The rapidly changing climate is escalating the frequency and intensity of extreme weather events in the Azores, Portugal. It is crucial to comprehend the dynamics of these events to mitigate them. Atmospheric water vapor data from the Global Navigation Satellite System (GNSS) and reanalysis products from an atmospheric general circulation model can be utilized to investigate the dynamics of weather fronts in the Azores Islands. A primary goal of our study is to conduct a comprehensive comparison between GNSS and MERRA2-based atmospheric reanalysis data and derive small-scale atmospheric structures with high-temporal resolution. Using statistical analysis, we will unveil the similarities and discrepancies between the two approaches in capturing atmospheric water vapor patterns. Emphasizing an exploratory methodology, we will showcase our findings using a restricted dataset that centers on specific instances of extreme precipitation witnessed in the Azores Islands.

The central islands of the Azores Archipelago in the North Atlantic straddle a diffuse zone of dextral transtension between the African and Eurasian plates, providing an ideal setting for studying the interplay between tectonics and magmatism. São Jorge is a narrow island dominated by a westward progression of past basaltic fissure eruptions, where fault zones act as volcanic rifts. After two inland eruptions with significant socioeconomic impact in 1580 and 1808, the most recent probable eruption occurred offshore in 1964, after two years of seismic activity. In March 2022, a seismic crisis began on São Jorge (magnitudes up to ML 3.8). Our analyses of InSAR and GNSS data are consistent with a dike intrusion that stalled at 2 km depth below sea level. Here, we use seismicity to probe the space-time evolution of the intrusion. The unique geography and near-coastal position of seismicity yield inherently uncertain locations. To address this, we supplemented on-land stations with 6 ocean-bottom seismometers (OBSs) around the island later in the crisis. We use NLL-SSST-coherence, a location method ideal for changing station density, to exploit later OBS data to form robust source-specific station terms that allow precise relocation of the earlier part of the seismic sequence when coverage was sparser. In a final step, we combine waveform coherence and location uncertainty stacks to enhance hypocenter location precision to <100m. Relocations of ~12,000 earthquakes show precursory, weak seismicity that started ~6 months before, starting offshore, south of São Jorge before migrating to shallower depth beneath the centre of the island. The main seismic crisis on 19 March 2022 started at shallower (<8 km) depth and moved north-westward and deeper before concentrating in the central zone at ~10 km depth. Intriguingly, nearly all the seismicity is located west of and deeper than the modelled dike intrusion, suggesting the intrusion was largely aseismic. Nevertheless, the agreement between the strike of the dike and the seismicity lineations suggests that the pre-existing Pico do Carvão Fault Zone guided melt ascent in the crust. However, moment tensors from polarity and waveform inversion show double-couple left-lateral strike-slip faulting along planes striking obliquely (by ~20°) to the dike and seismicity lineation, evidencing high fluid/melt pressures. The overall b-value is high (~2).Interpreting both the seismicity and near-field GNSS displacements, we discuss the intrusion’s evolution along the preexisting fault zone, particularly focussing on potential magmatic inflow and drainage beneath the main dike intrusion.We are grateful to the UK Ocean Bottom Instrument Consortium (OBIC) and SEIS-UK teams for providing the instrumentation and installation services. This work was also supported by Portuguese FCT/MCTES through the project GEMMA (https://doi.org/10.54499/PTDC/CTA-GEO/2083/2021).

