Seismica

Seismic characterization of the subsurface and anthropogenic noise at the LUNA Moon analog facility

2026-02-03T17:28:06-05:00

The increased interest in crewed and robotic lunar exploration results in a need for high-quality testbeds for instruments, experiments-including seismological ones-and procedures, and for operations training. The LUNA analog facility is a new large-scale testbed on the DLR campus in Cologne, Germany, i.e. located in an urban environment that includes traffic, heavy machinery, and a neighboring international airport. We perform the first characterization of the site and its ambient wavefield, with a focus on anthropogenic signals, as relevant background information for future users of LUNA. Combining active and passive seismic measurements, we derive velocity models for the site down to the bedrock at 152 ± 13 m depth. We provide a preliminary characterization of the ambient noise on campus and discuss and interpret examples of common anthropogenic signals in detail, demonstrating their use e.g. for traffic monitoring with a single station, or as a repeating seismic source.
This study showcases how relevant information for future seismological users of a planetary analog facility can be derived with comparatively limited means, the potential of single-station seismology for monitoring airborne and ground traffic, and hints at possible uses of the future permanent seismometer in LUNA.

Activating a Natural Fault Zone in the Swiss Alps

2026-02-12T06:17:10-05:00

One major hurdle for understanding earthquake mechanics are observational limitations. Important phenomena like strain localisation, fault dilation, and fault healing are readily studied in rock mechanical laboratory experiments and with numerical models. At the scale of natural earthquakes, however, these phenomena are often unresolvable, even by state-of-the-art observatories. To overcome this limitation, we are currently building the Earthquake Physics Testbed at the Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab), an experimental testbed where we can activate an extensively instrumented natural fault zone via hydraulic stimulation. The goal of the Fault Activation and Earthquake Rupture (FEAR) project is to induce earthquakes of up to M_w~1.0 on this exceptionally well characterised and instrumented fault zone. Here we summarize the main scientific goals and current FEAR project status, and present first results from conducted experiments. We discuss how this large-scale experimental approach may allow us to tackle both fundamental science as well as practical questions on earthquake physics, induced seismicity and seismic hazard.

Spatiotemporal Dynamics of Clusters in the Bridge Zone Linking L'Aquila 2009 and Central Italy 2016 Seismic Sequences

2026-01-07T20:03:51-05:00

We analyze the spatiotemporal evolution of earthquake clusters in the Campotosto area, located between the L'Aquila 2009 and Central Italy 2016 seismic sequences. This region has experienced several moderate earthquakes (Mw 5–5.5) and persistent low-level seismicity. Using a hierarchical density-based algorithm on high-resolution catalogs, we identify clusters lasting from days to months and migrating at rates of meters to kilometers per day. These clusters alternate between phases of rapid expansion and slower diffusion, reflecting complex interactions among fluids, aseismic slip, and seismic rupture across fault segments. Energy release within clusters is low, with effective stress drop ranging from 0.01 to 1 MPa, suggesting diverse driving processes. Clusters with larger spatial extents tend to exhibit lower effective stress drop, implying a significant aseismic slip component, consistent with remote sensing observations. Our findings indicate that deep fluids promote multiphase slip and fault reactivation, influencing seismicity across fault segments with varying criticality. Variations in fault orientation and dip further contribute to heterogeneous slip distribution, affecting both energy release and cluster formation.

Submarine seismicity monitoring with distributed acoustic sensing near Santorini and Kolumbo Volcano

2026-01-16T23:30:13-05:00

Submarine volcanoes and faults pose hazards to nearby populated islands, yet their inaccessibility limits monitoring efforts. The Christiana-Santorini-Kolumbo volcanic field is capable of generating devastating eruptions, earthquakes and tsunamis. The 2025 earthquake swarm near Kolumbo, causing the evacuation of thousands from their homes, underlines the need for accurate and real-time monitoring. We interrogate a 45 km dark fibre that extended from Santorini past the submarine volcano Kolumbo for two months in 2021, comparing the performance of the fibre with the existing monitoring network for earthquake detection and location. The detected quakes originated all over Greece, coming from any azimuth. We can reliably identify events, doubling the number of detections in the vicinity of the fibre and Kolumbo. For event location, the azimuthal coverage of the existing seismometer network outperforms the fibre, emphasising the importance of a nonlinear fibre layout. Our findings suggest that while the higher detection sensitivity of DAS leads to an information gain, the data analysis remains challenging. The data quality may be insufficient for automated workflows. The need for human input limits the potential of DAS for real-time monitoring, although the enhanced detection sensitivity in remote areas justifies the continued research of DAS for submarine volcano monitoring.

