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SUMMARY Passive seismic interferometry has become a popular technique towards monitoring. The method depends on the relative stability of background seismic sources in order to make repeatable measurements of subsurface properties. Such stability is typically assessed by examining the similarity of cross-correlation functions through time. Thus, techniques that can better assess the temporal similarity of cross-correlation functions may aid in discriminating between real subsurface processes and artificial changes related variable seismic sources. In this study, we apply agglomerative hierarchical clustering to cross-correlation functions computed using seismic networks at two volcanoes. This allows us to form groups of data that share similar characteristics and also, unlike common similarity measures, does not require a defined reference period. At Piton de la Fournaise (La Réunion island), we resolve distinct clusters that relate both to changes in the seismic source (volcanic tremor onset) and changes in the medium following volcanic eruptions. At Mt Ruapehu (New Zealand), we observe a consistency to cross-correlation functions computed in the frequency band of volcanic tremor, suggesting tremor could be useful as a repeatable seismic source. Our results demonstrate the potential of hierarchical clustering as a similarity measure for cross-correlation functions, suggesting it could be a useful step towards recognizing structure in seismic interferometry data sets. This can benefit both decisions in processing and interpretations of observed subsurface changes.

The Skaftár cauldrons are a pair of depressions at the surface of the Vatnajökull glacier that signify subglacial lakes caused by geothermal heat sources at the underlying bedrock. These subglacial lakes continuously grow in volume and produce fast-rising jökulhlaups (glacial lake outburst floods) at the glacier outlet 35-40 km away. Seismic tremor events coincide with these large floods, but the exact source of this tremor is unknown. To investigate the origin and characteristics of these tremors, network covariance matrix spectral width, real-time seismic amplitude measurements (RSAM), and spectral analyses were conducted for eight jökulhlaups spanning the years 2015 to 2021, with joint interpretation alongside hydrological and GPS data. Seismic data was aquired by the Icelandic Meteorological Office's network. The several-day time period spanning the flood propagation and deepening of the ice cauldrons was dominated by small icequakes along the subglacial flood path and emergent low frequency, temporally regular earthquakes with repetitive waveforms. Once most of the water had drained from the cauldrons, strong tremor bursts with a duration on the order of 10s of minutes and a dominant frequency range between 0.5 and 3 Hz were recorded by seismic stations both on and off of the glacier. This tremor occurred during seven out of the eight floods examined, exhibiting fewer than 10 clear tremor bursts per flood over approximately a 24-hour period. Preliminary results show that the amplitude of the tremor bursts correlates to the magnitude of the flood, with the strongest tremor occurring in 2015, during the largest recent flood from the eastern cauldron. Due to the similarities shared between the shape of this tremor’s seismic envelope and that of the tremor during hydrothermal explosions, we interpret the tremor seen after the Skaftá cauldrons have drained as steam explosions facilitated by a drop in the overlying water pressure.

Karstic zones are numerous on Earth and offer a particular field of study to evaluate the ground motion levels that occurred in the past in support of regional seismic hazard assessment. Indeed, some fine and slender candlestick stalagmites are intact and therefore indicate that a certain level of ground motion has not been exceeded since they exist. Many parameters must be considered in the behaviour of stalagmites to earthquakes such as their shape, their mechanical properties and their natural frequency. A good way to better understand and characterize the reaction of these stalagmites to earthquakes is to study their reaction to the current permanent ground motion. To do this, a study based on the measurement of ambient seismic noise is underway in the cave of Han-sur-Lesse (Ardenne, Belgium). The ambient seismic noise is measured both at the surface (above the limestone massif and in the nearest village), on the floor of the cave and on the stalagmites themselves. Different three-component seismic sensors are used in parallel: three SmartSolo IGU-16HR 3C and two Raspberry Shake 3D Personal Seismographs, one of which has been adapted to be easily attached to the stalagmites. This parallel configuration during two-week recording periods made it possible to determine the eigenfrequencies and the polarization of the associated movements of 16 stalagmites. In addition, daily and weekly variations in ambient noise and transient events are measured such as earthquakes, quarry explosions or flooding in the cave. The presence of sensors in different places over the same period also makes it possible to study the possible impact of the cave's local characteristics on these measurements.

