth, the geodynamic con-
text of the area may deeply influence the surface
composition. In fact, shallow-originating fluids
could mix with others coming from different
depths of the crust and/or from the upper mantle.
The mixing proportions may change with time
due both to seasonal variations and/or to the de-
velopment of seismogenesis (stress accumula-
tion, deformation, strain release, etc.).

The seismic event in itself, however, is not the
main goal of this research, although its forecasting
does in some ways represent a sort of final target.

The monitoring activity focuses on the evaluation
of the effects that the development of a seismo-
genic process has on the circulating fluids which
may or may not generate a «seismic shock».

Starting from the beginning of the seismic cri-
sis (September 26, 1997), thermal water samples
(Bagni di Triponzo, Parrano and Stifone) and
samples from gas vents (Montecastello di Vibio,
Umbertide, San Faustino) were collected on a
weekly basis during the period October 1997-July
1998 and at longer intervals thereafter (i.e. twice a
week, then monthly and then once every season).
The data collected during a rather quiet seismic
period (1999-2002) allowed us to identify the
background values for some geochemical param-
eters that could characterise the study area. All the
values outside the ±1σ interval were considered as
being real variations. However, the observed fluc-
tuations were wider than the ± 2σ interval.

Some previously published results have
pointed out that: a) the thermal waters are Ca-
SO4– enriched due to their circulation in the
deep evaporitic basement and have changed
their mixing proportions with cold waters typi-
cally equilibrated in shallow carbonate systems
(Favara et al., 2001); during the seismic period
deep-originating waters contaminated shallow
aquifers exploited for human purposes (Bazzof-
fia et al., 2002); b) the gases dissolved in the
thermal waters, CO2-dominated with the pres-
ence of CH4 and an excess of N2 in respect of
the atmosphere, displayed various influxes of
both deep-originating and atmospheric-derived
components (Italiano et al., 2004); the 3He/4He
ratios of venting gases showed that although the
region is located in a typical crustal environ-
ment, the contribution of a mantle-derived com-
ponent could be detected (Italiano et al., 2001).

This paper contains new data mainly collect-
ed at a single site, namely the thermal spring of
Bagni di Triponzo, where the emitted fluids have
undergone the widest geochemical modifications
of all the monitored sites (e.g., the temperature of
the thermal water, 29.6 ± 0.2°C constant over
time and with no seasonal effect, dropped by
2°C two weeks before and > 20°C two days be-
fore the occurrence of the March 26, 1998
shock). This paper focuses on the relationships
between the geochemistry of the circulating flu-
ids and the mechanical properties of the hosting

Fig. 1. Location of the selected sampling sites for
thermal waters (Umbria Region, Central Italy) and the
main boundaries between extensional and compres-
sional zones of the Central Apennines (after Barba and
Basili 2000). Squared symbols show the epicentral lo-
cations of the main shocks; in red the 26th March 1998
deep located event.

Long-term geochemical monitoring: case study of the Umbria Region (Central Italy)

rocks, highlighting the geochemical phenomena
related to extension versus those related to the
compressive behaviour of the faults involved.

2. Seismic activity and geotectonic settings

The Umbria Apennine is affected by frequent
seismic activity associated with an extensional
strain field. Earthquakes affecting the area gener-
ally occur in the sedimentary cover at a shallow
depth (Deschamps et al., 1984; Haessler et al.,
1988). At least 22 Ml ≤ 5 local earthquakes oc-
curred between 1279 and 1984 (Boschi et al.,
1998). Focal mechanisms of the main shocks oc-
curred in 1979, 1984 and 1997. This has also
been confirmed by further stress indicators
(Montone et al., 1997), highlighting the active
extension processes in the crust characterized by
NE-SW and E-W directions (Frepoli and Amato,
1997; Amato et al., 1998). Selvaggi and Amato
(1992) demonstrated that deep subcrustal seis-
micity also affects the Central and Northern
Apennines. Subcrustal seismicity may indicate
the ongoing activity of subductive processes.

