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\DOI{10.5802/crgeos.284}
\datereceived{2024-11-30}
\daterevised{2025-02-12}
\dateaccepted{2025-02-13}
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\begin{document}

\begin{noXML}

\TopicFR{Volcanologie}
\TopicEN{Volcanology}

\shortrunauthors

\CDRsetmeta{articletype}{research-article}

\title{Scientific response to the 2021--2022 seismic swarm in the Monts
Dore volcanic province (France): structural insights from punctual
surveys (1/2)}

\alttitle{R\'{e}ponse scientifique \`{a} l'essaim sismique
de 2021--2022 dans la province volcanique des Monts Dore (France) :
informations structurales issues de campagnes ponctuelles (1/2)}

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

\author{\firstname{Cl\'ement} \lastname{Grace}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Guillaume} \lastname{Boudoire}\CDRorcid{0000-0002-3148-3581}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}
\address{Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy}
\email[G. Boudoire]{Guillaume.BOUDOIRE@uca.fr}

\author{\firstname{Marc} \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}

\author{\firstname{Jean}\nobreakauthor\lastname{Battaglia}\CDRorcid{0000-0002-9513-1200}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\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}

\author{\firstname{Philippe} \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}

\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}

\author{\firstname{Jean-Michel}\nobreakauthor\lastname{Douchain}}
\address{Universit\'{e} Clermont Auvergne, CNRS, 
Observatoire de Physique du Globe de Clermont-Ferrand, 8 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Nicolas} \lastname{Cluzel}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Romain} \lastname{Guillard}\CDRorcid{0009-0009-8836-3052}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Valentin}\nobreakauthor\lastname{Fr\'{e}ret\nobreakauthor Lorgeril}\CDRorcid{0000-0003-4571-8641}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Cyril} \lastname{Aumar}\CDRorcid{0000-0002-7430-3413}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\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}
\curraddr[V. Rafflin]{Department of Earth Sciences, Royal Holloway, University of
London, Egham, TW20 0EX, UK}

\author{\firstname{Vincent} \lastname{Boulenger}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Sol\`ene} \lastname{Buvat}\CDRorcid{0009-0001-6340-4147}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Jehoji} \lastname{Kuzamba}}
\addressSameAs{1}{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, 
Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\author{\firstname{Erwan}\nobreakauthor\lastname{Th\'{e}bault}\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{4}{Universit\'{e} Clermont Auvergne, CNRS, 
Observatoire de Physique du Globe de Clermont-Ferrand, 
8 avenue Blaise Pascal, 63178 Aubi\`{e}re, France}

\keywords{\kwd{Long-dormant volcanic provinces}
\kwd{Seismicity}
\kwd{Magnetization and conductivity anomalies}
\kwd{Soil CO\textsubscript{2} degassing}
\kwd{Tectonic control}
\kwd{High permeability structures}}

\altkeywords{\kwd{Provinces volcaniques dormantes}
\kwd{Sismicit\'{e}}
\kwd{Contrastes d'aimantation et de conductivit\'{e}}
\kwd{D\'{e}gazage de CO\textsubscript{2} sol}
\kwd{Contr\^{o}le tectonique}
\kwd{Structures \`{a} haute perm\'{e}abilit\'{e}}}

\thanks{OPGC through the `P\^ole R\'egional d'Observation de
lactivit\'e Volcano-tectonique dAuvergne et dArd\`eche: The
Auvergne-Ard\`eche regional volcano-tectonic monitoring center'
(PROVA$^2$), FEDER Project `CAPRICE', CNES in the framework of the
projects Volcadrone and RMagEDrone ``Multi-scale geophysical imaging of
volcanic edifices'', ERDF (HADGR211022), BRGM, University of Clermont
Auvergne (contribution no. 676)}

\begin{abstract} 
Long-dormant 
volcanic provinces remain excellent proxies in studying
active edifices. The Monts Dore volcanic province (French Massif
Central) has been recently the site of a unique seismic episode.
Geophysical surveys were conducted at different horizontal and vertical
spatial scales. Magnetic anomalies highlight mechanical heterogeneities
consistent with the regional tectonic context. Low-conductive
structures imaged suggest the presence of fluid rising along main paths
at various depths. Although we cannot strictly exclude a purely
tectonic explanation, our data seem to support the origin of seismicity
as being linked to the injection of fluids at depth controlled by
internal structural constraints.
\end{abstract}

\begin{altabstract} 
Les provinces volcaniques depuis longtemps dormantes constituent
d'excellents analogues pour l'\'{e}tude des \'{e}difices actifs. La
province volcanique des Monts Dore (Massif Central fran\c{c}ais) a
r\'{e}cemment \'{e}t\'{e} le site d'un \'{e}pisode sismique unique. Des
\'{e}tudes g\'{e}ophysiques ont \'{e}t\'{e} men\'{e}es \`{a}
diff\'{e}rentes \'{e}chelles spatiales, tant horizontales que
verticales. Les anomalies magn\'{e}tiques r\'{e}v\`{e}lent des
h\'{e}t\'{e}rog\'{e}n\'{e}it\'{e}s m\'{e}caniques coh\'{e}rentes avec
le contexte tectonique r\'{e}gional. Les structures \`{a} faible
conductivit\'{e} mises en \'{e}vidence sugg\`{e}rent la pr\'{e}sence de
fluides remontant le long d'axes principaux \`{a} diff\'{e}rentes
profondeurs. Bien qu'une explication purement tectonique ne puisse
\^{e}tre totalement exclue, nos donn\'{e}es tendent \`{a} soutenir
l'hypoth\`{e}se d'une sismicit\'{e} li\'{e}e \`{a} l'injection de
fluides en profondeur, contr\^{o}l\'{e}e par des contraintes
structurales internes.
\end{altabstract} 

\maketitle

\vspace*{1pc}

\twocolumngrid

\end{noXML}

\defcitealias{Vidaletal1996}{ibid.}
\defcitealias{Merleetal2023}{ibid.}
\defcitealias{Pearceetal2020}{ibid.}
\defcitealias{BattagliaDouchain2017}{ibid.}

