The recent observation of neighboring earthquakes with anti-correlated waveforms, referred to as anti-repeating earthquakes, or anti-repeaters, suggests that almost opposite fault slip may occur consecutively along the same or nearby structures in Earth. These earthquakes have so far been identified in a few areas only, although involving different seismogenic contexts, like tectonic, volcanic, and induced seismicity, so they could be a more common phenomenon than previously thought. Here, we analyze large and moderate-sized earthquakes worldwide in a systematic manner, scanning the Global Centroid Moment Tensor catalog over 48 yr in search of pairs of earthquakes with neighboring centroid locations and nearly opposite moment tensors. The results are somewhat surprising: thousands of nearby earthquake pairs with opposite source orientations can be found, clustering at hundreds of locations on Earth, occurring at different depths, in different tectonic settings, as well as in volcanic environments. We validate the detections by their waveform anti-correlation and report a global catalog of opposite faulting, as a first atlas for this new type of seismological observation.

Abstract Despite decades of study, the detailed geometry of the Connector fault at the Alaska–Canada border remains poorly constrained. We investigate the rupture complexity and directivity of the 6 December 2025, Mw 7.0 Hubbard Glacier earthquake using hypocenter relocations, moment tensor inversions of the mainshock and aftershocks, and finite‐fault modeling. We model seismological data using bootstrap and Bayesian‐based probabilistic inversion. Observations indicate that the mainshock rupture was complex, nucleated with a right‐lateral strike‐slip mechanism and propagating northwestward, ultimately terminating in a restraining bend. Source inversion of the mainshock reveals shallow right‐lateral oblique‐slip on a fault striking 303° and dipping 63° NE. Relocated aftershocks delineate an ∼60 km‐long seismic zone characterized by strike‐slip mechanisms along the ruptured area and reverse/thrust at the left stepover of the fault. Our results provide a detailed mechanism for a buried Connector structure and highlight the role of fault bends in controlling rupture evolution.

The East Eifel region is a Cenozoic-Quaternary intraplate volcanic field, well known as the type locality for maar volcanism. Its most recent major eruption occurred at the Laacher See volcano about 13,000 years ago and is regarded as the largest late Quaternary volcanic eruption in central Europe. Despite its dormant state, ongoing regional mantle-derived CO2 degassing and persistent microseismicity, particularly within the Ochtendung Seismic Zone, indicate active subsurface processes, likely related to fluid migration. Using the Eifel large-N seismic network, we recorded over 500 local microearthquakes from September 2022 to August 2023. Earthquakes within the Ochtendung Seismic Zone exhibit distinct secondary S-wave phases, indicating strong impedance contrasts associated with fluid accumulations within the crust. By extracting travel-time differences between direct and reflected S waves and applying an eikonal-based reverse-time-migration-like approach, we image localized fluid-rich zones at depths of 12-14 km beneath the microearthquakes, suggesting the accumulation of magmatic fluids aligned near the seismic zone. Microearthquakes within the Ochtendung Seismic Zone occur as burst-like sequences reflecting successive ruptures. We propose that the sustained microseismicity results from slip reactivation induced by transient fluid flux, representing an example of natural hydraulic fracturing.

Large dikes are the main mechanism of crustal extension in volcanic areas, but the processes in the underlying magma system that supply the required volumes remain unclear. We show that 1.4 cubic kilometers of magma propagated under the Ethiopian rift in December 2024 and continued for ~3 months. Geodesy and seismicity reveal that the dike was fed from a network of magma reservoirs between 6 to 12 kilometers in depth with pathways rapidly forming between them. We calculate pressure changes in the reservoirs and show that underpressure developed in the deeper portion, creating the conditions to drain large magma volumes. We find that tectonic stress and availability of magma alone are not enough to drive intrusion of massive dikes. These events will start only after magma connectivity and deep underpressure develop. Similar conditions may be important for the transfer of large magma volumes from the mantle and the formation of large igneous provinces.