After a relatively long period of quiescence,
a seismic swarm, marked by the Ml 4.5 Massa
Martana mainshock, occurred in the Umbria
Region in May 1997 about 40 km south of the
village of «Colfiorito», which was the area
most damaged during the subsequent months of
September and October 1997. The strongest
event, which occurred in 1998 (Ml 5.7 on
March 26), had a subcrustal hypocenter located
at a depth of about 51 kms (Morelli et al.,
2000). A descending lithospheric slab, linked to
an ongoing subduction process, may account
for subcrustal events in the Northern-Central
Apennines (Selvaggi and Amato, 1992). The
entire seismogenic process caused a crustal de-
formation with a maximum horizontal co-seis-
mic displacement of 14 ± 1.8 cm and a maxi-
mum co-seismic subsidence of 24 ± 3 cm.
These were detected by means of SAR differ-
ential interferometry and GPS data (Stramondo
et al., 1999), while a post-seismic long-term
deformation process was detected by means of
levelling data (Basili and Meghraoui, 2001).

The inner part of the Umbria-Marche Apen-
nine was affected by a relatively extensional

regime able to generate a set of grabens during
Plio-Pleistocene times within the frame of a
possible, limited rifting process (Elter et al.,
1975; Liotta et al., 1998).

The basement of the Central Apenninic rock
is made up of phyllitic rocks from the Carbonif-
erous age, underlying conglomerates of the Ver-
rucano Group attributable to the Middle Triassic
age. The sedimentary cover continues with the
Burano anhydrites, Contorta limestone and Cal-
care Massiccio (Late Triassic-Early Lias). It is
followed by a pelagic, calcareous and marly
multilayer (middle Lias-Oligocene) and ends in
a flisch sequence (Middle to Upper Pliocene)
(Bally et al., 1986; Ponziani et al., 1995).

Travertine rocks occur in the vicinity of
CO2 gas emissions of the Umbria Apennine,
evidencing the ascent of deep fluids towards
the surface (Barbier and Fanelli, 1976). CO2
emissions are widely distributed across the
Umbria Region and can be related to the
slightly anomalous heat flow pattern (Cataldi
et al., 1995), that may be responsible for the
thermo-metamorphic processes at depth (Chio-
dini et al., 1999), while intense local faulting
(e.g., Cello et al., 1997) can be considered re-
sponsible for CO2 uprising. The existence of
the magmatic structures of San Venanzo (Stop-
pa and Sforna, 1995) in the vicinity of Monte-
castello di Vibio, and Colle Fabbri (Stoppa,
1988) in the vicinity of Massa Martana, pro-
vides clues to the possible increase in local
crustal permeability.

The Triponzo spring sources are located in
a limestone formation (Calcare Massiccio) of
the Hettangian-Sinemurian age characterized
by high porosity. This favours locally impor-
tant karst phenomena. Spring sources are lo-
cated at the bottom of a canyon generated by
an important regional fault, which cut the lime-
stone formations allowing the groundwaters to
reach the underlying evaporitic rocks (Gi-
aquinto et al., 1991).

3. Sampling and analytical methods

Water composition was determined by
both field measurements (temperature, pH,
electrical conductivity and Rn activity) and

Antonio Caracausi, Francesco Italiano, Giovanni Martinelli, Antonio Paonita and Andrea Rizzo

laboratory analyses. In the field the radon ac-
tivity was measured by an EDA-RD 200
Radon detector. A 130-ml aliquot of water
was degassed and the extracted Radon was
measured in a ZnS (silver activated) coated
cell. The analytical errors have been estimat-
ed to be ± 10%. The temperature was always
measured using the same mercury thermome-
ter (scale 0-50°C; resolution 0.1°C), while pH
and conductivity were measured by electron-
ic instruments calibrated in situ using buffer
solutions (error below ± 5%). The chemical
composition was determined by High-Pres-
sure Liquid Chromatography (HPLC, Dionex
2001) using a Dionex CS-12 and a Dionex
AS4A-SC column for cation and anion deter-
minations, respectively. The HCO3 content
was measured in the laboratory by standard
titration procedures on samples sealed in the
field following the same method for the dis-
solved gas analyses.