\section{Introduction}\label{sec1}
The characterization of the seismic hazard in volcanic regions stands
as a huge challenge with real both scientific and societal issues. It
implies understanding physical and chemical processes leading to
seismicity, potential fluid transfers and volcanic activity. Unlike
major high magnitude seismic events, earthquake swarms are
characterized by the lack of a dominant main shock. Such swarm-like
seismicity can be triggered by numerous physical mechanisms in various
contexts, especially in active volcanic areas where they are typically
associated with either hydrothermal fluids, magma movements or volcanic
eruptions  \citep{Mesimerietal2023}. Earthquake swarms also occur in
area without active volcanism in continental rifts (i.e.\ Rio Grande,
West Bohemia, and recently in the French Massif  Central;
\xcitealp{Ibs-vonSehtetal2008}{2008}) 
where seismicity is restricted to
deep-reaching zones of weakness and a heterogeneous stress field that
could favor fluid circulation in crustal layers. If the seismicity of
intracontinental regions is generally low compared to active margins
for example, seismicity is less constrained due to the relative rarity
of occurrences and the absence of dense sustainable monitoring 
networks  \citep{Larroqueetal2021}. Regarding the long-term history and
phenomena in such areas (significant long-lived structural heritage,
tectonics and erosion, diffuse seismicity and very rare strong
earthquakes), seismic hazard assessment, and especially the
characterization of associated sources and mechanisms, remains a
particularly difficult exercise. 

The French Massif Central (FMC) remains a large seismogenic area in
memory of its past and long-term volcano-tectonic  activity 
\citep{BattagliaDouchain2017}. For example, at the local scale of the
Monts Dore volcanic province, several seismic events of low magnitude
are regularly recorded, and an important seismic swarm recently
occurred in  2021--2022, remarkable both in terms of duration and
number of events detected. To provide a detailed spatial
characterization of this seismic swarm event, 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) deployed an operational response on the field during
the seismic crisis in order to provide at the first order structural
insights around this phenomenon. The relocated seismicity has evidenced
a seismic swarm clustered between 3 and 6~km-depth aligned following
two fault planes-oriented along main regional tectonic axes. Magnetic
anomalies mapping, 2D Electrical Resistivity Tomography (ERT) and
magnetotelluric (MT) soundings, highly sensitive to fluids contents,
fracturing and alteration processes, were carried out to image any
structural contacts that could link fluids circulation or storage at
various depths with structural weakness paths. Complementary soil
CO$_{2}$ and self-potential (SP) measurements have also been carried
out to highlight the possible circulation of fluids close to the
surface. Details on the spatio-temporal behavior of the seismic swarm
as well as dynamic insights from complementary temporal surveys are the
focus of a companion  paper  \citep{Boudoireetal2025}.

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig01}
{\vspace*{-5pt}}
\caption{\label{fig1}(a)~General context of the Monts Dore volcanic
province (colored areas) in the FMC and main tectonic features. The
rose diagram indicates the main orientations of the four families of
emblematic faults.  (b)~Zoom on the simplified geological and
structural scheme of the study Monts Dore volcanic province 
\citep[after][]{Chevremontetal2023}.}
{\vspace*{-5pt}}
\end{figure*}

\section{Context of the study}\label{sec2}
The recent seismicity in the French Massif Central is mainly
distributed in four main areas  \citep{BattagliaDouchain2017}: (1)~the
Monts Dore sector,  (2)~the Saint-Flour one,  (3)~the region of Ambert
(including a seismic swarm in 2013 and more recently in 2024) and to
the northwest of Clermont-Ferrand on a large area encompassing the
Combrailles zone (Figure~\ref{fig1}a). It is moreover important to
stress a virtual absence of seismicity under most of the volcanic
provinces (Cantal, Velay, Dev\`{e}s, Cha\^{i}ne des Puys), with the
exception of the Monts Dore area. Actually, this volcanic province is
also the site of the last eruptive activity known in mainland France
around 6700 years ago (Pavin's Group of Volcanoes;
\xcitealp{Villemantetal2016}{2016}). 
Seismic swarms in the Monts Dore volcanic
province (less than one hundred seismic events with an occurrence of a
few days to a few weeks) were documented in 1979, 1980, 1983, 1984,
2008 and 2013  
\citepalias{BattagliaDouchain2017}.  
A new very recent
seismic swarm has occurred in 2021--2022, highly remarkable by the
number of earthquakes detected
(more than 400 events on the
BCSF-R\'{e}nass catalog; \url{https://api.franceseisme.fr/fr/search};
\xcitealp{BCSF-Renass2022}{2022}) 
and for the longevity of seismic activity
(i.e.\ more than one year). In more detail, this latter is 
\mbox{concentrated}
on an 8-km$^{2}$ area with the first events initially 
\mbox{located} 2~km west
of the Beaune le Froid 
{village} and 2~km north-west of the
Chambon-sur-Lac town (Figure~\ref{fig1}b).

