Abstract. The western Peloponnese exhibits a continuum of earthquake faulting styles that reside in a complex stress field near the western termination of the Hellenic Subduction System. Here we present a detailed study of recent seismicity in western Peloponnese, including the spatiotemporal distribution of earthquake sequences and their clustering and migration properties to infer possible driving mechanisms. We build a detailed earthquake catalog from December 2023 – September 2024 with a magnitude-of-completeness Mc ~1 and a location precision on the order of 100 s of meters by combining new data from temporary seismic station deployments with publicly available data from permanent stations. Catalog statistical and clustering analysis shows increased background seismicity rates and seismic moment release in the northern part of the study area that is consistent with larger strain rates and a higher stressing rate reported from geodetic data. The seismicity distribution and focal mechanism solutions suggest that the predominantly north-south extensional regime in the north changes to east-west extension in the central-western Peloponnese near the town of Zaharo. Nearest-neighbor cluster analysis reveals mainshock-aftershock-type sequences in the northwest near the town of Vartholomio that are consistent with tectonic loading. Conversely, clustering properties in the central-western Peloponnese near Zaharo are consistent with swarm-like sequences driven by external forcing, such as pore-fluid pressure changes, and potential aseismic slip. Independent studies of slab dewatering suggest fluid-driven pore-pressure gradients that might be responsible for migrating seismicity and swarm-like behavior in the upper plate.

At the eastern end of Gakkel Ridge, Arctic Ocean, spreading rates drop below 5 mm/y near the termination of the active mid-ocean ridge in the Laptev Sea. A small-scale ocean bottom seismometer network deployed for one year at a volcanic center near Gakkel Ridge Deep in sea ice covered waters revealed abundant microseismicity despite the low spreading rate. In order to reveal spreading processes, we analyze a manually picked earthquake catalog refined by low-magnitude events detected by template matching. We attribute seismicity occurring randomly in time and space to tectonic stress release along the ridge. During short time periods of hours to days, seismicity is organized in time and densely clustered in space with signs of migration away from an aseismic area. In analogy to volcanic centers at Knipovich Ridge and in Iceland, we interpret the seismicity as signs of ongoing localized magmatism occurring even at the slowest spreading rates.

The Hellenic Subduction System (HSS) in the eastern Mediterranean is the oldest active subduction margin on earth. It is a segmented boundary that hosts the continuum of faulting styles over a ~200km range in depth and can generate large earthquakes with high tsunamigenic potential. The complexity of deformation styles and rates leave key aspects of the system poorly understood. For example, historical records of Mw

Earthquake swarms represent a particular mode of seismicity, not directly related to the occurrence of large earthquakes (e.g., aftershocks) but rather driven by external forcing such as aseismic deformation or fluid migration in fault systems. Sometimes their occurrence overlaps with observable geodetic signals in space and time, indicating a direct link. However, the low resolution of geodetic observations tends to obscure the small scale spatial and temporal dynamics of swarms. In this work, we automatically extract clusters of seismicity related to the 2014 Alto Tiberina swarm sequence (Italy) using an unsupervised clustering approach that exploits space and time information of the seismicity. The quantitative characterization of each cluster indicates that the overall swarm is composed of spatially and temporally confined (sub) swarms each of which could potentially be driven by small-scale aseismic deformation process. This observation aligns with similar findings during slow slip events in subduction zones. An earthquake swarm is characterized by an elevated rate of earthquake occurrences in a specific region, surpassing the typical seismic activity, without being preceded by a major seismic event. Earthquake swarms are thought to be driven by mechanisms like slow deformation or the migration of fluids. In this study we take a closer look into the seismicity of an earthquake swarm in the Apennines in Italy, exploiting unsupervised machine learning methods. This approach indicates that the swarm sequence consists of a general, smooth increase in seismicity, occasionally punctuated by short accelerations of seismic activity. The subsequent analysis of the accelerated seismicity suggests a connection with several small, slow deformation processes that collectively constitute a large-scale deformation process, as measured by geodetic data. Combining hierarchical and density-based clustering enables to automatically separate background and clustered seismicityThe quantitative analysis of the extracted clusters reveals a swarm-like characterThe swarm-like clusters further indicate relations to aseismic processes

The Alto Tiberina Fault system, located in the Northern Apennines (Italy), consists of a low angle normal fault (LANF) which radiates micro‐seismicity that can be explained by continuous creep. On top of the LANF, a network of syn‐ and antithetic high angle faults frequently hosts seismic swarms, one of which has been associated with a transient aseismic deformation signal. To study in detail the seismicity and its relationships with aseismic deformation processes occurring in this fault system, we apply template matching on seismic data recorded at an array of borehole stations, to derive a high‐resolution earthquake catalog. Thanks to the additionally detected events, we are able to reveal time periods of increased spatial‐ and temporal clustering during an aseismic deformation event. This reflects the complex evolution of aseismic slip together with the complexity of the shallow fault system. Along the LANF, we observe a bimodal type of seismicity, with diffuse seismicity active continuously, and short‐lived bursts of seismicity that could indicate rapid fluid releases. We additionally identify repeating earthquakes. These events not necessarily match a simple creep model and therefore open the possibility for new models to explain the seismicity along the LANF.