For the analysis of the dissolved gases,
samples were collected and sealed in the field
in 240 ml Pyrex bottles. Keeping the bottles
immersed, they were filled with the thermal
water, and then a gas-tight septum was fitted on
using special pliers.

The chemical gas-analyses were carried out
using the methodology based on the equilibri-
um partitioning of gases between the liquid and
gas phase (Sugisaki and Taki, 1987), and fol-
lowing the procedure previously suggested by
Capasso and Inguaggiato (1998). The chemical
analyses and helium isotope measurements
were made by gas-chromatography and mass
spectrometry, respectively; the sample was split
into two gas aliquots concomitantly extracted
from the same sampling bottle (Italiano et al.,
2004). Chemical analyses of He, O2, N2, CH4
and CO2 were carried out using a Perkin Elmer
8500 gas-chromatograph equipped with a 4 m
Carbosieve 5A column and double detector
(Flame Ionization Detector with methanizer
and Hot Wire Detector). The detection limits
were 5 ppmv for He, O2, and N2, and 1 ppmv
for CH4 and CO2. Analytical errors were ± 5%
for He, and ± 3% for the other species.

A 0.3 cm 3 aliquot was admitted at a known
pressure to the purification UHV line for heli-
um isotope analysis. Helium was purified in

three stages in separate sections of an all-metal
UHV preparation system. First, the reactive
gases were adsorbed into a charcoal trap held at
77 K; then two SAES getters worked simulta-
neously to adsorb residual nitrogen at 250°C
and hydrogen at room temperature. The third
step, after measuring the 4He/20Ne ratio and
checking for residual 40Ar on an in-line Quadru-
pole Mass Spectrometer (QMS, VG Quartz),
was to completely separate helium from neon
by means of a charcoal trap held at 40 K. The
isotopic analyses of the purified helium fraction
were performed with a modified static vacuum
mass spectrometer (VG5400TFT, VG Isotopes)
that allows for the simultaneous detection of
3He and 4He-ion beams, reducing the 3He / 4He
measurement error to very low values. Typical
uncertainties in the range of low-3He (radi-
ogenic) samples are below ± 5%.

4. Results and discussions

The chemical composition of the sampled
thermal waters was established to be a mixture
of a carbonate component, equilibrated with
calcite and dolomite, and a selenitic component
circulating in the Evaporitic Triassic basement
(Chiodini et al., 1982, 1999; Quattrocchi et al.,
2000). However, no information was available
regarding temporal modifications before the oc-
currence of the last seismic crisis (Favara et al.,
2001). The Langelier-Ludwig diagram (Lange-
lier and Ludwig, 1942) in fig. 2 highlights the
composition changes of the major constituents
during the seismic crisis. It should be noted that
not only did the composition change, but the
physical-chemical features and the flow rate of
the thermal waters did as well (Favara et al.,
2001; see also fig. 7). Figures 3 and 4a,b show
both the background anomalies and the record-
ed ones for some anions and cations of the in-
vestigated springs (Favara et al., 2001; Cara-
causi et al., 2002a).

Using the WATEQP software (Appelo, 1988)
we were able to calculate the Saturation Index
(SI) expressed as the logarithm of the ion activity
ratio, pertinent to a mineralogical phase, and the
equilibrium constant of its solubility product. The
results plotted in figs. 5a,b and 6 show the water-

Long-term geochemical monitoring: case study of the Umbria Region (Central Italy)

Fig. 2. Langelier-Ludwig classification diagram.
The plotted analyses show the modifications that oc-
curred over time at all the selected thermal springs.
The dates show the modifications that happened at
Triponzo at the end of March 1998. The arrows dis-
play the chemical modifications caused by cold wa-
ter mixing. Concentration units are in mEq/l.

rock interactions occurring at the thermal waters
of Parrano Stifone and Triponzo, respectively.