In March 2022, a seismic crisis was declared in São Jorge Island. Despite the regular seismotectonic activity observed in the Azores Central Group, São Jorge has not exhibited significant activity since the crisis associated with an eruption in 1964. Between the fall of 2021 and the end of 2022, approximately 12,000 earthquakes (magnitudes up to ML 3.8) have been recorded, with the seismicity and geodetic modelling pointing to a magmatic intrusion. Intrusions cause gas release, fluid circulation, and pressure perturbations in the subsurface volcanic system that often induce changes in seismic velocity. Here, we probe spatial-temporal changes in the seismic velocity structure beneath São Jorge using ambient noise interferometry.In this study, we analyzed data continuously recorded between January 2021 and December 2022 by two permanent stations (PMAN and ROSA) operated by the Instituto Português do Mar e da Atmosfera (IPMA) to investigate the presence of subsurface structural changes in response to the seismic crisis. Data were cut into 1-hr length files and filtered between 1 and 3 Hz for autocorrelation, and between 0.1 and 1.0 Hz for cross-correlation. We applied the Phase Auto- and Cross-Correlation (PAC and PCC) method to the filtered data. This method is based on phase coherence and is amplitude-unbiased. PAC and PCC functions were then linearly stacked over three days to achieve a stable noise response. To infer changes in the velocity structure, we analyzed the waveform similarity values for different time lag windows. We compared the waveform similarity results with meteorological data and ground deformation inferred from GPS. Additionally, relative velocity changes have been estimated. The two analyzed stations exhibit different waveform-similarity results. Preliminary interpretation of PMAN results (closer to the island center) show, in the second half of 2022, a very slight recovery of the waveform similarity at shorter lag times (shallower depths) that decreases again in the fall of the same year. Globally, data from this station exhibits a more systematic decorrelation when the crisis was declared, most likely due to perturbations in the seismic structure between < 8 - 10 km and deeper than 15 km. This work is a contribution to RESTLESS (DOI:10.54499/PTDC/CTA-GEF/6674/2020) and GEMMA (DOI:10.54499/PTDC/CTA-GEO/2083/2021). It was also funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).

Ambient noise correlation analyses are largely used to monitor temporal medium changes generally associated with stress field variations and/or fluid movement. Here, we analyze the 2021 eruption of La Palma, the most active island in the Canary archipelago, to study its effects on the structure in a post-eruptive stage. To date, most of the studies, whether in volcanic environments or other geological systems, focus on determining seismic velocity changes that can be associated with precursory signals. In our study, we are interested in localizing the medium changes that permit constraining the depth of the most affected structure and suggesting possible mechanisms capable of inducing such alterations. The auto- and cross-correlation functions were computed using the phase cross-correlation strategy. The correlations were linearly stacked using a 3-day sliding window. The combination of these two approaches proved to render the best results. The analysis of 3 years of data resulted in the detection of occasional decorrelation before the eruption, followed by a well-defined decoherence period after the eruption. In addition, the relationship between the waveform correlation and lag time, using autocorrelations from before and after the eruption, permits identifying phase shifts and waveform distortion, which are sensitive to different parameters and, thus, have great importance in inferring the possible mechanism. Phase shifts occur when there is only velocity change without changing the structure, whereas waveform distortion is caused by a structural (geological) change. We also inferred the depths at which the most significant medium alterations occur. We observed that the decorrelation occurs at lag times corresponding to changes localized in depth. The surface structure appears to not have undergone significant medium changes for depths until approximately 8 km, either before or after the eruption.

The island of São Miguel is among the most seismically active areas of the Azores archipelago. This work focuses on the most significant recent swarm, which occurred on February 2018. We set up an automated procedure to process continuous full seismic waveform data from local stations to generate high-quality earthquake information on the volcano unrest episode. First, we applied an automated detector software, next we located the detected events and then classified the earthquakes based on their waveform similarity, identifying three families of seismic events. We then extended the catalog by template matching. Finally, we computed moment tensors to investigate the source mechanisms of the largest earthquakes. Our results image the ∼2-week swarm evolution. The activity started with a precursory phase with low rate and low magnitude (ML < 2.0) seismicity and the activation of a deeper structure (∼10-15 km). After ∼1 week, a new earthquake family emerged at shallower depths (∼8–12 km) reaching magnitudes up to ML 3.4. Finally, a third slightly shallower family was activated. Moment tensors show mostly normal faulting mechanisms, striking ∼NW-SE, compatible with the orientation of the regional stress field. A surface deformation transient was recorded by geodetic stations, starting with the swarm, and continuing over the following ∼17 months, corresponding to either inflation or extension around the swarm region. The prolonged surface deformation implies a process that was initiated during the swarm and subsequently accommodated mostly aseismically. We interpret the seismicity observed at the early stage of deformation as indicating episodic fluid injection through the crust, related to the local hydrothermal or magmatic systems. We conclude that the Fogo-Congro region continues to be seismo-volcanically active, with both seismic and aseismic deformation observed and requiring close multidisciplinary monitoring. The proposed methology based on the automated analysis of continuous waveform data provides high-quality imaging of the spatio-temporal evolution of seismicity, which can be used elsewhere in the operational monitoring of seismo-volcanic crises to gain insight into the ongoing deformation processes, improve hazard assessment and help in the development of effective mitigation strategies.