Late February 2021, the Reykjanes Peninsula in southwest Iceland experienced severe seismicity associated with the development of a 9 km long dyke. Eight earthquakes of magnitude M≥5 were registered in the vicinity of Fagradalsfjall from February 24 until the onset of the Fagradalsfjall eruption in mid-March, which lasted for six months. Here, we analyze the temporal variations in crustal seismic wave velocities and the source characteristics of earthquakes during the dyke formation phase (February-March 2021).We apply ambient-noise seismic interferometry and compute seismic noise cross-correlations using the MSNoise software. Cross-wavelet analysis, a powerful technique that allows us to obtain frequency-dependence of velocity change, is used to investigate relative variations in seismic wave velocities (dv/v). Along with our wavelet-based dv/v results, we also present the stretching-based dv/v time-series that were calculated in real-time for volcano monitoring during the unrest. The Fagradalsfjall dyke intrusion induced temporal variations in seismic velocities and strong decorrelation that were picked up by the entire network across the peninsula. Beginning abruptly with the increased seismic activity, velocities at nearby seismic stations decreased by 1.5 percent. The amount of dv/v change was noticeably less than 1 percent at distant stations (15-30 km). The regional time-domain moment tensor inversion method (TDMT_INVC) was also applied to obtain earthquake mechanism solutions. Source parameters of 50 moderate-sized events with magnitudes Mw≥4.0 revealed predominantly normal and strike-slip faulting. We compare these to the deformation, dv/v and modeled Coulomb stress changes and present a joint interpretation.We provide a summary of the complex spatial and temporal evolution of crustal seismic velocity changes in the weeks preceding the effusive eruption. The understanding of the pre-eruptive geophysical signatures of the Fagradalsfjall volcano will contribute to better predict future volcanic activity in the area.

Most volcanoes on Earth host a volcano-hydrothermal system. Lying between the surface and magma reservoirs, they exert a dramatic influence on volcano dynamics. Understanding their behavior is however challenging because of the complex interplay between gas, liquid, and rocks. Volcanic gas can, for example, be completely scrubbed through interactions with groundwater whereas the kinetics of these reactions are controlled by thermodynamic conditions that are poorly constrained. While fluid circulation gives rise to a range of geophysical signals such as ground vibrations, self-potential, or variable resistivity observable at the surface, their complex dynamics complicate the isolation of pre-eruptive signals and the interpretation of the observed volcanic activity. Indeed, some volcanoes remain on alert for months or years without experiencing any eruption. Such situations severely affect the credibility of the agencies in charge of monitoring volcano activities.In this contribution, we will focus on multi-disciplinary efforts to better characterise the time evolution of these complicated systems. Our ultimate goal is to deconvolve the contribution of dynamic processes occurring in such systems (temperature, gas saturation, alteration, precipitation) to possibly facilitate real-time monitoring efforts. Our examples come from different parts of the world, in both hemispheres.