Designing small-aperture seismic arrays to enhance earthquake monitoring on ocean islands: Application to Antikythera island, Greece

2025-10-23T10:17:42-04:00

We present an optimisation strategy in order to design a seismic array at the Antikythera island (Greece), consisted of nine elements in total, namely, eight new and one permanent station. This new seismic array aims to improve the seismic event detection capability and location accuracy of the Hellenic Unified Seismic Network (HUSN) at the SW-end of Greece, for local and regional seismicity, which is constrained by the sparse station coverage between Peloponnese and Crete. Instead of simply being based on theoretical transfer function calculations, we set up a synthetic dataset of realistic seismic sources and we determine the backazimuth and slowness vectors based on array beamforming via a global optimisation scheme that takes into account several criteria, such as amplitude power, event mislocation, array shape and landscape restrictions. The result is a set of station coordinates whose positions shape different array configurations at each step of the optimisation process, affecting both the maximum amplitude beam of P and S wavefields, as well as the ability of each array configuration to successfully resolve the backazimuth of each seismic source. The optimal array is determined as the one associated with the minimum score of an objective function based on the above criteria, being an irregular shaped array with an aperture of ~4.0 km.

Focal mechanisms in the southeastern South Island of Aotearoa New Zealand indicate scale-dependent partitioning of transpressional strain

2026-01-19T11:35:42-05:00

The classic Andersonian model of faulting is difficult to apply to plate boundaries with oblique motion, as displacement is accommodated across oblique-slip faults, or it is partitioned into distinct strike-slip and dip-slip faults. Here, we investigate how faults accommodate oblique plate motion by using the focal mechanism solutions of 126 M_LV 1.3-4.3 earthquakes in the transpressional southeastern South Island of Aotearoa New Zealand. Focal mechanisms were assigned an A-D quality, and of the 91 C or better quality solutions, 57 are strike-slip. In addition, when incorporated into a stress inversion, these focal mechanisms indicate a strike-slip stress state with an WNW-trending maximum principal compressive stress. By contrast, constraints on active crustal-scale faulting from the New Zealand Community Fault Model indicate reverse faulting in this region. A high stress shape ratio can partly account for the coexistence of reverse and strike-slip faults. However, we also propose that the focal mechanisms are typically sampling slip on optimally-oriented small-scale faults in intact crust, while the larger magnitude reverse faulting reflects local stress rotations within pre-existing faults and shear zones in the southeastern South Island. Our study therefore demonstrates how inherited structures influence the scale and orientation of faults onto which transpressional strain is partitioned.

Optimized Workflows with a Single Phase-Only Response Correction for Building Empirical Green's Functions for Ambient-Noise Tomography

2025-08-27T04:58:46-04:00

We present simple and optimized workflows for computing empirical Green's functions in ambient-noise tomography that enhance computational efficiency and numerical stability. A key improvement is a phase-only instrument-response correction applied only once after stacking instead of to the raw data before correlation. This prevents instability in spectral division, simplifies computations, and reduces execution time. While some of the additional optimizations we employ are already in use within the ambient-noise tomography community, we provide a detailed description along with systematic benchmarks that quantify their actual impact on runtime and stability. Key improvements include reducing redundant Fourier transforms and combining spectral equalization, cross-correlation, and stacking into a single frequency-domain step. An additional optimization reuses spectral representations of individual stations across multiple station pairs, maintaining linear complexity. We also propose a completely new optimization: applying a phase-only instrument-response correction only once after stacking instead of before correlation. This prevents instability in spectral division, simplifies computations, and reduces execution time. We validate the workflows using datasets from Southern California, Brazil, and Uganda. For individual station pairs, our primary optimized workflow (WF2) reduces execution time by approximately 67–75% (speed-up factors of 3.0–3.9), closely matching theoretical expectations (~5.1). A more scalable variant (WF3) achieves speed-up factors of 15–60 for moderate-sized networks. Furthermore, we demonstrate that a partial implementation into existing codes, requiring only minimal modifications, yields about 10% execution-time savings and improved numerical stability. The proposed workflows produce EGFs nearly indistinguishable from conventional methods and are particularly suitable for large-scale ambient-noise tomography in computationally limited environments.