Regarding its overall context, the Monts Dore volcanic province covers
an area of approximately  500~km$^{2}$ with about  220~km$^{3}$ of
emitted products \citep{Brousse1971}. The volcanic system is divided
into two complex units: the Gu\'{e}ry stratovolcano (or  ``Mont Dore'';
3.1 to 1.8~Ma) and the Sancy stratovolcano  
\citep[1.1 to 0.24~Ma;][]{Cantagreletal1999,Chevremontetal2023}.
It is also the site of
the last eruptive activity known in mainland France around 6700 years
ago  \citep[Pavin Volcanoes Group;][]{Villemantetal2016}.
Thermomineral spring waters are currently widespread  in the area 
\citep{Boudoireetal2023,Braueretal2017,Riccietal2024} that make it of
peculiar interest for geothermal  prospects 
\citep{Pauwelsetal1997,Varetetal1980}.  From a geological and tectonic
point of view, the Monts Dore volcanic province is marked by
sedimentary and volcanic formations outcropping on a highly fractured
Variscan-age plutonic basement. It is located 
{between} the two regional
major tectonic accidents of the Limagne gap to the east (Oligocene) and
the Sillon Houiller to the west. According  to  \citet{Merleetal2023},
the Variscan faults  (${&gt;}$300~Ma) can be divided, based on their
orientations, into four families named after emblematic faults of the
FMC \mbox{(Figure~\ref{fig1}a):}
\begin{itemize}
\item
The Oligocene N20 direction (Sillon Houiller Fault family), which acted
in distension, and mainly conditioned the regional tectonics in horsts
and  grabens  \citep{Hulot1988,Varetetal1980}.
\item
The NE--SW direction (C\'{e}vennes Fault family), a Variscan
intracontinental strike-slip fault that was reactivated during the
Tertiary.
\item
The Pleistocene N160-170E direction (Villefort Fault family), which
played a distensional role feeding the late phases of volcanism  
\citep[Monts Dore, Cha\^{i}ne des Puys;][]{Hulot1988,Varetetal1980}.  
It replays the Hercynian and Oligocene faults in shear and reactivates
them until the present time as suggested by thermal and mineral
activity along all these weak  directions 
\citep{Besson1978,Varetetal1980}.
\item
The late-Hercynian N110-140E direction (Armorican Shear Zone family)
exhibits varying dips depending on their behavior, ranging from
high-angle (sub-vertical) for strike-slip faults to steeper dips for
normal faults.
\end{itemize}
These brittle directions have been characterized by recent reactivation
(i.e.\ Tertiary and Quaternary), accommodating extension and uplift
processes  \citep{Merleetal2023,Varetetal1980}. The reactivation of
these structures likely continues at present, considering the ongoing
uplift of the  FMC  \citep{DelfauLenotre1992} and the presence of
thermal activity emerging notably through these fractures 
\citep{Besson1978,Varetetal1980}. While these major discontinuities
have been highlighted at a regional scale, it is noteworthy that they
are also found at smaller  scales  \citep{Aumar2022,Merleetal2023},
consisting of multiple faults maintaining a general orientation rather
than a single linear fault  \citep{Merleetal2023}. Accordingly, around
the seismic swarm area, several interesting structures have been
identified in the literature, which may belong to the families defined
by  \citetalias{Merleetal2023} (Figure~\ref{fig1}b):
\begin{itemize}
\item
The Jassat strike-slip fault  \citep{Chevremontetal2023} that is part
of the Armorican Shear Zone family, whose faults network forms the
boundary between the Monts Dore and the Plateau des D\^{o}mes 
\citep{Merleetal2023}.
\item
Closer to the swarm, around Chambon Lake and in the western part of the
study area, a fault network was defined by direct observation or 
gravity measurements  \citep{Glangeaudetal1965} with a consistent
orientation to the Sillon Houiller Fault family. A similar oriented
faults network is also observed to the east in the substrate
outcropping at Saint Nectaire  
(Veyre-Monton geological map edited by
the BRGM;  \xcitealp{Chevremontetal2023}{2023};  
Figure~\ref{fig1}b).
\end{itemize}
Furthermore, the presence of numerous recent volcanic vents  
\citep[cones, diatremes, maars;][]{Chevremontetal2023} 
evidenced along a 
${\sim}$N--S alignment in the center of our study area suggests an
organization of eruptive vents along Variscan fault structures, as
proposed by  \citet{Merleetal2023} in the Cha\^{i}ne des Puys. A late
feeding of Monts Dore volcanic activity along the N170 direction is
also suggested by  \citet{Varetetal1980}.

\section{Methods}\label{sec3}
In such a complex tectonic province and well-developed hydrothermal
system, several hypotheses can be proposed to explain the seismic
activity: (1)~a tectonic readjustment along the previously described
faulting directions and/or (2)~a disturbance of an underlying
hydrothermal system by endogenous (arrival of deep fluids) and/or
exogenous (drainage) phenomena. To better characterize its origin and
address potential issues raised by a renewal of the seismicity in local
volcanic provinces as a whole, several actions have been initiated
under the aegis of the P\^{o}le R\'{e}gional d'Observation de
l'activit\'{e} Volcano-tectonique d'Auvergne et d'Ard\`{e}che
\citep[PROVA$^{2}$;][]{Boudoireetal2025}. Initially set up for
structural investigations, the team carried out a series of geophysical
and geochemical measurements at the scale of the area considered in
response to this exceptional seismic activity  (Figure~\ref{fig2}). 
\looseness=-1

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}(a)~Diagram summarizing the main goal of each
approach and their complementarity in the study context.  (b)~Location
of the geophysical measurements superimposed to the relocated seismic
events (zoom on area located in Figure~\ref{fig1}).}
\vspace*{-6pt}
\end{figure*}

We mainly focus on contrasts in magnetization deduced from magnetic
anomalies and electrical resistivity at various scales since those
parameters depend on numerous factors (mechanical heterogeneities,
alteration, fluids storage and circulation) that accumulate in time,
especially in long-dormant volcanic provinces. Beyond the overall area
concerned, several key sites were also investigated in more detail. The
purpose is to better characterize the overall context at various scales
(both laterally and in depth), in order to bring additional constraints
on the origin of the seismic crisis, and its potential for coupling
tectonics with hydrothermal circulation at various depths.

\subsection{Seismic data}\label{sec3.1} 
The Monts Dore region experienced a significant seismic episode
starting from late 2021. BCSF-R\'{e}nass detected the first events on
November 16 using RESIF-Epos (now Epos-France) seismological stations.
Seismic activity intensified from March 19, 2022, reaching 441
earthquakes detected and localized by BCSF-R\'{e}nass by year-end, with
a peak of about 30 events recorded on July 24 and on November 19, 2022
(Figure~\ref{fig3}a). In order to better characterize the spatial
distribution of seismic events and thus the evolution of the swarm,
three additional temporary seismic stations were deployed from April 4,
2022, together with a network of seismic\break nodes.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig03}
\vspace*{-2pt}
\caption{\label{fig3}(a)~Number of earthquakes per month 
(BCSF-R\'{e}NaSS vs.\ Fine-tuning localizations workflow 
\protect\xcitealp{GrunbergLambotte2024}{2024}) 
with visualization of installations of
temporary nodes and additional seismic stations.  (b)~Relocation of
seismic events using the double-difference method, highlighting
principal directions that distinguish two main families of seismic
events.  (c)~Time evolution of the hypocenters related to the seismic
swarm (relocated data BCSF-R\'{e}NaSS).}
\vspace*{-2pt}
\end{figure*}

To enhance the quality of this seismicity catalog, BCSF-R\'{e}NaSS
incorporated artificial intelligence tools, specifically an automatic
phase picking method based on deep learning. The Seisbench/Phasenet 
model  \citep{Woollametal2022}  was employed, trained on manually phase
picked from the BCSF-R\'{e}NaSS catalog covering 2016 to early 2024.
This approach significantly improved the detection and localization of
seismic events, particularly for low-magnitude earthquakes previously
undetectable using SeiScomP  picker \citep{SeisComP2008}.