Shallow, light to moderate magnitude earthquakes in stable continental regions can have a damaging impact on vulnerable surface constructions. In the coal area of the Hainaut province in Belgium, a century of shallow seismic activity occurred from the end of the 19th century until the late 20th century. This seismicity is the second largest source of seismic hazard in northwestern Europe, after the Lower Rhine Embayment. The present study synthesises the impact and damage caused by this unique shallow seismicity. Reviewing intensity data provided in official macroseismic surveys held by the Royal Observatory of Belgium, press reports, and contemporary scientific studies resulted in a complete macroseismic intensity dataset. The strong shaking of five seismic events with moment magnitudes Mw around 4.0, which occurred on 3 June 1911, 3 April 1949, 15 December 1965, 16 January 1966, and 28 March 1967, locally caused widespread moderate damage to buildings corresponding to maximum intensity VII in the EMS-98 scale. For 28 earthquakes, detailed macroseismic maps were created. Our study highlights the capability of shallow, small-magnitude earthquakes to generate damage. Subsequently, using the Hainaut intensity dataset, we modelled a new Hainaut intensity attenuation law and created relationships linking magnitude, epicentral intensity and focal depth. Using these relationships, we estimated the location and magnitude of pre-1985 earthquakes that occurred prior to deployment of the modern digital Belgian seismic network. Estimated focal depths allowed discriminating between two different types of earthquakes. Some events were very shallow, only a few hundred metres deep, suggesting a close link to mining activities. Other earthquakes, including the largest and most damaging events, occurred at depths greater than 2 km but no deeper than 6 km, which would exclude a direct relationship with mining, but yet still might imply a triggering causality. This work results in a new updated earthquake catalogue including 123 seismic events. Our attenuation modelling moreover suggests that current hazard maps overestimated ground motion levels in the Hainaut area due to the use of inadequate ground motion prediction equations. Our Hainaut attenuation model is hence useful to evaluate the potential impact of current and future, e.g. geothermal energy, projects in the Hainaut area and other regions with a similar geological configuration.

Icequakes are the result of processes occurring within the ice mass or between the ice and its environment. Studying icequakes provides a unique view on ice dynamics, specifically on the basal conditions. Changes in conditions due to environmental or climate changes are reflected in icequakes. Counting and characterizing icequakes is thus essential to monitor them. Most of the icequakes recorded by the seismic station at the Belgian Princess Elisabeth Antarctica Station (PE) have small amplitudes corresponding to maximal displacements of a few nanometres. Their detection threshold is highly variable because of the rapid and strong changes in the local seismic noise level. Therefore, we evaluated the influence of katabatic winds on the noise measured by the well-protected PE surface seismometer. Our purpose is to identify whether the lack of icequake detection during some periods could be associated with variations in the processes generating them or simply with a stronger seismic noise linked to stronger wind conditions. We observed that the wind mainly influences seismic noise at frequencies greater than 1 Hz. The seismic noise power exhibits a bilinear correlation with the wind velocity, with two different slopes at a wind velocity lower and greater than 6 m s−1 and with, for example at a period of 0.26 s, a respective variation of 0.4 dB (m −1 s) and 1.4 dB (m −1 s). These results allowed a synthetic frequency and wind-speed-dependent noise model to be presented that explains the behaviour of the wind-induced seismic noise at PE, which shows that seismic noise amplitude increases exponentially with increasing wind speed. This model enables us to study the influence of the wind on the original seismic dataset, which improves the observation of cryoseismic activity near the PE station.

The use of Nodal systems based on autonomous geophone-based instruments entered the field of Seismology only recently. These lightweight solutions revolutionized seismological fieldwork through lightweight and wholistic instruments that are faster to deploy and easier to handle. The IGU-16HR series of SmartSolo® is one example, but yet lacking a thorough lab-based performance analysis. Here, we fill the knowledge gap, by performing a series of lab and field-based tests that focus on the sensors performance. The investigated parameters are the instruments transfer function, self-noise and overall performance to classical seismometer-based instruments. In the real-world application we show examples of H/V measurements of ambient vibrations in urban environments and the performance ranges with teleseismic waveform recordings. Under lab conditions, the nodal systems perform equally well as standard seismometers (e.g., Lennartz 3D/5s), even in the frequency range down to 0.2Hz, way below their natural frequency. The restitution can be carried out correctly with manufacturer given transfer function. At least for the vertical component, the instruments self-noise reaches the lower boundary of the global minimum noise level, confirming the ability to properly record teleseismic phases down to 0.1 Hz. In ambient noise studies the instrument limits are already reached at 0.8 Hz, but still resolve the fundamental frequencies within the methods uncertainty ranges, based on classical instrument data. These versatile and easy-to-use nodal systems are useful and reliable for a wide range of seismological applications. In addition, their installation is faster and reduced prices open the doors towards Large N installations and research studies for groups that face limited financial budgets.