All the Calcite-Dolomite SI show that the
discharged thermal waters are normally from
under-saturated to close-to-saturation with re-
spect to both of the two mineralogical species,

Fig. 3. Temporal variation of SO4 and Mg concentra-
tion (mEq/l) at the thermal spring of Parrano between
June 1997 and June 2002. The occurrence of seismic
events with Ml > 4 is reported as a bar diagram.

Fig. 4a,b. Temporal variation of Ca and Cl concen-
tration (mEq/l) at the thermal springs of Triponzo
«pipe» and «pond» between June 1997 and Decem-
ber 2001 and their relationship to the seismicity as
shown by the associated bar diagram. Even though
the two springs are located several dozen meters
apart at the same thermal area, they followed very
different trends during the monitoring period. The
«pipe» spring is the one referred to by Favara et al.
(2001), while the «pond» site is the one sampled by
Quattrocchi et al. (2000).

Fig. 6. Saturation indices calculated for the thermal
water samples of Triponzo «pipe». The faulting be-
haviour that marked the seismic events of September-
October 1997 and March 1998, respectively, induced
significant and different effects that can be recognized
in the paths followed by the water-bodies when re-
turning to initial conditions, i.e. chemical equilibrium.
The dates refer to the month of March 1998.

Antonio Caracausi, Francesco Italiano, Giovanni Martinelli, Antonio Paonita and Andrea Rizzo

Fig. 5a,b. Calcite versus Dolomite and Calcite-Gyp-
sum Saturation Indices (SI) calculated for the thermal
springs of Parrano (a) and Stifone (b). The lowest SI
are related to the event that occurred in March 1998.

as this variability is related to seasonal varia-
tions. However, they varied simultaneously as
time passed (figs. 5a,b and 6) to become deci-
sively under-saturated over a short time-scale.
The Calcite-Dolomite SI pattern is similar for
all the thermal waters, but Parrano and Stifone
(fig. 5a,b) show variable SI even for gypsum.
These results are consistent with an initial equi-
libration within evaporitic (selenitic) reservoirs,
then thermal waters mix with shallower cold
waters equilibrated in carbonatic reservoirs dur-
ing their uprising towards the surface.

All the sampled thermal waters displayed
the lowest SI values during the earlier phase of
the seismic crisis (September-October 1997),
in agreement with the extension that marked
the faulting activity. Then the SI increased as
time passed.

Normal faulting (Barba and Basili, 2000) in-
creased crustal permeability allowing a higher
CO2 flow rate to the surface (Heinicke et al.,
2000; Italiano et al., 2001), thus inducing a high-
er aqueous CO2 concentration in the thermal wa-
ters. As a consequence of this, the aggressiveness
of the groundwaters increased, thereby causing
more limestone to dissolve. However, consider-
ing the chemical reaction kinetics, the equilibri-
um with the hosting rocks was far from being
achieved and the calculated SI displayed a water-
body that became less and less under-saturated in
both calcite and dolomite (fig. 6). The evidence
that the gypsum saturation index and the emis-
sion temperature remained unchanged means
that the crustal permeability increase did not en-
hance the thermal waters uprising from the
Evaporitic basement. The gypsum SI at Tripon-
zo shows that the water is constantly gypsum-un-