For earthquakes previously localized, manual picks were supplemented
with automatic picks obtained through deep learning, using waveforms
from all Epos-France stations within a 150~km 
{radius.} Data from a
temporary network specifically 
{deployed} to monitor this seismic swarm
was also 
\mbox{incorporated.} This network included six short-period seismic
nodes, operational between 30/06/2022 and 08/08/2022, along with three
broadband stations whose data are retrieved since 13/04/2022 (XX.CHBN),
06/16/2022 (XX.BNLF), and 06/22/2022 (XX.VRNF).

The catalog construction followed several key steps. First, picks
closely spaced in time were clustered to reduce redundancy,
prioritizing manual picks. Next, seismic phase association was
performed to form events, combining the HDBSCAN  algorithm
\citep{McInnesetal2017}, to regroup picks close in time and space, with
PyOcto \citep{Munchmeyer2024}, which discarded picks not conforming to
typical travel-time curves. The third step involved event localization
using the NonLinLoc  algorithm  \citep{Lomaxetal2000} with a five-layer
1D regional velocity model  \citep{Mazabraudetal2005} suited for FMC.
Finally, when possible, moment magnitude (Mw) was estimated through
waveform spectral fitting using a modified version of  SourceSpec
\citep{Satriano2021}. In order to further refine earthquake locations,
a double-difference relocation was conducted using the HypoDD 
algorithm  \citep{WaldhauserEllsworth2000}. This method incorporated
both catalog-based travel times and differential times derived from
waveform cross-correlation, minimizing relative travel-time residuals
between nearby events. By improving the relative positioning of
clustered seismicity, this approach enabled a more accurate
reconstruction of fault structures.

The combination of deep learning for automatic phase picking,
traditional seismological methods, and data from temporary seismic
stations significantly improved the quality and completeness of this
seismicity catalog. The refined catalog now includes 1616 earthquakes,
compared to the 441 previously identified by BCSF-R\'{e}nass from
November 2021 to the end of 2022. Additional details regarding the
temporary stations are described in the companion paper 
\citep{Boudoireetal2025}.

Final relocation of seismic events (as discussed hereafter) using the
double-difference method, highlighting principal directions that
distinguish two main families of seismic events, as well a time
\mbox{evolution}
of the hypocenters related to the seismic swarm are presented
in  Figures~\ref{fig3}b and~\ref{fig3}c respectively.

\subsection{Associated geophysical and geochemical one-off
surveys}\label{sec3.2} 

\subsubsection{Magnetic field survey}
A ground magnetic survey was performed in 2022  (Figure~\ref{fig4}a) to
derive a high-resolution 
\mbox{magnetic} anomaly map at the scale of the
seismic area (i.e.\ about 25~km of cumulated profiles; 
Figure~\ref{fig2}b) using a portable Overhauser proton-precession
magnetometer (GEM GSM19) in walking mode (0.5 s sampling interval),
with a sensor mounted on a backpack at a height of about 1.60~m above
the ground. The absolute instrumental accuracy is commonly specified at
${\pm}$0.1~nT, and the \mbox{position} was determined simultaneously while
surveying with an integrated GPS with a fairly precise 
{location} of the
magnetic measurements (i.e.\ around a few 
{meters)} even without any
post-processing. To~{provide} 
a more homogeneous and detailed map at the
scale of the overall area of interest, we also used data extracted from
the Bureau des Recherches G\'{e}ologiques et Mini\`{e}res (BRGM)
helicopter-borne geophysical campaign---HADGR211022 project, carried out
in 2023 at an altitude of 80~m above the ground  (Figure~\ref{fig4}b).
The overall dataset of this latter is available at
\url{https://data.gouv.fr/fr/datasets/donnees-geophysiques-aeroportees/}
where more details can be found (so called E-sector in the data
statement). This large-scale survey is conveniently oriented N--S, which
is complementary to our further E--W oriented ground survey 
(Figures~\ref{fig2}b,~\ref{fig3} and~\ref{fig4}a). Note that stitching
together such overlapping surveys acquired at different times,
altitudes and spatial resolutions should be carefully considered. For
this, we use a dedicated algorithm based on a Rectangular Harmonic
Analysis (RHA) described  in  \citet{ThebaultGailler2023}, with which
field measurements are automatically corrected for the global main
field \citep[CHAOS-7 model;][]{Finlayetal2020} to derive an
interpolated homogeneous magnetic anomaly grid leveled at a constant
altitude above the ground, i.e.\ 80~m  (Figure~\ref{fig4}c). The final
interpretative dataset 
\mbox{(Figure~\ref{fig4}c)} was interpolated using the
minimum curvature method in order to derive the smoothest possible grid
that fits the given data values. The method is particularily suitable
(1)~for data from both regional and local surveys, and (2)~is more
efficient when data are randomly distributed, as our present case. It
was defined by how closely the solution surface matches the initial
values, for a final interpolation versus extrapolation of 20~m in cell
size and a blanking distance of 260 respectively (Figure~\ref{fig4}d).
Here again, the goal was to derive a first order and qualitative
regional map.

