Seismically triggered microbial methane production relating to the Vogtland

Geochemical Journal, Vol. 39, pp. 441 to 450, 2005
Seismically triggered microbial methane production relating to the Vogtland
—NW Bohemia earthquake swarm period 2000, Central Europe
KARIN BRÄUER,1* H ORST KÄMPF,2 ECKHARD FABER,3 ULRICH K OCH,4
HORST-M ICHAEL N ITZSCHE5 and GERHARD STRAUCH1
1
UFZ Centre for Environmental Research Leipzig-Halle, Department of Hydrogeology,
Theodor-Lieser-Str. 4, D-06120 Halle, Germany
2
GeoForschungsZentrum Potsdam, Department Chemistry of Earth, Telegrafenberg, D-14473 Potsdam, Germany
3
Federal Institute of Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany
4
ˇ
Saxon Academy of Sciences at Leipzig, Fritz Rsdiger
Haus, D-08646 Bad Brambach, Germany
5
Center of Non-Classical Chemistry e.V. (INC), Permoserstr. 15, D-04318 Leipzig, Germany
(Received March 23, 2004; Accepted December 14, 2004)
Long-term radiometric and hydrological investigations at the Wettinquelle mineral spring in Bad Brambach demonstrated the existence of a fluidal connection to the currently most frequent earthquake-swarm hypocentre at Novy´ Kostel,
10 km east of Bad Brambach. The gas composition and δ13CCH4 values of this mineral spring were monitored from May
2000 until October 2003, i.e., before, during and after the protracted swarm earthquake period from late August until late
December 2000. About eight weeks after the beginning of the seismically active period, we observed an increase in the
methane concentration (from ≈40 up to ≈250 ppmv) accompanied by a decrease in the methane δ13C values from ≈–50 to
≈–70‰. For more than two years, such periods of variations were repeatedly observed before returning to the “baseline”
signature. It is assumed that this additional methane was microbially produced in the granite-enclosed aquifer using H2,
which was released (seismically triggered) from the fissured granite in which the Wettinquelle spring capture is located.
The additional methane production might have started as a co-seismic event, with only the migration from the deep
granite to the surface being responsible for the eight-week delay.
Keywords: gas geochemistry, carbon isotopes, microbial methane production, earthquake swarms, Central Europe
decreased sub-Moho P-wave velocities of 7.6–7.7 km/s
(Hemmann et al., 2003), and increased reflectivity in the
lower crust (Trappe and Wever, 1990) beneath the western Eger rift area, coinciding with a currently active CO2
mantle degassing field characterised by He isotope signatures (5.9 Ra) as found from the European subcontinental mantle, demonstrating the deep origin of these
gases (Bräuer et al., 2004). Isotopic shifts in fluids derived from the upper mantle (CO2, He) several months
after swarm earthquake activity indicate the admixture
of crustal fluids derived from the hypocentre to the permanent mantle volatile flux (Weise et al., 2001; Bräuer
et al., 2003). Finally, fluid/magma ascent beneath the
western Eger rift is thought to be connected to the occurrence of earthquake swarm activity with shallow focal
ˇ
ˇ
depths (5–15 km) (Spic̈ák
et al., 1999; Spic̈ák
and Horálek,
2001; Bräuer et al., 2003).
Pre-, co- and post-seismic anomalies of gas and isotope compositions have been measured all over the world
and interpreted to stem from both the seismically induced
release of fluids and changes in fluid migration paths (e.g.,
Sugisaki, 1987; Tsunogai and Wakita, 1995; Hilton, 1996;
Sano et al., 1998; Toutain and Baubron, 1999). Molecu-
INTRODUCTION
It was more than 100 years ago that Credner (1900)
coined the German term “Schwarmbeben” (swarm earthquake) to characterise a certain kind of seismicity usually occurring in the area of Vogtland and NW Bohemia.
This type of seismicity is typified by numerous smallmagnitude events within a short space of time with shallow focal depths (<15 km). Such earthquake swarms usually occur in volcanically and/or tectonically active areas
(Hill, 1977).
Recent geophysical and geochemical findings indicate
the existence of active magmatism and magma/fluidfilled-reservoirs at the base of the continental crust and
the crust/mantle boundary that may be responsible for the
periodic earthquake swarm activity in Vogtland and NW
Bohemia. In detail they comprise an asthenospheric
ˇ
upwelling (Babuska
and Plomerová, 2001), a Moho
updoming from 31 km to 27 km (Geissler et al., 2005),
*Corresponding author (e-mail: [email protected])
Copyright © 2005 by The Geochemical Society of Japan.
