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200‐m‐deep earthquake swarm in Tricastin (lower Rhône Valley, France) accounts for noisy seismicity over past centuries


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In the lower Rhône Valley (France), the Tricastin area was struck in 2002–2003 by an earthquake swarm with a maximum ML-magnitude of 1.7. These shocks would have gone unnoticed if they had not occurred beneath habitations and close to the surface, some events being only 200-m deep. A several months’ monitoring of the seismic activity by a 16-station mobile network showed that earthquakes clustered along a N–S-trending, at least 5-km long, shallow rupture zone, with no corresponding fault mapped in the surface. Half of the seismic events occurred in a massive, c. 250-m-thick, Lower Cretaceous limestone slab that outcrops near by. Since the late eighteenth century, several much more severe earthquake swarms have struck Tricastin. The 1772–1773 and 1933–1936 swarms were prolific and protracted, with reports of numerous detonations and even damage. Obviously, the abnormal noises that caused panic in the past centuries can be explained by the shallowness of the phenomena, a 200-m focal depth being perhaps a record value for tectonic earthquakes.
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200-m-deep earthquake swarm in Tricastin (lower Rho
ˆne Valley,
France) accounts for noisy seismicity over past centuries
Franc¸ois Thouvenot, Liliane Jenatton and Jean-Pierre Gratier
Laboratoire de ge
´ophysique interne et tectonophysique (CNRS UJF), Observatoire de Grenoble, France
Between the French Massif Central to
the west and the Alps to the east, the
ÔSillon Rhoˆ danienÕ(Fig. 1) is the
southern branch of the European
Cainozoic Rift System that dislocated
western Europe from the North Sea to
the Mediterranean (De
`zes et al.,
2004). In its middle part, midway
between Valence and Avignon, the
Tricastin area has long been recog-
nized as the seat of long-lasting earth-
quake swarms: besides the classical
way of releasing seismic energy
through mainshock–aftershock se-
quences, earthquake swarms are char-
acterized by long series of large and
small shocks, with no outstand-
ing principal event. The term
ÔSchwarmbebenÕ(i.e. Ôswarm quakeÕ)
was first used by Knett (1899) to
describe the random seismic activity
observed in the border region between
Germany and the Czech Republic
(Vogtland NW Bohemia), where this
phenomenon is frequent.
Swarms are common in volcanic
regions such as Japan, Central Italy,
Afar or oceanic ridges where they
occur before and during eruptions.
They are also observed in zones of
Quaternary volcanism such as Vogt- land NW Bohemia, where fluid
migration in a magmatic environment
can be invoked (e.g. Hainzl and
Fischer, 2002). In intraplate regions
´k, 2000) or orogenic belts, for
instance in the western Alps (Jenatton
et al., 2007), the dynamic evolution of
earthquake swarms remains more
mysterious, even if fluid migration is
a likely regulating factor (Daniel
et al., 2009).
Although hydrothermal sources are
documented, Tricastin is clearly not a
volcanic region. The boundary be-
tween Eurasia and the colliding Adri-
atic microplate, usually likened to the
In the lower Rho
ˆne Valley (France), the Tricastin area was struck
in 2002–2003 by an earthquake swarm with a maximum
magnitude of 1.7. These shocks would have gone unnoticed if
they had not occurred beneath habitations and close to the
surface, some events being only 200-m deep. A several monthsÕ
monitoring of the seismic activity by a 16-station mobile
network showed that earthquakes clustered along a N–S-
trending, at least 5-km long, shallow rupture zone, with no
corresponding fault mapped in the surface. Half of the seismic
events occurred in a massive,
250-m-thick, Lower Cretaceous
limestone slab that outcrops near by. Since the late eighteenth
century, several much more severe earthquake swarms have
struck Tricastin. The 1772–1773 and 1933–1936 swarms were
prolific and protracted, with reports of numerous detonations
and even damage. Obviously, the abnormal noises that caused
panic in the past centuries can be explained by the shallowness
of the phenomena, a 200-m focal depth being perhaps a record
value for tectonic earthquakes.
Terra Nova, 21, 203–210, 2009
Correspondence: Franc¸ ois Thouvenot,
Laboratoire de ge
´ophysique interne et
tectonophysique, Maison des ge
BP 53, 38041 Grenoble Cedex, France.
Tel.: +33 (0) 4 76 63 51 50; fax: +33 (0) 4
76 63 52 52; e-mail:
Fig. 1 Simplified map of the southern Rhoˆ ne Valley, with main geological contours
after Service de la Carte ge
´ologique de la France (1969): cross pattern, French Massif
Central; shaded, Mesozoic; blank, Cainozoic and Quaternary; barbed lines, main
thrusts; other faults mainly involve strike-slip motion. Dash-dotted lines in the
eastern part of the map: fold axes from Gratier et al. (1989). Box: study area of Fig. 2.
2009 Blackwell Publishing Ltd 203
doi: 10.1111/j.1365-3121.2009.00875.x
Piedmont seismic arc in Italy, is
located 200 km to the east (Thouve-
not and Fre
´chet, 2006). The Provenc¸ al
domain of the Alpine belt begins just
to the east of Tricastin, but the most
active part of the orogen in terms of
deformation and seismicity is located
150 km farther east. Thus, although
Tricastin is sited in the Sillon Rhoˆ da-
nien, which indeed has suffered exten-
sion since the Cainozoic, one may
rather classify it as an intraplate
Two of the earthquake swarms that
struck Tricastin in 1772–1773 and
1933–1936 have been described in
testimonies as accompanied by explo-
sion noises similar to cannonades.
´(1936) first evoked shallow
seismicity to explain these auditory
phenomena, but he lacked reliable
seismic data to argue this point, and
anyway shallow focal depths were
considered with scepticism at that
time when earthquakes were believed
to be usually seated much deeper in
the crust.
