Publications

Scientific exploration of the UK and Ireland’s subsurface is important for both economic and population growth, sustainable use of natural resources, storage of greenhouse gas, and to inspire curiosity about the Earth beneath our feet. This article outlines a vision for an array of seismological instruments spanning the UK and Ireland (UKI-Array), augmented by other types of geophysical sensors, to maximise the value offered by existing equipment pools. The mission is to research natural phenomena in the deep and shallow Earth, to solve problems concerning hazards and resources, to connect scientists to school pupils and the broader public, and thus to inspire a new generation to learn about geophysics. This vision was created through a community driven process of engagement and participation. This paper describes the concept and design of UKI-Array; the companion paper discusses potential applications.

In 2024 werd door De Vlaamse Waterweg NW (DVW) een ontwerpstudie opgestart voor de vervanging en verbreding van de linkeroever van het kanaal Brussel-Charleroi ter hoogte van de spoorwegbruggen Hoge Snelheidslijn (HSL) L94 en L96 over het kanaal in het zuiden van Halle (België). In dit expertise rapport rapporteren we de resultaten van een seismische ruiscampagne die werd uitgevoerd langs de linkeroever van het kanaal Brussel-Charleroi (Halle, Vlaams Brabant) met als doel om het contrast tussen zachte sedimenten en het sokkelgesteente van het Brabant Massief in kaart te brengen.

Seismic ambient noise spectra ubiquitously show two amplitude peaks corresponding to distinct oceanic wave interaction mechanisms called primary (seismic period (T) ~ 14 s) and secondary (T ~ 7 s) microseism. Seismic noise records are used in a wide range of applications including crustal monitoring, imaging of the Earth's deep interior using noise correlations, and studies on the coupling between oceans and solid Earth. All of these applications could benefit from a robust knowledge of spatiotemporal dynamics of microseismic sources. Consequently, seismologists have been studying how to model microseismic sources of ambient noise with the recent improvements in ocean wave models. Global sea state and its derivative products are now covering the past decades in models such as the WAVEWATCHIII hindcast. This paper introduces the Wave Model Sources of Ambient Noise (WMSAN, pronounced [wam-san]) Python package. This modular package uses standardized wave model outputs to visualize ambient noise source maps and efficiently compute synthetics of seismic spectrograms and cross-correlations for surface waves (Rayleigh) and body waves (P, SV), in a user-friendly way.

Open-conduit basaltic volcanoes are susceptible to sudden transitions from mild activity to violent explosive eruptions with little to no warning. Such was the case at Stromboli in the summer of 2019, when two paroxysmal explosions occurred within approximately two months (July 3 and August 28). We apply coda wave interferometry to identify possible transitions in behavior in the build-up to these events, computing seismic velocity changes using five broadband seismic stations on the volcano between 2013–2022. This timeframe encompasses a range of volcanic activity including effusive activity, major explosions and paroxysms. Cross-correlation functions are computed both between pairs of stations and single-station cross-components in multiple frequency bands that allow the sampling of different depths (between approximately 100–1000 m) within the plumbing system. Shallow velocity changes (1–2 Hz and 2–4 Hz) reveal mid-to-long term precursors prior to the paroxysms in 2019. For example, we observe that 2–4 Hz velocities recorded at the station closest to the active crater show an increase of 0.2–0.3 % relative to velocities recorded at other stations. This increase is largely accumulated from mid-2017, coinciding with previously observed heightened activity at the volcano, peaking approximately one month prior to the first paroxysm. A long-term decrease is also observed in deeper velocity changes (0.5–1.0 Hz) during the same time interval. It is hypothesized that these changes represent greater magma overpressure from increased volatile input from depth. The different response in the shallow subsurface may reflect a local response due to the same source within the vicinity close of the crater terrace. These findings illustrate how coda wave interferometry can provide meaningful insights into the evolving dynamics of open-conduit basaltic volcanoes.

The quality control process usually followed at broad-band seismic networks includes the calculation of the power spectral density and their probability density functions. These results do not make possible a quick estimation of temporal variations that can result from non-continuous sources of noise, meteorologic phenomena, etc. We propose the use of the SeismoRMS package, originally developed to analyze the seismic amplitude variations associated with the COVID19 lockdown, to monitor the time evolution of seismic noise sources in a permanent network, using as a case example the dataset collected during 2023 by the CA network in NE Iberia. Frequencies above 1 Hz show remarkable differences between the stations, despite sharing similar installation settings. Most of the sites show day/night and working day/weekend variations, suggesting a relevant contribution of anthropic sources, but the amplitude of such variations differs strongly among the sites. Our study allows us to identify specific sources of noise affecting some sites during short and regular time periods, an aspect that needs to be taken into account when evaluating the overall quality of each site. We conclude that a systematic analysis of the amplitude variations at different frequency bands can be a tool of interest for the management of a broad-band seismic network.

