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\DOI{10.5802/crgeos.285}
\datereceived{2024-12-01}
\daterevised{2025-02-12}
\dateaccepted{2025-02-24}
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\begin{document}

\begin{noXML}

\TopicEN{Volcanology}
\TopicFR{Volcanologie}

\CDRsetmeta{articletype}{research-article}

\shortrunauthors

\title{Scientific response to the 2021--2022 seismic swarm in the Monts
Dore volcanic province (France): dynamic insights from temporal surveys
(2/2)}

\alttitle{R\'{e}ponse scientifique \`{a} l'essaim sismique de
2021--2022 dans la province volcanique des Monts Dore (France) :
perspectives dynamiques issues des suivis temporels (2/2)}

\author{\firstname{Guillaume} \lastname{Boudoire}\CDRorcid{0000-0002-3148-3581}\IsCorresp}
\address{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire
Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France} 
\address{Universit\'{e} Clermont Auvergne, CNRS, Observatoire de
Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\address{Istituto Nazionale di Geofisica e Vulcanologia, Sezione di
Palermo, Italy} 
\curraddr[G. Boudoire, L. Gailler, J. Battaglia]{Laboratoire Magmas et
Volcans, Campus universitaire des C\'{e}zeaux, 6 Avenue Blaise Pascal,
63170 Aubi\`{e}re, France}
\email[G. Boudoire]{guillaume.boudoire@uca.fr}

\author{\firstname{Lydie} \lastname{Gailler}\CDRorcid{0000-0002-8132-2428}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[L. Gailler]{lydie.gailler@uca.fr}

\author{\firstname{Jean} \lastname{Battaglia}\CDRorcid{0000-0002-9513-1200}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[J. Battaglia]{jean.battaglia@uca.fr}

\author{\firstname{Aude} \lastname{Beauger}\CDRorcid{0000-0002-0911-0500}}
\address{Universit\'{e} Clermont Auvergne, CNRS, GEOLAB, F-63000 Clermont-Ferrand, France} 
\email[A. Beauger]{aude.beauger@uca.fr}

\author{\firstname{Martial}\nobreakauthor\lastname{Bontemps}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[M. Bontemps]{martial.bontemps@uca.fr}

\author{\firstname{Val\'{e}rie} \lastname{Bosse}\CDRorcid{0000-0003-1688-0949}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[V. Bosse]{valerie.bosse@uca.fr}

\author{\firstname{Vincent} \lastname{Breton}\CDRorcid{0000-0001-8197-7080}}
\address{Universit\'{e} Clermont Auvergne, CNRS, LPC, F-63000 Clermont-Ferrand, France} 
\email[V. Breton]{Vincent.BRETON@uca.fr}

\author{\firstname{Danielle} \lastname{Briot}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[D. Briot]{Danielle.BRIOT@uca.fr}

\author{\firstname{Philippe}\nobreakauthor\lastname{Cacault}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[P. Cacault]{Philippe.CACAULT@uca.fr}

\author{\firstname{Val\'{e}rie} \lastname{Cayol}\CDRorcid{0000-0001-9225-7441}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[V. Cayol]{valerie.cayol@uca.fr}

\author{\firstname{Nicolas} \lastname{Cluzel}\CDRorcid{0000-0002-2171-8789}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[N. Cluzel]{nicolas.cluzel@uca.fr}

\author{\firstname{Emmanuel} \lastname{Delage}\CDRorcid{0000-0003-2366-5788}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[E. Delage]{emmanuel.delage@uca.fr}

\author{\firstname{G\'{e}raldine}\nobreakauthor\lastname{Del\nobreakauthor Campo}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[G. Del Campo]{g.delcampo@opgc.fr}

\author{\firstname{Catherine} \lastname{Deniel}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[C. Deniel]{Catherine.DENIEL@uca.fr}

\author{\firstname{Jean-Luc} \lastname{Devidal}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[J.-L. Devidal]{j-luc.devidal@uca.fr}

\author{\firstname{Jean-Michel}\nobreakauthor\lastname{Douchain}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[J.-M. Douchain]{J-Michel.DOUCHAIN@uca.fr}

\author{\firstname{Alexandre} \lastname{Faissal}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[A. Faissal]{A.Faissal@opgc.fr}

\author{\firstname{Patrick} \lastname{Freville}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[P. Freville]{patrick.freville@uca.fr}

\author{\firstname{Francesco} \lastname{Frondini}\CDRorcid{0000-0002-7539-9541}}
\address{Department of Physics and Geology, University of Perugia, piazza Universit\`{a}, 06100 Perugia, Italy} 
\email[F. Frondini]{francesco.frondini@unipg.it}

\author{\firstname{Pierre-Jean} \lastname{Gauthier}\CDRorcid{0000-0003-0029-9973}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[P.-J. Gauthier]{Pierre-Jean.GAUTHIER@uca.fr}

\author{\firstname{Nicola} \lastname{Genzano}\CDRorcid{0000-0001-8184-5635}}
\address{Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, Via Ponzio 31, 20133 Milan, Italy} 
\email[N. Genzano]{nicola.genzano@unibas.it}

\author{\firstname{Cl\'{e}ment} \lastname{Grace}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[C. Grace]{Clement.GRACE@uca.fr}

\author{\firstname{Fausto} \lastname{Grassa}\CDRorcid{0000-0001-5043-792X}}
\addressSameAs{3}{Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy} 
\email[F. Grassa]{fausto.grassa@ingv.it}

\author{\firstname{Marc}\nobreakauthor\lastname{Grunberg}\CDRorcid{0000-0002-1307-7790}}
\address{EOST, UAR 830, Universit\'{e} de Strasbourg, CNRS, 5 rue Ren\'{e} Descartes, 67084 Strasbourg Cedex, France} 
\email[M. Grunberg]{marc.grunberg@unistra.fr}

\author{\firstname{Yannick}\nobreakauthor\lastname{Gu\'{e}henneux}\CDRorcid{0000-0002-0318-9713}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[Y. Gu\'{e}henneux]{Yannick.GUEHENNEUX@uca.fr}

\author{\firstname{Lucia} \lastname{Gurioli}\CDRorcid{0000-0002-5066-5153}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[L. Gurioli]{lucia.gurioli@uca.fr}

\author{\firstname{Andrew} \lastname{Harris}\CDRorcid{0000-0001-9137-8997}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[A. Harris]{Andrew.HARRIS@uca.fr}

\author{\firstname{Philippe}\nobreakauthor\lastname{Labazuy}\CDRorcid{0000-0002-4518-3328}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[P. Labazuy]{Philippe.LABAZUY@uca.fr}

\author{\firstname{Didier}\nobreakauthor\lastname{Laporte}\CDRorcid{0000-0003-1965-2351}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[D. Laporte]{didier.laporte@uca.fr}

\author{\firstname{Manfredi} \lastname{Longo}\CDRorcid{0000-0002-9465-2653}}
\addressSameAs{3}{Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy} 
\email[M. Longo]{manfredi.longo@ingv.it}

\author{\firstname{Francesco} \lastname{Marchese}\CDRorcid{0000-0001-7590-5638}}
\address{National Research Council, Institute of Methodologies of
Environmental Analysis, C.da S. Loja, Tito Scalo, Potenza 85050, Italy} 
\email[F. Marchese]{francesco.marchese@cnr.it}

\author{\firstname{Gilles} \lastname{Mazet-Roux}}
\address{CEA, DAM, DIF, Arpajon, France} 
\email[G. Mazet-Roux]{Gilles.MAZET-ROUX@cea.fr}

\author{\firstname{Etienne}\nobreakauthor\lastname{M\'{e}dard}\CDRorcid{0000-0002-7040-7442}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[E. M\'{e}dard]{etienne.medard@uca.fr}

\author{\firstname{Charley} \lastname{Merciecca}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[C. Merciecca]{Charley.MERCIECCA@uca.fr}

\author{\firstname{Marianne} \lastname{M\'{e}tois}\CDRorcid{0000-0002-1489-0513}}
\address{Universit\'{e} Claude Bernard Lyon 1, CNRS, ENS de Lyon, Universit\'{e}
Jean Monnet, LGL-TPE UMR5276, Villeurbanne, France} 
\email[M. M\'{e}tois]{marianne.metois@univ-lyon1.fr}

\author{\firstname{Nicola} \lastname{Pergola}\CDRorcid{0000-0001-7619-6685}}
\addressSameAs{9}{National Research Council, Institute of Methodologies of
Environmental Analysis, C.da S. Loja, Tito Scalo, Potenza 85050, Italy} 
\email[N. Pergola]{nicola.pergola@cnr.it}

\author{\firstname{Manon}\nobreakauthor\lastname{Pouget}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[M. Pouget]{Manon.POUGET@uca.fr}

\author{\firstname{Victoria} \lastname{Rafflin}\CDRorcid{0000-0001-8000-9643}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\address{Department of Earth Sciences, Royal Holloway, University of
London, Egham, \mbox{TW20 0EX,} UK} 
\email[V. Rafflin]{victoria.rafflin@uca.fr}

\author{\firstname{Edouard} \lastname{Regis}}
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[E. Regis]{edouard.regis@uca.fr}

\author{\firstname{Lisa} \lastname{Ricci}\CDRorcid{0000-0002-2780-2367}}
\addressSameAs{6}{Department of Physics and Geology, University of Perugia, piazza Universit\`{a}, 06100 Perugia, Italy} 
\address{Geosciences and Earth Resources (IGG) -- National Research
Council of Italy (CNR), 56124 Pisa, Italy} 
\email[L. Ricci]{lisa.ricci58@gmail.com}

\author{\firstname{Andrea\nobreakauthor Luca}\nobreakauthor\lastname{Rizzo}\CDRorcid{0000-0003-2225-3781}}
\address{Dipartimento di Scienze dell'Ambiente e della Terra,
Universit\`{a} degli Studi di Milano-Bicocca, Piazza della Scienza 4,
20126 Milano, Italy} 
\address{Istituto Nazionale di Geofisica e Vulcanologia, Sezione di
Milano, Italy} 
\email[A. L. Rizzo]{andrealuca.rizzo@unimib.it}