441
Fig. 1. Investigation area (mineral spring Bad Brambach) at the north western rim of the Tertiary Cheb Basin, western Eger rift,
central Europe. Geology and major faults according to Mahel
´ et al. (1984), Zoubek et al. (1990) and Bankwitz et al. (2003).
°
Quaternary volcano: KH, Komorní Hurka/Kammerbühl;
Ellipse: Epicentral region Novy´ Kostel.
lar hydrogen was released in connection with tectonic
stress resulting in seismicity (e.g., Sugisaki and Sugiura,
1985) and may act as an indicator for fault activity (Wakita
et al., 1980; Sato et al., 1986). The experimental studies
of Kita et al. (1982) give evidence that H2 is generated
by a mechano-chemical reaction between groundwater
and fresh surfaces of crushed rock material.
Seismically related changes in the concentration and
δ 13CCH4 values have in particular been found in, for instance, New Zealand (Whitehead and Lyon, 1999), the
Caucasus region (Voitov, 2000, 2001), and central
(Quattrocchi et al., 2000) and southern Italy (Italiano et
al., 2001).
The present work reports on the gas composition and
δ 13 C CH4 values from a weekly sampling at the
Wettinquelle mineral spring in Bad Brambach (Vogtland/
Germany) between May 2000 and October 2003. The sampling period included the strong earthquake swarm period lasting from August to December 2000. During these
monitoring studies, seismically induced changes of fluid
composition were impressively demonstrated by the varying concentration and isotopic composition of methane.
These results are discussed in connection with seismically
triggered geochemical processes in the crust.
442 K. Bräuer et al.
BACKGROUND
Figure 1 shows the position of the Wettinquelle mineral spring (Bad Brambach) in relation to its geological
background. Bad Brambach is located on the northern
ˇ
flank of the Fichtelgebirge/Smrciny
granite massif
(Variscan). According to Hecht et al. (1997), the average
thickness of the granite is approximately 2–3 km.
The results of long-time radiometric investigations at
the Wettinquelle mineral spring demonstrated a fluidal
connection to currently the most frequent earthquakeswarm hypocentre at Novy´ Kostel, 10 km east of Bad
Brambach (Heinicke et al., 1995; Koch and Heinicke,
1996; Heinicke and Koch, 2000). Before the swarm earthquake period, at the end of July 2000, significant hydrological effects were recorded at groundwater gauge
drillings in the Cure Park and at the Wettinquelle mineral
spring. The anomalous water level increase lasted more
than four weeks and was followed by the strongest swarm
earthquake period since the 1985/86 events in the
Vogtland/NW Bohemia area (Koch et al., 2003).
The most recent period of strong swarm earthquakes
started in late August 2000 and lasted until late December 2000. More than 10,000 micro-earthquakes in the
Fig. 2. Time series of CO2 content, water temperature and the discharge of the Wettinquelle, Bad Brambach, between May 2000
and October 2003. The gray-signed range mark the period of seismicity after Fischer (2003) and the dashed lines mark the time
span where the methane concentrations and their δ13C value are below the values of the “baseline” signature (see Fig. 3). The
included picture shows the correlation between water discharge and the CO 2 concentrations (black squares) that do not correlate
with water temperature.
Novy´ Kostel focal zone with magnitudes up to 3.3 were
recorded by local seismic networks. Nine swarm phases
were distinguished; five of them include event magnitudes
greater than 3.0. The events were distributed over a steeply
dipping planar area about 3 × 3 km in size. The hypocentre
has a lower boundary at about 10.5 km, whereas its top
margin varies between 7.6 and 8.5 km (Fischer, 2003;
Fischer and Horálek, 2003). The process of swarm
seismicity is reflected by the temporal variation of the
frequency of events and their magnitudes. Focal mechanisms for the 12 strongest events were used to determine
the regional stress field. The stress field in Vogtland/NW
Bohemia does not substantially differ from the overall
stress field in western and central Europe. Dislocations
of the seismic events seem to be controlled by the western European stress field. However, the swarms may have
been triggered and maintained by local sources (Klinge
et al., 2003; Plenefisch and Klinge, 2003). Parotidis et
al. (2003) assume that ascending fluids derived from the
upper mantle trigger earthquakes by the mechanism of
pore-pressure diffusion. This view is supported by their
analysis of the 2000 swarm data.