International audience

Climate plays a key role in shaping population trends and determining the geographic distribution of species because of limits in species’ thermal tolerance. An evaluation of species tolerance to temperature change can therefore help predict their potential spatial shifts and population trends triggered by ongoing global warming. We assessed inter- and intraspecific variations in heat resistance in relation to body mass, local mean temperatures, and evolutionary relationships in 39 bumblebee species, a major group of pollinators in temperate and cold ecosystems, across 3 continents, 6 biomes, and 20 regions (2386 male specimens). Based on experimental bioassays, we measured the time before heat stupor of bumblebee males at a heatwave temperature of 40 °C. Interspecific variability was significant, in contrast to interpopulational variability, which was consistent with heat resistance being a species-specific trait. Moreover, cold-adapted species are much more sensitive to heat stress than temperate and Mediterranean species. Relative to their sensitivity to extreme temperatures, our results help explain recent population declines and range shifts in bumblebees following climate change., SCOPUS: ar.j, info:eu-repo/semantics/published

The city of Auckland, New Zealand (Tāmaki Makaurau, Aotearoa), sits on top of an active volcanic field. Seismic stations in and around the city monitor activity of the Auckland Volcanic Field (AVF) and provide data to image its subsurface. The seismic sensors – some positioned at the surface and others in boreholes – are generally noisier during the day than during nighttime. For most stations, weekdays are noisier than weekends, proving human activity contributes to recordings of seismic noise, even on seismographs as deep as 384 m below the surface and as far as 15 km from Auckland's Central Business District. Lockdown measures in New Zealand to battle the spread of COVID-19 allow us to separate sources of seismic energy and evaluate both the quality of the monitoring network and the level of local seismicity. A matched-filtering scheme based on template matching with known earthquakes improved the existing catalogue of five known local earthquakes to 35 for the period between 1 November 2019 and 15 June 2020. However, the Level-4 lockdown from 25 March to 27 April – with its drop in anthropogenic seismic noise above 1 Hz – did not mark an enhanced detection level. Nevertheless, it may be that wind and ocean swell mask the presence of weak local seismicity, particularly near surface-mounted seismographs in the Hauraki Gulf that show much higher levels of noise than the rest of the local network.

We present a comprehensive study of the 23 February 1828 earthquake that caused severe damage in the central part of the present Belgian territory. We synthesised and critically reassessed the information provided by contemporaneous scientific studies and newspapers of the Low Countries. In addition, a systematic survey of official, private, and religious historical sources was undertaken to improve our knowledge of the earthquake effects in the most affected region. In total, we collected information for 185 localities in the present territory of Belgium, France, Germany, The Netherlands, Grand Duchy of Luxembourg, and southern England. The complex spatial repartition of earthquake testimony locations is related to the population density, the types of historical sources, and the regional geological context. Our 1828 earthquake intensity analysis demonstrates that the shaking strength diminished more slowly along the WSW-ENE strike in this part of the Brabant Massif compared to the surrounding Rhenish Massif and Paris Basin. Moreover, the northward increasing thickness of sediments on top of the Brabant Massif controlled the observed strong intensity attenuation. Using the intensity dataset, we located the earthquake at 50.70° N and 5.12° E, with uncertainty around 10 km. The magnitude is estimated as MW = 5.1 ± 0.3 and the average focal depth optimising the location inversion is 10 km with a possible range between 3 and 15 km. Our study illustrates the benefit of historical seismicity studies on the knowledge of the impact of rare destructive earthquakes in stable continental regions like Western Europe.