In February-March 2025 a seismic sequence occurred in the western sector of the Aeolian Archipelago (Southern Tyrrhenian Sea, Italy), a seismotectonic complex region located along the Africa-Eurasia plate boundary and mainly controlled by their NW-trending convergence. The seismicity, located similar to 20 km south of Alicudi Island and similar to 40 km north of the coast of Sicily, started on February 7 with an earthquake of magnitude Mw 4.7 that was followed in the next month by 42 events with local magnitudes between 1.2 and 3.4. Notwithstanding its moderate energy, this recent seismicity offers a unique opportunity to investigate seismogenic processes in a region for which a seismic potential of similar to M7 or even more has been suggested and a relevant data paucity mainly related to its offshore location was widely recognized. We tackle the limitations of not-optimal network configuration, by designing an ad-hoc approach, which integrates different advanced techniques. Specifically, we combine Bayesian methodology for accurate absolute hypocentre locations, machine learning techniques for detection of weaker events, distance geometry solvers for relative locations and a probabilistic inversion tool for source mechanism estimation. Our analysis led us to strongly enrich the data set of detected earthquakes, and to define the causative source of the 2025 sequence as a NE-SW trending N-dipping thrust faulting structure. The proposed source agrees with the regional seismogenic stress field and with the structural architecture of the southern Tyrrhenian portion of the Africa-Eurasia plate margin by also adding new constraints in a sector where no known fault segments were previously reported. This study provides new insights on seismogenic processes in the investigated area, while proving the effectiveness of the employed combined approach for characterizing seismogenic sources in poor network configurations.

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.

The understanding of the magma system beneath intracontinental volcanic fields depends critically on our ability to resolve small-sized anomalies distributed over large areas of hundreds of kilometres. Magmatic reservoirs co-exist at different depths in the upper mantle and crust and may consist of extensive zones of crystal mush, swarms of sills and dikes of different ages and states, pore space saturated by volatiles or melt, or larger-volume, differentiated magma. Passive seismological experiments with a large number of sensors deployed with small interstation spacings, combining different types of sensors and fibre-optic sensor technology, have great promise for addressing the resolution to capture the distributed magmatic system. We report on a one-year, large-N experiment in the Quaternary volcanic fields of the Eifel, Germany, where more than 494 seismic stations were deployed and combined with a 64-km-long DAS cable and permanent stations. A cloud-based, open-source GIS system was implemented to address logistical challenges and ensure data quality combined with seismological analysis and visualisation tools. We present initial results to test the potential of such an extensive waveform database and automated processing for locating small earthquakes and imaging crustal and upper mantle anomalies using techniques such as ambient noise cross-correlation, receiver functions, and SKS splitting.

In the realm of seismic and microseismic event detection and localization, our research marks a significant step forward by integrating machine learning with advanced waveform-stacking techniques for detecting, locating and characterising seismic events. This integration is crucial for unravelling the complex spatio-temporal patterns of seismicity sequences. Our study addresses the challenges posed by noise-dominant microseismic events, which are typically overlooked by conventional detection methods. Building upon the foundational work on migration and stacking, we have developed an automated, data-driven method utilising a neural network trained in seismic phase arrival identification. This approach, underpinned by stacking and migration techniques, is enhanced by the incorporation of a spatial octree to precisely and efficiently localise seismic sources. These enhancement gives insights into complex seismic sequences, such as volcanic swarms and regional tectonic sequences. The software framework facilitates extensive feature extraction, such as local and moment magnitudes, enabling the study of seismic events across various scales and tectonic settings. This is exemplified in our validation studies using data from the Eifel region, Germany, and the Reykjanes Peninsula, Iceland. These regions, known for their diverse seismic activities including tectonic earthquakes and fluid-induced swarm activity, provide a rich dataset for testing our method's efficacy in different geological contexts. Our research contributes to the session's overarching goal of understanding the physical processes behind complex seismic sequences. By enhancing detection and localization capabilities, we aim to offer new perspectives and tools for the geophysical community to investigate the triggering mechanisms of these sequences with unprecedented resolution.

From 2021 to 2024, unusually strong seismic activity, including multiple sequences with five 3.8 <= ML <= 4.6 shocks, struck the Vienna basin and Mur-M & uuml;rz fault systems in Eastern Austria. Three earthquakes (ML 4.6, 4.5, and 3.8) occurred between March and May 2021. These earthquakes were followed by an increase in seismic activity in March 2023, with an ML 4.3 earthquake to the southwest of the 2021 events. In February 2024, an ML 4.5 earthquake occurred 10 km southwest of the ML 4.3 in 2023. We investigate activated faults relying on nonlinear relocations of the seismic sequences and probabilistic moment tensor inversion. The seismic sequences' temporal evolution indicates a migration of seismic activity and a decrease of focal depths from northeast to southwest, suggesting a potential reactivation of the major fault systems in the region. The joint interpretation of the results shows distinct clusters along fault segments and focal mechanisms that match the region's complex tectonic activity.