S AMPLING AND METHODS
The gas composition of the Wettinquelle mineral
spring (50,2207 N, 12,3038 E, Fig. 1) was monitored for
more than three years from May 2000 until October 2003,
weekly for the first two years and fortnightly in the last
year. The δ 13C values of methane were analysed at intervals of four and eight weeks, respectively.
A diagram showing the technical details of
Wettinquelle was given by Koch et al. (2003). The gas
samples were collected in glass vessels with two stopcocks. The vessels were filled with spring water, which
was replaced by the free gas bubbling out of the water in
the glass vessel. Duplicate samples were taken to measure the gas composition and the δ 13C values of methane.
The CO2 content was determined volumetrically, while
other components such as N2, O2, Ar, He, H 2, and CH4
were measured by gas chromatography after CO2 absorption in KOH solution (Weinlich et al., 1998).
For carbon isotope analysis, methane was separated
in a gas chromatograph, combusted to CO2 and analysed
in an isotope ratio mass spectrometer (MAT 252). This
Seismically triggered microbial methane production relating to the Vogtland 443
Fig. 3. Time series of methane concentrations and the associated methane δ 13C values of the Wettinquelle, Bad Brambach,
between May 2000 and October 2003. The gray-signed range marks the period of seismicity after Fischer (2003) that can be seen
in the inlet picture in more detail.
online system (Faber et al., 1998), for very small methane quantities, is based on a conventional system (Dumke
et al., 1989). Carbon isotope ratios are given as δ-values
relative to the CO 2 from the PDB-carbonate. Precision,
determined by repeat analysis of air, is about 0.6‰ (Faber
et al., 1998).
RESULTS
Compositional variation of CO2, CH4 and H 2
The water temperature of the Wettinquelle mineral
spring displays a strong seasonal trend and followed the
air temperature delayed at about two months (Fig. 2). The
free gas phase of this spring is dominated by CO2 and
always contains more than 99 vol.% CO 2. The CO2 concentration correlates with the water temperature and follows the seasonal trend of the water temperature from
the beginning of our monitoring (May, 2000) until the
end of January 2001. The lower the water temperature,
the lower the CO2 concentration of the free gas phase due
to the degree of temperature-dependent CO2 solubility.
Between February 2001 and March 2003, there are several periods which do not correlate with the seasonal trend.
At first sight the periods which differ from the solubilitydependent trend seem to correlate with water discharge
(Fig. 2). However, on analysing these data in more detail
444 K. Bräuer et al.
(Fig. 2, inlet picture) it can be seen that only few data (if
any at all) really follow this trend.
The variations in the methane concentration do not
correlate with either the water temperature or the discharge (Fig. 3). Starting in late October 2000, a sharp
increase in the methane concentration was observed, accompanied by a decrease in the δ13CCH4 values. Although
the variations from the “baseline” signature started at different times, their duration was nearly the same. The level
of hydrogen concentration reached a maximum of 3 ppmv
but often remained below the limit of detection. Interestingly, between January and April 2001, when the highest
methane concentrations were observed, hydrogen could
not be detected. The main component CO2 is characterised by a δ13C value of –4.7‰ and the 3He/4He ratio yields
2.4 (R/Ra).
DISCUSSION
Origin of gas components
Weinlich et al. (1999) presented an extensive characterisation of free gas exhalations in Vogtland and NW
Bohemia, including several mineral springs in Bad
Brambach. CO2 is the main component of all the locations there, and it was shown that the CO2 stems from the
upper mantle. Bräuer et al. (2004) used monitoring re-
Table 1. Gas and water characteristic of the Wettinquelle mineral spring,
Bad Brambach
Parameter
Units
Discharge
ϑH2 O
Conductivity
pH-value
L/h
Gas data
CO2
N2
O2
Ar
He
H2
CH4
C2 H6
δ1 3 CCH4
mS/cm
vol.-%
vol.-%
ppmv
ppmv
ppmv
ppmv
ppmv
ppmv
‰
Number of data
35
35
35
35
Mean value
Variance
between 100 and 270
10.5
0.8
1.77
0.06
5.83
0.1
35
35
35
35
35
23
35
35
12
99.68
0.30
122
72
1.8
0.9
35
0.03
51.2
0.10
0.10
41
23
0.5
1.0
8
0.07
1.2
Means without data from 12.10.00 until 30.12.02.
sults from the Eisenquelle, Bad Brambach to evaluate the
origin of the gas components in more detail. A comprehensive study of the chemical and isotopic composition
of the gas, including the light noble gas isotope ratios
(He, Ne, Ar) and δ15N, proved that the CO 2 stems from
the upper mantle, whereas most of the nitrogen and argon are derived from the atmosphere. The MORB-type
helium content given by Weinlich et al. (1999) was confirmed.