International audience; Climate change is related to an increase in frequency and intensity of extreme events such as heatwaves. It is well established that such events may worsen the current world-wide biodiversity decline. In many organisms, heat stress is associated with direct physiological perturbations and could lead to a decrease of fitness. In contrast to endotherms, heat stress resistance has been poorly investigated in heterotherms; especially in insects, in which the internal physiological mechanisms available to regulate body temperature are almost negligible making them sensitive to extreme temperature variations.Wild bees are crucial pollinators for wild plants and crops. Among them, bumblebees are experiencing a strong decline across the world. Therefore, the ongoing global decline of these insect pollinators partly due to climate change could cause major economic issues.Here we assess how simulated heatwaves impact fertility and attractiveness (key parameters of sustainability) of bumblebee males. We used three model species: Bombus terrestris, a widespread and warm-adapted species, B. magnus and B. jonellus, two declining and cold-adapted species.We highlight that heat shock (40 degrees C) negatively affects sperm viability and sperm DNA integrity only in the two cold-adapted species. Heat shock can also impact the structure of cephalic labial glands and the production of pheromones only in the declining species.The specific disruption in key reproductive traits we identify following simulated heatwave conditions could provide one important mechanistic explanation for why some pollinators are in decline through climate change.A free Plain Language Summary can be found within the Supporting Information of this article.

International audience; Human population growth has increased demand for food products, which is expected to double in coming decades. Until recently, this demand has been met by expanding agricultural area and intensifying agrochemical-based monoculture of a few species. However, this development pathway has been criticised due to its negative impacts on the environment and other human activities. Therefore, new production practices are needed to meet human food requirements sustainably in the future. Herein, we assert that polyculture practices can ensure the transition of aquaculture towards sustainable development. We review traditional and recent polyculture practices (ponds, recirculated aquaculture systems, integrated multi-trophic aquaculture, aquaponics, integrated agriculture-aquaculture) to highlight how they improve aquaculture through the coexistence and interactions of species. This overview highlights the importance of species compatibility (i.e. species that can live in the same farming environment without detrimental interactions) and complementarity (i.e. complementary use of available resources and/or commensalism/mutualism) to achieve efficient and ethical aquaculture. Overall, polyculture combines aspects of productivity, environmental protection, resource sharing, and animal welfare. However, several challenges must be addressed to facilitate polyculture development across the world. We developed a four-step conceptual framework for designing innovative polyculture systems. This framework highlights the importance of (i) using prospective approaches to consider which species to combine, (ii) performing integrated assessment of rearing environments to determine in which farming system a particular combination of species is the most relevant, (iii) developing new tools and strategies to facilitate polyculture system management, and (iv) implementing polyculture innovation for relevant stakeholders involved in aquaculture transitions.

International audience; Recent improvements in taxonomy consider multiple operational criteria. Integrative taxonomy provides a methodological framework merging these multisource approaches. Bumblebees are considered as a uniform group where their taxonomy remains one of the most difficult. Here, we investigate the taxonomic statuses inside a monophyletic group including six taxa (B. inexspectatus, B. mlokosievitzii, B. ruderarius, B. sylvarum, B. velox and B. veteranus)in the most diverse subgenus of bumblebees: Thoracobombus. We used an integrative approach based on mitochondrial and nuclear genetic markers and eco-chemical traits commonly used in bumblebee taxonomy. For all species, our study shows a clear differentiation in DNA and eco-chemical traits. However, we conserve the subspecies status of B. ruderarius simulatilis and B. sylvarum daghestanicus from the east of Turkey and Iran according to their degree of genetic and eco-chemical differentiation