What seismologists now consider as
Ôshallow seismicityÕis of course a
matter of scale. At a global scale,
ÔshallowÕearthquakes are those that
occur in the first 40 km of the EarthÕs
interior. Man-made or volcanic seis-
micity documents events much closer
to the surface. In mines or gas fields,
seismicity usually occurs between the
surface and the depleted layers, and
focal depth values of a few hundred
metres are common. Earthquakes ow-
ing to dam filling usually occur much
deeper in the crust beneath the reser-
voir. Dry-rock experiments show that
fracturing occurs in a several hundred-
metre zone over and below the
injection point, which is usually sev-
eral-kilometre deep (e.g. Phillips,
2000). However, a thorough search
in the literature for shallow tectonic
earthquakes does not provide any
evidence for foci shallower than 1 or
2 km. In the following, the term Ôultra-
shallow seismicityÕwill be used to refer
to events with focal depths shallower
than 1 km.
The Tricastin 2002–2003 earth-
quake swarm revived memories of
the conflagration-like noises heard
in the previous episodes. When we
became aware of this phenomenon, we
judged that it offered a rare opportu-
nity to understand its origin, all the
more so as it could evolve into a
sequence of destructive events, in the
close vicinity (10 km) to major nuclear
installations on the western bank of
the Rhoˆ ne river. This article details
information gained by the deploy-
ment of a temporary seismic net-
work, and insists on the fact that
observed events were indeed ultra-
Stratigraphy and tectonics
Our study area is 4420.5¢N–4426¢N
and 444¢E–450.5¢E (Box, Fig. 1).
Between the widely stretched-out
Rhoˆ ne Valley to the west and the
Visan Miocene Basin to the east,
Tricastin emerges as a region of Cre-
taceous and Cainozoic jagged hills,
with outcropping stratigraphy ranging
from Barremian (118–106 Ma) to
Burdigalian (20–15 Ma). The Upper
Barremian stage is characterized, as
elsewhere in southeast France, by the
Urgonian facies. This massive, hard,
white, reef limestone formation has an
approximate thickness of 250 m (Ser-
vice de la Carte ge
´ologique de la
France, 1975). In the study area, it
outcrops only in a limited flat zone, to
the southeast of La Garde-Adhe
(Fig. 2). Seismic and electric explora-
tion in the whole Tricastin area indi-
cates that the top of the Urgonian
slab, often only a few tens of metres
deep, is slashed by a complex fault
network (Service de la Carte ge
que de la France, 1964).
Major faults, west of the Rhoˆ ne
River, involve mostly strike slip along
the N40–N60E Hercynian direction
(Fig. 1). Some probably extend far-
ther to the east, where they meet the
Alpine domain, here characterized by
north- and south-verging thrusts, and
N–S-striking faults. Most probably
because of their poor preservation
within loose sedimentation, few tec-
tonic fractures are known in the study
area. However, seismic exploration
recognized several Upper Miocene
N–S-striking normal faults in the
Rhoˆ ne Valley. With throws reaching
several hundred metres, their juxtapo-
Fig. 2 Temporary seismological stations (triangles) and 38
relocated earth-
quakes, with a lighter shade for events shallower than 200 m. Symbol size is
proportional to the magnitude. Station CLAN marked by a white dot; Chabrelet is
the hypothetical epicentre of the 1936 earthquake swarm. Topographical contours are
at 10-m vertical intervals.
200-m-deep earthquake swarm in Tricastin (France) F. Thouvenot
et al.
Terra Nova, Vol 21, No. 3, 203–210
204 2009 Blackwell Publishing Ltd
sition makes this zone a real rift
(Service de la Carte ge
´ologique de la
France, 1964). In the study area, other
minor features affect Aquitanian lime-
stones to the northeast of La Garde-
´mar, where several conjugate
faults striking NW–SE and SW–NE
are documented.
Historical seismicity
The earthquake swarm that visited
Tricastin between June 1772 and
December 1773 is particularly well
documented by a contemporaneous
four-page report (Revol, 1773) and a
geological investigation (Faujas de
Saint-Fond, 1781). It affected the
whole study area shown in Fig. 2
(Clansayes, Sole
´rieux, Chantemerle,
Valaurie, Les Granges-Gontardes),
with more than 60 felt events (Boisse,
1936; Rothe
´, 1941). The old village of
Clansayes, perched on an outlier, had
its church tower knocked down by the
strongest event of the sequence (23
January 1773, maximum MSK inten-
sity I
= VII–VIII). According to
Revol (1773), the epicentral area
seems to have migrated afterwards,
and houses at Saint-Raphae
¨l (Sol-
´rieux), to the southeast of Clansayes,
suffered cracking damage from sub-
sequent events. Faujas de Saint-Fond
(1781) conversely states that, at the
end of the swarm in 1773, earthquakes
were more felt in villages to the
northwest of Clansayes. Throughout
the 19 months of the swarm, under-
ground noises similar to cannon
explosions were reported, whereas
earth vibrations did not seem to be
systematically noticeable.
In 1933–1936, another swarm vis-
ited the same area. This time most of
the underground noises were reported
in the northern villages of Les
Granges-Gontardes and La Garde-
´mar (Rothe
´, 1936) – although
this statement might be biased by the
detailed observations left by Abbe
Boisse who precisely exercised his
priesthood at Les Granges-Gontardes.
The swarm was active between Octo-
ber 1933 and December 1934. After
10 months of quiescence, the activity
burst again in October 1935 till
August 1936. The total swarm activity
amounted to 24 months, with a climax
being reached by mid-May 1934 when
shocks were reported to be felt every
minute during the night of the 11–12
May 1934 (Boisse, 1936). A few hours
later, a stronger shock damaged sev-
eral churches and houses at Vallaurie,
Roussas and La Garde-Adhe
with chimneys and one church
tower knocked down (12 May 1934,
= VII). Further slight damage
was reported at Clansayes on 11
January 1936 (I
= V), and at La
´mar and Les Granges-
Gontardes on 13 February 1936 (I
= VI; Rothe
´, 1939a).