\author{\firstname{Thierry} \lastname{Souriot}\CDRorcid{0000-0003-3303-8016}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[T. Souriot]{thierry.souriot@uca.fr}

\author{\firstname{Luca} \lastname{Terray}\CDRorcid{0000-0003-4708-0995}}
\addressSameAs{5}{Universit\'{e} Clermont Auvergne, CNRS, LPC, F-63000 Clermont-Ferrand, France} 
\email[L. Terray]{luca.terray@uca.fr}

\author{\firstname{Valerio} \lastname{Tramutoli}\CDRorcid{0000-0003-3875-7909}}
\address{School of Engineering, University of Basilicata, 85100 Potenza, Italy } 
\email[V. Tramutoli]{valerio.tramutoli@unibas.it}

\author{\firstname{Cristina}\nobreakauthor\lastname{Trull-Hernandis}\CDRorcid{0000-0002-4182-6808}}
\address{Universitat Polit\`{e}cnica de Val\`{e}ncia, Instituto
Universitario de Seguridad Industrial, Radiofisica y Medioambiental, Spain} 
\email[C. Trull-Hernandis]{critruhe@upv.es}

\author{\firstname{Benjamin}\nobreakauthor\lastname{van\nobreakauthor Wyk de Vries}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[B. van Wyk de Vries]{Ben.VANWYK@uca.fr}

\author{\firstname{Olivier} \lastname{Voldoire}\CDRorcid{0000-0003-1306-3054}}
\addressSameAs{4}{Universit\'{e} Clermont Auvergne, CNRS, GEOLAB, F-63000 Clermont-Ferrand, France} 
\email[O. Voldoire]{Olivier.VOLDOIRE@uca.fr}

\author{\firstname{Erwan} \lastname{Thebault}\CDRorcid{0000-0002-7038-2855}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC,
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\addressSameAs{2}{Universit\'{e} Clermont Auvergne, CNRS, Observatoire
de Physique du Globe de Clermont-Ferrand, 4 avenue Blaise Pascal, 63178
Aubi\`{e}re, France} 
\email[E. Thebault]{erwan.thebault@uca.fr}

\keywords{\kwd{Seismic swarms}\kwd{Monitoring}\kwd{Volcanic
provinces}\kwd{Risk mitigation}
\kwd{ECRS}}

\altkeywords{\kwd{Essaims sismiques}\kwd{Surveillance}\kwd{Provinces
volcaniques}\kwd{Pr\'{e}vention des risques}
\kwd{ECRS}}

\thanks{French Government Laboratory of Excellence initiative 
(ANR-10-LABX-0006), Region Auvergne, European Regional Development Fund
through the project DegazRift}

\begin{abstract} 
During years 2021--2022, an unusual seismic swarm was recorded at crustal
level beneath the Monts Dore volcanic province (France). Complementary
field and remote measurements were performed. Together with the time
series recorded on the seismological and GNSS national networks, these
measurements were fundamental for monitoring the evolution of the seismic
swarm and deciphering its origin. Although a potential vertical
migration of the seismic events is suggested, the complementary 
measurements presumably discard the hypothesis of magma intrusion at shallow 
crustal level. The ascent of a CO\tsub{2}-rich fluid originating from the
mantle might instead have reacted with the hydrothermal system beneath the
Monts-Dore since at least the summer 2021 leading to the reactivation
of pre-existing tectonic structures with known associated seismicity.
Feedback on the management of the 2021--2022 seismic swarm prompts for
several recommendations that should be considered in future to better
face and address at the national level the issues raised at 
long-dormant volcanic provinces in mainland France.
\vspace*{-2pt}
\end{abstract}

\begin{altabstract} 
Entre 2021 et 2022, un essaim sismique inhabituel a \'{e}t\'{e}
enregistr\'{e} \`{a} l'\'{e}chelle crustale sous la province volcanique
des Monts Dore (France). Des mesures compl\'{e}mentaires, \`{a} la fois
sur le terrain et par t\'{e}l\'{e}d\'{e}tection, ont \'{e}t\'{e}
effectu\'{e}es. Associ\'{e}es aux s\'{e}ries temporelles
enregistr\'{e}es sur les r\'{e}seaux nationaux sismologiques et GNSS,
ces donn\'{e}es ont \'{e}t\'{e} essentielles pour suivre
l'\'{e}volution de l'essaim sismique et en comprendre l'origine. Bien
qu'une migration verticale potentielle des \'{e}v\'{e}nements sismiques
soit sugg\'{e}r\'{e}e, les mesures ont rapidement permis d'\'{e}carter
l'hypoth\`{e}se d'une intrusion magmatique \`{a} un niveau crustal peu
profond. L'ascension d'un fluide riche en CO\tsub{2} d'origine
mantellique pourrait avoir interagi avec le syst\`{e}me hydrothermal
sous les Monts Dore depuis au moins l'\'{e}t\'{e} 2021, entra\^{i}nant
la r\'{e}activation de structures tectoniques pr\'{e}existantes et une
sismicit\'{e} associ\'{e}e. Le retour d'exp\'{e}rience sur la gestion
de l'essaim sismique de 2021--2022 met en lumi\`{e}re plusieurs
recommandations \`{a} prendre en compte pour mieux anticiper et traiter
les probl\'{e}matiques soulev\'{e}es par de telles activit\'{e}s dans
les provinces volcaniques en sommeil de la France m\'{e}tropolitaine. 
\end{altabstract} 

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\defcitealias{Gailleretal2025}{ibid}
\defcitealias{Braueretal2017}{ibid}

\section{Introduction}\label{sec1}
Seismic swarms are ``localized surges of earthquakes, with no one shock
being conspicuously larger than all other shocks of the swarm''
\citep{Govonietal2013}. They are often considered as a mechanical
response to stress history of the geological structures where
fluid-driven mechanisms  \citep{Hill1977,Miller2013} and/or fault-creep
or slow-slip  \citep{OkutaniIde2011,RolandMcGuire2009}  mechanisms are
involved. The question of seismic swarms trigger becomes even more
critical when they occur in volcanic context. There, seismic swarms
induced by dyke propagation or by the pressurization of magmatic
reservoirs are frequent and may be precursors of potential subsequent
volcanic eruptions  \citep{Acocellaetal2024,Passarellietal2018,
RomanCashman2018}. In long-dormant volcanic provinces, such as in the
Monts Dore, monitoring networks can be sparse and generally not
designed for local and multidisciplinary analyses. In such a context,
detecting volcano-tectonic activity and deciphering its causes are
challenging tasks. For such a reason, seismic swarms in long-dormant
volcanic provinces are often a major source of concern for both
scientists, authorities, and local population as recently evidenced in
the West Halmahera (Indonesia) in 2015, in the Campi Flegrei (Italy)
since 2005, or in Mayotte (France) in 2018
\citep{Astortetal2024,Chiodinietal2015,Fallouetal2020,Passarellietal2018}. 

The European Cenozoic Volcanic Province (ECVP) that accommodates the
European Cenozoic Rift System (ECRS) extends from Spain, France,
Germany, Czech Republic, to Poland over more than 1500~km 
\citep{Martietal2017,MerleMichon2001}. Related volcanism is mainly
confined to four volcanic regions which are the Bohemian Massif, the
Rhenish Massif, the French Massif Central, and the Valencia Trough
\citep{Martietal2017,Ulrychetal2011}  (Figure~\ref{fig1}a). In the
Bohemian Massif, the last known eruptive activity dates back to about
290,000 years \citep{Ulrychetal2011} but a repeated occurrence of
seismic swarms characterized the Novy Kostel focal zone. The
hypocentres of these seismic swarms \mbox{(6--15~km-depth)} indicate a
west-dipping fault plane 70--80\textdegree\ in accordance with
regional tectonics  \citep{Bankwitzetal2003,Tanneretal2020}. These
seismic swarms are considered as a shallow response (fluid circulation
and hydraulic fracturing) to the ascent of magma from the deeper
lithospheric mantle reservoir  \citep{Braueretal2011,Spicaketal1999}. 
In the Rhenish Massif, the last eruption occurred about 11,000 years
ago in the Eifel Volcanic Field. Seismic swarms were recently
documented beneath the Laacher See volcano. They cluster along an
80\textdegree\ dipping line between 10 and 40~km-depth that suggested
interactions between shear cracks and fluids potentially coming from
magmatic reservoirs  \citep{Braueretal2013,Henschetal2019}. In the
French Massif Central, the last known eruption occurred about 7000
years ago (i.e., the most recent eruptive activity of the ECVP) at the
Pavin maar-crater  \citep{JuvigneMialler2016}. The volcanic edifice, now
filled with a lake, is located at the southern extremity of an
alignment of about 80 monogenetic volcanoes along a N--S orientation
forming the Cha\^{i}ne des Puys volcanic chain. Geographically, the
eruptive site is set on the slope of the Monts Dore stratovolcano (last
eruptive phase about 200,000 years ago)  \citep{Nomadeetal2014} 
(Figure~\ref{fig1}b,c). At the end of 2021, a seismic swarm started
beneath the Monts Dore stratovolcano raising some concerns about the
origin of this unusual seismicity  (Figure~\ref{fig1}d).  The
scientific response to this event provided on the field by the local
scientific and technical staff of the Observatoire de Physique du Globe
de Clermont-Ferrand (OPGC) and the Laboratoire Magmas et Volcans (LMV)
in collaboration with the Bureau Central Sismologique
Fran\c{c}ais--R\'{e}seau National de Surveillance Sismique
(BCSF-R\'{e}NaSS \url{https://renass.unistra.fr};
\url{https://doi.org/10.17616/R31NJNC4}) allowed a detailed spatial
characterization of the seismic swarm  
\citep{Gailleretal2025}. The
seismic swarm clustered between 3 and 7~km-depth along two fault
planes-oriented N173-85W and N13-68W in accordance with the regional
tectonic setting (Figure~\ref{fig1}e). Although the spatial definition
of the seismic swarm and its link with the reactivation of pre-existing
tectonic structures are now constrained, the origin of this
reactivation is still subject of debate. 