The Volcán de Colima is one of the most active volcanoes in North America, but it still has a poorly constrained upper crustal structure. We used ambient seismic noise tomography to generate the highest-resolution three-dimensional shear-wave velocity model of the volcano to date. We measured group velocity dispersion curves of Rayleigh and Love waves extracted from the records of two distinct networks deployed on the Colima Volcanic Complex. Those were regionalized into 2-D velocity maps and then locally inverted using a neighborhood algorithm and an anisotropic parametrization to obtain an accurate shear-velocity model down to 4 km below sea level. The resulting model highlights a network of deeply rooted NE-SW low-velocity zones oriented along a local fault system. This low-velocity zone also roughly aligns with the north-south trend associated with the gradual trenchward shift of the magmatic front of the quaternary Colima Volcanic Complex. An overlapping negative radial anisotropy indicates that magma follows vertically oriented structures, such as interfingered dikes, faults, or cracks with a substantial vertical component. Our results also highlight the difference between the former active system, filled with solidified dikes and sills, and the current one, associated with a network of fluid-filled dikes.

On 13 May 2008, a strong eruption took place along a fracture opened on Mt. Etna's eastern flank accompanied by significant seismic activity. Recent studies show that seismic ambient noise is an effective method to observe the fluctuations of seismic wave velocities due to volcanic and seismic activities. In this study, we analyze the temporal seismic velocity changes (dv/v) from January 2007 to June 2008 at Mt. Etna by using seismic ambient noise. We retrieve a large dataset to calculate dv/v because of the large number of permanent seismic stations located at Mt. Etna. Daily cross-correlation functions (CCFs) are computed at 15 stations for 105 interstation pairs in the 1-2 Hz frequency band by using vertical components. We observe that the dv/v displays a gradual reduction and reaches down to -0.3% associated with the aforementioned volcanic activity. We compared our results with volcano-tectonic earthquakes and volcanic tremor. Our study shows a significant relationship between the magma intrusion and decrease of the dv/v.

Our study presents temporal seismic velocity changes (dv/v) associated with the May 2008 Ölfus earthquakes by computing the cross-correlations of ambient noise. The 2008 Ölfus doublet (M5.8 and M5.9, with a composite magnitude of M6.1) occurred in the South Iceland Seismic Zone, which is a highly active transform zone that accommodates plate motion with major earthquake sequences (e.g., 1896, 1912, 2000). We investigate co-seismic and post-seismic response of the crust in the epicentral area, to the 2008 Ölfus doublet. For our analysis, we used three-component continuous data from three stations of the SIL national seismic network operated by the Icelandic Meteorological Office. Using the MSNoise software package (http://www.msnoise.org), we calculated single station ambient noise cross-correlations and utilized the stretching approach to quantify relative seismic velocity variations. We found the highest co-seismic velocity decrease (

Seismica is an effort to establish a Diamond Open-Access (OA) journal for the community of researchers working in the (broadly defined) fields of seismology and earthquake science. Diamond OA journals are free to read, do not charge authors to publish, and thus promise to widen access to published research to all. Such efforts require broad community participation beyond editing and reviewing articles, as all functions typically fulfilled by paid staff, including administration, production, typesetting and copy editing, must be undertaken by volunteers.In December 2020, the Seismica Task Force, a spontaneously-organized group of seismologists and earthquake scientists, connected on Twitter. Inspired by the example of the established Diamond OA journal Volcanica, we began to explore the possibility of founding such a journal in our own field. Through outreach via social media, blog posts, conference presentations and disciplinary mailing lists, we have built a community of over 180 interested participants. With broad input, this group has developed a roadmap (https://doi.org/10.31223/X5304V) for the build-out of the journal, including an editorial structure and policies, broad outreach and structures to ensure a diverse editorial team, a logo and visual branding, and agreements with the McGill University Library to host and support the journal.In this presentation, we will introduce our first editorial board, and solicit our first article submissions for publication in 2022. We will also update the broader community on these and other developments in the journal-building process, and share our experiences and lessons learned.