From the report of underground
noises, Rothe
´(1936) estimated that
the epicentre was situated at Chabr-
elet, southeast of Les Granges-Gon-
tardes (Fig. 2). He first tried to
determine the focal depth of an event
that occurred during the night of the
11–12 May 1934, and which was
particularly well recorded by four
seismological observatories (Clermont
and Strasbourg in France; Neuchaˆ tel
and Zurich in Switzerland). The clos-
est instrument (Clermont) being
200 km away, this attempt was
doomed to failure. However, Rothe
believed a 0-km focal depth better
fitted observed arrival times. He also
tried (Rothe
´, 1939b) to use isoseismal
curves observed for the 13 February
1936 event, but the various empiric
relations he used provided scattered
values (between 4 and 18 km). He
judged them unrealistic. In one last
attempt, Rothe
´(1939b) made use of
records provided for the 1936 active
period by two horizontal Mainka-
SOM seismographs, which had been
installed at Les Granges-Gontardes in
July 1934. From the average S–P
interval of 1.2 s, and taking into
account that the station was 2.5 km
away from his preferred epicentral
zone, he computed a focal depth of
3 km. However, when we scrutinized
original seismograms recorded on
smoked paper with a drum speed of
0.25 mm s
, we found such minute
S–P intervals hardly discernable.
From all these attempts, one con-
cludes that the shocks were shallow,
even if one cannot prove that they
were ultra-shallow.
The 2002–2003 earthquake swarm
The 2002–2003 earthquake swarm ini-
tiated at the beginning of Decem-
ber 2002 by shocks perceived as
explosions by the inhabitants of a
c. 20-house hamlet close to Clansayes.
These abnormal sounds were not at
once identified as earthquakes by the
inhabitants because local earthquakes
are inexistent in the inter-swarm qui-
escence periods, and – to our knowl-
edge – the latest felt swarm dates back
to 1933–1936. A temporary velocimet-
ric station (CLAN) was installed in
the basement of one of the houses at
the end of December 2002.
On several seismograms recorded
by this station, we observed events
with an S–P interval of only 45 ms
(Fig. 3), which implies a very shallow
focus. In the first minutes of the New
YearÕs Day, 2003 (31 December 2002
UTC), two stronger (and felt) earth-
quakes occurred at 23:19 (M
= 1.3)
and 23:20 (M
= 1.7), both with S–P
intervals of about 100 ms. It
prompted us to install another 15
mobile stations (Fig. 2): 11 were fitted
with velocimeters and 4 with acceler-
ometers from the French mobile ac-
celerometric network. Although this
network has been operated for
8 months, seismic events were de-
tected only during the first three
months. During this period (10 Janu-
ary–7 April 2003), we located 51
events with magnitudes ranging from
Fig. 3 Tricastin swarm earthquake
recorded by three-component station
CLAN in the epicentral area (see posi-
tion in Fig. 2). Seismometers have a
2-Hz natural frequency. Amplitude win-
dow for each component (vertical, N–S
and E–W) is ±100 lms
. Three-sec-
ond time scale; sampling frequency is
200 Hz. The minute 45-ms S–P interval
is clearer on the E–W component, bot-
tom signal.
Terra Nova, Vol 21, No. 3, 203–210 F. Thouvenot
et al.
200-m-deep earthquake swarm in Tricastin (France)
2009 Blackwell Publishing Ltd 205
)0.7 to 1.4. For the present study,
earthquakes were first picked and
located using the
´chet and Thouvenot, 2000), which
enables an interactive control of picks.
We then used
, a modified version of the
programme (Lee and Lahr,
1975), with a one-dimensional velocity
model consisting of two 100-m thick
layers, with P-wave velocities of 2 and
, and a 5.3 km s
underneath (S-wave velocities were
derived by assuming a V
of 1.71). This model was built on
velocity measurements obtained for
similar sedimentary series at the Eguil-
les borehole, to the south of Avignon
(Mari, 1977). We eventually formed
travel time differences from P- and
S-picks and used the
gramme (Waldhauser and Ellsworth,
2000; Waldhauser, 2001) to improve
location precision.
Out of the initial 51 events, 38 only
were relocated (Fig. 2) because relo-
cation demands a higher data quality,
and events recorded by too few
stations are excluded. We prefer
relocated events which, although few-
er, are more reliable (Jenatton et al.,
2007). Although relocation involves
relative positioning, we have a good
control here on how the centroid of
the relocated swarm is positioned: the
largest-magnitude event (1.4) that
occurred on 26 January 2003 (Fig. 4)
is relocated right beneath the station
CLAN (triangle with white dot in
Fig. 2), in accordance with what could
be ascertained from a P-wave almost
exclusively recorded on the vertical
component at that station. However,
we note a slight vertical discrepancy
between the 400-m relocated depth
(relative to sea level), and the 45-ms
S–P interval observed at that station,
which rather corresponds to a 200-m
focal depth (relative to the sea level,
with a mean surface elevation of
100 m). We have not attempted to
correct this, which means that depth
values used in the following might
be overestimated (but by 200 m at
Although activity was maximal
right beneath CLAN (Fig. 2), other
shocks were detected along a N–S-
trending zone whose length reaches at
least 5 km, even if one excludes the few
events that occurred at Roussas and
´rieux. Half of the events occurred
in the 0–200-m depth range (Fig. 5);
half of the remaining foci clustered in
the 400–600-m depth range; few others
occurred at a depth of about 1000 m.
Migration of epicentres with time is
uneasy to detect. However, earlier
events in the series were deeper and
more clustered in the central part of
the active zone (Fig. 6). While becom-
ing shallower, activity has migrated
southwards and northwards since the
beginning of February.
Figure 7 shows the complete time
series for the earthquake swarm.
Station CLAN, installed in December
2002, recorded many small-magni-
tude shocks whose epicentres cannot
be located. For most of them, P- and
S-wave arrivals can be read on
seismograms. We considered that an
event belonged to the earthquake
swarm whenever the observed S–P
interval was smaller than 500 ms.
These earthquakes daily recorded at
CLAN provide an estimate of the
swarm activity. For 79 events that
could not be located, we estimated
the M
magnitude by assuming that
earthquakes were beneath CLAN.
Figure 7 also includes magnitudes
for the 51 earthquakes located by
the temporary network, so that the
magnitude series totals 130 events,
with magnitude ranging from )1.3 to
1.7. Activity was variable in the
course of the 4 monthsÕperiod. The
two New YearÕs Day shocks in-
disputably generated aftershocks,
whereas other ÔlargeÕshocks in Janu-
ary and March did not. For located
events, we observed variable intervals
between consecutive events, ranging
from less than 1.5 s to more than
10 days.