\begin{figure*}
\includegraphics{fig01}
\bcaption{\label{fig1}}{0}{(a)~Map of the European Cenozoic Rift System
(ECRS) showing the main Cenozoic faults (black lines), rift-related
sedimentary basins (light gray: BG, Bresse Graben; EG, Eger Graben; LG,
Limagne Graben; LRG, Lower Rhine Graben; URG, Upper Rhine Graben),
volcanic fields (black), and the recent seismic swarms (green) within
the European Cenozoic Volcanic Province (ECVP). Modified from 
\citet{Dezesetal2004}. Epicenters of the seismic events detected and
localized by the BCSF-R\'{e}NaSS (b,c)~between January 1, 2018 and
December 31, 2020 and (d,e)~between January 1, 2021 and June 30, 2023
(\url{https://api.franceseisme.fr/fr/search}). The main urban areas
(white bold italic), places of interest discussed in this study
(white), the most recent eruptive site (white star), the Jassat
strike-slip fault (black line), and the surface projection of the fault
planes accommodating the seismic swarm (dashed white lines) are shown.
Source data from QGIS (IGN-G\'{e}oportail, BD Carthage, ASTER DEM).}
{\vspace*{-2pt}}
\end{figure*}

In this study, we evaluate the time series of monitoring records from
both permanent and temporary networks and surveys in order to better
decipher the processes involved in such a seismic activity beneath the
most recent stratovolcano of the French Massif Central. In 
Section~\ref{sec2}, we summarize the geological context of the Monts
Dore volcanic province. In Section~\ref{sec3}, we present some details
about the seismic, remote satellite and geochemical data that were
collected and on the data processing applied for the global analysis of
this seismic swarm. Additional local measurements and analyses based on
dedicated geophysical campaign surveys are compiled and analysed in the
companion paper  \citep{Gailleretal2025}. In  Section~\ref{sec4}, we
present the results and discuss them in  Section~\ref{sec5}, with a
special focus on the chronology of the seismic swarm and on lessons
learned from the scientific and operational management at the national
level. We finally conclude in \mbox{Section~\ref{sec6}.}

\section{Context of the study}\label{sec2}
The Monts Dore volcanic province spreads on about 600~km$^{2}$ and is
located immediately South of the Cha\^{i}ne des Puys volcanic chain,
the most recent volcanic field in mainland France 
\citep{BoivinThouret2014}. Its eruptive activity spans a period ranging
from 3~Myr to 0.2~Myr \citep{Nomadeetal2014} and its current morphology
was acquired through successive glacial erosion periods. However,
several recent  (${&lt;}$15~kyr) eruptive cones and maars (cf.\ Tartaret,
Montcineyre, Montchal, Pavin) were identified on the slope of the
stratovolcano  \citep{JuvigneMialler2016}.  While their ages are
contemporaneous of the last eruptive phase of activity in the
Cha\^{i}ne des Puys, their location as well as the chemical fingerprint
of the eruptive products \citep{Boudoireetal2023,Villemantetal2016}
still raises debates about their affiliation.

Current evidence of a potential reminiscent magmatic activity in the
mantle beneath the Monts Dore include the presence of numerous
springwaters (bicarbonate, bicarbonate-chloride, and acid-sulfate) that
contain partly mantle-derived  CO$_{2}$ and noble gases
\citep{Boudoireetal2023,Braueretal2017,Pauwelsetal1997,Riccietal2024}. 
\citet{Braueretal2017} noted that the fingerprint of ``fluid transport
in deep reaching faults from the magmatic reservoir to the surface'' is
more marked for the Monts Dore volcanic province than anywhere else in
the French Massif Central. The area is also characterized by soil 
CO$_{2}$ emissions \citep{Boudoireetal2023,Galetal2018,GalGadalia2011}.
Finally, high average heat flux  \citep{LucazeauVasseur1989}, high
electrical conductivity body identified by magnetotellurics at depth
greater than 8~km  \citep{Dupisetal1980,Gailleretal2025}, and the water
chemistry and thermometry support the presence of a hydrothermal system
beneath the Monts Dore volcanic province  \citep{Boschetal1980}.

In the French Massif Central, about 5000 earthquakes were recorded
since 1962, i.e., 110 earthquakes per year, on average 
\citep{BattagliaDouchain2016}.  The depth of hypocentres usually does
not exceed 11~km. With the exception of few historical high magnitude
earthquakes $(M &gt; 4.5)$, like those of the fifteenth century (1450,
1477, 1490) that generated major damages, earthquakes are mostly unfelt
by the population  \citep{Doreletal1995}. The seismicity mostly
clusters at the Northwest of Clermont-Ferrand (cf.\ ``Combrailles'' on
Figure~\ref{fig1}), in the vicinity of Saint-Flour and Ambert (out of
the map on Figure~\ref{fig1}), and in the Monts Dore (cf.\ ``Monts
Dore'' on Figure~\ref{fig1}).

Between 1962 and 2017, just over 290 earthquakes were localized in the
Monts Dore with only five of magnitude (ML) higher than 3. This seismic
activity mostly occurred on restricted time intervals 
\citep{BattagliaDouchain2016}:  (1)~10 earthquakes in June 1979, (2)~79
earthquakes in March--April 1980, (3)~7 earthquakes in January 1983,
(4)~37 earthquakes in April 1984, (5)~6 and 16 earthquakes in April and
September 2008, respectively, and (6)~14 earthquakes in June 2013. To
date (before the 2021--2022 seismic swarm), the main period of seismic
activity in the Monts Dore refers to 1979--1980 (89 earthquakes, some
of which were close to magnitude of~3) and 1982--1983 (44 earthquakes,
37 of which were recorded within 4 days). These earthquakes mostly
occurred on the North-eastern rim of the caldera that is the place of
the 2021--2022 seismic swarm too. \citet{BattagliaDouchain2016}
reported that these previous seismic swarms do not present the usual
appearance of a main earthquake followed by aftershocks, which
generally characterizes tectonic earthquakes occurring on a
well-defined fault system. However, we note that these previous seismic
swarms were recorded by a monitoring network that was probably too
sparse at that time to detect and locate all events.

The BCSF-R\'{e}NaSS catalogue of seismic events detected by the RAP
(R\'{e}seau Acc\'{e}l\'{e}rom\'{e}trique Permanent) and RLBP
(R\'{e}seau Large Bande Permanent) national seismic networks
(integrated in the EPOS-France research infrastructure;
\url{https://www.epos-france.fr/en/about-epos-france/epos/}) listed 8
earthquakes in the Monts Dore between 2018 and 2020 
(Figure~\ref{fig1}b,c). Following this period, the first earthquake in
the area in 2021 was detected on November 16 and marked the beginning
of the 2021--2022 seismic swarm characterized by more than 400
earthquakes catalogued listed by the BSCF-R\'{e}NaSS catalogue. All are
located at the Northwest of the town of Chambon-sur-Lac within a
3--7~km depth range above a high electrical conductivity body
identified at depth by magnetotellurics  \citep{Gailleretal2025} 
(Figure~\ref{fig1}d,e). Hypocentres clustered along two fault planes
N173-85W and N13-68W linked to regional structures (Jassat joint fault
and Glangeaud fault). These fault directions are reminiscent of those
of the two main Variscan crustal faults families cutting the Plateau
des D\^{o}mes (i.e., the basement of the Cha\^{i}ne des Puys volcanic
chain)  \citep{Merleetal2023}: the Villefort fault family (N160-170E)
and the Sillon Houiller fault family (N20E).

\section{Data and methods}\label{sec3}
Time series of monitoring signals were obtained by merging data
available from (1)~national permanent networks (seismology, ground
deformation), (2)~remote sensing (ground deformation, thermal imagery),
and (3)~specific measurements and temporary networks (fluid
geochemistry, seismology, ground deformation) deployed during the
seismic swarm event  (Figure~\ref{fig2}). The main objective was to
compare time series of these physical 
\mbox{quantities} measured before,
during, and after the seismic swarm in order to better understand the
chronology and then to constrain involved processes.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig02}
\vspace*{-2pt}
\caption{\label{fig2}Map of the national permanent seismic (RAP, RLBP)
and GNSS networks (RENAG, RGP, Orph\'{e}on) and punctual network/survey
deployed during the seismic swarm. Source data from QGIS
(IGN-G\'{e}oportail, BD Carthage, ASTER DEM). The orange square
delimits the geographical extent of the areas covered for thermal
imagery (LANDSAT) investigations.} 
\vspace*{-2pt}
\end{figure*}

\subsection{Seismology}\label{sec3.1}
The BCSF-R\'{e}NaSS, from which the seismic data for this study are
processed (\url{https://renass.unistra.fr}), integrates seismic signals
from permanent velocimetric (RLBP) and accelerometric (RAP) national
networks, which are part of the national EPOS-France research
infrastructure (so-called R\'{e}sif before 2023 according to the FDSN
nomenclature). Seismic events are detected and localized at the
national level in near-real time, first using Seiscomp's automatic pick
detection  \citep{Weberetal2007}, then refined, completed, and
validated manually by human operators. Locations are obtained using the
LocSAT earthquake location program  \citep{BrattNagy1991} with a
five-layer 1D regional velocity model well suited for the French Massif
Central  \citep{Mazabraudetal2005}. In addition, the BCSF-R\'{e}NaSS
catalogue provides the local magnitude (MLv) of each event 
(Figure~\ref{fig3}) and a preprocessing tool designed to discriminate
between earthquakes and quarry blasts 
\citep{Grunbergetal2023,Renouardetal2021}, the latter representing a
significant amount of recorded seismic events.