The calculation of the tropopause height is crucial to the investigation of fundamental interactions between the troposphere and stratosphere, playing an essential role in areas such as climatology, geodesy, geophysics, ecology, and aeronautics. Since the troposphere and stratosphere have many distinct features, it is possible to define the boundary between them using different variables, such as temperature lapse rate, potential vorticity and chemical concentrations. However, according to the chosen variable, different tropopause definitions are created, each one with some limitations. Using 41 years of European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) data, we examined the variability of the tropopause for the north and south hemispheres and developed two models, both based on blending the potential vorticity and thermal tropopauses. One model (based on a sigmoid function, named STH) depends only on latitude and day of the year, while the other model (based on bilinear interpolation, named BTH) requires an additional look-up table. In order to account for the different behaviors of the tropopauses in the north and south hemispheres, we estimated two sets of model coefficients (one for each hemisphere). When compared against a benchmark of estimated tropopause heights during three years of radiosonde data, we obtained an average RMSE for the differences of 0.88 km for the STH model and 0.67 km for the BTH model. A similar comparison for alternative models available in the literature shows that the new models have superior performance and represent a significant improvement in tropopause height determination.

A variação da velocidade de rotação da Terra é um tema de grande complexidade e que tem merecido a atenção da comunidade científica ao longo do último século. Existe naturalmente uma relação intrínseca entre a velocidade de rotação da Terra e a duração do dia baseada nesse movimento: quanto maior for a velocidade de rotação da Terra, menor será a duração do dia. Com a evolução permanente das técnicas de observação e de novas técnicas de processamento e análise de dados, tem sido possível estabelecer correlações entre diferentes fenómenos geofísicos e algumas das variações na duração do dia.

The neutral atmospheric delay is one of the major error sources in Space Geodesy techniques such as Global Navigation Satellite Systems (GNSS), and its modeling for high accuracy applications can be challenging. Improving the modeling of the atmospheric delays (hydrostatic and non-hydrostatic) also leads to a more accurate and precise precipitable water vapor estimation (PWV), mostly in real-time applications, where models play an important role, since numerical weather prediction models cannot be used for real-time processing or forecasting. This study developed an improved version of the Hourly Global Pressure and Temperature (HGPT) model, the HGPT2. It is based on 20 years of ERA5 reanalysis data at full spatial (0.25° × 0.25°) and temporal resolution (1-h). Apart from surface air temperature, surface pressure, zenith hydrostatic delay, and weighted mean temperature, the updated model also provides information regarding the relative humidity, zenith non-hydrostatic delay, and precipitable water vapor. The HGPT2 is based on the time-segmentation concept and uses the annual, semi-annual, and quarterly periodicities to calculate the relative humidity anywhere on the Earth’s surface. Data from 282 moisture sensors located close to GNSS stations during 1 year (2020) were used to assess the model coefficients. The HGPT2 meteorological parameters were used to process 35 GNSS sites belonging to the International GNSS Service (IGS) using the GAMIT/GLOBK software package. Results show a decreased root-mean-square error (RMSE) and bias values relative to the most used zenith delay models, with a significant impact on the height component. The HGPT2 was developed to be applied in the most diverse areas that can significantly benefit from an ERA5 full-resolution model.

The Global Navigation Satellite System (GNSS) meteorology contribution to the comprehension of the Earth’s atmosphere’s global and regional variations is essential. In GNSS processing, the zenith wet delay is obtained using the difference between the zenith total delay and the zenith hydrostatic delay. The zenith wet delay can also be converted into precipitable water vapor by knowing the atmospheric weighted mean temperature profiles. Improving the accuracy of the zenith hydrostatic delay and the weighted mean temperature, normally obtained using modeled surface meteorological parameters at coarse scales, leads to a more accurate and precise zenith wet delay estimation, and consequently, to a better precipitable water vapor estimation. In this study, we developed an hourly global pressure and temperature (HGPT) model based on the full spatial and temporal resolution of the new ERA5 reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The HGPT model provides information regarding the surface pressure, surface air temperature, zenith hydrostatic delay, and weighted mean temperature. It is based on the time-segmentation concept and uses the annual and semi-annual periodicities for surface pressure, and annual, semi-annual, and quarterly periodicities for surface air temperature. The amplitudes and initial phase variations are estimated as a periodic function. The weighted mean temperature is determined using a 20-year time series of monthly data to understand its seasonality and geographic variability. We also introduced a linear trend to account for a global climate change scenario. Data from the year 2018 acquired from 510 radiosonde stations downloaded from the National Oceanic and Atmospheric Administration (NOAA) Integrated Global Radiosonde Archive were used to assess the model coefficients. Results show that the GNSS meteorology, hydrological models, Interferometric Synthetic Aperture Radar (InSAR) meteorology, climate studies, and other topics can significantly benefit from an ERA5 full-resolution model.