der-saturated with (SI = − 0.4). Furthermore, the
calcite SI is close to saturation and varies as time
passes (fig. 6). The thermal water from Triponzo
showed the highest calcite and dolomite SI in the
sample collected four days before 26th March,
Ml = 5.7, deep located event (Morelli et al.,
2000). The water samples taken during the days
that followed showed some very interesting be-
haviour: all the calcite and dolomite SI dropped
to typical under-saturated values, whereas the
sample taken on 1st April was again in the range
recorded at the beginning of the seismic crisis.
Since the observed modifications were consis-
tent with other observations that denoted a
transient disappearance of the thermal water
component (Favara et al., 2001), they reveal a
compressive phenomenon that involved vari-
ous crustal layers. The compression was al-
ready acting on the deep crustal layers many
days before the seismic shock, obstructing the
rise of CO2. The water sample dated 22nd
March shows a decrease in the dissolved CO2
that modified the equilibrium conditions of the
water body toward lower SI values. The sam-
ples taken in the days that followed exhibit a
trend toward a new chemical equilibrium, with
a wider decrease in the dolomite SI in respect
of the calcite SI. The sample collected on the
28th, two days after the shock, shows the low-
est SI for both the mineralogical species. The
trend in gypsum-calcite SI (fig. 6) highlights
another aspect of the compressive mechanism:
gypsum SI did not change very much before
the earthquake, however afterwards it dis-
played the lowest value (sample dated 28th
March). This is related to the transient disap-
pearance of the thermal water component, as
pointed out by the dramatic water temperature
drop, meaning that the compression obstructed
the uprising pathways located at the level of
the evaporitic basement (where waters become
«thermal») at the end of the compression. An
alternative explanation, based on the occur-
rence of mixing phenomena, has to be ruled
out due to the very low temperature (8.4°C)
reached by the thermal waters that cannot be a
mixture of a thermal component, having a tem-
perature of about 30°C, with water colder than
the infiltrating waters (normally as cold as 8-
10°C).

The graph log pressure CO2 versus temper-
ature (fig. 7) shows that, two weeks before 26th
March seismic shock, the thermal component
was already mixing with cold waters or, alter-
natively, that the relative flow-rates were
changing. The 20°C drop occurred two days be-
fore the shock and the time the temperature
took to return to the same pre-seismic range
(i.e. some months) is consistent with a long-
lasting process (compression and relaxation).

Figure 8 shows the radon activity trend ver-
sus pH. The different behaviour of Rn at the
times of the two seismic periods in 1997 and
1998 is clearly evident. In fact, during the for-
mer, Rn increased, reaching its peak activity,
while, coincident with the latter, it decreased to
its lowest value. The activity of Rn returned to
background values about 15 weeks after the re-
markable geochemical changes observed in
concomitance with the last, deep seismic shock
(March 1998).

The pH variation pattern is similar, al-
though antithetical. A 0.3 pH-unit decrease,
that lasted for 6 months, was observed at the

Long-term geochemical monitoring: case study of the Umbria Region (Central Italy)

Fig. 7. Log pCO2, temperature and seismicity ver-
sus time at the Triponzo «pipe» spring. Data refer to
the sampling period that lasted from June 1997 to
August 1999.

Fig. 9. Radon activity (Bq/l) and 3He/4He isotope
ratio in the thermal water of Triponzo «pipe». During
the March 1998 seismicity the radon activity dis-
played a marked antithetical trend in respect of the
3He/4He ratio, showing that radon content and 4He
are related to a common radiogenic origin. The in-
crease in the R/Ra values is a consequence of the dis-
solution of an atmospheric component in the thermal
waters. The displayed helium isotope ratios, ex-
pressed as R/Ra values (R = sample ratio; Ra = at-
mospheric ratio = 1.39 × 10− 6), have not been cor-
rected for atmospheric contamination.

Antonio Caracausi, Francesco Italiano, Giovanni Martinelli, Antonio Paonita and Andrea Rizzo

Fig. 8. Radon activity (Bq/l), flow-rate (litres per
second) and pH versus time at Triponzo «pipe» ther-
mal spring. The flow rate changes because of sea-
sonal effects but does not affect the radon activity,
the pH or the temperature (see fig. 7). In coincidence
with the deep event of 26th March 1998, both the
flow rate and radon concentration dropped to the
lowest values ever recorded (see text).

beginning of the seismic crisis. The values of
pH increased towards the pre-seismic/back-
ground values at the end of February 1998, and
exhibited a further, sudden increase of the or-
der of 0.8 units in concomitance with the low-
est radon activity, the lowest water temperature
and flow-rate, and the highest 3He/4He ratio
(i.e. the variations related to the 26th March
1998 shock). It then went back to within the
± 2σ background interval, displaying varia-
tions, probably due to seasonal oscillations,
consistent with those of Rn, pCO2, etc.