\begin{figure*}
\includegraphics{fig03}
\vspace*{-2pt}
\caption{\label{fig3}Time evolution of the seismic swarm beneath March
2022 and July 2023 (cf.\ zoom on the January 2021--July 2023 period):
(a)~Time difference of arrival between the S- and P-waves, 
$t_{\mathrm{S}}-t_{\mathrm{P}}$, at
CHBN (PhaseNet algorithm; the horizontal red line highlights the
average time). (b)~Magnitude of the seismic events reported in the
BSCF-R\'{e}NaSS catalogue. (c)~Number of seismic events per day: in
black, the counting from the BCSF-R\'{e}NaSS catalogue (automatic
detection system computed by the BCSF-R\'{e}NaSS including over time
the three temporary seismic stations whose installation dates are
indicated by the red arrows); in green, the number of detections on the
CBHN temporary seismic station, only (automatic detection on Phasenet
of seismic events with both P- and S-waves separated by less than 
2~s). P1, P2, P3, P4 highlight the 4 periods of seismic activity
described in the main text.} 
\vspace*{-4pt}
\end{figure*}

At local scale (i.e., within and in the vicinity of the Monts Dore),
the seismic monitoring network includes 8 accelerometers (RAP) and 14
broadband velocimeters (RLBP) maintained by the Observatoire de
Physique du Globe de Clermont-Ferrand (OPGC). Accelerometric stations
are equipped with Nanometrics Centaur digitizers and Kinemetrics
Episensor FBA ES-T accelerometers. All the closest velocimetric
stations are equipped with Nanometrics Centaur digitizers and Trillium
120PH sensors installed in 5- to 10-m-deep boreholes. In March
2022, these stations, and the ones belonging to the CEA seismic network
identified a long-lasting seismic swarm beneath the Monts Dore volcanic
province. The nearest permanent stations, however, are an \mbox{accelerometer}
(OCMD from the RAP network) and a velocimeter (CMPS from the RLBP
network) located around 6.5~km and 17~km from the swarm, respectively.
As the average distance between stations in the region is around
30--40~km, a first temporary velocimetric station (CHBN) was installed
right above the swarm on April 4, 2022, to improve the detection
threshold for monitoring small seismic events. It is equipped with a
Kinemetrics Q330 digitizer and a Nanometrics Trillium 120PA sensor
buried. Given the persistence of the seismic swarm, two additional
stations (BNLF and VRNN) were installed on June 15 and 20, 2022. These
are equipped with Centaur digitizers and Nanometrics Trillium Compact
120s sensors. As data from the CHBN and BNLF stations are transmitted
in real time, they are used by BCSF-R\'{e}NaSS to monitor seismicity in
real time, unlike VRNN, whose data are only available once they have
been collected in the field. VRNN data are therefore only used to
complete data on events that have already been previously identified by
BCSF-R\'{e}NaSS.

For a more comprehensive view of the number of events, particularly
those with magnitudes too low to be detected automatically by the
BCSF-R\'{e}NaSS, we apply the PhaseNet algorithm  \citep{ZhuBeroza2019}
to the continuous recordings of the CHBN station. The algorithm
identifies potential P and S phases and provides associated
probabilities. We used a version of the program trained with the
Californian data set with no preliminary filtering. We then consider
that the station detects a local event if it shows a P phase with a
probability greater than 0.90, an S phase with a probability greater
than 0.50 and a difference in arrival times 
$t_{\mathrm{S}}-t_{\mathrm{P}}$ less than 2~s 
(Figure~\ref{fig3}). All detected events forming the seismic swarm had
P and S waves (see Supplementary Material for an \mbox{example} of
the typical seismic waveform detected) and no tremors or Long Period
events have been identified until now.

\subsection{Ground deformation}\label{sec3.2}
In order to seek for large-scale deformation of the Monts Dore area
that could have occurred together with the seismic swarm, we compute
baseline time-series of open-access daily GNSS data (``daily-rapid''
Rinex) from three geodetic quality {continuous} {stations} of the national
RGP (R\'{e}seau GNSS Permanent) and RENAG
(\url{https://renag.resif.fr/fr/}) networks that both contribute to the
national EPOS-France research infrastructure. The closest {stations}
(PDOM, SJPL, STGN) from the area marked by the seismic activity 
(Figure~\ref{fig2}) that have a good data availability over the period
of interest (2021--2023) are located more than 20~km from the seismic
swarm location. ``Daily-rapid'' Rinex were processed for these three
{stations} with the IGN PPP calculation {service}
(\url{https://rgp.ign.fr/SERVICES/ppp_online.php}) by using the default
parameters of the BERNESE 5.2 software. 3D distances between each
station\break {(PDOM-SJPL,} PDOM-STGN and SJPL-STGN) were then computed. 

One temporary GNSS station made of a TOPCON GB1000 receptor with a
PG-A1 antenna (equipped with a one-meter steel mast sunk in the soil to
a depth of 70~cm) available at the Laboratoire Magmas et Volcans (LMV)
was deployed in a pasture from July 6 to November 2, 2022. The pasture
is in the area marked by the seismic activity and was targeted due to
geological evidence of past landslide above the D996 road 
(Figure~\ref{fig2}). Data were recorded at 30~s intervals and
converted in daily Rinex. Two post-processing methods were tested. The
first post-processing method (so-called ``BERNESE'' in
Supplementary Material) was carried out online on the IGN
website (\url{https://rgp.ign.fr/SERVICES/calcul_online.php}), by using
the default parameters of the BERNESE 5.2 software. For the second
post-processing method (so-called ``GIPSYX'' in Supplementary
Material), Rinex were reduced to daily static positions using the
GIPSYX software  \citep{Bertigeretal2020} in a precise point
positioning mode applied to ionospheric-free carrier phase and
pseudorange data \citep{Zumbergeetal1997} and using JPL's final
no-net-rotation GPS orbit products. We use the Vienna Mapping Function
to estimate tropospheric gradients and delays every 5~min, and
compute ocean loading from the FES2012 tidal model
(\url{http://holt.oso.chalmers.se/loading}). Ambiguities were fixed
using wide lane and phase bias method  \citep{Bertigeretal2010}.
Finally, we convert our coordinates in IGS2014 reference frame using
the daily parameters provided by the JPL. No significant difference
between the time series obtained by the BERNESE and GIPSYX softwares
was observed (Supplementary Material).

The data used in this study are published by the RENAG network
\citep{EposFrance2023}. ORPHEON GNSS data were provided to the authors
for a scientific use in the framework of the GEODATA-INSU-CNRS
convention. They are distributed by the RENAG data center 
\citep{re3data.org2022}.

\subsection{Geochemistry}\label{sec3.3}
A quadrimestrial sampling of four thermo-mineral bubbling springs and
mofettes was performed since April 2021 in the Plaine de la Limagne and
Monts Dore volcanic province  (Figure~\ref{fig2})
(Supplementary Material). Springs and mofettes were firstly
targeted for gas chemistry (since April 2021) for the purpose of local
research projects focused on the degassing mechanisms in continental
rifting, then applied progressively for characterizing the water
chemistry (since February 2022) in response to this unprecedented
seismic activity.

Bubbling gaseous samples were collected by accumulation in a funnel and
pumped into a 100~cm$^{3}$ syringe. By using three-ways valves, gas was
then injected into a two-ways stainless steel tube with Swagelok valves
at both ends for noble gases analysis. Air was purged away from
containers by means of several cycles of pumping and injection.
Analyses were made at the Istituto Nazionale di Geofisica e
Vulcanologia--Sezione di Palermo (INGV Palermo). The abundance and
isotope composition ($\mbox{}^{3}$He, $\mbox{}^{4}$He) of helium (He)
were determined by a split flight tube mass spectrometer (Helix
SFT-GVI). Neon (Ne) abundance and isotope composition
($\mbox{}^{20}$Ne) were determined by a Helix MC Plus Thermo. Argon
(Ar) abundance and isotope composition ($\mbox{}^{40}$Ar,
$\mbox{}^{38}$Ar, $\mbox{}^{36}$Ar) were measured in a multicollector
mass spectrometer (Helix MC-GVI). The analytical errors of the He, Ne,
and Ar isotopes were less than 1.4\%, 0.13\%, and 0.8\%, respectively.
The analytical procedure and correction for air-contamination of noble
gases are the same as the ones described in  \citet{Boudoireetal2020}. 
\looseness=1

Temperature, conductivity, and pH measurements of the water were
obtained by using a multiparameter WTW probe FC Multi 340i. Water
samples were collected for analysis in major ions. Concentrations
(mg/L) of various anions and cations (lithium, sodium, ammonium,
potassium, magnesium, calcium, fluoride, chloride, nitrite, nitrate,
phosphate, and sulphate) were analyzed using high-pressure ion
chromatography. For cations, a Thermo Scientific Dionex ICS1100 system
was used, and for anions, a Thermo Scientific Dionex Aquion system
(Thermo Fisher Scientific) was used. The uncertainty is lower than 5\%
for all anions and cations. Water total alkalinity was measured onsite
by using a HI3811 HANNA alkalinity test kit (0--300~mg/L). The Total
Dissolved Inorganic Carbon (TDIC) of water was calculated from pH,
temperature, and total alkalinity by assuming that the total alkalinity
was equivalent to the carbonate alkalinity (phosphate, borate,
silicate, and hydroxide ions contribution being negligeable). 
The~maximum uncertainty (2$\sigma$) on the calculation of the TDIC is
6~mmol/L.

Additionally, the 100~m-spacing soil CO$_{2}$ transect measurements
crossing the seismic swarm described in \citep{Gailleretal2025} was
reiterated 5 times between May 2021 and June 2023 to study the time
evolution of diffusive degassing in the area (Supplementary
Material). Measurements were performed by combining a West Systems
C-type accumulation chamber and a PP Systems EGM-5 NDIR gas analyzer
device (with temperature and pressure compensation). The EGM-5 device
may measure CO$_{2}$ content up to 50,000 ppm with an uncertainty
lower than 1\% over the calibrated range. Soil CO$_{2}$ gas was also
collected in glass vials by using a similar three-ways valves procedure
as described above. In this case a probe was inserted in the soil at
50~cm-depth on 5 sites along the whole profile. Gas samples from soil
were successively analysed for carbon isotopes
($\mbox{}^{13}$C/$\mbox{}^{12}$C) of CO$_{2}$ by using a Thermo
(Finnigan) Delta Plus XP CFIRMS connected to a Trace GC gas
chromatograph and a Thermo (Finnigan) GC/C III interface installed at
INGV-Palermo (Supplementary Material). The absolute
$\updelta\mbox{}^{13}$C analytical error is
${&lt;}$0.15\permil\ (1$\sigma$).