Repeating earthquakes have overlapping rupture patches, similar focal mechanism and magnitudes. They are often detected based on highly similar waveforms using template matching techniques, which help to reconstruct complex sequences and swarms. Here, we investigate earthquakes with highly anti-correlated waveforms. Such poorly known observation implies the occurrence of reversed seismogenic processes at close hypocentral locations. We introduce the terms true and quasi anti-repeating earthquakes to denote cases affecting the same rupture patch or neighboring patches, respectively. We report about a number of observations of anti-repeating earthquakes in different environments, such as volcano, induced and intermediate-depth seismicity, and then review conceptual models to explain them. Some of these observations occurred during seismicity unrests, in the form of seismic sequences and swarms. Both true and quasi anti-repeating earthquakes are indicators for stress perturbation transients or local stress heterogeneities, often controlled by fluid migration processes. Therefore, their analysis may help the identification and tracking of fluids in the subsurface.

The Campi Flegrei caldera has been experiencing volcanic unrest since 2005, rising concern in the population and in local and national authorities. On May 20, 2024, the largest local earthquake ever instrumentally recorded up to that time produced substantial damage, forcing the evacuation of tens of buildings west of the epicenter. At Campi Flegrei, M > 4 earthquakes are rare and their analysis is crucial to understand the unrest dynamics and the relation between rupture and ground shaking pattern, which is essential to mitigate the damage of future earthquakes. We analyse seismic waveforms at local to regional distances to reconstruct the source geometry and kinematics. We estimate millimetric to submillimetric coseismic surface subsidence - below the sensitivity of any standard geodetic technique - which, compared to the general uplift, highlights the crucial role of deep pressurized fluids in earthquakes' generation. Our results also indicate that rupture directivity and local amplification determined the damage distribution.

We introduce a data-driven method and software for detecting and locating earthquakes in large seismic datasets. By combining seismic phase arrival annotations, delivered by neural network phase pickers, and waveform stacking with an adaptive octree search, we can automatically detect and locate seismic events even in noise-dominant seismic data. The resolution of the search volume is iteratively refined toward the seismic source location; this strategy facilitates an efficient, fast, and accurate search. We present a user-friendly and high-performance open-source software framework based on established frameworks, featuring event detection in layered 1D and complex 3D velocity models and event feature extraction capabilities, such as moment and local magnitude calculation from peak ground motions. We incorporated station-specific corrections and source-specific station terms into the search to enhance the location accuracy. We demonstrate and validate our approach by extracting extensive earthquake catalogs from large seismic datasets in different regions and geological settings: (1) Reykjanes Peninsula, Iceland; (2) Eifel volcanic region, Germany; and (3) Utah FORGE, USA. We capture seismic events from tectonic activity, volcanic swarms, and induced microseismic activity with magnitudes ranging from -1 to 5. Such precise and complete earthquake catalogs contribute to the interpretation and understanding of otherwise hidden subsurface processes.

In January-February 2023 a seismic sequence took place in the Central Mediterranean Sea, ~90 km south of the island of Malta, and ~200 km from the coast of Sicily. The seismicity started in Mid January, in a region that experienced only sparse seismicity in the past. In the following days several M4+ events occurred. The largest earthquake, with moment magnitude Mw 5.3, took place a couple of weeks after the unrest onset, on January 30. The seismicity continued for several weeks, before fading down. Later seismicity was observed and still going on. This recent, unusual seismicity offers a unique opportunity to investigate seismogenic processes in this region with unprecedented detail. However, analyzing seismic sequences in offshore environments presents significant challenges due to the absence of optimal seismic monitoring conditions. These limitations compromise the effectiveness of conventional data analysis techniques, hindering the characterization of offshore seismic sequences. We tackled these limitations through the adoption of advanced, waveform-based seismic data analysis techniques that allow to investigate offshore seismic sequences, with the aim to provide insights into their origin. We combine full-waveform based detection and template matching methods to enhance the detection of events, advanced location techniques based on Distance Geometry Solvers (DGS), and probabilistic waveform-based methods for seismic source characterization. We combine the seismic source analysis for the 8 largest earthquakes in the sequence, with magnitude exceeding ML 4.5, with waveform-based and statistical analysis of the seismicity. About 500 events are identified. Their locations map a narrow lineament extending ~NW-SE. Full moment tensors for the largest events identify normal faulting mechanisms with a similar orientation, and shallow centroids of ~5 km depth. This result, combined with a waveform similarity analysis, suggests a predominant mechanism for the entire sequence. Using different seismicity indicators we classify the 2023 sequence as a seismic swarm. Indeed, the largest events in the sequence occur weeks after the unrest onset. Compared to previous seismicity, the sequence was outstanding in terms of maximum magnitude, seismicity rate and moment rate. While normal faulting earthquakes are not unusual in the Central Mediterranean, they differ from the few focal mechanisms previously proposed for the swarm focal region, which has important implications, considering that normal faulting earthquakes at shallow depth pose a tsunami hazard in the region.