Damaging earthquakes happened in the Hainaut coal basin (Belgium) in the 19th and 20th centuries but stopped after coal mining. We relocated 54 Hainaut earthquakes of the period 1965 and 1985 by using regional and local seismic phase measurements from recordings in and around Belgium. These new results finalise the Hainaut earthquake catalogue, derived from macroseismic data for the 1887–1965 period and instrumental records after 1965. This updated database allows us to discuss the origin of this intriguing seismicity. Computed focal depths show that the strongest earthquakes before 1985 occurred inside the Upper Palaeozoic part of the Brabant Parautochton, probably below the deepest parts of the coal mines. The shallowness of this seismicity, its spatial link with the coal basin, the compressive to strike-slip focal mechanisms of the stronger earthquakes between 1965 and 1985 and the large quantity of coal mass removal all match with worldwide observations of miningtriggered earthquakes. The earthquake mechanisms agree with the local, compressive to transpressive stress regime derived from differences in upper crustal density and topography that make the Hainaut area more sensitive to fault reactivation at shallow depths, as observed in similar tectonic settings elsewhere in the world.

Co-eruptive volcanic tremor during the 2021 Fagradalsfjall eruption in Iceland (19 March–18 September 2021) is characterized using seismic and visual data recorded close to the eruption site and across the Reykjanes Peninsula. An automatic seismic network-based approach reveals several tremor patterns associated with seven phases of the eruption, including (a) continuous tremor located beneath the eruption site and attributed to pressure changes in the shallow vent system(s) in phases I, III, and VII, and (b) two patterns of minute- and hour-long intermittent tremor in May (phase II) and July–August (phases IV–VI), respectively. The first intermittent pattern of minute-long tremor bursts associated with pulsating lava fountains in May is attributed to magma degassing in a shallow reservoir (top 100 m) connected to a top-conduit. The progressive enlargement of both the top-conduit and shallow reservoir with time is estimated quantitatively using a collapsing foam model. Sudden changes of their geometries, as detected from tremor characteristics, are systematically associated with observed crater collapse events. The second intermittent pattern of cyclic hour-long tremor episodes associated with episodic effusive activity in July–August is attributed to magma flowing and cooling in the feeder dike connected to a sill at 5 km depth. The sill is fed by a constant influx of magma from the deeper plumbing system and stores magma during low discharge periods. The observed cyclicity of both the eruptive activity and the tremor is interpreted quantitatively with a sill-dike model accounting for magma cooling and induced cyclic viscosity changes in the dike.

A re-assessment of the macroseismic intensity data was conducted for the second-largest instrumentally recorded event in the Netherlands: the 20 November 1932 Uden earthquake. This event was felt across the Netherlands, Belgium and Germany. Intensity values on the EMS98 scale were assigned based on original data (reports/enquiries/letters), manually for the Netherlands and automatically for Belgium, with existing German data added for completeness. The updated dataset was used to calculate macroseismic location and magnitude using the Bakun and Wentworth (1997, 1999) algorithm. To capture epistemic uncertainty, four newly calibrated intensity attenuation relations were applied and their results averaged. The results using only intensity data from the Netherlands provided stable solutions within the region of maximum observed intensity (Imax = VII). However, including Belgian and German data shifted the source location outside this region and becomes less reliable, likely due to differences in data collection methods and local/regional site effects. Comparison of confidence levels from the BW method with bootstrap modelling showed that almost all bootstrap results fall within the 50% confidence region. A more realistic estimate for the location uncertainty was derived from the bootstrap analysis. The revised source parameters are 51.63°N and 5.61°E ± 7.3 km for the source location and ML = 5.1 ± 0.3 (MS 4.9 ± 0.3) for the magnitude.