\subsection{Thermal imagery}\label{sec3.4}
Thermal imagery from Landsat 8 and 9 satellite platforms (courtesy of
the U.S. Geological Survey). was obtained on a well-defined area
covering the major part of the Monts Dore  (Figure~\ref{fig2}). Images
from Landsat 8 (Collection 2 Tier 1 and 2) and Landsat 9 (Collection 2
Tier 1) were collected, analysed, and processed with Google Earth
Engine (GEE) through online JavaScript algorithms. In the absence of
high temperature hotspot in the area, the spectral band 13 of the OLI
and OLI-2 (TIR; 10.60--11.19~$\upmu$m) was considered to explore
possible faint temperature variations  \citep{Harris2013}. 

The Robust Satellite Technique (RST)
\citep{Genzanoetal2021,Tramutoli1998}  is based on a multi-temporal
approach that identifies statistically significant variations,
considered as anomalies, in the signal at pixel level. In this work,
the RST was applied on two thermal indices: ALICE 
\citep{Tramutoli1998,Tramutoli2007} and RETIRA 
\citep{Eleftheriouetal2016,Tramutolietal2005} (Supplementary
Material). The anomalous pixels are highlighted once the baseline
signal and its statistical variability are identified. For this
purpose, the mean $(\mu)$ and standard deviation $(\sigma)$ of each
pixel were computed from a series of thermal images covering every
month since 2013. The images were sorted according to the month of year
and time of day after careful removal of cloudy pixels. This provided
us with reference field \mbox{images} $(\mu, \sigma)$ for each date  $(t)$
from which the ALICE values for each pixel  $(x,y)$ were computed
through Equation~(\ref{eq1}):
{\begin{equation}
\mathrm{ALICE} (x,y,t)= \frac{T(x,y,t)-\mu (x,y)}{\sigma (x,y)}\label{eq1}
\end{equation}}\unskip
where  $T(x,y,t)$ is the brightness obtained for each pixel  $(x,y)$ of
the thermal image available at a date $(t)$.  RETIRA values were
computed in a similar way but now considering the brightness variation
$\Delta T (x,y,t)$ defined as (Equation~(\ref{eq2})): 
{\begin{equation}
\Delta T (x,y,t)= T (x,y,t)- T_{\mathrm{average}} (t) \label{eq2}
\end{equation}}\unskip
where  $T_{\mathrm{average}}(t)$ is the mean value of  $T(x,y,t)$ over
all pixels for a given date $(t)$. This index is useful to mitigate the
effect of occasional large scale warming on generating false positives.
Equation~(\ref{eq1}) relies on the Gaussian statistic hypothesis from
which we identify the anomalous pixels as those corresponding to ALICE
or RETIRA values larger than ${+}$2.5; i.e., thus corresponding to
positive brightness values and variations outside the 99.5\% confidence
interval around the means.

\section{Results}\label{sec4}

\subsection{Time series of the seismicity}\label{sec4.1}
The BCSF-R\'{e}NaSS catalogue shows that the first seismic events in
this study were detected in the area on November 16, 2021 
(Figure~\ref{fig3}). This was followed by an almost continuous series
of daily seismic events (up to 22 events on May 18, 2022) from March
18, 2022, to July 15, 2022 (aseismic periods not exceeding 5 days).
From then, the seismic activity concentrated on narrower time windows:
(i)~68 seismic events on July 24--26, 2022, (ii)~16 events on September
27, 2022, (iii)~15 events on October 22, 2022, and (iv)~34 events on
November 19, 2022. From January 5, 2023, the seismic activity becomes
anecdotal with 13 seismic events detected up to July 1, 2023. The
magnitude of the seismic events $(M &lt; 2.5)$ seems to decrease in time. 
\looseness=-1

However, it is worth noting that there is a bias in the time evolution
of the seismic swarm extracted from the BCSF-R\'{e}NaSS catalogue. In
fact, the algorithm of automatic detection is not based on a fixed
network of seismic stations but includes also progressively the signals
obtained from the temporary stations (CHBN, BNLF, VRNN) set in the
seismic area (see Methods). It means that the ability to 
\mbox{detect} the
lowest magnitude seismic events increased in time. Consequently, a
better fit is obtained between the BCSF-R\'{e}NaSS catalogue and the
CHBN catalogue in time, i.e., when all three stations are included in
the BCSF-R\'{e}NaSS detection algorithm  (Figure~\ref{fig3}). It is
important to keep this bias in mind as it suggests that the number of
daily seismic events detected at the beginning of the seismic swarm
(prior to installing the CHBN station in April 2022 and the BNLF-VRNN
stations in June 2022) is a lower limit of the real number of seismic
events. Consequently, the most intense seismic activity cannot be
excluded to have happened prior April 2022. 

The time difference of arrival between the S- and P-waves on the CHBN
temporary station remained mostly stable since the station was
installed in April 2022 (Figure~\ref{fig3}) suggesting at first order
an absence of large-scale (i.e.\ from mantle to crustal depths) vertical
migration of the seismic swarm. A similar conclusion was preliminary
reached when looking at the hypocenters of the seismic events
automatically detected from the BSCF-R\'{e}NaSS catalogue that are
randomly distributed between 2 and 9~km-depth. Further investigations
based on the deployment of seismic nodes in the area (not considered in
the BCSF-R\'{e}NaSS catalogue) allowed a posteriori relocation of the
seismic events (methods and dataset available in
\citet{Gailleretal2025}). Results highlight a preferential clustering
of the seismic events between 3 and 7~km-depth along two preferential
directions  (Figure~\ref{fig1}e)  \citepalias{Gailleretal2025}. This
clustering is characterized by a slight and non-linear decrease of the
depth of the hypocenters until June 2022. Similarly, the time
difference of arrival between the S- and P-waves on the CHBN temporary
station slightly decreases between April and June 2022 
(Figure~\ref{fig3}).

Considering these analytical limits, we distinguish 4 periods of
seismic activity making up the swarm:
\begin{enumerate}[(1)]
\item 
November 16, 2021--February 3, 2022 (P1 on  Figure~\ref{fig3}) marked
by the first casual detections of seismic events (11 days with no more
than 2 events per day on the BCSF-R\'{e}NaSS catalogue).
\item 
March 18--July 15, 2022 (P2 on  Figure~\ref{fig3}) characterized by an
almost daily detection of seismic events (262 seismic events on the
BCSF-R\'{e}NaSS catalogue). The seismic activity apparently culminated
on May 18--21, 2022, but we cannot exclude that the peak of seismic
activity was reached on March 28--April 5, 2022. A slight non-linear
decrease of the depth of the hypocenters seems specific to this period
\citepalias{Gailleretal2025} with the shallowest seismic events
(3.5--5.5~km-depth) clustering along the N13-68W structure and the
deepest ones (5--6.5~km-depth) along the N173-85W structure.\looseness=-1
\item 
July 24, 2022--January 4, 2023 (P3 on  
Figure~\ref{fig3}) characterized
by 4 narrow time windows of seismic activity not exceeding 3 days and
counting for 133 seismic events of the 175 ones detected on the full P3
period. Over this period, seismic detection on CHBN with Phasenet is
still almost continuous. 
\item 
January 5, 2023 (P4 on  Figure~\ref{fig3})--July 1, 2023 (P4 on 
Figure~\ref{fig3}) characterized by only punctual seismic detections on
both the BCSF-R\'{e}NaSS catalogue (13 seismic events) and on CHBN with
Phasenet.
\end{enumerate}

\subsection{Time series of ground deformation}\label{sec4.2}
Daily baselines time series (from 25 to 43~km of distance) between the
three closest GNSS stations from the RGP and RENAG networks (PDOM,
SJPL, STGN on  Figure~\ref{fig2}a) show variations of several
millimeters on a daily, monthly or even multiannual basis. It is
therefore impossible to detect any significant ground deformation of
the Monts Dore volcanic province on the period 2021--2023 
(Figure~\ref{fig4}a--c). Because of the large distance between the
permanent GNSS stations and the heart of the volcanic province, this
kind of baseline calculation cannot detect localized and/or low
amplitude deformation and is rather prescribed to retrieve large
deformation of the entire massif. Therefore, we cannot totally discard
that very localized surface deformation occurred before, during, or
after the swarm based on the GNSS data.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig04}
{\vspace*{-3pt}}
\bcaption{\label{fig4}}{3}{Baselines of ground deformation obtained from 
(a)~PDOM-SJPL, (b)~PDOM-STGN, and (c)~SJPL-STGN GNSS stations. The
period of deployment of the temporary GNSS station at Champsiaux is
marked by the shaded blue area. The general trend of evolution of the
seismic swarm (Figure~\ref{fig3}; BCSF-R\'{e}NaSS) is reported for
comparison. The daily position (with uncertainty) of each station is
available in Supplementary Material.}
\end{figure*}

Meanwhile, during the management of the seismic swarm (see below), the
ISDeform national service provided InSAR time series to the LMV-OPGC
that also concluded in an absence of detectable \mbox{deformation} when the
seismic swarm occurred (unpublished data from internal PROVA$^{2}$
technical report on July 4, 2022). Additionally, the GNSS station
temporary deployed between July and November 2022 at the Champsiaux
site, i.e., in the zone of the seismic swarm  (Figure~\ref{fig2}b), did
not reveal significant position changes (Supplementary
Material).