In September 2021, two jökulhlaups were released into the Skaftá river from western Vatnajökull icecap (Iceland). Such floods have been known since 1955. These jökulhlaups originate from two subglacial lakes under 1–3 km wide and 50–150 m deep depressions in the glacier surface, commonly referred to as ice cauldrons, formed by geothermal melting. The average time interval between jökulhlaups from each cauldron is ~2 a. The jökulhlaups travel ~40 km under a glacier that reaches maximum thickness of ~750 m and emerge in the Skaftá river at the terminus of Skaftárjökull outlet glacier. In addition to increased subglacial water flow and river discharge, these floods are responsible for seismicity and tremor onset, inside the cauldrons, along the subglacial channels, at the outlet where the subglacial channels meet the river, as well as on the river path. We used seismic interferometry or cross-correlation of seismic noise to analyse data from 2015 to the end of 2021. We computed cross-correlation functions for 27 seismic stations and for frequencies between 0.5 and 8 Hz. To characterize these floods, we calculated the propagation velocities based on the cross-correlation functions and for each frequency band. We located seismic signatures both during the floods period and previous times by using a grid-search method based on the approach of Ballmer et al. 2013, which calculates theoretical differential times and provides probabilities of locations as the summed stack amplitudes of correlograms. These daily location grids allowed us to analyse the spatial evolution of probability of location during these floods as well as compare them with previous “non-flood” periods. Furthermore, based on these location grids, we were able to compute temporal series for isolated locations (pixels) such as ice cauldrons, a hydrological station located on the river path, or any other target, allowing us to analyse the temporal evolution of location probability during the floods as well as compare it with the last six years of data. The resolution of this temporal evolution ranges from monthly to hourly. By using six years of seismic data, we were also able to compare the 2021 floods with floods due to the same ice cauldrons, for example in August 2018 and September 2019.This study provides an insight on how relevant seismic interferometry can be in the monitoring of such processes, with the purpose of being fully automatic in the near future.

Malta, a small island nation in the centre of the Mediterranean, is deemed as the European country facing the highest stress on its water resources. Malta has a semi-arid climate with approximately 550 mm of annual rainfall over an area of ∼315 km2 and a very high population density. Consequently, 80% of the water used in the Maltese agricultural sector is directly abstracted from groundwater resources via boreholes or underground galleries. To improve the groundwater monitoring in Malta, which currently depends on a network of in situ borehole readings, we analyse ambient seismic noise data recorded on the Malta Seismic Network (MSN) as part of the project SIGMA (Seismic Imaging of Groundwater for Maltese Aquifers). We investigate temporal changes in seismic velocity as an indication of the variability of water in underground rocks. Water-saturated rocks have an increased pore pressure, which, in turn, leads to the opening of cracks in the rock that reduces the contact area between different grains of rock leading to a decrease in seismic velocity. We compiled the seismic data from each station of the MSN (Galea et al., 2021, SRL) and the FASTMIT experiment (Bozionelos et al., 2019, Xjenza) consisting of a combination of eight broadband and six short-period, three-component seismic stations for the years 2017-2020. The data was pre-processed by demeaning, tapering and merging into a 1-day long trace, which were then band-pass filtered and decimated or downsampled. Power Spectral Density (PSD) charts for the data show that most microseisms energy has a period range of 1-10s. We therefore test different filtering bands encompassing this frequency range. We perform auto and cross-correlation of noise data from 78 station pairs. We perform stacking for 1, 5 and 10 days for smoother cross-correlation functions. We then compute the time delays using the Moving-Window Cross-Spectral analysis (Clarke et al., 2011, JGI). Finally, the change in velocity (dv/v) is determined from the calculated time delays. The algorithm was run via the software package MSNoise (Lecocq et al., 2014, SRL).We find that the changes in the dv/v time series (~±0.01%) have seasonal patterns, where a negative dv/v in the winter period and a positive dv/v in summer is observed. We compare the auto and cross-correlations with the time series of groundwater measurements from nearby boreholes (ranging from 0.25-3.3m above mean sea level) to investigate the correlation between them. We also take into consideration the NW-SE geology of the island, distinguished by an impermeable layer between the geological strata (Blue Clay) in the north. We find that long paths traversing across different geological layers show weak correlations. We present the tests performed and the results showing the extended spatial coverage for groundwater monitoring in conjunction with the borehole data for the Maltese Islands. Project SIGMA is financed by the Energy and Water Agency under the National Strategy for Research and Innovation in Energy and Water (2021-2030).