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}(a)~Total magnetic field extracted from the 2023
BRGM helicopter-borne survey (HADGR211022 project).  (b)~Detailed
magnetic ground measurements performed in 2022.  (c)~Combined map of
magnetic anomaly computed at an altitude of 80~m above the ground. 
(d)~Reduced to the pole magnetic anomaly computed using the averaged
magnetic field vector at the time of the survey. White text indicates
village names.  (e)~Analytic signal computed from the RTP map. Seismic
events (black spheres) and the main reported fault (dashed line) are
shown in background.}
{\vspace*{-3pt}}
\end{figure*}

A Reduction To the Pole (RTP;  Figure~\ref{fig4}d) of the magnetic
anomalies was lastly applied to reduce the dipolar appearance of the
anomalies and shift them to lie above their  sources 
\citep{Baranov1957}. This transformation requires the directions of the
apparent magnetization and the ambient field to be specified
(Inclination-I and Declination-D), here the induced magnetization
vector collinear with the ambient field (i.e.\ the averaged magnetic
field vector at the time of the survey:  $I={-}61.3\text{\textdegree}$
$D=1.8\text{\textdegree}$) was used. Note that such an assumption remains
acceptable since the study area does not span the Brunhes--Matuyama
reversal as detailed hereafter. In order to reveal major magnetization
contrasts we also computed the analytic signal  (Figure~\ref{fig4}e), a
post-processing that transforms the shape of a magnetic anomaly to a
positive one centered above the  source 
\citep{Nabighian1984,Nabighian1972}. The absolute value of the analytic
signal is obtained by taking the square root of the squared sum of the
vertical and two horizontal derivatives of the magnetic field. The
analytic signal was computed on the RTP grid and upward continued at
100~m in order to reduce the noisy very short wavelengths of the
magnetic field commonly observed in such a volcanic and anthropic area.
Independent of the direction of both the ambient magnetic field and the
source magnetization, the RTP exhibits maxima over magnetization
contrasts and is helpful for highlighting mechanical lithologic
variations. Magnetic data treatment, RTP and analytic signal maps were
computed using Oasis Montaj software (\tsup{\text{\textcopyright}}Geosoft).

\subsubsection{ERT-SP-CO$_2$ reference profile}
For a more detailed investigation, a 5~km long reference profile (red
line in  Figure~\ref{fig2}b) was conducted in the northern part of the
seismic swarm including Self-Potential (SP, i.e.\ passive measurements
of disturbances in electrical currents relevant in mapping subsurface
preferential flow paths) as together with soil CO$_2$ measurements (50
and 100~m spacing respectively). Soil CO$_2$ measurements were
performed using a MEGA multi-analyzer designed at the Istituto
Nazionale di Geofisica e Vulcanologia  (INGV)---Sezione di Palermo,
equipped with two Gascard NDIR  CO$_{2}$ sensors 
\citep{Boudoireetal2020}. The ``dynamic concentration'' method was
applied  for this  survey  \citep{GurrieriValenza1988}. An Electrical
Resistivity Tomography (ERT) profile was also acquired along a
collocated but more restricted profile in an area where soil CO$_{2}$
and magnetic anomalies had been previously detected. The measurements
were performed using an ABEM Terrameter LS2 resistivity meter with 64
electrodes 10~m spaced, and a roll along made up of a half of the total
cable length to increase the investigated profile length (i.e.\ $630 +
315$~m long). We used a Schlumberger field protocol which has the
advantage of being \mbox{sensitive} to both horizontal and vertical
structures, down to a depth of about 100~m below the surface in its
central part, given the 10~m spacing between electrodes. After an
initial data analysis and filtering of residual spurious measurements,
we computed a 2D {inversion} model of the electrical resistivity
distribution at depth using RES2DINV  commercial software 
\citep{LokeBarker1996}.

\subsubsection{Magnetotelluric soundings}
For greater depth investigation (i.e.\ deeper than 8~km),
magnetotelluric soundings were carried out at several key sites 
(Figure~\ref{fig2}) over 4 to 5 days each, with two main goals:  (i)~to
characterize the resistivity of the formations at the depth of the
seismic swarm and to image any potential presence of crustal  fluids
\citep{Pateletal2020,Pearceetal2020},  (ii)~to record any temporal
variation that could be linked with the seismic activity. Since we
focus here on structural insights, we do not discuss this latter point
in the present study. The survey was conducted from December 2022 to
February 2023 using a broadband Phoenix Geophysics instrument (MTU-5C).
The station was successively installed in strategic areas regarding the
seismic swarm: three above the main area of seismicity, one in its
northern part, and the last one outside (south-west) where previous MT
measurements were available in Vall\'{e}e de Chaudefour 
\citep[Figure~\ref{fig2}b;][]{Hulot1988} in order to ensure the
consistency of resistivity measurements through time.\looseness=-1

Data processing was carried out in a single station using so-called
``robust'' algorithms  \citep{EgbertBooker1986}. Static-shift effects
that commonly \mbox{affect} MT  \citep{PellerinHohmann1990} data were
corrected by conducting simultaneous and collocated electromagnetic
soundings (TDEM for Time Domain ElectroMagnetic). A careful analysis of
the phase tensor was also carried out as discussed hereafter, \mbox{allowing}
to graphically determine the strike axis of the main regional
conductive structure. In terms of modelling, 2D inversions were then
performed to model the best resistivity distribution along the main
area of interest, i.e.\ a profile selected perpendicularly to the main
fault of the area (grey line in  Figure~\ref{fig3}a). In order to
maximize the impedance tensor's off-diagonal elements and minimize its
diagonal elements, data were rotated to the strike axis (N140). The 2D
inversions were run with the RLM2D  code  \citep{RodiMackie2001}  that
uses a finite-difference modeling algorithm based on the approach of
Tikhonov  regularization  \citep{TikhonovArsenin1977}. The model was
segmented into 2D rectangular blocks measuring 25 by  25~meters at the
surface and in the central part of the profile, from which the block
dimensions increased laterally by a factor of 1.5 and vertically by a
factor of 1.06 with depth. In terms of initial constraints, an a-priori
model calculated using a constant conductivity value of 100~$\Omega{\cdot}$m has
been fixed as an initial step, and resistivity bounds have been also
set from 5000 to 1~$\Omega{\cdot}$m, which is consistent in such geological
contexts. Several methodologies were tested in terms of data rotation,
providing similar results so we consistently used the best fit on
inversion using the regional strike rotation (i.e.~N140).