\subsection{Time series of geochemical data}\label{sec4.3}
The isotope chemistry of the fluids released by bubbling springwaters
and the Escarot mofettes is variable on the 2021--2022 period 
(Figure~\ref{fig5}). Maximum $\mbox{}^{3}$He/$\mbox{}^{4}$He values in
the CO$_{2}$-rich gas  \citep{Braueretal2017} are reached in September
2021  (Figure~\ref{fig5}a), i.e., before the seismic swarm. At the
Escarot mofettes, the $\mbox{}^{3}$He/$\mbox{}^{4}$He value (up to 6.54
Ra) is even greater than the ones measured by  \citetalias{Braueretal2017}
in 2011--2015 (up to 6.38 Ra). On the course of 2022,
$\mbox{}^{3}$He/$\mbox{}^{4}$He decreased towards minimum values in
May--June (during the seismic swarm). This evolution is mirrored by a
slight decrease of the $\mbox{}^{4}$He/$\mbox{}^{40}$Ar$^*$ 
(Figure~\ref{fig5}b) from 2.49 to 2.35 at the 
Escarot\unskip\break mofettes.

\begin{figure*}
\includegraphics{fig05}
\vspace*{-4pt}
\caption{\label{fig5}Time evolution of
(a)~$\mbox{}^{3}$He/$\mbox{}^{4}$He (expressed as Rc/Ra where Rc is the
$\mbox{}^{3}$He/$\mbox{}^{4}$He corrected from atmospheric
contamination and Ra the ratio in the air; the range of values from a
Sub-Continental Lithospheric Mantle (SCLM) source is highlighted) 
\citep{GautheronMoreira2002} and
(b)~$\mbox{}^{4}$He/$\mbox{}^{40}$Ar$^*$ in the gas and of  (c)~TDIC
and (d)~$\mathrm{SO}_{4}^{2-}$ content in the water related to bubbling
springwaters. Shaded horizontal bars highlight the range of values
before 2021 in literature at Escarot 
\citep[in red;][]{Braueretal2017}, La Gargouill\`{e}re (in purple)
(\protect\xcitealp{Moreiraetal2018}{2018};
\protect\xcitealp{Riccietal2024}{2024}; unpublished data GEOLAB),
Sainte-Anne (in orange) 
(\protect\xcitealp{Braueretal2017}{2017}; unpublished
data GEOLAB), and Fontaine Goyon (in grey) 
(\protect\xcitealp{Braueretal2017}{2017};  unpublished data GEOLAB). The
uncertainty on the measurements (2$\sigma$) is smaller than the size of
the square symbols.} 
\vspace*{-6pt}
\end{figure*}

The chemistry of the associated waters presents maximum TDIC (Total
Dissolved Inorganic Carbon) values in February 2022 
(Figure~\ref{fig5}c), i.e., at the beginning of the main seismic period
(P2 on Figure~\ref{fig3}): up to 127 and 99~mmol/L at Fontaine Goyon
and La Gargouill\`{e}re, respectively. These values are two- to
three-times greater than past measurements performed in 2015--2018 and
in July 2021 \citep{Riccietal2024}. They highlight a process of
carbon-enrichment in the springwaters since at least the beginning of
2022. Over the course of 2022 (during the seismic swarm), TDIC values
decrease to reach values similar to the ones measured in 2015--2018.
Similarly, the composition in other chemical elements (e.g.,
$\mathrm{SO}_{4}^{2-}$,  Cl$^-$, F$^-$) and physical parameters (e.g.,
pH, conductivity) is the same in 2022--2023 than in 2015--2018 for the
springwaters in the Monts Dore (Sainte-Anne, Fontaine Goyon).

Along the transect of soil CO$_{2}$ flux crossing the area of the
seismic swarm, $\updelta\mbox{}^{13}$C values of soil  CO$_2$ are in the
range ${-}$19.9/${-}$22.1\permil\  in May 2022 and ${-}$20.3/${-}$23.2\permil\
in June 2023. At the first order, these values are close to the mixing
line between air and organic endmembers. No significant variation is
\mbox{observed} between Zone 1 (medium- to- high flux) and Zone 2 (low flux),
neither in time (during and after the seismic swarm) 
(Figure~\ref{fig6}a,b).

\begin{figure*}
\includegraphics{fig06}
\bcaption{\label{fig6}}{5}{(a)~Transect of soil CO$_{2}$ flux (May 6, 2022)
with a 100~m-spacing crossing the area of the seismic swarm
\citep{Gailleretal2025}  (Figure~\ref{fig2}b). Vertical dashed lines
highlight the theoretical projection of the fault planes at the
surface. The black arrows highlight the sample sites for the analysis
of the $\updelta\mbox{}^{13}$C of soil CO$_{2}$ (from which the two sites
394 and 430 reiterated in time). Zones 1 and 2 delimit the areas
characterized by medium- to- high flux and low flux, respectively.
(b)~Mixing plot of the $\updelta\mbox{}^{13}$C of soil CO$_{2}$ between
three endmembers: atmospheric  \citep{Gravenetal2020}, organic
\citep{Morandietal2016}, and mantellic  \citep{Boudoireetal2023}.
CO$_{2}$ content from GC analysis of the gas sampled in glass vials
(see Methods). Averaged  $\updelta\mbox{}^{13}$C values of soil CO$_{2}$
from C3--C4 plants are also shown \citep{Staddon2004}. The numbering
highlights the two sample sites reiterated in time (cf.\ (a)). (c)~Time
evolution of the averaged soil CO$_{2}$ flux along the transect
(calculated from the 48 sites of measurements). The vertical dotted
lines highlight the date of the gas sampling for the analysis of the
$\updelta\mbox{}^{13}$C of soil CO$_{2}$. The general trend of evolution
of the seismic swarm  (Figure~\ref{fig3}; BCSF-R\'{e}NaSS) is reported
for comparison. The uncertainty on the measurements (2$\sigma$) is
smaller than the size of the square and circle symbols.}
\vspace*{4pt}
\end{figure*}

The average of raw soil  CO$_2$ flux reached a maximum
$(25.4~\mathrm{gm}^{-2}{\cdot}\mathrm{d}^{-1})$ in July 2022 and
decreased in the following months to reach a minimum 
$(6.6~\mathrm{gm}^{-2}{\cdot}\mathrm{d}^{-1})$ in November 2022 
(Figure~\ref{fig6}c). Another high average of raw soil CO$_{2}$ flux is
measured in June 2023. Even if only 5 reiteration surveys were
performed between May 2022 and June 2023, this time evolution (with
maximum flux during the summer) seems mirroring a seasonal effect 
\citep{Boudoireetal2017,Viveirosetal2008}. To investigate this effect,
we applied a correction  (Equation~(\ref{eq3})) to each measurement of
raw soil  CO$_2$ flux\unskip\break
$(\phi_{{\mathrm{CO}_{2}}_{\mathrm{raw}}}(t))$:
{\begin{equation}
\phi_{{\mathrm{CO}_{2}}_{\mathrm{corrected}}}
(t)=
\frac{\mu_{{\mathrm{CO}_{2}}_{\mathrm{raw}}}(t)}
{\mu_{{\mathrm{CO}_{2}}_{\mathrm{raw}}}\ 
(\mathrm{May}~2022)}
\times 
\phi_{{\mathrm{CO}_{2}}_{\mathrm{raw}}}
(t)\label{eq3}
\end{equation}}\unskip
where
$\mu_{{\mathrm{CO}_{2}}_{\mathrm{raw}}}(t)$ is the average of the soil
CO$_{2}$ flux obtained from the 9 sites of measurement located in Zone
2 (i.e., the area of low soil CO$_{2}$ flux more prone to be affected
by seasonal variations) and $\mu_{{\mathrm{CO}_{2}}_{\mathrm{raw}}}$ (May
2022) is the average obtained from the Zone 2 in May 2022. This
corrective model highlights a maximum average of corrected soil
CO$_{2}$ flux reached in May 2022 followed by a continuous decrease
until the last survey in June 2023  (Figure~\ref{fig6}c). 

\subsection{Times series from remote thermal imagery}\label{sec4.4}
Thermal satellite imagery on the period 2015--2023 highlights a peak of
anomalous pixels on August 2015 for ALICE (15,823 anomalous pixels in
the area of interest;  Figure~\ref{fig7}a) and July 2019 for RETIRA (1891;  
\mbox{Figure~\ref{fig7}b).} Anomalous pixels are also detected in
May--July 2020 for ALICE (up to 2838 anomalous pixels) but unmatched
for RETIRA that is less sensitive to weather conditions and in
particular to anomalous thermal pixels during the summer\unskip\break
(see Methods).

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Time evolution of thermally anomalous pixels
(spatial resolution is  $30\times30$~m in Google Earth Engine)
identified with (a)~ALICE and (b)~RETIRA indices (see Methods) with
99.5\% of confidence (2.5$\sigma$). The period of seismic activity is
highlighted with the grey shaded square.  (c)~Location of anomalous
hotspots from satellite imagery (LANDSAT) on July 13, 2022, in the area
of interest.} 
\vspace*{-2pt}
\end{figure*}

During the seismic swarm, no significative thermal anomalies are
detected, even less in the zone immediately above it. A detailed
analysis of a clear weather satellite image obtained on July 13, 2022,
reveals 5 anomalous pixels for RETIRA, only (areas R1 and R2 on
Figure~\ref{fig7}c). They belong essentially to areas marked by a
contrast of vegetation. For instance, the geographical extension of
A1/R1 overlaps the area where trees have recently been pruned for
commercial purposes. It argues in favour of developing indices taking
into account the influence of vegetation and soil heterogeneity on the
detection of thermal anomalies. Considering the reference field images
(i.e., $\mu$ and $\sigma$) used to compute the RETIRA index (see
Methods), one can argue with 99.5\% of confidence (2.5$\sigma$) that
no anomalous temperature greater than 5~\textdegree C were detected in
the area during the seismic swarm.