Rn and water flow rate followed very similar
trends (fig. 8) that occurred at an almost con-
stant water temperature (apart from the 20°C-
drop preceding the 26th March 1998 seismic
event). The absence of significant flow-rate-re-
lated temperature variations implies that local
hydrological settings control the flow rate, thus
its temporal variation does not represent vari-
able mixing proportions between the deep se-
lenitic thermal component and the shallow cold,
carbonatic water, but variable hydrostatic loads.

Since the mixing ratios of the thermal water
are almost the same on a temporal scale, the pH

decrease recorded at the beginning of the seismic
crisis cannot be related to compositional varia-
tions but to the massive CO2 output in the area.
This evidence leads to the consideration that at the
beginning of the seismic crisis the excess radon
release was triggered by the CO2 outflow (i.e.
Dongarrà and Martinelli, 1995 and references
therein). Rn reached its lowest value when only
the carbonatic water was outflowing, highlighting
how the background of this noble gas is kept at
levels of about 40 Bq/l by the thermal component.

Figure 9 shows the inverse correlation be-
tween the trends of the radon activity and the
3He/4He isotope ratio. This pattern, that gives
support to the hypothesis of an enhanced
crustal permeability that allowed CO2 to be
released and to act as carrier for the heavy,
low-mobile radon, highlights the same radi-

15th, 1988), the epicentre of which was located
about 80 km southwards, on the same main tec-
tonic structure. The results show that the geo-
chemistry of the fluids in a seismic area reflects
the faulting behaviour and the properties of the
hosting rocks, so it is closely related to the local
tectonic, geologic and hydrologic settings, and
that the findings collected in one selected area can-
not be applied directly to another. Taking into con-
sideration all the results we obtained, we argue
that the geochemical variations are linked to a
more general seismogenic process, involving
crustal deformation, rather than to single events.
The results strongly outline that without long-term
geochemical monitoring, it is impossible either to
interpret or to create a model of the geochemical
anomalies and the temporal variations occurring
during the development of a seismogenic process.

Acknowledgements

The authors wish to thank Dr. H. Woith, Dr.
J. Heiniche and an anonymous referee for the
helpful comments and suggestions that greatly
improved the manuscript. Mrs. Penelope Dyer
is thanked for having revised the English.

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ogenic origin for 4He and Rn: they both origi-
nate from the deep crustal levels involved in
the tensive and compressive phenomena of the
seismogenetic process.

5. Conclusions

The long-term monitoring program, which
started in July 1997, allowed us to interpret the
geochemical information in such a way as to
model the circulation pattern of the fluids re-
leased in the area. Geochemical variations dis-
closed changes in crustal permeability that are
to be referred to deformations associated with
subductive processes that characterize the
Apennine chain. The most significant geo-
chemical changes were apparently linked to 5
events characterized by Ml ≤ 5 and limited to
the period September 1997-April 1998; con-
versely the long-term geochemical trends show
the persistence of long-lasting processes that
have no relationship with the seismic shocks.
The results collected reasonably denote exten-
sive crustal movements at the beginning of the
seismic crisis, in full agreement with the ob-
servations that the normal fault, known as
Colfiorito Fault, was the first structure to move
(Basili and Barba, 2000). The enhanced CO2
release was one of the consequences that in-
duced modifications in the chemical equilibri-
um of the water bodies. Coincident with the 51
km-deep seismic shock that occurred on 26th
March 1998, a dramatic permeability drop, ca-
pable of modifying the deep thermal water cir-
cuit, was responsible for the most extensive
modifications observed during the entire seis-
mic crisis. Nevertheless the focal mechanism
of the event was a combination of normal fault-
ing and strike-slip (Morelli et al., 2000),
whereas the observed geochemical modifica-
tions reveal an opposite effect at shallower lev-
els. It is worth noting that the variations oc-
curred some days before and continued some
days after the event, thereby supporting the hy-
pothesis that the permeability drop was not a
consequence of the earthquake itself.

Some strong geochemical variations also oc-
curred in the period May-August 1998, when there
was only one significant event (Ml ≥ 4.8, August

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Long-term geochemical monitoring: case study of the Umbria Region (Central Italy)