In typical subduction systems, plate convergence is subperpendicular to the trench. The Gibraltar Arc System is exceptional, with its narrow subduction arc oriented N‐S and laterally “squeezed” by the NNW‐SSE tectonic convergence between Nubia and Iberia. The extent to which the slab is still coupled to the surface and how it interacts actively with the surrounding mantle is a matter of ongoing debate. Here, we analyze new densely spaced GPS data, together with crustal and mantle observations, to better understand the slab kinematics, plate dynamics, and mantle flow. In light of previous and current research, we find that subduction below the Gibraltar Arc is currently in the middle of a disruption process, with parts of it already detached and others yet coupled to the surface. In particular, the slab seems to be detached to the north of the Gibraltar Strait, with a small portion still attached to the surface or in the process of detaching below the western Betics. South of Gibraltar, the slab is still coupled to the overriding plate, although the subduction seems to be very slow or stopped. Flow of mantle material around the detached portions of the slab causes most of the surface uplift and a positive residual topography anomaly. Our findings show that the interplay between slab dynamics, mantle flow, and plate convergence explains much of the observed residual topography, surface motion, seismicity, and mantle structure.

In this study, we estimate vertical land motion for 35 stations primarily located along the coastline of Portugal and Spain, using GPS time series with at least eight years of observations. Based on this set of GPS stations, our results show that vertical land motion along the Iberian coastline is characterized, in general, by a low to moderate subsidence, ranging from -2.2 mm yr(-1) to 0.4 mm yr(-1), partially explained by the glacial isostatic adjustment geophysical signal. The estimates of vertical land motion are subsequently applied in the analysis of tide gauge records and compared with geocentric estimates of sea level change. Geocentric sea level for the Iberian Atlantic coast determined from satellite altimetry for the last three decades has a mean of 2.5 +/- 0.6 mm yr(-1), with a significant range, as seen for a subset of grid points located in the vicinity of tide gauge stations, which present trends varying from 1.5 mm yr(-1) to 3.2 mm yr(-1). Relative sea level determined from tide gauges for this region shows a high degree of spatial variability, that can be partially explained not only by the difference in length and quality of the time series, but also for possible undocumented datum shifts, turning some trends unreliable. In general, tide gauges corrected for vertical land motion produce smaller trends than satellite altimetry. Tide gauge trends for the last three decades not corrected for vertical land motion range from 0.3 mm yr(-1) to 5.0 mm yr(-1) with a mean of 2.6 +/- 1.4 mm yr(-1), similar to that obtained from satellite altimetry. When corrected for vertical land motion, we observe a reduction of the mean to E1.9 +/- 1.4 mm yr(-1). Actions to improve our knowledge of vertical land motion using space geodesy, such as establishing stations in co-location with tide gauges, will contribute to better evaluate sea level change and its impacts on coastal regions.

The development of the next-generation of very long baseline interferometry (VLBI) systems, known as VLBI Global Observing System (VGOS), is well underway. Co-location of VGOS stations with instruments from the other space geodetic techniques (i.e., GPS, SLR, and DORIS) is essential for synergistic, robust global reference frame realization. The local ties between reference points of geodetic instruments at co-location sites effectively connect the various techniques together. Precise ties are required for a multi-technique reference frame that is suitable for high-accuracy geophysical applications such as global sea-level change. Unfortunately, the uncertainties of local tie vectors remain above 3 mm. We are investigating approaches that could improve the accuracy of relative positions estimates of the co-location sites. These approaches use external constraints based on local atmospheric structure at core geodetic sites where multi-techniques are co-located. The challenge is to add information to the geodetic solution based on our knowledge of atmospheric structure without biasing the estimates of the intersite vectors. Here, we present preliminary results from tests wherein atmospheric structure is used to enhance the strength of geodetic solutions and combinations. In this study, we use data from existing small-scale GPS networks as a proxy for co-location sites instrumented with next-generation geodetic systems. 1. MIT Haystack Observatory, Westford, MA 01851 2. Lamont-Doherty Earth Observatory, Palisades, NY 01964 3. Institut National de l’Information Géographique et Forestière (IGN), Paris, France 4. IDL, Faculdade de Ciências da Universidade de Lisboa, Lisbon, Portugal