\section{Discussion}\label{sec5}

\subsection{Seismic activity triggered by fluid
circulation}\label{sec5.1}
The comparison of the distinct physico-chemical signals and/or
measurements performed over the 2021--2023 period is fundamental to
investigate the processes linked to the seismic swarm 
(Figure~\ref{fig8}).

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}Comparison between the evolution of the distinct
time series of physico-chemical signals acquired in the frame of this
study. No data for the absence of measurements. Low (/) and high $(++)$
levels highlight qualitatively, during the considered period,
steady-state background conditions and maximum amplitude of identified
anomalies described in the text, respectively. Medium levels highlight
either the increase of the monitored signals from background conditions
to maximum amplitude identified anomalies  $(+)$ or its decrease
$(-)$.} 
\end{figure*}

The chemistry of the gas phase released by bubbling springwaters spread
in the Monts Dore volcanic province and its vicinity changed at the end
of the summer 2021. In particular, an increase of the
$\mbox{}^{3}$He/$\mbox{}^4$He was measured together with a \mbox{potential}
slight increase of the $\mbox{}^{4}$He/$\mbox{}^{40}$Ar$^*$. At the end
of the autumn 2021, the first seismic events are detected (P1 period on
Figure~\ref{fig3}). While the gas chemistry came back to more classical
values during the winter 2021--2022, the water chemistry of the
bubbling springwaters became marked by a clear increase of the
\mbox{dissolved} carbon without significant evolution of other species such as
$\mathrm{SO}_{4}^{2-}$,  Cl$^-$, F$^-$. The most intense seismic
activity (P2 period  on Figure~\ref{fig3}) is not associated with any
anomalous values of physico-chemical parameters monitored in the
bubbling springwaters. However, we note that this period is
characterized by the highest soil CO$_{2}$ flux measured above the
seismic swarm. In parallel, local testimonies reported the occurrence
of new degassing areas (bubbling) on some hiking paths of the
Vall\'{e}e de Chaudefour  (Figure~\ref{fig2}), close to the Sainte-Anne
springwater. Soil CO$_{2}$ flux progressively decreased during the
summer 2022 in parallel to the decrease of the seismic activity (P3
period on Figure~\ref{fig3}). No global ground deformation of the
volcanic massif was detectable on GNSS baselines during the whole
period. From the beginning of 2023, the seismic activity became only
residual (P4 period on Figure~\ref{fig3}) with no significant
variations of the other physico-chemical parameters monitored.
Interestingly, some seismic events were also detected in the area of
the Joze springwaters, more than 40~km far from the area of the seismic
swarm  (Figure~\ref{fig1}). This observation supports the idea of a
close relation between fluid circulation and seismicity, not only
within the Monts Dore volcanic province but rather at regional scale.

The absence of notable ground deformation, of enrichment in
S--Cl--F-bearing species in springwaters, and of more enriched
$\updelta\mbox{}^{13}$C signature of the CO$_2$ release from the soil
tend to discard the hypothesis of magma intrusion at crustal level.
Constraining the expected ground deformation induced by magma injection
is challenging due to several unknown input parameters (magma volume
and pressure, crustal rheology,\,{\ldots} \citep{Townsend2022} for a
review) required to build thermo-mechanical models. If we recognize
that future work is needed to model such expected changes, it is worth
noting that dyke propagation or magma refilling of crustal reservoirs
at this depth range (3--7~km) generally trigger inflation/deflation
processes characterized by horizontal and/or vertical plurimillimetric
displacements 
\citep{Beauduceletal2020,Sigmundssonetal2018,Wadgeetal2006} that should
have been detected by InSAR, and are not detected in this study, even
in the epicenter zone (Champsiaux GNSS temporary station;
Supplementary Material). Similarly, magma ascent towards the
surface favors the degassing of gaseous species with a higher
solubility in magma (i.e., SO$_{2}$, H$_{2}$S, HCl, HF) that could in
turn modify the chemistry of local springwaters
\citep{Federicoetal2023} and/or the 
\mbox{enrichment} in $\mbox{}^{13}$C in
CO$_{2}$ emissions in the epicenter zone or in its vicinity 
\citep{Boudoireetal2020}. These geochemical markers are not detected in
our study.\looseness=-1

The seismic swarm clustered along tectonic structures where
magnetotelluric surveys revealed relatively higher electrical
resistivity values $({\sim}100~\Omega{\cdot}\mathrm{m})$ compared
to the low electrical resistive body 
$({&lt;}10~\Omega{\cdot}\mathrm{m})$ present at depth greater than
8~km \citep{Gailleretal2025}. This observation raises the question of
what triggered the reactivation of these structures, bearing in mind
that the reactivation was presumably not caused by magma injection.
Considering the temporal sequence of variations reported above, we
propose that this seismic swarm was induced by a perturbation of the
hydrothermal system beneath the Monts Dore due to the arrival of a
CO$_{2}$-rich fluid originating from the mantle. Undated magma
intrusion at mantle depth  (Figure~\ref{fig9}a) could have generated
the forming of a foam at the top of a magma reservoir by early
degassing or second boiling  (Figure~\ref{fig9}b) where overpressure
would be able to release a CO$_{2}$-rich gaseous phase. A similar
process has already been proposed to explain recurrent seismic swarms
in several volcano-tectonic provinces in continental rifts 
\citep{Lindenfeldetal2012},  including the ones of the European
Cenozoic Rift System \citep{Audinetal2002,FischerHoralek2003} as the
Novy Kostel area (Eger Rift, Czech Republic; \mbox{Figure~\ref{fig1}a)} 
\citep{Braueretal2011,Fischeretal2010,Fischeretal2014} 
and the Laacher See Volcano (Eifel, Germany)  \citep{Henschetal2019}.

\begin{figure*}
\includegraphics{fig09}
{\vspace*{1pt}}
\caption{\label{fig9}Model of the time evolution of the processes 
(a--e)~involved in the 2021--2022 seismic swarm in the Monts Dore
volcanic province and described in the main text 
\citep[modified from][]{Boudoireetal2023}.}
{\vspace*{1pt}}
\end{figure*}

In the Monts Dore volcanic province, the initial increase of the
$\mbox{}^{3}$He/$\mbox{}^{4}$He values in the gas phase of bubbling
springwaters during the summer 2021 may track such changes in magma
dynamics and degassing within the Sub-Continental Lithospheric Mantle 
\citep{Boudoireetal2023,Braueretal2017}  (Figure~\ref{fig9}c). The
isotopy of noble gases ($\mbox{}^{3}$He/$\mbox{}^{4}$He;
$\mbox{}^{4}$He/$\mbox{}^{40}$Ar$^*$) is consistent with gas ascent
from mantle depth rather than from crustal reservoirs 
\citep[][and this study]{Boudoireetal2023}. Gas--water reaction
triggered by the arrival of mantle CO$_{2}$ at crustal level could have
increased the total dissolved carbon content in the aqueous phase of
the springwaters in the following months  (Figure~\ref{fig9}d). The
presence of a high electrical conductivity body at depth greater than
8~km beneath the Monts Dore supports the existence of a hydrothermal
system at depth that may favor fluids circulation and reaction 
\citep{Gailleretal2025,Pauwelsetal1997}.  This perturbation of the
hydrothermal system may have been balanced by the seismic reactivation
of pre-existing shallower structures  (Figure~\ref{fig9}d) acting as
preferential fluid pathways (as suggested by self-potential and
electrical resistivity tomography surveys,  \citep{Gailleretal2025})
and the increase of soil CO$_{2}$ flux. This synchronous response of
soil degassing (without change of the $\updelta\mbox{}^{13}$C of
CO$_{2}$;  Figure~\ref{fig6}b) to stress change at depth is perfectly
compatible with a pulsating degassing mechanism 
\citep{Boudoireetal2018}.  
This process is reminiscent (on a shorter
timescale and with a much smaller magnitude) of that occurring in
active calderas, such as the Campi Flegrei (Italy), where seismic
activity at shallow level is partly explained by a thermofluid
dynamical model where deep fluid injections other than magma may be
involved but with minor changes in the temperature 
\citep{Chiodinietal2003,Chiodinietal2012}. At the same time, the
decrease of the $\mbox{}^{3}$He/$\mbox{}^{4}$He during the seismic
swarm may reflect a preferential release of crustal-derived components
due to rupturing  \citep{Braueretal2011}.\looseness=1

\subsection{Insights for future monitoring}\label{sec5.2}
The first communication about the occurrence of a seismic swarm in the
Monts Dore area was made by the Commissariat \`{a} l'Energie Atomique
(CEA; in charge of seismic alerting in mainland France) to the
Observatoire de Physique du Globe de Clermont-Ferrand (OPGC) and the
Laboratoire Magmas et Volcans (LMV) on April 12, 2022. Once the
communication was received at OPGC, the first temporary seismic station
(CHBN) was installed in the area of the seismic swarm to densify the
national network and therefore to increase our ability to detect and
localize the events at depth with more accuracy  
\mbox{(Figure~\ref{fig3}).} A
concerted effort between the OPGC and the LMV towards deploying
scientific and technical human resources with instrumental and
logistical supports made it possible 30 dedicated field surveys as of
July 2023 (without counting the maintenance routine operations). The
OPGC internal school also temporarily reorganized its practical courses
in geology and instrumentations. This allowed involving some of its
undergraduate and master's students, as part of their initial training,
on a scientific issue requiring rapid intervention. In addition, the
data center of OPGC created a dedicated catalogue to aid monitoring and
planning survey missions and to share and archive the data on the long
term. This action, articulated between research, technical
developments, teaching, and the national facilities, allowed to collect
multidisciplinary data to characterize the seismic swarm 
\citep[][and this study]{Gailleretal2025}.