The installation of the temporary, large-N Swath-D seismic network in the years 2017-2019 (Heit et al., 2021) provided the basis for the recent compilation of a high-resolution, consistently processed seismicity catalogue for the eastern and southern Alps (Hofman et al., 2023). The catalogue contains more than 6,000 earthquakes with magnitudes down to −1.7 ML. In the present study, we analyse in more detail several of the newly detected microseismic clusters in the study area, which includes the most active parts of the Alps as well as particularly quiet regions with very little previously reported seismicity. We combine inter-event waveform similarity clustering, catalogue statistics and rupture mechanisms to characterise the clustered seismicity swarms and mainshock-aftershock sequences. We apply a relative location technique based on differential Ts-Tp arrival times to better resolve the seismogenenic structures. For subgroups of microseismic events with magnitudes Mw 1.2-3.0, we obtain moment tensor solutions using the flexible probabilistic inversion framework Grond, which allows to combine different fitting targets and frequency bands, while providing meaningful estimates of uncertainties (Heimann et al., 2018, Petersen et al., 2021). This adds to resolve subtle, but systematic variations of the inner-cluster seismicity.Thanks to the outstanding network density, we can report a variability of seismic sequences and microseismic event mechanisms across the study area and interpret them with in terms of long-term tectonic and intermediate triggering processes.

On September 16, 2023, a landslide collapsed in the Dickson Fjord, a remote area of East Greenland. The collapse triggered a tsunami that hit Ella Island, which lies to the east of the fjord. The mass movement is identified in high resolution Planet Labs Dove mini-satellite imagery, and the generated seismic signals were recorded at both regional and teleseismic distances. The seismic records reveal a first strong transient signal (0.02-0.06 Hz) around 12:35:00 UTC, which we attribute to the landslide, followed by a long-lasting (~50 hours) monochromatic (~0.01 Hz) signal at teleseismic distances. We perform full waveform inversions using moment tensor and single force models to characterize the source of both signals. At regional distances, the first transient signal is well reproduced by single and double source models and is consistent with the landslide process. The long-lasting oscillation is modeled by a damped dipole oscillator, which is in agreement with the Love and Rayleigh waves radiation patterns observed at different azimuths. Using multiple different data and source models we are able to characterize the complex source process.

The equatorial Atlantic transform faults are among the largest and most complex in the world’s oceans. Among them, the St. Paul Transform System (SPTS) is a large multi-faulted system formed by four slow-slipping transform faults (Transforms A, B, C, and D), accumulating ∼630 km of axial offset (Maia et al., 2016). Transform A is the northernmost fault which contains the Atoba Ridge Zone (ARZ), a large transpressive ridge formed at a large stepover (Maia et al., 2016). St. Peter and St. Paul (SPSP) islets sit at the ARZ summit, where is installed a single broadband seismograph (ASPSP) recording local seismicity since 2011 (de Melo and do Nascimento., 2018). Here, we produce a new catalog of the local seismicity recorded around the ARZ between 2011 and 2016. For the epicenter location, we process an initial picking of the P and S waves referent to 359 earthquakes identified manually using SEISAN package applying a 2-12 Hz band-pass filter. Next, we follow a single station approach to locate the epicenters. We estimate the source-receiver distance based on the differential S-P time and the back azimuth from the polarization of P wave recordings, whenever this shows a high rectilinearity coefficient (Montalbetti and Kanasewich., 1970; Cesca et al., 2022). A total of 245 earthquakes were cataloged again using the new improved location process. 54 earthquakes were also identified also by EquatorialAtlanic hydroacoustic catalog (Parnell-Turner et al., 2022) and 12 by the International Seismological Centre. Our results reveal that a large part (174 earthquakes) of the local seismicity is clustered on the west flank of the ARZ, located from 2.18 to 22.58 km southwest of the SPSP islets. Rocks sampled along the ARZ are peridotite mylonites exhumed during the transpressional push-up tectonism of the ARZ. Other minor events are located on the east flank of the ARZ where sample deformation shows strong control of seawater fluid percolation (Bickert et al., 2023), enabling weakening of the rheology and possibly contributing to maintain an aseismic behavior on the faults. Bickert, M., Kaczmarek, M. A., Brunelli, D., Maia, M., Campos, T. F., & Sichel, S. E. (2023). Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults. Nature Communications, 14(1), 4087.Cesca, S., Sugan, M., Rudzinski, Ł., Vajedian, S., Niemz, P., Plank, S., ... & Dahm, T. (2022). Massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica. Communications Earth & Environment, 3(1), 89.de Melo, G. W., & Do Nascimento, A. F. (2018). Earthquake magnitude relationships for the Saint Peter and Saint Paul archipelago, equatorial atlantic. Pure and Applied Geophysics, 175, 741-756.Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., ... & Oliveira, P. (2016). Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience, 9(8), 619-623.Montalbetti, J. F., & Kanasewich, E. R. (1970). Enhancement of teleseismic body phases with a polarization filter. Geophysical Journal International, 21(2), 119-129.Parnell‐Turner, R., Smith, D. K., & Dziak, R. P. (2022). Hydroacoustic monitoring of seafloor spreading and transform faulting in the equatorial Atlantic Ocean. Journal of Geophysical Research: Solid Earth, 127(7), e2022JB024008.