Fig. 4 Example of normalized amplitude signals recorded by stations of the
temporary network for the M
-1.4 26 January 2003 earthquake. Thirteen-second
time scale; epicentral distances range from 0.2 km (top) to 2 km (bottom). Amplitude
windows range from ±300 lms
(top) to ± 9 lms
200-m-deep earthquake swarm in Tricastin (France) F. Thouvenot
et al.
Terra Nova, Vol 21, No. 3, 203–210
206 2009 Blackwell Publishing Ltd
Earthquake populations are classi-
cally characterized by the Gutenberg–
Richter law (Gutenberg and Richter,
log10 N¼abM ;
where Nis the number of earthquakes
with magnitudes larger than or equal
to M. Figure 8 shows the frequency–
magnitude distribution for the 130
events of the magnitude series. The
deviation from the Gutenberg–Rich-
ter law for negative magnitudes obvi-
ously results from our catalogue being
incomplete for small-magnitude earth-
quakes. The bvalue of 1.0 ±
0.3, a figure similar to that found for
the western Alps as a whole (0.95 ±
0.03), was estimated by a maximum-
likelihood analysis (Aki, 1965; Utsu,
1966). The large uncertainty for b
partly results from the Tricastin series
being limited in size, but also from
significant variation with time: if we
split the time series into two and
analyse its two halves, bdecreases
from 1.2 ± 0.4 at the beginning of the
swarm (ÔdeepÕevents) to 0.8 ± 0.3 at
its termination (events shallower than
0.2 km).
Discussion and conclusions
No fault has ever been mapped at the
surface where the approximately N–S-
trending, c. 5-km long, 0- to 1-km
deep rupture zone imaged by the 38
relocated events was identified. One
reason is that most brittle Lower
Cretaceous series that could be used
as tectonic markers are obliterated in
the study area by looser sediments.
However, we mentioned that seismic
exploration recognized several Upper
Miocene N–S-striking faults in the
Rhoˆ ne Valley and that electric explo-
ration reveals extensive fracturing of
the top of the Urgonian slab. N–S-
oriented topography in the central
part of Fig. 2 between Clansayes and
Valaurie can also be noticed. The
identification of such a widespread
rupture zone instead of a pinpointed
focal zone can explain the impression
of migrating events reported by inhab-
itants during episodes of the past
The main peculiarity of the Trica-
stin swarm is its ultra-shallowness,
other swarms being usually deeper-
seated (Table 1). For station CLAN,
we indeed observed S–P intervals of
only 45 ms. If we assign a P-wave
velocity of 5.3 km s
to the massive
Urgonian limestone slab that outcrops
1.5 km from CLAN, and if we use a
ratio of 1.71, the hypocen-
tral distance would be c. 300 m.
This would be the focal depth value
(relative to the surface) for a focus
right beneath the station; a still
shallower value would be obtained
Half of the events occurred in the
0–200-m depth range (Fig. 5), which
very likely corresponds to the 250-m
thick Urgonian slab that outcrops
nearby. Seismic foci also cluster
400 m below in the 400–600-m depth
range, in the midst of the Lower
Cretaceous sediments where relatively
(a) (b) (c)
Fig. 5 Depth histograms for the 38 relocated earthquakes. (a) All events; (b) events
before 1 February 2003; (c) events after 2 February 2003.
Fig. 6 As a function of time (vertical
axis), position of epicentres along the N–
S axis (km 0 is station CLAN, dotted
triangle in Fig. 2). Each event is repre-
sented by a circle with radius propor-
tional to the magnitude. Solid circles
represent events with focal depth larger
than 200 m and open circles indicate
events with focal depth shallower than
200 m.
Fig. 7 Full-time series for the 2002–2003
Tricastin earthquake swarm. Histogram
shows daily number of earthquakes
detected by station CLAN. An M
magnitude was computed for events that
could be located (plotted as circles with
radii proportional to the magnitude);
some smaller events recorded at CLAN
were also assigned a magnitude under
the assumption that the focus was right
beneath the station. Note that, although
monitoring continued till August 2003,
no event could be located later than
Fig. 8 Cumulated frequency–magnitude
distribution for the 130 events of
the series yields a Gutenberg–Richter
b-value of 1.0 ± 0.3.
Terra Nova, Vol 21, No. 3, 203–210 F. Thouvenot
et al.
200-m-deep earthquake swarm in Tricastin (France)
2009 Blackwell Publishing Ltd 207
rigid limestones alternate with marls.
A third cluster is sited at a depth of
about 1000 m. This is precisely where
another rigid limestone slab can be
expected in the stratigraphy (the so-
called Tithonian facies characteristic
of Upper Jurassic series in southeast
France). Thus, the upper and lower
clusters occurred in brittle limestone
slabs and the intermediate cluster in a
relatively rigid part of the series.
We could not evidence any migra-
tion of epicentres with time for the
beginning of the swarm (before 1
February 2003), when only the central
part of the rupture zone was active
(Fig. 6). After that date, activity seems
to have spread southwards and north-
wards by several kilometres. We prob-
ably lack a detailed description of the
swarm at earlier times (i.e. in Decem-
ber 2002) to understand this pheno-
menon. As a result of its suddenness,
it cannot be attributed to fluid diffu-
sion from a single source point, as is
sometimes observed for other deeper
swarms (see, e.g. Hainzl and Fischer,
2002; Daniel et al., 2009).
Figure 6 also shows that, before 1
February 2003, the swarm involved
ÔdeepÕearthquakes (with focal depths
larger than 200 m). In contrast, all
later earthquakes but three clustered
in the first 200 m. Thus, the extension
of seismic activity we observed along
the rupture zone in the last part of the
swarm series was accompanied by the
upward migration of seismic foci. This
difference between the two halves of
the swarm series can also be evidenced
with regard to the bvalue, which
decreased from 1.2 (deeper, earlier
events) to 0.8 (shallower, later events).