In parallel with the scientific surveys, communication to the national
authorities (Direction G\'{e}n\'{e}rale de la Pr\'{e}vention des
Risques: DGPR), to the regional authorities (Direction R\'{e}gionale
de l'Environnement, de l'Am\'{e}nagement et du Logement: DREAL), and
to the local authorities (Direction D\'{e}partementale des Territoires: 
DDT) about the seismic swarm were initiated on June 13, 2022, by the
OPGC and LMV managers and their representatives. This was made under
the umbrella of the Centre National de la Recherche Scientifique
(CNRS). Public communications to the regional press were also organized
to minimize the propagation of fake news among local inhabitants about
a potential renewal of the eruptive activity in the Monts Dore.
Additional meetings with public authorities were organized to raise
awareness about the necessity to reinforce the national monitoring
networks in the Monts Dore, and more generally in the volcanic
provinces in mainland France. These efforts led to~the signing of a
5-year agreement (ReODORE)\unskip\break 
between the 
Universit\'{e} Clermont Auvergne
(UCA) and the Direction D\'{e}partementale des Territoires (DDT) in
order to strengthen the national observation networks in the Monts Dore
and to facilitate in turn the delivery of reports to the local
authorities. The DDT through this agreement provided funding for one
permanent seismic station, two permanent GNSS stations, one permanent
soil CO$_{2}$ flux station, and for regular sampling and analysis of
five springwaters  (Figure~\ref{fig10}a). 

\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig10}
{\vspace*{3pt}}
\caption{\label{fig10}(a)~Reinforcement of the monitoring network
(seismology, geodesy, geochemistry) in the Monts Dore planned in the
frame of the ReODORE agreement between the UCA and the DDT. 
(b)~Organisation chart of the PROVA$^{2}$ consortium led by the
OPGC-LMV.}
{\vspace*{3pt}}
\end{figure*}

On August 29, 2022, this collaborative experience led to the creation
by the OPGC and the LMV of a local initiative, the P\^{o}le
R\'{e}gional d'Observation de l'Activit\'{e} Volcano-tectonique
d'Auvergne et d'Ard\`{e}che (PROVA$^{2}$) led by scientists at LMV in
which, the scientific long-term questions, the 
operational resources
engaged to address them, and the communication channels to local
authorities are now structured in working groups 
(Figure~\ref{fig10}b). This consortium includes the support of national
and international collaborators and is designed to address five
objectives: (1)~to promote programs strengthening the observation of
local volcano-tectonic activity, (2)~to support the structuration of
field measurements and monitoring signals into database and metadata,
(3)~to deploy a scientific response in the event of significant
variations in activity, (4)~to promote research and student formation
based on the geological wealth in the local territory, (5)~to
strengthen the communication mission of the OPGC towards the regional
authorities and populations, and (6)~to address specific information
requests from local authorities. Furthermore, the new French national
research infrastructure EPOS-France created in November 2023 represents
an opportunity to bring together and federate the scientific
communities responsible for monitoring the French territory (mainland
and overseas), to strengthen links of solidarity and instrumental
pooling, and to establish multidisciplinary actions in solid earth
sciences with broad thematic contours. The PROVA$^{2}$ initiative is in
line with this effort and supports the consideration of volcanic
provinces in mainland France as main subjects of scientific interest
for the EPOS-France community.

\subsection{Insights for risk mitigation strategies}\label{sec5.3}
Two main recommendations emerge from the management of the 2021--2022
seismic swarm in the Monts Dore volcanic province. Firstly, a concerted
protocol at the national level regarding the detection and the rapid
reporting of seismic swarms on mainland France could be consolidated in
favor of quick scientific and operational responses. In a similar way,
an emergency response strategy with the various Services Nationaux
d'Observation (SNO), EPOS-France and international partners would
enable rapid access to complementary information that appeared crucial
during this seismic swarm, such as satellite imagery. For instance, an
effort should be made to extract multi-year InSAR time series of ground
deformation at the scale of the entire massif to identify potential
displacements over the long term and not only during seismic swarms (as
documented in this study). In parallel, a scientific effort should be
also led to develop physics-based models of expected change in
monitoring geodetic signals in case of volcano unrest 
\citep{Acocellaetal2024}.  This is particularly important for the
geographical area covered by long-quiescent volcanic provinces and
where in-situ monitoring networks are sparse. Secondly, we support
initiatives for the elaboration of a general transverse management
protocol between the academic and societal worlds regarding unusual
volcano-tectonic activity in mainland France. In France, the ORSEC
plans (Organisation de la R\'{e}ponse de SEcurit\'{e} Civile) define
the general organization of civil protection to cope with natural or
industrial disasters. Plans to trigger a rapid and appropriate response
to volcano-tectonic activity exist in the French overseas departments
but not, to our knowledge, for mainland France. We recognize that the
probability of an eruption in mainland France may appear low but we
also note the presence of several basanitic and basaltic eruptive cones
within or close to the seismic area with ages more recent than the end
of the Monts Dore stratovolcano's activity \citep{Boivinetal2017}.
Following the definition of the French Ministry of Ecological
Transition (in charge of preparing and implementing the government's
policy regarding environmental \mbox{questions)} a major risk ``is
characterized by its low frequency and its enormous severity''. As
such, discussions regarding the relevance of including volcano-tectonic
activity as a potential risk for the volcanic provinces of the French
Massif Central could be at least conducted with the civil protection.

\section{Conclusion}\label{sec6}
The evolution of the 2021--2022 seismic swarm in the Monts Dore
volcanic province was monitored by coupling (1)~time series from
permanent stations of the seismic and GNSS national networks, (2)~time
series from temporary seismic and GNSS stations deployed by the
OPGC-LMV, (3)~field measurements of the physico-chemical parameters of
local springwaters and soil CO$_{2}$ flux transect, and (4)~remote
sensing based on thermal imagery. Results indicate an absence of global
ground deformation during the seismic swarm. No anomaly in the content
of S--Cl--F-bearing species in springwaters, and no thermal anomaly
before, during, and after the seismic swarm was identified. Therefore,
there is so far no evidence supporting the direct causal link between
the seismic swarm and magma intrusion at shallow crustal level.

However, $\mbox{}^{3}$He-enrichment in the gaseous phase of local
springwaters at the end of the summer 2021 followed by an increase of
the Total Dissolved Inorganic Carbon (TDIC) in the aqueous phase during
the winter 2021--2022, preceded the most intense period of seismic
activity in March--July 2022. During this period, soil CO$_{2}$ flux
was at its highest. Based on these results, we proposed a model in
which a CO$_{2}$-rich fluid ascending from the mantle reacted in a
hydrothermal system beneath the Monts Dore at crustal level. Fluid
circulation and change in the fluid composition in this mixing zone
would have triggered the seismic activity and the increase of soil
CO$_{2}$ flux along pre-existing tectonic structures. Note that when
finalizing this article (in May--June 2024), new seismic events were
detected in the same area.

The management of this unusual seismic activity in the Monts Dore
volcanic province raises several recommendations about the monitoring
of long-dormant volcanic provinces on mainland France, among which
(1)~the development of a national protocol of detection and rapid
reporting of seismic swarms in mainland France, especially in active
volcano-tectonic areas, (2)~the creation of an operational protocol for
the management and the communication of any unusual activity in
long-dormant volcanic provinces, and (3)~the promotion of joint
research initiatives to better investigate 
\mbox{expected} change in
monitoring signals as well as hazard assessment related to potential
eruptive scenario. Similar seismic swarms detected in other volcanic
provinces of the European Cenozoic Rift System (Novy Kostel, Laacher
See) as well as inferences about the presence of a persistent magmatic
activity at mantle depth beneath these provinces should especially
simulate joint research and civil protection European initiatives. 

\section*{Data availability}
Supplementary Material related to the manuscript is available through a
.zip file the OPGC data center PROVA$^{2}$
\citep{ObservatoiredePhysiqueduGlobedeClermont-Ferrand2021}.
The Supplementary Material is an Excel file with seven sheets:
``Seismic database'' compiles the daily seismic number of events from
the BCSF-R\'{e}NaSS catalogue in the area of interest; ``Seismic
waveform'' show typical waveform of volcano-tectonic events forming the
seismic swarm; ``GNSS permanent stations'' compiles the data from the
PDOM, SJPL, and STGN stations of the RENAG and RGP networks used to
calculate the baselines; ``GNSS temporary station'' compiles the East,
North and vertical component of the Champsiaux station; ``Bubbling
springwaters'' compiled the chemical dataset used in this study for the
four monitored sites; ``Soil degassing'' related to the soil
CO$_{2}$ flux measurements and related carbon isotopy made
along the transect; ``Thermal imagery'' compiles the time series of
anomalous pixels in the area of interest by using both ALICE and RETIRA
indexes. 

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgements}
We would like to thank all the inhabitants and the municipalities of
the Chambon-sur-Lac, Le-Vernet-Sainte-Marguerite, and Murol for their
availability and, above all, for allowing us to carry out measurements
on their properties. We are in debt with\unskip\break 
\mbox{M.~Tantillo} (INGV Palermo), Y.
Oliveri (INGV Palermo), and F. Salerno (INGV Palermo), for their
assistance during gas analysis in laboratory. We acknowledge the staff
of the French Services Nationaux d'Observation (SNO) for sharing
information during the seismic crisis and especially the ISDeform one
for providing InSAR time series. This research is part of the
PROVA$^{2}$ (P\^{o}le R\'{e}gional d'Observation de l'activit\'{e}
Volcano-tectonique d'Auvergne et d'Ard\`{e}che) initiative held at the
OPGC-LMV and has benefited from the Memorandum of Understanding
established between the UCA and the INGV. This research was financed by
the French Government Laboratory of Excellence initiative no\ 
ANR-10-LABX-0006, the Region Auvergne, and the European Regional
Development Fund through the project DegazRift. This is contribution
no.\ 677 of the ClerVolc program of the International Research Center
for Disaster Sciences and Sustainable Development of the University of
Clermont Auvergne. 

\CDRGrant[ANR]{ANR-10-LABX-0006C}

\back{}

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

\CDRsupplementaryTwotypes[application/zip]{supplementary-material}{\cdrattach{crgeos-285-suppl.xlsx}}

\printbibliography
\refinput{crgeos20240982-reference.tex}

\end{document}
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