The Wettinquelle discharges in the same fissured granite only about 50 m away from the Eisenquelle. Accordingly, the CO2 from Wettinquelle ought also to be derived
from the upper mantle. Its δ 13C value and 3He/4He ratios
are comparable with those of the neighbouring
Eisenquelle mineral spring (Weise et al., 2001; Bräuer et
al., 2003, 2004). Related to the MORB signature, the free
gas of the Wettinquelle mineral spring contains approximately 30% mantle-derived helium. Based on the neighbourhood of Eisenquelle and Wettinquelle, the origin of
Ar and N2 should also be comparable. The N2/Ar ratio
(42) of the Wettinquelle is only somewhat higher than
that of dissolved air. Adapted from the 40Ar/36Ar ratio of
301 and regard to the results from the Eisenquelle, we
assume that most of the nitrogen and argon are derived
from the atmosphere. The N2/O2 ratio (≈24.5) is clearly
higher (compared to the atmosphere), indicating that most
of the oxygen was consumed by microbial and/or chemical reactions in the groundwater.
The average methane concentration yielded ≈35 ppmv
and the “baseline” 13C signature of the methane is –51.2‰
(Table 1). This isotopic signature does not allow methane formation by the thermogenic degradation of organic
matter (Schoell, 1988) to be distinguished from micro-
bial methane production using 13C-enriched CO2 (Sherwood Lollar et al., 1993a; Kotelnikova, 2002) as well as
abiogenic generation of methane can not be ruled out
(Sherwood Lollar et al., 1993b; Potter and KonnerupMadsen, 2003; Ward et al., 2004).
The Wettinquelle mineral spring is originally fed by a
fissured aquifer in the Fichtelgebirge granite. The northern boundary of the granite massif is tangent to Bad
Brambach. The methane concentration is low and not
comparable with the methane concentration in natural gas
reservoirs (Jeffrey and Kaplan, 1988). In the free gas of
the Wettinquelle, the concentration of higher hydrocarbons is extremely low, and as a result the C1/C2 (CH 4/
C2H6) ratio is higher than expected for the exclusively
thermogenic degradation of organic matter (Riedel et al.,
2001). Sherwood Lollar et al. (2002) use the carbon isotope ratios of higher hydrocarbons to distinguish between
thermogenic and abiogenic formed hydrocarbons. In the
case of thermogenic produced hydrocarbons the higher
hydrocarbons are more enriched in δ13 C compared to
methane, whereas in the case of abiogenic generation the
δ13C values of the higher hydrocarbons are depleted in
comparison with methane. In the free gas of the
Wettinquelle the concentration of higher hydrocarbons as
ethane was below the detection limit, and also the ethane
concentration was too low for the analysis of δ 13CC2H6.
Therefore, we can not apply this criterion. But, C1/C2 ratios higher than 1000 let us assume a dominantly biogenic methane generation.
A mixture of methane formed biogenically near the
surface (δ13C ≈ –80‰) and highly 13C-enriched upper
mantle methane (13C ≈ –15‰; e.g., Welhan, 1988) could
also result in the δ13C-methane value measured. The mag-
Seismically triggered microbial methane production relating to the Vogtland 445
matic reservoir of the Bublak gas (Bräuer et al., 2003,
2004) also feeds the mineral springs of Bad Brambach.
The gas consists of CO2 derived from the upper mantle
and minor amounts of trace elements (He, N2 and possibly CH 4), and represents the mostly unchanged magmatic
signature of the free gas exhalations in Vogtland/NW
Bohemia, even though it contains only ≈3 ppmv methane. Therefore, the admixture of such methane derived
from the upper mantle must be of minor relevance.