Hainzl and Fischer (2002), when anal-
ysing the Vogtland NW Bohemia
swarm, that is in contrast much deeper
(8.5 km), also observed similar b-
value variations.
As past swarm episodes that were
marked by detonation-like sounds, the
2002–2003 Tricastin earthquake
swarm was noticed for its many felt
events. This can be surprising for
magnitudes that did not exceed 1.7.
However, a recent study in the south-
ern French Jura (Thouvenot and Bou-
chon, 2008) showed that earthquakes
sited at a depth of c. 900 m could be
felt even for negative magnitude val-
ues (down to magnitude )0.7). In
Tricastin, as all relocated events but
two have magnitudes larger than )0.7,
and as focal depths are much shal-
lower, practically all shocks could
have been felt or – more probably –
Finally, as with all earthquake
swarms, the most puzzling problem
remains that of the initiation and
duration of the phenomenon. In Tri-
castin, just like in Ubaye, there is an
interesting common belief that earth-
quake swarms are often the conse-
quence of flooding. Seasonal
groundwater recharge and rainfall
have effectively been described as
triggering agents for some swarms
(e.g. Saar and Manga, 2003; Kraft
et al., 2006; Husen et al., 2007). Miller
(2008) theorized that Ôunambiguous
rain-triggered seismicity will only oc-
cur in karst regionsÕ. This hypothesis
is appealing in our particular case
because, where exposed in southeast
France, the Urgonian slab indeed
presents karstic features. However,
daily rainfall at the nearby Monte
mar weather station (Fig. 9) does not
reveal any exceptional variations prior
to the observed swarm activity. In
September 2002, the catastrophic
storm that swept the Avignon region,
50 km to the south of Tricastin, and
whose rainfall is held by Rigo et al.
(2008) as responsible for an increase in
seismicity rate on faults between
ˆ mes and Avignon cannot be traced
in Fig. 9, probably because of its local
– although devastating – character.
Unless we hypothesize that the weath-
er station used here and situated
20 km to the north missed a similar
heavy rain episode in Tricastin, we
Table 1 Mean depth for a selection of instrumentally studied earthquakes swarms.
Depth is referred to the surface where the source text is explicit about surface
elevation, assumed to be referred to the surface otherwise.
Location Date
(km) Reference
Imperial Valley, California (USA) 1975 6 Johnson and Hadley (1976)
Reykjanes Peninsula (IS) 1972 3.5 Klein
et al.
Arkansas (USA) 1982 5.5 Chiu
et al.
Remiremont, Vosges (F) 1984–1985 7 Haessler and Hoang-Trong (1985)
Mammoth Mt., California (USA) 1989 7.5 Hill
et al.
Steigen, Nordland (N) 1992 6.5 Atakan
et al.
Crested Butte, Colorado (USA) 1986 6.5 Bott and Wong (1995)
Izu Peninsula, Honshu (J) 1997 4.5 Aoki
et al.
Manchester (GB) 2002 2.5 Baptie and Ottemoeller (2003)
Colfiorito, Umbria-Marche (I) 1997 6 Chiaraluce
et al.
Vogtland NW Bohemia (D CZ) 1985–2001 8.5 Fischer and Hora
´lek (2003)
Mt. Hood, Oregon (USA) 1980–2002 4.5 Saar and Manga (2003)
Campi Flegrei, Campania (I) 2000 2 Bianco
et al.
Usu Volcano, Hokkaido (J) 2000 5.5 Zobin
et al.
Mt. Hochstaufen, Bavaria (D) 2002 2 Kraft
et al.
Ubaye, western Alps (F) 2003–2004 7 Jenatton
et al.
Obsidian Buttes, California (USA) 2005 5 Lohman and McGuire (2007)
Tricastin, Rho
ˆne Valley (F) 2002–2003 0.5* This study
*With 50% of events in the 100–300-m depth range.
CZ, Czech Republic; D, Germany; F, France; GB, Great Britain, I, Italy; IS, Iceland; J, Japan; N, Norway; USA,
United States of America.
Fig. 9 Daily rainfall at Monte
weather station (Me
´o-France), 20 km
north of Tricastin, over a 10-year period
(1994–2003). Daily rainfall exceeded
100 mm several times over this period
(1994, 1998, 1999, 2000 and 2003), with
a maximum in autumn 1999
(c. 220 mm), but no triggered seismic
activity was ever detected in Tricastin.
The 2002–2003 earthquake swarm fol-
lows a rainy episode very similar to
other autumnal heavy rain events.
200-m-deep earthquake swarm in Tricastin (France) F. Thouvenot
et al.
Terra Nova, Vol 21, No. 3, 203–210
208 2009 Blackwell Publishing Ltd
still lack any clear triggering phenom-
enon that could explain why Upper
Jurassic and Lower Cretaceous series
can be healed for so many tens of
years before suddenly bursting out in
a veritable cannonade.
The Conseil Ge
´ral de lÕIse
`re, the De
gation aux Risques Majeurs (French Min-
istry of the Environment), the Institut
National des Sciences de lÕUnivers (CNRS)
and the Conseil Re
´gional Rhoˆ ne-Alpes
funded the Sismalp network. The Bureau
Central Sismologique Franc¸ ais, the Obser-
vatoire de Grenoble and several Conseils
´raux (Ise
`re, Alpes-de-Haute-Prov-
ence, Haute-Savoie, Ain and Savoie)
supported its running costs. The Conseil
´ral de la Droˆ me granted special funds
for studying the earthquake swarm. The
authors are indebted to M. Garin, Mayor
of Clansayes, and to the inhabitants of
Clansayes and nearby villages who facili-
tated field work between December 2002
and August 2003. Robert Guiguet was
involved in station maintenance and data
processing; accelerometric stations were
installed by several other colleagues from
LGIT whose help is acknowledged. Julien
´chet (IPG Strasbourg) provided the
authors with the 1936 records at Les
Granges-Gontardes. Figures of this article
were drawn by using the GMT software
(Wessel and Smith, 1998). Three anony-
mous reviewers who provided helpful com-
ments are also acknowledged.