The GeoCamb project is set within the context of the energy transition aimed at reducing Belgium's dependency on fossil fuels and achieving the European Union's climate objectives for 2030. These objectives include a significant increase in the share of renewable energy, with a particular focus on decarbonizing heating and cooling, which account for 50% of energy consumption. In this framework, geothermal energy, particularly the use of Cambrian formations in the Brabant Massif, offers strong potential. However, the development of such systems remains limited due to among others geological complexity, lack of data on the basement rocks and important upfront costs. The primary objective of GeoCamb was to assess and demonstrate the geothermal potential of Cambrian rocks beneath Brussels and the Brabant provinces. This project aimed to characterize these formations from geological, geophysical, and hydrogeological perspectives while evaluating the energy needs of public buildings that could benefit from these systems. By employing a multidisciplinary approach, GeoCamb combined subsurface exploration, 3D modelling, hydrogeological testing, and the analysis of the thermal demand profiles of buildings. Concurrently, the project focused on collaboration with public and private partners to optimize the use of available data and expand scientific knowledge. Special attention was given to public buildings, which represent strategic opportunities for demonstrating geothermal energy as a sustainable energy source. The project followed several key phases. First, an extensive collection of existing data was carried out. This information was consolidated into a database that gathered a total of 107 boreholes in the Cambrian bedrock. In parallel with the geological survey, an extensive seismic noise campaign was conducted using seismic nodal sensors. This allowed to better map the top of the Brabant Massif below Brussels region, especially under sites where the bedrock was unexplored by drillings. Next, a participatory "win-win" approach enabled collaboration with ongoing projects to pool resources and results. Among the flagship initiatives, five case studies were conducted, such as the Paul Henri Spaak (PHS) building and the Gandhi site in Molenbeek, to assess the feasibility of geothermal systems. Parallel to these efforts, modelling studies were performed at the Tour & Taxis site to examine potential interactions between open systems located in different aquifers. The project's results highlighted the strong geothermal potential of the Cambrian basement. Exploratory boreholes, including the one at Molenbeek, confirmed that the basement depth gradually increases from south to north below Brussels and that the basement rock exhibits a high thermal conductivity, reaching 3.8 W/mK in some areas. Analyses of pumping tests revealed variable hydraulic flows, reflecting heterogeneous fracturing of the basement. These characteristics are favorable for implementing geothermal systems, whether closed-loop (BTES) or open-loop (ATES). However, closed-loop systems appear to be better suited to dense urban environments like Brussels, while open-loop systems are more appropriate for larger-scale projects where accessible aquifers are available. The case studies demonstrated the effectiveness of these technologies. At the Gandhi site in Molenbeek, although the study was limited to a pre-feasibility assessment, the results provided crucial insights for considering a geothermal system as part of future renovations. Finally, an impact study and simulations at Tour & Taxis showed limited interactions between systems in different aquifers but emphasized that thermal balance at the building level is essential to maintain long-term efficiency. In conclusion, the GeoCamb project underscores the untapped potential of the Cambrian basement for shallow geothermal energy in Belgium. Its findings highlight the need for continued exploration and an enhanced, centralized database of geothermal project parameters (hydro-geological, thermal, energetical, …). GeoCamb also advocates for increased awareness among policymakers and private actors to encourage support and investment in these technologies. Lastly, the project emphasizes the importance of pilot systems in public buildings as a lever to promote sustainable energy and reduce Belgium’s carbon footprint.

Understanding volcanic eruption triggers is critical toward anticipating future activity. While internal magma dynamics typically receive more attention, the influence of external processes remains less understood. In this context, we explore the relationship between seasonal snow cycles and eruptive activity at Ruapehu, New Zealand. This is motivated by apparent seasonality in the eruptive record, where a higher than expected proportion of eruptions (post-1960) occur in spring (including the two previous eruptions of 2006 and 2007). Employing recent advancements in passive seismic interferometry, we compute sub-surface seismic velocity changes between 2005 and 2009 using the cross-wavelet transform approach. Stations on the volcano record a higher velocity in winter, closely correlated with the presence of snow. Inverting for depth suggests these changes occur within the upper 300 m. Notably, we observe that the timing of the previous two eruptions coincides with a period associated with an earlier velocity decrease at approximately 200–300 m depth relative to the surface. Reduced water infiltration (as precipitation falls as snow) is considered a likely control of seasonal velocities, while modeling also points to a contribution from snow-loading. We hypothesize that this latter process may play a role toward explaining seasonality in the eruptive record. Our findings shed light on the complex interactions between volcanoes and external environmental processes, highlighting the need for more focused research in this area. Pursuing this line of inquiry has significant implications toward improved risk and hazard assessments at not just Ruapehu, but also other volcanoes globally that experience seasonal snow cover.