Long-lasting harmonic tremor signals are frequently observed in spectrograms of seis-mological data. Natural sources, such as volcanoes and icebergs, or artificial sources, such as ships and helicopters, produce very similar harmonic tremor episodes. Ocean-bottom seismometer (OBS) records may additionally be contaminated by tremor induced by ocean-bottom currents acting on the OBS structure. This harmonic tremor noise may severely hinder earthquake detection and can be misinterpreted as volcanic tremor. In a 160-km-long network of 27 OBSs deployed for 1 yr along the Knipovich ridge in the Greenland Sea, harmonic tremor was widely observed away from natural sources such as volcanoes. Based on this network, we present a systematic analysis of the char-acteristics of hydrodynamically induced harmonic tremor in OBS records to make it dis-tinguishable from natural tremor sources and reveal its generation processes. We apply an algorithm that detects harmonic tremor and extracts time series of its fundamental frequency and spectral amplitude. Tremor episodes typically occur twice per day, starting with fundamental frequencies of 0.5-1.0 Hz, and show three distinct stages that are characterized by frequency-gliding, mode-locking, and large spectral amplitudes, respectively. We propose that ocean-bottom currents larger than similar to 5 cm/s cause rhythmical Karman vortex shedding around protruding structures of the OBS and excite eigenvibrations. Head-buoy strumming is the most likely source of the dominant tremor signal, whereas a distinctly different tremor signal with a fundamental fre-quency similar to 6 Hz may be related to eigenvibrations of the radio antenna. Ocean-bottom current velocities reconstructed from the fundamental tremor frequency and from cross correlation of tremor time series between stations match observed average current velocities of 14-20 cm/s in this region. The tremor signal periodicity shows the same tidal constituents as the forcing ocean-bottom currents, which is a further evidence of the hydrodynamic nature of the tremor.

Foreshocks in the form of microseismicity are among the most powerful tools to study the physical processes that occur before main earthquakes. However, their detection and precise characterization is still sparse, especially for small-to-moderate-size earthquakes (Mw < 6). We present here a detailed foreshock analysis for the 7 November 2019, Balsorano, Italy, normal fault earthquake (Mw 4.4). To improve the detection of the microseismicity before and after the mainshock, we use six three-component broadband receivers at distances of less than 75 km from the targeted seismicity, through template matching. To improve the understanding of the physical mechanism(s) behind the earthquake initiation process, as well as other accompanying phenomena, we also detail the spatiotemporal evolution of the sequence associated with this medium-sized earthquake, using waveform clustering and hypocenter relocation. Clear differences between foreshocks and aftershocks are revealed by this analysis. Moreover, five distinct spatiotemporal patterns associated with the different seismic activities are revealed. The observed spatiotemporal behavior shown by the foreshocks highlights a complex initiation process, which apparently starts on an adjacent unmapped antithetic fault. Finally, the aftershock activity comprises four different clusters with distinct spatiotemporal patterns, which suggests that the different clusters in this sequence have distinct triggering mechanisms.

The Alto Tiberina fault (ATF) system (Northern Apennines, Italy) is dominated by a low-angle normal fault with syn- and antithetic splay faults located in the hanging wall. Starting in August 2013 the hanging wall has been affected by a swarm-like sequence that lasted until the end of 2014. Within this period more than ~20k events are listed to have nucleated along the same fault segment with the largest events having magnitudes of ~Mw 3.9. In this study we aim to constrain the physical forces driving the swarm-like sequence (e.g. pore pressure diffusion, transient slow slip) in this fault segment by combining a template matching approach with continuous seismic data from a borehole array deployed in the near field of the ATF. This array approach helps us to identify small events which are hidden in the background noise and usually undetected with conventional picking approaches. We are able to extend the preexisting catalog by a factor > 5. The new detected events decrease the magnitude of completeness and the inter-event time resolution. We use the extended catalog to analyze the spatio-temporal evolution, scaling properties and statistical behavior to enhance insights on the physical forces driving this swarm like sequence.