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200-m-deep earthquake swarm in Tricastin (France) F. Thouvenot
et al.
Terra Nova, Vol 21, No. 3, 203–210
210 2009 Blackwell Publishing Ltd
... Instrumental data have shown that earthquakes in the VRGB occur within the uppermost 2 km of the crust [10], which provides a reasonable explanation for the acoustic effects and consequent observations by local residents. Similar observations have been reported in Southeastern France, where swarm events occur close to the ground surface [36]. By contrast, in West Bohemia and Vogtland in Central Europe, the focal depths of prolific earthquake swarms have been determined to be between 6.5 and 11 km, or even somewhat deeper clusters [37]. ...
... The 1751 sequence is the only occurrence known for the VRGB in the entire century. One reason behind the attention it received was most likely the prolonged duration, which increased the sensitivity of the population [36]. However, the literacy rates of the local residents were extremely low at the time [46]. ...
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This investigation examines the contemporary documentation of a sequence of low-magnitude earthquakes at the fringes of the Kingdom of Sweden, today Southeastern Finland, in 1751–1752. A total of 11 pages of original correspondence sent from the target village of Svenskby to the Swedish capital Stockholm are reviewed. Newspaper accounts from Sweden and Russia are included in the analysis, and a timeline of the reporting is constructed. A newly created catalog shows over 30 distinct events between the end of October and December 1751 (Julian calendar). The assignment of macroseismic intensity to the earthquakes is hampered by loud acoustic effects that accompany and/or constitute the observations. Maximum intensities are assessed at IV–V (European Macroseismic Scale 1998), and maximum macroseismic magnitudes in the range of MM1.9–2.4, and were probably observed at short epicentral distances close to the ground surface. Comparisons to macroseismic data related to instrumentally recorded earthquakes in the region support the notion of low magnitudes. The data from 1751 provide an analog to modern macroseismic observations from geothermal stimulation experiments. Such experiments have acted as a spur for considering seismic risk from low-magnitude earthquakes whose consequences have seldom previously been a matter for concern.
... Recently, two swarms were observed in 2002-2003 to the east of La Garde-Adhémar, with about 50 localized earthquakes. Several events were felt despite their very low magnitude (ML max = 1.7, the two felt earthquakes being ML = 1.3 and ML = 1.7) because of their extremely shallow depth, down to 200 m [Thouvenot et al., 2009]. ...
... The influence of a large quarry in the immediate vicinity of the Rouvière fault is one of the hypotheses explaining the triggering and the very shallow focal depth of the focus for the Le Teil earthquake , Liang and Ampuero, 2020and De Novellis et al., 2020and author correction, 2021. Unfortunately, apart from the Le Teil earthquake and the swarm studied by Thouvenot et al. [2009], the depth of the earthquakes in the region listed in the instrumental catalogues, including the SI-Hex reference catalogue, cannot be considered as reliable data due to the low density of stations and to the distances from the nearest station, often several tens of kilometres. ...
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The Le Teil earthquake (south France, 11 November 2019, Mw 4.9, 1 km depth, about 4 km of surface rupture) was felt at a distance up to about 300 km. We estimate the EMS98 intensity in each of the affected localities by collecting macroseismic observations via both individual forms, filled in by citizens (2094 testimonies), and collective forms, filled in by authorities (388 localities), and by conducting a field survey in the epicentral zone (24 most damaged cities). Field observations and communal surveys remain essential in the case of structural damage. Intensities deduced from public surveys are preliminary, and their consideration in the final estimates must be limited. The maximum intensity (EMS98) observed is VII–VIII in Le Teil, and 30 localities experienced an intensity VI. The earthquake generated damage () up to about 50 km away and was felt in at least 568 localities.
... 1-D ray-theoretical models (Arrowsmith et al., 2010) and 3-D models based on the parabolic equations of infrasound propagation across regional and global distances (Collins, 1993;Evers et al., 2014) include atmosphere properties and resolve dynamics between the ground and heights up to 120 km altitude in the thermosphere. In contrast, near-surface audible earthquake noise patterns excited by small to moderate earthquakes (Sylvander and Mogos, 2005;Thouvenot et al., 2009;Mäntyniemi, 2022) on the here relevant local scales are often approximated using simple relationships between vertical ground motion and induced sound pressure (Hill et al., 1976;Tosi et al., 2000;Lamb et al., 2021). ...
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Irritating earthquake sounds, reported also at low ground shaking levels, can negatively impact the social acceptance of geo-engineering applications. Concurringly, earthquake sound patterns have been linked to faulting mechanisms, thus opening possibilities for earthquake source characterisation. Inspired by consistent reports of felt and heard disturbances associated with the weeks-long stimulation of a 6 km-deep geothermal system in 2018 below the Otaniemi district of Espoo, Helsinki, we conduct fully-coupled 3D numerical simulations of wave propagation in solid Earth and the atmosphere. We assess the sensitivity of ground shaking and audible noise distributions to the source geometry of small induced earthquakes, using the largest recorded event in 2018 of magnitude ML=1.8. Utilizing recent computational advances, we are able to model seismo-acoustic frequencies up to 25 Hz therefore reaching the lower limit of human sound sensitivity. Our results provide for the first time synthetic spatial nuisance distributions of audible sounds at the 50-100 m scale for a densely populated metropolitan region. In five here presented 3D coupled elastic-acoustic scenario simulations, we include the effects of topography and subsurface structure, and analyse the audible loudness of earthquake generated acoustic waves. We can show that in our region of interest, S-waves are generating the loudest sound disturbance. We compare our sound nuisance distributions to commonly used empirical relationships using statistical analysis. We find that our 3D synthetics are generally smaller than predicted empirically, and that the interplay of source-mechanism specific radiation pattern and topography can add considerable non-linear contributions. Our study highlights the complexity and information content of spatially variable audible effects, even when caused by very small earthquakes.
... At depth, the NW-trending La Lance anticline is underlain by a northeast-dipping antithetic normal fault in the basement, inherited from the Jurassic rifting. As La Lance anticline is located close to the southeastern extremity of a shallow coaxial seismogenic domains known as the Tricastin and Marsanne swarms (Thouvenot et al. 2009), we assume that they are co-genetic and relate to the Neogene to Present reactivation of the deep Marsanne Fault imaged on this seismic profile. ...