\section{Results}\label{sec4}

\subsection{Details on the seismic swarm and tectonic
implications}\label{sec4.1} 
The 3D geometric processing of the relocated events, the temporal
analysis of their succession and the calculation of a few focal
mechanisms, allow us to envisage the effect of a  ``flower structure''
type tectonic system. Two main faults orientations associated with two
main families of seismic events can be therefore distinguished 
(Figure~\ref{fig3}a):
\begin{itemize}
\item
A N173-85W dextral strike-slip fault including the deeper seismic
events at a depth between 3900~m and 5600~m below the surface 
(Figure~\ref{fig3}b). This fault corresponds to a shear zone
approximately 350~m wide.
\item
A N13-68W normal fault located above the shear zone, including the
shallower 
\mbox{seismic} events at a depth between 2600~m and 4400~m below the
surface  (Figure~\ref{fig3}b).
\end{itemize}
Note that the dextral nature of the deep shear zone is not solely based
on focal mechanism calculations, but is also supported by field
evidence (in the form of cataclasites within the Variscan basement and
the formation of Cenozoic sedimentary basins), as well as by rock
mechanics calculations that are note detailed here. These calculations
further allow for the determination of the maximum horizontal regional
stress, which had previously been interpreted only in subsurface. The
same applies to the normal faulting observed at the surface.

This tectonic system is in line with the regional fracturing of the
Variscan basement, made up here of plutonic and metamorphic rocks. The
shear zone thus appears as the conjugate fault of the Jassat  fault 
\citep{Vidaletal1996}.  
Finally, it should be stressed that this seismic swarm confirms the
current tectonic stress in the northern Massif Central, with a
direction of around N148 for the maximum horizontal component, in
agreement with the homogeneous maximum horizontal component at N150
recorded below a depth of 500~m in the Chassolles, Echassi\`{e}res,
Mayet de Montagne and Auriat  boreholes  \citepalias{Vidaletal1996}.
\looseness=1

\subsection{Qualitative description of the magnetic anomaly
map}\label{sec4.2}
Basically, spatial variations in magnetization mainly depend on rocks
ages (i.e.\ if the concerned area spans a magnetic reversal),
composition, fluids contents, mechanical heterogeneities (i.e.\ 
fracturing and destabilization structures) and associated alteration
processes  \citep[e.g.][]{Bouligandetal2020,GaillerKauahikaua2017} 
but are also largely affected by anthropic artifacts that should be
considered. The combined airborne  (Figure~\ref{fig4}a) and ground 
(Figure~\ref{fig4}b) magnetic anomaly map (Figure~\ref{fig4}c) clearly
highlights the strong effect of the main villages (especially
Beaune-le-Froid and Bressouleille) that generate ``anthropic'' negative
anomalies. However, an interesting long wavelength negative anomaly
axis oriented about N170 appears in the central part of the study area,
where the seismic swarm occurred. In long-dormant and complex volcanic
provinces, numerous phenomena could generate such negative magnetic
anomalies. Regarding the low elevation of the combined magnetic dataset
as well as the overall small wavelength of the magnetic signal, we will
consider that the sources of the associated anomalies are \mbox{concentrated}
within the shallow pile of the study area, which does not span the
negative\break reversal.

If thermomagnetic effects are to be proscribed in such an ancient
volcanic context, stress variations (due for example to fracturing and
hydrothermal activity) are commonly associated with piezomagnetic
effects that could contribute to rocks demagnetization and therefore
negative magnetic anomalies. Electrokinetic variations due to fluid
circulation  
\citep[e.g.][]{Lenatetal2012} as well as slow natural or
hydrothermal alteration processes would also locally affect the
magnetization. The observation of an anomalous magnetic axis is even
more striking on the RTP  (Figure~\ref{fig4}d) and analytic signal 
\mbox{(Figure~\ref{fig4}e)} maps. There, the highest values of the analytical
signal delimit a N140 to N170 corridor encompassing the seismicity.
Discrete increase of the analytical signal is also associated with the
family of seismic events aligned along the N13-68W. These maxima
therefore suggest the presence of magnetization contrasts probably
associated with lithological and/or mechanical heterogeneity at depth
where the seismicity may (at least partly) cluster.

\subsection{Combined analysis of the ERT-SP-CO$_2$ reference
profile}\label{sec4.3}
SP and soil CO$_{2}$ measurements highlight two main trends along the
northern reference profile  
\mbox{(Figure~\ref{fig5}a;} red line in 
Figure~\ref{fig2}b):  (i)~positive SP anomalies correlating to highest
soil CO$_{2}$ contents (well above 1.0\%) and  (ii)~negative (or low)
SP anomalies in areas where soil CO$_{2}$ content is low (mostly lower
than 0.2\%). These trends can be attributed to two different behaviors
in fluid circulation. In such a volcanic context, SP anomalies are
mainly linked to electrokinetic phenomena, water--rock interactions
leading to positive charges in the direction of  flow 
\citep{Reviletal2003,Reviletal1999,RevilPezard1998}. 
Therefore, negative SP anomaly, without notable soil CO$_{2}$ \mbox{degassing}
can be attributed to meteoric infiltration, the water \mbox{buffering} the
CO$_{2}$ content in the soil. A similar process was described by 
\citet{Boudoireetal2018} coupled with carbon isotopes fractionating by
a Rayleigh 
{condensation} process. Such influence of meteoric water
infiltration along the CO$_{2}$ channels has been also referenced in
different  areas  \citep[e.g.][]{Flechsigetal2008}. Conversely,
positive SP anomalies coupled with high soil  CO$_{2}$ content argue in
favor of a rise of fluids  
\citep[see][and references therein for a review]{Reviletal2023}.