Em 1929, na sua obra “A Origem dos Continentes e Oceanos” Alfred Wegener deixava um desafio à Geodesia, que permitiria provar a sua teoria da deriva continental: “Isto deve ser deixado para os geodesistas. Não tenho dúvidas de que num futuro não muito distante teremos sucesso em fazer uma medição precisa da deriva da América do Norte em relação à Europa.”

S U M M A R Y This study addresses long-term sea level variability in Macaronesia from a holistic perspective using all available instrumental records in the region, including a dense network of GPS continuous stations, tide gauges and satellite observations. A detailed assessment of vertical movement from GPS time series underlines the influence of the complex volcano-tectonic setting of the Macaronesian islands in local uplift/subsidence. Relative sea level for the region is spatially highly variable, ranging from −1.1 to 5.1 mm yr−1. Absolute sea level from satellite altimetry exhibits consistent trends in the Macaronesia, with a mean value of 3.0 ± 0.5 mm yr−1. Typically, sea level trends from tide gauge records corrected for vertical movement using the estimates from GPS time series are lower than uncorrected estimates. The agreement between satellite altimetry and tide gauge trends corrected for vertical land varies substantially from island to island. Trends derived from the combination of GPS and tide gauge observations differ by less than 1 mm yr−1 with respect to absolute sea level trends from satellite altimetry for 56 per cent of the stations, despite the heterogeneity in length of both GPS and tide gauge series, and the influence of volcanic-tectonic processes affecting the position of some GPS stations.

A GPS-based crustal velocity field for the SW Portuguese territory (Algarve region, SW Iberia) was estimated from the analysis of data from a network of campaign-style GPS stations set up in the region since 1998, complemented with permanent stations, covering an overall period of 16.5 years. The GPS monitoring sites were chosen attending to the display of the regional active faults, in an attempt to detect and monitor any related crustal straining. The residual horizontal velocities relative to Eurasia unveil a relatively consistent pattern towards WNW, with magnitudes that noticeably increase from NNE to SSW. Although the obtained velocity field does not evidence a sharp velocity gradient it suggests the presence of a NW-SE trending crustal shear zone separating two domains, which may be slowly accumulating a slightly transtensional right-lateral shear strain. Based on the WNW velocity differential between the northeastern block and the southwestern block, a shear strain rate accumulation across the shear zone is estimated. This ongoing crustal deformation is taken as evidence that a nearby major active structure, the São Marcos − Quarteira fault, may be presently accumulating strain, therefore being potentially loaded for seismic rupture and the generation of a large magnitude earthquake. Further inferences are made concerning the interseismic dynamic loading of other major onshore and offshore active structures located to the west.

The SERVIR Continuous Operation Reference Stations (CORS) network was implemented in 2006 to facilitate land surveying with Global Navigation Satellite Systems (GNSS) positioning techniques. Nowadays, the network covers all Portuguese mainland. The SERVIR data is provided to many users, such as surveyors, universities (for education and research purposes) and companies that deal with geographic information. By middle 2012, there was a significant change in the network accessing paradigm, the most important of all being the increase in the responsibility of managing the network to guarantee a permanent availability and the highest quality of the geospatial data. In addition, the software that is used to manage the network and to compute the differential corrections was replaced by a new software package. These facts were decisive to perform the quality control of the SERVIR network and evaluate positional accuracy. In order to perform such quality control, a significant number of geodetic monuments spread throughout the country were chosen. Some of these monuments are located in the worst location regarding the network geometry in order to evaluate the accuracy of positions for the worst case scenarios. Data collection was carried out using different GNSS positioning modes and were compared against the benchmark positions that were determined using data acquired in static mode in 3-hour sessions. We conclude the geospatial data calculated and provided to the users community by the network is, within the surveying purposes, accurate, precise and fits the needs of those users.