Despite the occurrence of large, past earthquakes in the Central Betic Range (Southern Spain), seismicity recorded with digital seismographs is limited to small magnitude events. Here we are interested in the three strongest events (MW 4.5 to 5.0), of which source models are unclear for different reasons: The June 20th 1979 and June 24th 1984 earthquakes are still characterized by a lack of regional recordings, while the August 12th 2021 earthquake occurred during a teleseismic M8 event. We use beamforming at distant seismic arrays and waveform modelling of depth phases to estimate source parameters for seven earthquakes altogether. The technique is successful at reproducing P-waveforms and at estimating the depth of four recent (1997-2021) earthquakes with M-W > 4. In addition, it is also used along with an inverse scheme that yields source mechanisms similar to regional moment tensor solutions. Inversion suggests normal faulting at depths of 7 km and 9 km for the 1984 and 2021 events, which is consistent with our understanding of regional seismotectonics. Beamforming has been able to extract the 2021 waveforms from the M8 coda wavefield, and could be a suitable approach also for other cases of earthquake coincidence. The most noteworthy result is a strike-slip mechanism at 60 km depth for the 1979 earthquake, which is a singular subcrustal event in this area and might be related to tearing at the edge of the Gibraltar slab.

Volcanic crises, driven by renewed magma inflow and migration, result in surface deformation and seismicity that can provide unique insights into the structure of volcanic systems and magmatic processes. Although the highly explosive volcanoes of Santorini and Kolumbo1,2 in the Greek Aegean Sea are just 7 km apart, their potentially coupled deep magmatic feeding systems are only poorly understood3,4. The 2025 volcano-tectonic crisis of Santorini simultaneously affected both volcanic centres, providing insights into a complex, multistorage feeder system. Here we integrate onshore and marine seismological data with geodetic measurements to reconstruct magma migration before and during the crisis. Gradual inflation in the Santorini caldera, beginning in mid-2024, preceded the January 2025 intrusion of a magma-filled dike sourced from a mid-crustal reservoir beneath Kolumbo, indicating a link between the two volcanoes. Joint inversion of ground and satellite-based deformation data indicates that approximately 0.31 km3 of magma intruded as an approximately 13-km-long dike, reactivating principal regional faults and arresting 3-5 km below the seafloor. The 2024-2025 resurgence of magmatic activity beneath both volcanic centres and their apparent coupling provides insights into the dynamic interplay of magma storage, transport and reservoir failure beneath neighbouring volcanoes.

The southern and eastern Alps are a fascinating target region for a seismological study because they include the deformation front of Adria-Europe convergence with historically significant events (e.g., M 6.0 Friuli 1976) as well as areas where seismicity seems more or less absent despite geologically mapped large fault systems and past deformation fronts. The large-N installations of the Swath-D (2017-2019) and AlpArray (2016-20219) seismic networks provide unmatched opportunities to study the microseismicity in the Eastern Alps in unprecedented detail. For the first time in the study area, the homogeneous station spacing allows a consistent analysis of seismicity across the entire area. These detailed seismological analyses provide the opportunity to characterize deformation in the upper 15 km of the crust.We show how a combined workflow, including clustering, relocations, and MT inversions, sheds light on the seismicity and the ongoing active deformation. We observe strong zonations of seismic activity rates, sequence characteristics, and rupture mechanisms, coinciding with dominant tectonic deformation styles and subsurface properties such as Qp attenuation. We identify and characterize multiple likely unknown fault systems that experience local stresses deviating from the regionally dominant Adria-Europe convergence. Our findings agree well with the occurrence of large historical earthquakes while simultaneously shedding light on much smaller seismogenic features.