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The shallow earthquake of November 11, 2019, at Le Teil and the catalogue of historic seismicity in the SE Basin, together record diffuse seismicity along high-angle faults associated with rift structures of the Tethyan margin. In addition to several evidences of Quaternary surface co-seismic ruptures, archeogeological data suggest at least one highly damaging earthquake in Roman times. Regional seismic profiles currently help define the complex tectonic structure and the Plio-Quaternary slip history of these faults. The surface trace of these faults is not connected to off-sets in the basement level owing to several decollement levels within the Triassic salt, deeper Carboniferous coal measures and shallower Jurassic and Cretaceous blackshale horizons. Analogous to the Northern Apennines, brittle failures along the faults of the SE Basin may occur only within the deep crystalline basement and shallow carbonate layers, while salt and shale horizons deform ductilly. In this context, the salt diapir beneath the Châteauneuf-du-Pape has caused progressive uplift and bending of alluvial terraces throughout the Quaternary. These faults pose a potential danger to nuclear plants and several cities. Regional deep seismic profiles would be required to define the architecture of the fault system within the infra-Triassic substratum. The acquisition of high resolution seismic profiles in the areas where these faults are covered by thick Plio-Quaternary deposits would also provide data to date and quantify the successive episodes of fault reactivations and deformations. Long-term GPS measurements within the different fault blocks also would be required to quantify both the vertical and horizontal motion of each block relative to the others. Ultimately, a denser seismological network is needed to properly locate the focal mechanisms at depth and plot them on an accurate 3D structural model of the regional fault system.
... Jomard et al., 2021). The 2002The -2003 swarm was recorded by a local seismicity network (Thouvenot et al., 2009). More details about seismicity is the Tricastin region can be found in the Supplementary Material. ...
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The so-called site effects caused by superficial geological layers may be responsible for strong ground motion amplification in certain configurations. We focus here on the industrialized Tricastin area, in the French Rhône valley, where a nuclear site is located. This area lies above an ancient Rhône Canyon whose lithology and geometry make it prone to site effects. This study presents preliminary measurements to investigate the local seismic amplification. We deployed three seismic stations in the area for several months: two stations were located above the canyon, the third one was located on a nearby reference rock site. The recorded seismicity was analysed using the Standard Spectral Ratio technique (SSR). The estimated amplification from weak motions reaches a value of 6 for some frequencies. These first results confirm the possibility of estimating seismic amplification using earthquakes recorded for less than one year, in this highly anthropogenic and industrialized environment, despite the local low-to-moderate level of seismicity. Noise-based SSR, that presents an obvious interest in such seismic context, shows also promising results in the area. To complement this empirical approach, we estimated the amplification using 1D wave propagation modelling. This numerical estimate is based on shear wave velocity profiles resulting from geophysical characterization campaigns. Comparison of the two approaches at low frequency, where numerical estimate is considered as the most representative, tends to suggest that edge-generated surface waves may have a strong influence in the local seismic response. This interpretation will be further investigated in the future.
... last accessed February 4, 2020) ( Figure 1a). Thouvenot et al. [2009] found that the 2002-2003 swarm (M max = 1.7), was also very superficial (1 km depth at most). ...
... Swarms are observed in different contexts, such as in regions with a low rate of tectonic deformation [Thouvenot et al., 2009, Jenatton et al., 2007, Hainzl, 2004, like the French Alps. The Alpine region is considered one of the most seismically active areas in France [Drouet et al., 2020]. ...
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In 2017–2019, a seismic swarm was triggered in the Maurienne valley (French Alps), with more than 5000 events detected by the regional SISmalp network. The population, who asked SISmalp to provide information on the processes and the associated risk, felt many earthquakes. In a post-L’Aquila trial context, we conducted a reflection on the scientific and social operational management of the crisis. The geological and tectonic analysis, the deployment of a temporary seismic network, an automatic double-difference relocation procedure (HypoDD) after clustering earthquakes, as well as the interactions with the population and the risk managers, have been carried out jointly. The length and unpredictability of the sequence complicated crisis management and the relations between local authorities and civil protection. The involvement of SISmalp, beyond its main scientific and observation prerogatives, has contributed to moderate the fears of the population by providing scientific explanations. © Académie des sciences, Paris and the authors, 2021. Some rights reserved.
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The Tricastin region in the lower Rhône Valley (France) is affected by an atypical seismic activity characterised by the development of long-lasting and recurrent seismic swarms. Indeed, since the 16th century, hundreds of seismic events sometime associated with underground noises of the explosion have been reported by local inhabitants. However, to date, none of the many scenarios of earthquake generation proposed for the area, involving either tectonics and/or hydrological forcings, appears consensual. To overcome that lack of comprehension, we compile and analyse an 880 seismic-events catalogue derived from both historical macroseismicity and instrumental records. The earthquakes appear to occur at shallow depths similar to those determined below a local network in 2002–2003. We confront to this catalogue models involving hydrological mechanisms, including aquifers elastic loading and karst-drains responses, as well as tectonic mechanisms, including transient aseismic processes and their related effects on the fold hinges or on the local fault planes. Most of the earthquakes are located at short distances from karst drains and fractured fold hinges, possibly affected by transient hydrological changes.