In addition to microbial methane formation in the granite, either thermogenically formed methane derived from
buried organic material of unknown origin or methane
formed by abiogenic methanogenesis involving FischerTropsch reaction mechanisms (e.g., Sherwood Lollar et
al., 1993b) needs to be taken into account.
To sum up, the “baseline” methane signature arriving
at Wettinquelle may be microbially originated with admixture of thermogenic/abiogenic (shallow and/or deeper
granitic) formed methane. The exact mixing ratios cannot yet be determined. Traces of methane derived from
the upper mantle cannot be ruled out.
Seismically related temporal variations in the gas composition and δ13C signature of methane
Isotope-geochemical changes in the free gas phase
(3He/4He, δ13CCO2) were observed for the first time in the
Bad Brambach area after the small swarm earthquake on
4–5 December 1994 at the neighbouring Eisenquelle mineral spring (Weise et al., 2001).
Similarly, a few months after the beginning of the 2000
swarm earthquake period (August 2000) at Wettinquelle,
anomalies were observed due to a decreasing CO 2 concentration and an increase in the non-acid gas components, respectively (Fig. 2). There are several periods of
such variations that started at the beginning of February
2001 and lasted until late March 2003. The relative decrease in CO 2 is connected to an increase in nitrogen, argon, helium and methane.
We showed that most of the nitrogen and argon may
be derived from dissolved air. However, given the simultaneous He increase, it is likely that the additional N 2 and
Ar may also stem from a deeper source. The isotopic variations ( 13CCO2, He) established at Eisenquelle after the
small swarm quake event in December 1994 could be
explained by the admixture of crust-derived fluids released by rupturing processes inside the hypocentre and
transported with the flux of CO 2 derived from the upper
mantle (Weise et al., 2001).
Following this argumentation, the increased He, Ar
and N 2 concentrations measured several months after the
beginning of the swarm quake period could be interpreted
as follows. Because the hypocentre migrates during the
different seismic phases (Fischer, 2003), the stored
volatiles were mobilised from different crustal areas and
446 K. Bräuer et al.
admixed to the upper mantle derived CO2 flux. The first
volatiles mobilised in this manner arrived at Wettinquelle
after about 150 days, this time span being in the same
order of magnitude as that observed after the swarm quake
event on 4/5 December 1994. Sugisaki et al. (1996) argued that the duration of post-seismic gas anomalies
seems to correlate with seismic intensity.
The sharp rise in the methane concentration was observed just eight weeks or so after the start of the swarm
earthquake period in 2000 at the end of October 2000.
Due to the simultaneous decrease in the δ13CCH4 values,
the additional methane has to be of microbial origin. Explaining the more negative δ13CCH4 values with the rising methane concentration (Fig. 3) at the Wettinquelle
mineral spring would indicate that the fractionation factor αCO2-CH4 of microbial methane formation in the granite must be higher than commonly found (Whiticar, 1999).
In fact, Fey et al. (2004) observed that microbial conversion from CO2 to CH4 may be connected to relatively large
fractionation factors (αCO2-CH4 = 1.073–1.083).
Valentine et al. (2004) studied the carbon and hydrogen isotope fractionation by methanogens using a flowthrough bio-reactor and found a variation dependant upon
the reaction conditions resulting in a range of the
fractionation factor αCO2-CH4 between 1.023–1.064.
Because the variation of the fractionation may depend
on the environmental conditions, the “baseline” δ13CCH4
signature could be caused dominantly by microbial produced methane and the increased methane generation after the swarm earthquake happened under changed environmental conditions and consequently result in another
fractionation.
Based on the extreme CO2 excess in the water, the
CO2 concentration monitored does not indicate the additional CO2 consumption. The variations provoked by seasonal effects are stronger than those of additional microbial CO2 consumption.
For some years the research of the deep biosphere
becomes more and more important. One topic is the “hydrogen driven deep biosphere” that means the production of organic matter takes place independent of photosynthesis using hydrogen and carbon dioxide from the
deep crust of the earth as energy and carbon sources, respectively (Pedersen, 1997, 2000, 2001; Chapelle et al.,
2002). The limitation for an active microbial life might
be the availability of H2 as energy source (Pedersen,
1999).