Our knowledge about the physics behind the initiation process of large or small earthquakes remains limited. The current understanding of this process suggests that an earthquake occurs when increasing stress causes a pre-existing fault to fail suddenly (e.g. Dieterich 1992). Models such as the pre-slip instability growth or the triggered cascade of events have been proposed in order to theoretically explain this preparation stage (Dodge and Beroza, 1996; Ellsworth and Bullut, 2018; Bouchon et al., 2011). However, the mechanisms behind this process are still unknown. This debate is mainly due to the lack of direct observations of the subsurface shear stress evolution at the area of interest before and after an earthquake. Considering that the shear stress evolves through time until the moment of failure, indirect observations of this change might be available but hidden inside the continuous seismic data. In this work, we analyze in detail the evolution of the seismic activity of a small (Mw 4.4) normal fault earthquake which occurred in Central Italy on 7th November 2019 at the middle lower crust (16 km depth). We first analyze the available continuous data using the Fast Matched Filter (Beauc\'e et al., 2017). Then, every new detected event is spatially localized with respect to the other events through the Double Difference algorithm (DD). As a result, we obtain the spatio-temporal evolution of the foreshock and aftershock sequences of that event. The results from this analysis shed light on the patterns that the shear-stress spatio-temporal evolution follows before and after a given event. Therefore, we expect that this study will contribute to improve our understanding of the physics behind the earthquake initiation process.

In December 2018, the NASA InSight mission deployed a seismometer on the surface of Mars.In preparation for the data analysis, in July 2017 the Mars Quake Service initiated a blind test, in which participants were asked to detect and characterize seismicity embedded in a one Earth year long synthetic dataset of continuous waveforms.Synthetic data were computed for a single station, mimicking the streams that will be available from InSight as well as the expected tectonic and impact seismicity, and noise conditions on Mars (Clinton et al. 2017).In total, 84 teams from 20 countries registered for the blind test and 11 of them submitted their results in early 2018.The collection of documentations, methods, ideas and codes submitted by the participants exceeds 100 pages.The teams proposed well established as well as novel methods to tackle the challenging target of building a global seismicity catalogue using a single station.This paper summarizes the performance of the teams, and highlights the most successful contributions.

Etude détaillée de la sismicité liée à une faille normale à bas angle dans les Apennins du Nord de l'Italie Dans les contextes tectoniques d’extension, où la croûte se sépare et brise, on peut trouver des registres géologiques concernant les failles normales avec un angle de pendage inférieur à 30°, appelées failles normales à bas angle (LANF en anglais). Le consensus général ce que ces failles accueillent une grand partie de l’extension de la croûte terrestre. Bien qu’elles ont été l’objet d’études intensives au cours des dernières années dans différents contextes géologiques, les questions fondamentales par rapport aux LANF restent d’actualité. Par exemple, il y a encore un débat par rapport à la manière précise dont l’extension est prise en compte, soit en forme de déformation lente subie directement au long de la surface du LANF, soit en forme des failles inverses secondaires dans l’éponte supérieure. Une question associé est de savoir si les LANF ont la capacité de produire de grands séismes.La faille de l’Alto Tiberina, dans les Apennins du Nord (Italie), est une LANF bien étudiée. La surface de la LANF est bien décrit par sa production en continu de micro-sismicité. Au-dessus du LANF, dans l’éponte supérieure (HW en anglais), un réseau de failles syn- et antithétiques à angle élevé héberge fréquemment une sismicité transitoire en essaims. Au cours des dernières décennies, un réseau dense de capteurs multiparamétriques (GNSS, sismique) a été installé dans la région et fournit des mesures de haute qualité.Dans ce travail de thèse, j'exploite les données à haute résolution produites par ce réseau dense pour essayer de répondre aux questions posées ci-dessus. Dans le chapitre 3, j'applique une approche par filtres-adaptés pour obtenir un catalogue a haute résolution de séismes qui sera analysé sous différents aspects. Dans le HW, l'analyse révèle des périodes d'augmentation des regroupements spatiaux et temporels lors d'un événement de déformation asismique. Cela correspond à l'évolution hétérogène du glissement asismique et la complexité du système de failles peu profondes. Tout au long du LANF, la sismicité a un comportement bimodal, avec une sismicité diffuse active en continu et des épisodes intermittents de fortes intensités. De plus, on trouve dans le catalogue élargi des pistes montrant des séismes répétitifs (REs), ce qui semble difficile à concilier avec un modèle de déplacement simple basé sur une analyse préliminaire.Afin d’extraire automatiquement des informations significatives du catalogue à haute résolution, au chapitre 4, j’utilise les informations spatiales et temporelles de la sismicité détectée et j’applique des méthodes de re-groupement non supervisées. Cela permet de trouver et d’extraire des groupes des séismes liés aux essaims sismiques. L’analyse quantitative des essaims sismiques indique que l’essaim général du HW pourrait être composé de (sous) événements de glissement asismique limités dans l’espace et dans le temps.Finalement, dans le chapitre 5, je réévalue les REs trouvés dans le chapitre 3. Les REs relocalisés et les paramètres de source améliorés révèlent des groupes d’événements avec des temps inter-événements courts difficiles à explique avec le fluage comme mécanisme moteur. C’est pourquoi je propose un mécanismes alternatif pour répondre aux observations.La thèse révèle que d’autres mécanismes à part la déformation lente au long du LANF pourraient être à l’origine de l’extension dans la zone d’étude. En effet, les résultats indiquent que certaines déformations pourraient être libérées au long de failles normales avec des angles élevés dans le HW peu profond, ce qui est en accord avec d’autres modèles.