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The 2019, Mw4.9 Le Teil earthquake occurred in south-eastern France, causing substantial damage in this slow deforming region. Field observations, remote sensing and seismological studies following the event revealed that coseismic slip concentrates at shallow depth along a ∼5 km long rupture associated with surface breaks and a thrusting mechanism. We further investigate this earthquake by combining geological field mapping, 3D geology, InSAR time series analysis and a coseismic slip inversion. From structural, stratigraphic and geological data collected around the epicenter, we first produce a 3D geological model of the region surrounding the rupture using the GeoModellerTM software. Our model includes the geometry of the geological layers and of the main faults, including the La Rouvière Fault, the Oligocene normal fault that ruptured during the earthquake. We generate a time series of surface displacement from Sentinel-1 SAR data ranging from early January 2019 to late January 2020 using the NSBAS processing chain. The spatio-temporal patterns of surface displacement for this time span show neither a clear pre-seismic signal nor significant post-seismic transient deformation. We extract the coseismic displacement pattern from the InSAR time series, highlighting along-strike variations of coseismic surface slip. The maximum relative displacement along the Line-Of-Sight is up to ∼16 cm and is located in the southwestern part of the rupture. We invert for the slip distribution on the fault from the InSAR coseismic surface displacement field. Constraining our fault geometry from the geological model, acceptable fault dip ranges between 55° and 60°. Our model confirms the reactivation of La Rouvière fault, with reverse slip at very shallow depth and two main slip patches reaching respectively 30 cm and 24 cm of slip, both around 500 m depth. We finally discuss how the 3D fault geometry and geological structure may have impacted the slip distribution and propagation during the earthquake. This study is a step to reassess the seismic hazard of the many faults similar to the La Rouvière one along the Cévennes fault system, in a densely populated area hosting several sensitive nuclear sites.
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Earthquakes are frequently accompanied by public reports of audible low-frequency noises. In 2018, public reports of booms or thunder-like noises were linked to induced earthquakes during an Engineered Geothermal System project in the Helsinki Metropolitan area. In response, two microphone arrays were deployed to record and study these acoustic signals while stimulation at the drill site continued. During the 11 day deployment, we find 39 earthquakes accompanied by possible atmospheric acoustic signals. Moment magnitudes of these events ranged from $$-0.07$$ - 0.07 to 1.87 with located depths of 4.8–6.5 km. Analysis of the largest event revealed a broadband frequency content, including in the audible range, and high apparent velocities across the arrays. We conclude that the audible noises were generated by local ground reverberation during the arrival of seismic body waves. The inclusion of acoustic monitoring at future geothermal development projects will be beneficial for studying seismic-to-acoustic coupling during sequences of induced earthquakes.
This supersedes Paper 1 (Gutenberg and Richter, 1942). Additional data are presented. Revisions involving intensity and acceleration are minor. The equation log a = I/3 − 1/2 is retained. The magnitude-energy relation is revised as follows: (20) log ⁡ E = 9.4 + 2.14 M − 0.054 M 2 A numerical equivalent, for M from 1 to 8.6, is (21) log ⁡ E = 9.1 + 1.75 M + log ⁡ ( 9 − M ) Equation (20) is based on (7) log ⁡ ( A 0 / T 0 ) = − 0.76 + 0.91 M − 0.027 M 2 applying at an assumed point epicenter. Eq. (7) is derived empirically from readings of torsion seismometers and USCGS accelerographs. Amplitudes at the USCGS locations have been divided by an average factor of 2 1/2 to compensate for difference in ground; previously this correction was neglected, and log E was overestimated by 0.8. The terms M2 are due partly to the response of the torsion seismometers as affected by increase of ground period with M, partly to the use of surface waves to determine M. If MS results from surface waves, MB from body waves, approximately (27) M S − M B = 0.4 ( M S − 7 ) It appears that MB corresponds more closely to the magnitude scale determined for local earthquakes. A complete revision of the magnitude scale, with appropriate tables and charts, is in preparation. This will probably be based on A/T rather than amplitudes.
The characteristics of earthquake swarms can neither be described by simple laws nor are the underlying mechanisms presently understood. Swarm activity is often assumed to be caused by an intrusion of fluids into the seismogenic zone. We have studied the earthquake catalog of the large earthquake swarm that occurred in the year 2000 in Vogtland, SE-Germany and NW-Bohemia, an area well known for its episodic swarm generation. We observe a significant decrease of the Gutenberg-Richter b value during the swarm evolution as well as a fractal temporal clustering of the earthquakes. The spatial spreading of the swarm's activity, which is approximately confined to one plane, cannot simply be explained by a process of fluid diffusion. Instead, we observe a simple relationship between the spatial spreading and the seismic moment release, which is in good agreement with empirical relationships derived for tectonically driven earthquakes and theoretical crack growth models. This observation points to a progressively growing main fracture underlying the swarm activity. In addition, we find that the swarm earthquakes themselves trigger aftershocks near the border of their rupture area. The stickslip behavior of the rupture propagation can be explained by stress transfers and induced fluid flows due to earthquakes in a fluid-permeated critically loaded fault zone. However, during the first phase, the temporal behavior is found to be different, pointing to intrusion of fluids initiating the swarm activity.
This supersedes Paper 1 (Gutenberg and Richter, 1942). Additional data are presented. Revisions involving intensity and acceleration are minor. The equation log a = I/3 − 1/2 is retained. The magnitude-energy relation is revised as follows: log E = 9.4 + 2.14 M – 0.054 M^2 (20). A numerical equivalent, for M from 1 to 8.6, is log E = 9.1 + 1.75 M + log (9-m) (21). Equation (20) is based on log (A_0/T_0) = -0.76 + 0.91 M – 0.027M^2 (7) applying at an assumed point epicenter. Eq. (7) is derived empirically from readings of torsion seismometers and USCGS accelerographs. Amplitudes at the USCGS locations have been divided by an average factor of 2 1/2 to compensate for difference in ground; previously this correction was neglected, and log E was overestimated by 0.8. The terms M2 are due partly to the response of the torsion seismometers as affected by increase of ground period with M, partly to the use of surface waves to determine M. If MS results from surface waves, MB from body waves, approximately M_S – M_B = 0.4 (M_S – 7) (27). It appears that MB corresponds more closely to the magnitude scale determined for local earthquakes. A complete revision of the magnitude scale, with appropriate tables and charts, is in preparation. This will probably be based on A/T rather than amplitudes.
The probability density function for the value of b in the formula log n(M)=a-bM estimated by the author's method has been obtained in an exact form. This function leads to a simple method to test the statistical significance of the difference between two b-values. Application of this method to Suyehiro's data on foreshocks and aftershocks of the great Chilean earthquake of 1960 indicates a significant difference in b-value between the foreshocks and the aftershocks at the 98% confidence level. © 1966, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.