There are several potential processes in the earth’s
crust that result in the generation of H2. That may be, for
instance, the decomposition of methane to graphite and
hydrogen at temperatures of more than 600°C, the
radiolysis of water by radioactive isotopes (U, Th and
their daughters) and further numerous water-rock interaction processes, including the catalysis of silicates un-
der stress in the presence of water (e.g., Pedersen, 2001;
McCollom and Seewald, 2001; Potter et al., 2004). Kita
et al. (1982) give evidence for the H2 generation by reaction of freshly crushed rock material and water due to a
mechanism of H2 degassing along active fault zones.
Pedersen (e.g., 1997, 2000, 2001) reported the presence of autotrophic micro-organisms in deep granitic aquifers that utilise hydrogen as a source of energy for methane production. Autotrophic methanogens produce methane from hydrogen and CO 2 . In granitic rocks, the
radiolysis of water represents a plausible process for permanent H2 generation due to the steady decay of radioactive elements such as uranium, thorium or radium that
could supply the energy for such a process. Additionally,
radon gas is produced.
The Wettinquelle at Bad Brambach is one of the most
radioactive springs on earth (≈26 kBq/L). Uranium/radium accumulations in the granite where the Wettinquelle
discharges are the source of the high radioactivity (Koch
et al., 2003). The radiation produced by the decay of these
elements may generate sustainable hydrogen. In this sense
the subsurface radon resource of the Wettinquelle may
be an indication of the presence of hydrogen formed by
radiolysis.
Supposing the hydrogen supply is the limiting factor
in microbial methane production, the rise of the methane
concentration and the simultaneous decreasing trend of
methane δ13C values has to be explained by the increasing generation of microbial methane. This in turn may be
prompted by an additional hydrogen reservoir. During the
swarm earthquake period, nine separate phases of events
could be distinguished between late August and late December 2000 (Fischer, 2003). Evaluating the variations
in methane concentration, several peaks can be seen and
it took more than two years before the primary methane
concentration and the associated δ13 CCH4 values were
reached again.
Pre/co-seismic stress changes inside the Bad
Brambach granitic aquifer may result in releasing of hydrogen being permanently produced and stored in the fissured granite. Additional compression could be produced
in connection with the triggering of different phases of
seismicity (Parotidis et al., 2003).
The majority of the hydrogen (trapped) may be released during the first phase of the swarm earthquake,
explaining why the highest gain in microbially produced
methane is observed at this time. Later swarm event
phases would then be due to smaller effects concerning
the concentration and the δ 13CCH4 values. Why the increasing methane concentration at Wettinquelle is delayed
by only about eight weeks could be explained by the time
needed by the methane to get from the area of formation
(deep granite) to the Wettinquelle mineral spring. If so,
the observed variation in the methane concentration would
have occurred pre-/co-seismically. Hydrological stressinduced indications before the beginning of the period of
seismicity (Koch et al., 2003) back up the assumption
that the additional microbial methane production may
have been started pre/co-seismically.
Hydrogen anomalies with large seismic events have
been reported previously (e.g., Sugisaki and Sugiura,
1986; Sato et al., 1986). In addition, when monitoring
the gas composition of groundwater Ito et al. (1999)
proved that the release of H2 was related to micro-earthquakes (M < 3). These studies indicate that such
seismically related geochemical signals may even be observed in segments outside the final rupture zone.
CONCLUDING R EMARKS
The weekly monitoring of the free gas of the
Wettinquelle mineral spring provided indications of seasonal effects and seismically related changes in the gas
composition.
Several phases of the relative increase in crust-derived
components (He, N2, Ar) were observed to start about 150
days after the beginning of the strong swarm quake period in 2000.
The increase in methane concentration started only
about 60 days after the beginning of the swarm quake
period, indicating the occurrence of a number of
seismically related processes. The rise in methane concentration was accompanied by decreasing δ13C values
due to additional microbially formed methane in the granitic aquifer of Bad Brambach.
Although both seismically related variations were
observed post-seismically, the change in the methane concentration and its isotope composition can probably be
attributed to the pre/co-seismic hydrogen escaping due
to crustal stress changes before seismic events. By contrast, the higher N2 and He concentrations may be coseismic in origin, given the rupturing processes in the
hypocentral area.
Acknowledgments—We would like to thank J. Tesar for measuring the gas composition. Thanks are due to Yuji Sano and
Donald Thomas for their helpful comments. This project was
kindly funded by the German Research Foundation (Deutsche
Forschungsgemeinschaft, grant nos. Str 376/6 and Ka 902/7).
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