The eigenfrequencies of speleothems are fundamental parameters in the study of their response to earthquakes. To study these, the seismic ambient noise is measured by three-component seismic sensors adapted to the geometry of the speleothems. This method is currently being studied in the Han-sur-Lesse cave (Ardenne, Belgium).A previous study (Martin et al. 2020) was carried out with a SmartSolo IGU-16HR 3C sensor on an imposing 4.5 m tall stalagmite. This approach demonstrated the feasibility and interest of studying the eigenfrequencies of stalagmites from ambient noise. However, this sensor was too heavy for use on thin and slender stalagmites. The challenge was to find and adapt a lighter sensor able to record very weak movements while being easily adjustable to the various shapes of the stalagmite and securely attachable on these to reduce the impact of the sensor on frequencies measurements and the risks for the fragile structure.A solution was found by using a Raspberry Shake 3D Personal Seismograph (RS) that initially integrates three orthogonal velocity sensors (Sunfull PS-4.5B), the digitizer, and the Raspberry Pi computer into a single plexiglass box​. The RS has the advantage of being less heavy while being composed of three weak motion geophones. After a comparison study, this sensor gives similar results for eigenfrequency and polarization analyses. However, the use of this new sensor on thin and slender stalagmites requires the creation of suitable support. The RS was split and distributed around the stalagmite. The geophone wiring was modified and extended to separate the geophones from the acquisition system. A 3D-printed support was created to guarantee the orthogonality of the horizontal sensors while reducing the stresses by distributing the weight of the sensor around the stalagmite.This new configuration allowed determining the eigenfrequencies of 16 thin and slender stalagmites in the Han-sur-Lesse cave (Ardenne, Belgium) and the polarization of the motions associated with these frequencies. Moreover, a two-week recording period allows to measure the daily and weekly variation of ambient noise and transient events like earthquakes, quarry blasts or flooding events in the cave.Reference: Martin, A.; Lecocq, T.; Hinzen, K.-G.; Camelbeeck, T.; Quinif, Y.; Fagel, N. Characterizing Stalagmites’ Eigenfrequencies by Combining In Situ Vibration Measurements and Finite Element Modeling Based on 3D Scans. Geosciences 2020, 10, 418. https://doi.org/10.3390/geosciences10100418

The Whakaari/White Island volcano, located ~ 50 km off the east coast of the North Island in New Zealand, has experienced sequences of quiescence, unrest, magmatic and phreatic eruptions over the last decades. For the last 15 years, seismic data have been continuously archived providing potential insight into this frequently active volcano. Here we take advantage of this unusually long time series to retrospectively process the seismic data using ambient noise and tremor-based methodologies. We investigate the time (RSAM) and frequency (Power Spectral Density) evolution of the volcanic tremor, then estimate the changes in the shallow subsurface using the Displacement Seismic Amplitude Ratio (DSAR), relative seismic velocity (dv/v) and decorrelation, and the Luni-Seismic Correlation (LSC). By combining our new set of observations with the long-term evolution of earthquakes, deformation, visual observations and geochemistry, we review the activity of Whakaari/White Island between 2007 and the end of 2018. Our analysis reveals the existence of distinct patterns related to the volcano activity with periods of calm followed by cycles of pressurization and eruptions. We finally put these results in the wider context of forecasting phreatic eruptions using continuous seismic records.