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Understanding underground mass redistribution in geothermal fields is fundamental to assess the harnessing in geothermal reservoir. Here we apply the hybrid gravimetry method at Theistareykir geothermal field (Northern Iceland). We collected absolute and discrete microgravity measurements yearly within the geothermal field at fixed locations jointly with continuously recorded gravity time series with two superconducting gravimeters (SGs). Data acquisition started in 2017 at the onset of the anthropogenic perturbation. We present and interpret the discrete and continuous gravity datasets from 2017 until 2024 in an attempt to characterize fluid redistribution within the subsurface. The hybrid gravimetry dataset reveal a gravity decrease nearby the extraction area with rates of about-20 mu Gal per year for the 2017-2022 period, that reduces to few mu Gal per year in the last two years (2023-2024). An increase in gravity is observed towards the injection area (around 5 to 10 mu Gal per year). Time-lapse gravity maps reveal a localized gravity decrease (with a maximum of-60 mu Gal), not collocated with the zone of largest extraction and furthermore an additional trendline of gravity increase (+15 mu Gal) is observed towards the North. The first suggests a possible lower permeability subsurface zone within the geothermal field, the latter, is in accordance with the direction of the fault system that crosses Theistareykir field, suggesting potential underground fluid pathways. From 2023, although extraction and injection rates did not change, we evidence a change in the gravity trends outside the western part of the geothermal field possibly associated with the 2023 magmatic intrusion.
Swarm-like seismicity manifests as earthquake clusters driven by aseismic transients. The investigation of the mechanisms behind their occurrence is generally based on automatic detection and characterization of swarm-like clusters. In this study, we investigate four different (de-)clustering algorithms to identify earthquake clusters, and then classify these clusters as either swarm-like or mainshock–aftershocks sequences. The classification uses the clusters’ distribution of seismic moment over time, quantified by standardized central moments. Synthetic catalogs from an epidemic-type aftershock sequence model are used to establish confidence bounds for swarm classification. The workflow is applied to the swarm-dominated regions Húsavík–Flatey fault, Iceland, and Pollino range, Italy. Our workflow effectively detects/classifies earthquake clusters, but their inherent variability in duration and seismic moment release can bias automated swarm/mainshock–aftershocks labeling. The results provide benchmarks for future swarm-like seismicity analyses, highlighting the importance of a posteriori careful inspection of clusters to understand the underlying physical mechanisms.
Very high-resolution SAR sensors, such as ICEYE, Umbra, Capella Space, COSMO-SkyMed, and TerraSAR-X, are enabling new algorithms and applications. Unlike an instantaneous snapshot, a SAR image is formed over acquisitions that can last up to tens of seconds along the satellite orbit, so targets are observed over a non-negligible time interval and from slightly different viewing angles. When this is exploited with advanced processing, a single SAR image can provide multiple types of information about the scene, including target motion (e.g., boat or vehicle speed), building height and 3D structure, and, most importantly here, the vibration frequencies and amplitudes of buildings and infrastructure. These capabilities open new perspectives for wide-area, relatively low-cost structural monitoring and for supporting disaster prevention activities. This paper explains the basic principles and recent developments in satellite SAR Micro-Doppler technology for measuring characteristic structural vibrations. It analyzes the range of measurable vibrations and the accuracy achievable with current satellite missions, and it presents initial tests and promising validation results. Finally, potential applications are discussed, also in the context of the EU Directive 2022/2557 on Critical Entities Resilience (CER).
Determining the maximum possible magnitude of fluid-induced earthquakes requires to understand rupture arrest within or outside a fluid-pressurized patch. Recent studies have highlighted the importance of incorporating rupture physics into the study of injection-induced earthquakes. We perform 3D dynamic simulations of spontaneous ruptures propagating across a pressurized fault, stimulated by fluid injection within the nucleation zone. Our simulations unveil two end-member models describing a fluid-induced micro-earthquake: a self-arresting rupture that decelerates spontaneously and a run-away rupture that terminates abruptly at the fault edge. We compute synthetic waveforms radiated from both models and invert them using a probabilistic spectral inversion approach to identify characteristics that distinguish between the two rupture types. We find that self-arresting ruptures radiate less high-frequency waves (with gamma> 3) and lack the typical P/S corner frequency shift. In contrast, run-away ruptures conform to the omega(2) model (gamma similar to 2, f (p)(c)/f (S)(c) similar to 1.3). We interpret these differences as primarily arising from the rupture arrest mechanism, smooth arrest results in gradual variations in the moment-rate function, whereas abrupt arrest at a barrier causes a sharp changes in the moment-rate function. This abrupt arrest generates high-frequency radiation and a back-propagating stopping phase, playing a critical role in controlling the rupture duration and the radiated seismic waves. Our results demonstrate that spectral features such as high-frequency decay and P/S corner frequency shift may provide observational diagnostics to distinguish rupture arrest mechanisms, even in the absence of direct evidence from rupture kinematics.
The Monteluco di Roio mountain (L’Aquila, Italy) presents a compelling case study of unexpected seismic amplification. Based on its geophysical and geotechnical features, characterized by stratigraphy primarily composed of carbonate bedrock, significant seismic amplification was unexpected. Nevertheless, during the April 6, 2009, earthquake, a reinforced concrete building within the Monteluco di Roio university complex sustained severe structural damage. To isolate the effects of topography, shallow stratigraphy, and internal bedrock geometry on seismic response, we conducted several numerical simulations across various scenarios. The results indicate that topographic amplification is negligible and stratigraphic amplification is limited to high frequencies. In contrast, the internal geometry of the carbonate substrate, characterized by tilted stratification intersected by sub-vertical faults, is responsible for significant amplification across a broad frequency range. Such geological configurations are common in Apennine chain or other orogenic belts worldwide. Given that broad-range frequency amplification can potentially impact a high number of buildings during a seismic event and considering that current seismic guidelines often lack requirements for detailed bedrock characterization, this study highlights critical warnings for seismic safety assessments and territorial planning.