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}(a)~SP and soil CO$_{2}$ measurements along the
5~km long reference profile located in Figure~\ref{fig2}b. 
(b)~Resistivity distribution along the ERT profile, along with SP, soil
CO$_{2}$ and magnetic signals. Light shaded vertical rectangles
highlight the main area of SP and soil CO$_{2}$ positive anomalies.
Dark shaded rectangles highlight the areas of negative RTP magnetic
anomalies.}
\end{figure*}

Electrical resistivity being highly sensitive to fluid circulation, the
ERT profile can be used to reveal inherited weakness paths at depth
that could favor an uprise fluid circulation highlighted by both soil
CO$_{2}$ and SP positive anomalies at the surface 
\mbox{(Figure~\ref{fig5}b).}
Similarly to the qualitative analysis of magnetic
anomalies, the highly resistive layer observed at the surface
(${&gt;}$500~$\Omega{\cdot}$m) can be attributed to massive, stable, and unaltered
formations as detailed in various  contexts  \citep{Portaletal2019}.
Two main conductive structures are evidenced deeper, in the western and
central parts of the profile. This latter seems to rise toward the
surface along two vertical conductive axes east of the profile. Note
that these two axes are associated, at least in relative, to short
wavelength but high amplitude negative magnetic anomalies
(Figure~\ref{fig5}b). Accordingly, the correlation between conductive
and probably demagnetized axes rising toward the surface in areas where
both SP and soil CO$_{2}$ positive anomalies are measured could argue
for a preferential rise of fluids from shallow weakness paths inherited
from fracturing, alteration, and hidden structures\break at depth.

\subsection{Deep information from magnetotelluric (MT)
investigation}\label{sec4.4} 
The analysis of the MT data using the phase tensor method 
(Figure~\ref{fig6}a) reveals a N170 orientation at depth below the
study area with high phase tensor value suggesting the presence of a
conductor. This direction is fully consistent with the main trend
evidenced by the magnetic surveys and by the family of the deepest
seismic events (oriented\break N173-85W).

\begin{figure*}[t!]
\includegraphics{fig06}
\caption{\label{fig6}(a)~Map showing phase tensor of the four MT
stations located in the seismogenic area (black dots), and location of
the 2D profile used for inversions (black line). (b)~2D cross-section
along Peyre L\'{e}vade-Coujat soundings (West and East, respectively),
and associated fit for each sounding. The black dots represent the
relocated seismicity within 400~m around the profile. Northing (m) and
Easting (m) WG84 UTM zone 31N and iso-altitude (m).}
\end{figure*}

The resistivity distribution on the best 2D inversion result (i.e.\ RMS
${=}$ 2) along a selected \mbox{profile} crossing the main regional inferred
structures highlights three main features  (Figure~\ref{fig6}b): 
(i)~In the central part of the model, a conductive sub-vertical body
dips first to the west and then to the east as depth increases
(indicated by iso-contour 24~$\Omega{\cdot}$m on  
\mbox{Figure~\ref{fig6}b).}  (ii)~This
conductive structure is encompassed by resistive borders
(100--1000~$\Omega{\cdot}$m).  (iii)~A~deep conductive body (with resistivity
values lower than 10~$\Omega{\cdot}$m) is evidenced at depth (deeper than 6~km).
Based on the geology of the area, even if the resistive surrounding
could be interpreted as granitic formations that are more or less
altered and water-saturated  
(i.e.\ between 300 and 1000~$\Omega{\cdot}$m according to  
\citet{Hulot1988} and \citet{Dupisetal1980} 
\mbox{respectively,} who studied the area),
the nature of the conductive bodies needs to be discussed in more
detail. Another interesting feature is that the distribution of
seismicity around the profile appears consistently located at the
resistive/conductive contact, as it will be discussed hereafter.

\section{Discussion and conclusions}\label{sec5}

\subsection{Links between seismicity and local tectonics}\label{sec5.1}
The magnetic anomaly maps, especially the long and short wavelengths,
observed in the analytic signal, clearly evidence some limits of
mechanical heterogeneities (mostly N140-to-N170) in accordance with
regional tectonics  (Figure~\ref{fig4}). The presence of mechanical and
lithological weakness paths inherited from fracturing and alteration is
confirmed at shallow depth by the combination of ERT, SP and soil
CO$_{2}$ measurements  (Figure~\ref{fig5}). The 2D magnetotelluric
model shows the extension of these shallow heterogeneities in greater
depth with a main conductive axis dipping west and extending down to
5--7~km-depth. Both geophysical and geochemical surveys thus evidence
the presence of a significant anomalous structure that extends in\break
depth.

We therefore derive a synthetic interpretation based on geological
constraints and our multi-method approach in order to propose a
structural scheme for this seismogenic area  (Figure~\ref{fig7}). This
synthetic interpretation is refined by  \citet{Boudoireetal2025} 
taking into account the temporal evolution of the processes involved in
the 2021--2022 seismic swarm in the Monts Dore volcanic province. Our
analysis of geological data indicates that the main shallow conductive
structure could be associated with the Variscan Jassat strike-slip 
fault
\mbox{(Figure~\ref{fig7})}.  \citet{Chevremontetal2023} described this
formation as composed of ultra-mylonite rocks with a N140-85SW
foliation that is consistent with our geophysical observations. This is
also consistent with the main long wavelength evidenced in the analytic
signal, which could mark such a shear zone. It should be noted that
Variscan faults are known to have a control on the recent volcanic
activity in the Monts Dore and Cha\^{i}ne des Puys volcanic  provinces 
\citep{Merleetal2023,Varetetal1980}. Actually, the eastern border of
the ultra-mylonite corridor is marked by numerous volcanic
manifestations (i.e.\ springs, maars, flows and domes according to the
geological map;  Figure~\ref{fig1}b). These findings suggest that, even
nowadays, the Variscan faults, such as the Jassat fault in the present
study  (Figure~\ref{fig7}b), are still major paths for channeling
fluids from depth to the surface. 

\begin{figure*}
\includegraphics{fig07}
\vspace*{-2pt}
\caption{\label{fig7}Combined synthetic schemes of structural insights
derived from our multi-methods analysis in the area of the 2021--2022
seismic swarm. The rose diagram recalls the main orientations of the
four families of emblematic faults.}
\vspace*{-1pt}
\end{figure*}

\subsection{A seismic swarm triggered by fluids
circulation?}\label{sec5.2} 
The nature of the deeper part of the conductive body evidenced at about
8--10~km in depth is more conspicuous.  \citet{Hulot1988} evidenced and
interpreted a deep conductor at a depth of about 10~km below the Sancy
volcano as an ancient magma chamber. The recent volcanic events (i.e.\ 
springs, maars, flows and domes;  Figure~\ref{fig1}b) along the Eastern
border of the fault, such as the recent Tartaret eruption (about 15~ka;
Figure~\ref{fig1}), could support the idea of the existence of an
ancient magma chamber, possibly a mushy zone area in relation with this
deep conductor body. Further investigations are required to decipher
the depths of the magma ponding zones and the eruptive dynamics related
to this eruption. However, the current gas and water chemistry from
thermomineral springs in the Monts Dore volcanic province does not
support the idea of current magma storage and degassing at crustal 
depth  \citep{Granetetal1995} as it may be observed for other active
volcanoes 
\citep{Boudoireetal2025,Braueretal2017,Pauwelsetal1997,Riccietal2024}.