Macroseismic intensity data have been gathered continuously in Belgium since the start of the twentieth century. In this study, we review the applied survey practices used over the previous century: from small-scale ad-hoc improvised surveys to the mass distribution of collective questionnaire versions sent to local authorities. The variety of survey methodologies causes a high degree of heterogeneity within the resulting macroseismic data, increasing the uncertainty in macroseismic applications that rely on homogeneous data. We thoroughly re-evaluated the original source material and intensity scale conversions in order to create the Belgian Traditional Macroseismic (BTM) database, a comprehensive compilation of 20th-century macroseismic data in which all sources are properly referenced. The BTM database currently consists of 23,950 intensity data points (IDPs) on the European Macroseismic Scale for 80 felt earthquakes, ranging from 2.4 to 5.8 on the local magnitude (M L) scale. Each IDP is provided with a source type and each earthquake is attributed a data quality parameter that indicates the level of uncertainty associated with its IDP source quality. The publication of the BTM database facilitates the use of Belgian macroseismic data for a variety of seismological purposes and allows us to summarize the overall seismic impact on Belgium for the duration of a century. Like in many other parts of the world, traditional procedures have practically been discontinued in Belgium in favour of an online enquiry. However, the potential for high-quality data following a traditional survey methodology is still large and we strongly recommend its continuation.

In South Iceland, populated and agricultural areas are at risk of earthquakes due to their location within the South Iceland Seismic Zone (SISZ). In 2008, two moderate-sized earthquakes (M5.8 and M5.9) occurred in Ölfus, the western end of this highly active transform zone. We analyze temporal seismic velocity variations (dv/v) related to the Ölfus earthquake doublet, using cross-correlations of ambient noise in the frequency range of 0.1–3.0 Hz. The two mainshocks decrease the average velocity by 0.8% at the nearest stations. The co-seismic changes are most noticeable from 0.7 to 1.7 Hz and affect a 40 km wide region. We present a first-time comparison of dv/v to crustal deformation, seismicity, co-seismic volumetric stress changes and reported PGA distribution for the Ölfus doublet. Ground accelerations caused by mainshocks at intermediate distances suggest that strong shaking-related damage may contribute to the co-seismic dv/v decrease. A rapid velocity increase (0.3%) in a month after the co-seismic drop indicates crustal rock healing. We find 3-months of post-seismic decorrelation, followed by a nearly permanent velocity decrease (0.2%) confined to a shallow layer (1 km) until the end of the observation period. Afterslip and pore fluid effects in the near-source region are likely to influence post-seismic dv/v. We demonstrate that seismic interferometry can contribute to future fault-zone monitoring operations in the SISZ by detecting small changes in velocity.

Climate change is increasingly predisposing polar regions to large landslides. Tsunamigenic landslides have occurred recently in Greenland (Kalaallit Nunaat), but none have been reported from the eastern fjords. In September 2023, we detected the start of a 9-day-long, global 10.88-millihertz (92-second) monochromatic very-long-period (VLP) seismic signal, originating from East Greenland. In this study, we demonstrate how this event started with a glacial thinning–induced rock-ice avalanche of 25 × 106 cubic meters plunging into Dickson Fjord, triggering a 200-meter-high tsunami. Simulations show that the tsunami stabilized into a 7-meter-high long-duration seiche with a frequency (11.45 millihertz) and slow amplitude decay that were nearly identical to the seismic signal. An oscillating, fjord-transverse single force with a maximum amplitude of 5 × 1011 newtons reproduced the seismic amplitudes and their radiation pattern relative to the fjord, demonstrating how a seiche directly caused the 9-day-long seismic signal. Our findings highlight how climate change is causing cascading, hazardous feedbacks between the cryosphere, hydrosphere, and lithosphere.

In 2024 start De Vlaamse Waterweg NW (DVW) met de uitvoering van de vervanging van de Bospoortbrug en de IJzerwegstraat in het centrum van Halle (Vlaams Brabant). In dit expertise rapport werd een seismische ruiscampagne uitgevoerd langs beide oevers van de Bospoortbrug (Halle, Vlaams Brabant) om het contrast tussen zachte sedimenten en het sokkelgesteente van het Brabant Massief in kaart te brengen.