Another explanation for the deep conductor would be the existence of a
fluid reservoir at this depth. We observed that the seismic activity
occurred at the contrast between high and low resistivity formations
that is characteristic of fluid injection-related seismicity as
proposed  by  \citet[and references therein]{Pearceetal2020}. 
These authors suggest that this phenomenon is linked to the migration
of fluids in a permeable zone (mechanically weak, with low resistivity)
adjacent to an impermeable zone (mechanically strong, with high
resistivity), resulting in the accumulation of fluid pressure and in
successive rock failure and/or fault reactivation. Based on the data
presented here, we hypothesize that the impermeable granitic borders
and permeable discontinuity of the Jassat strike-slip fault allow
hydrothermal fluids to channel and accumulate pressure, leading to the
fault reactivation by reducing the effective normal stress 
\citepalias{Pearceetal2020}.

This hypothesis is also sustained by studies in similar contexts such
as the Nov\'{y} Kostel region (Czech Republic) located  in the Eger
Graben  \citep{Fischeretal2014}, that has experienced numerous seismic
swarms since the  1980's. The most recent events show a clear trend of
vertical migration of seismicity in time and space, interpreted as a
progressive rupture along a major fault  plane 
\citep{Bachuraetal2021,Fischeretal2017,HainzlFischer2002}  consistent
with the regional stress field \citep{Fischeretal2010}. To explain
them,  \citet{Fischeretal2014} reject the tectonic hypothesis, because
the accumulation of stress sufficient to cause such recurrent activity
seems very unlikely regarding the tectonics of Central Europe. These
authors propose that these seismic swarms result from the reactivation
of faults, caused by the migration toward the surface of CO$_{2}$-rich
fluids coming from a deep mantellic reservoir where they have
accumulated pressure. This model also explains periods of quiescence
between swarms as reflecting the time needed for the ``deep reservoir''
to re-pressurize after a rupture. Such an interpretation is relevant in
our present case, as the seismic swarms in the Monts Dore volcanic
province are very similar to those in the Nov\'{y} Kostel region, in
terms of geodynamic  context  \citep{MerleMichon2001},  recurrence,
duration, magnitude, number of events, and behavior. It suggests that
magmatic processes are still active in the mantle beneath the Monts
Dore volcanic province as supported by the isotopic signature of the
gas emissions in the area
\citep{Braueretal2017,Boudoireetal2023,Boudoireetal2025}. 

\subsection{Concluding remarks}\label{sec5.3} 
According to a detailed seismic analysis combined with  (1)~geological
information and (2)~complementary geochemical and geophysical surveys,
we \mbox{presented} a spatial definition of the 2021--2022 Monts Dore large
seismic swarm recorded beneath the most recent stratovolcano of the
French Massif Central.

We evidenced its link with the reactivation of pre-existing tectonic
structures and suggested the influence of a circulation of fluids at
depth as commonly inferred in such a complex structural context.
However, the origin of this reactivation is still subject to question
and requires further evaluation of time series of monitoring records
from both permanent and temporary networks/surveys. The companion paper
of  \citet{Boudoireetal2025} aims at better deciphering the dynamic
processes involved in this particular seismic zone, through highly
complementary information derived from temporal surveys.

\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*{Open research}
The ground geophysical data (i.e.\ magnetic field measurements, ERT
profile and MT soundings) presented in this study are available at
\url{https://doi.org/10.25519/PROVA2}. 

The helicopter-borne magnetic data are extracted from the BRGM
HADGR211022 project and available at
\url{https://data.gouv.fr/fr/datasets/donnees-geophysiques-aeroportees/}
(ETALAB-Licence-Ouverte-v2 0 ID 65412d94ef687b2a614553f8).

Licensed commercial softwares have been used for geophysical data
modeling and visualization: EmPower (2022), Geotools (2022), Geosoft
(2022), Golden Software Surfer (2015), Matlab (2020) and Seequent
Limited (2020).

\section*{Funding}
This work was partly funded by the OPGC through the ``P\^{o}le
R\'{e}gional d'Observation de l'activit\'{e} Volcano-tectonique
d'Auvergne et d'Ard\`{e}che: The Auvergne-Ard\`{e}che regional
volcano-tectonic monitoring center'' (PROVA$^{2}$), the FEDER Project
``CAPRICE'', and the CNES in the framework of the projects Volcadrone
and RMagEDrone ``Multi-scale geophysical imaging of volcanic
edifices''.

The HADGR211022 project was financed by ERDF funds for the Auvergne and
by BRGM.

This is contribution no. 676 of the ClerVolc program of the
International Research Center for Disaster Sciences and Sustainable
Development of the University of Clermont Auvergne. 

\CDRGrant[ERDF]{HADGR211022]}

\section*{Acknowledgements} 
We are grateful to the Town halls of Chambon sur Lac, le Vernet
Sainte-Marguerite and Murol as well as all the landowners. We also
acknowledge Georeva and Phoenix Geophysics for their scientific and
logistic support as well as the 2023--2024 Master 1 promotion,
Ad\`{e}le, Maxime, for their help during the ERT profile acquisition.

We would like to thank the tho anonymous referees for their
constructive comments.

\back{}

\printbibliography
\refinput{crgeos20240998-reference.tex}

\end{document}
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