A quasi-real-time system for automatic local event monitoring in Germany

2026-02-10T16:51:41-05:00

We present TieBeNN, a wrapper that integrates open-source, state-of-the-art seismic monitoring tools, including advanced machine learning--based approaches, to enhance the German Federal Seismological Survey’s (EdB) automatic real-time earthquake monitoring system. TieBeNN extends the existing workflow by adding automatic, probabilistic focal depth estimation using NonLinLoc and introduces a Location Quality Score (LQS) to quantify location reliability with a single metric. In testing, TieBeNN’s automated locations approach the accuracy of human analyst solutions, demonstrating comparable performance in well-instrumented regions. By automating depth determination and providing immediate quality assessment, the system reduces analysts’ daily workload, allowing them to focus on events flagged as low-quality or complex. The LQS effectively distinguishes well-constrained event locations from those with large uncertainties or poor network geometry, enabling rapid identification of high-quality automatic results versus those requiring review. However, events below the Moho depth (i.e., deeper than apaproximately 30 km), which are rare in Germany, remain challenging: their uncertainties are larger, and LQS values tend to be lower, indicating limitations in the current calibration. Overall, these enhancements significantly advance real-time local seismic event monitoring in Germany by increasing both the speed and reliability of automatic event characterization.

A repeating earthquake catalog for the Atacama Segment in North Chile (24.5°S-30.5°S)

2026-01-27T03:39:30-05:00

Repeating earthquakes are close to identical seismic events that recur on the same fault patch with consistent focal mechanisms. They can be used to estimate local fault slip and their magnitude-dependent recurrence intervals can serve to infer patterns of aseismically creeping zones at depth. We have constructed a long-term catalog of repeating earthquakes for the Atacama segment of the Chilean subduction zone. Using waveforms from approximately 60,000 earthquakes as templates, we performed GPU-based template matching on continuous data from 25 permanent seismic stations from 2014 to 2024 and one additional station with data from 1998 to 2024. The resulting catalog includes 4781 repeating events grouped into 1142 families, showing variability in size and behavior, from long-lasting sequences to short-term bursts. Several magnitude M5.7+ earthquakes significantly influenced the occurrence rates of nearby repeaters. The dataset is dominated by the 2015 M_W 8.3 Illapel megathrust earthquake. This catalog aims to support detailed analysis of rupture processes, source structures, and the spatiotemporal evolution of slow slip at depth. The production and reporting style are similar to a previous article by Folesky et al. (2025) on the adjacent northern Chile repeater catalog.

A compilation of elastic anisotropy measurements from metamorphic rocks

2025-12-18T08:02:18-05:00

Preamble

Nikolas Christensen, a pioneer in the study of the elasticity of crust and upper mantle lithologies, passed away on May 19, 2022 (see https://rock.geosociety.org/net/documents/gsa/memorials/v51/Christensen-NI.pdf). Prior to his passing, Nik had been working on a manuscript that described and interpreted his extensive data base of seismic anisotropy measurements. He provided the manuscript materials to one of us (M.G.B.) for comment with the intent of eventual submission. Although the manuscript was not completed, the initial sections, which provide context, describe the methodology, and summarize the measurements, were reasonably self contained. More importantly, the extensive tables of painstakingly taken measurements that form the basis of the work had been prepared in publication-ready form and as spreadsheets. Owing to the profound and invaluable contributions of Nik's previous compilation papers on crustal composition (Christensen & Mooney, 1995) and the isotropic elasticity of common lithologies Christensen, 1996) to the seismological and geological communities, it is our opinion that the present work should be published for the benefit of future scientific investigations of lithospheric anisotropy. We have elected to submit the manuscript on Nik's behalf as a research note to Seismica. The title has been changed from the original ``Metamorphism and crustal seismic anisotropy: A global perspective'' to the present one, but aside from this and minor editorial revisions, it is a faithful representation of the original draft. Note that this manuscript is accompanied by an independent commentary by Douglas Schmitt in this issue of Seismica.
-- Michael G. Bostock, Simon M. Peacock, Matthew S. Tarling. The University of British Columbia.

Abstract

An increasing number of seismic investigations have reported convincing evidence for the widespread existence of crustal anisotropy in a variety of tectonic regions. Interpretations of these observations, as well as future seismic studies designed specifically to investigate crustal anisotropy, require detailed knowledge of anisotropic wave propagation in rocks which have undergone deformation and accompanying recrystallization. Of particular importance are the symmetries and magnitudes of P- and S-wave anisotropies and S-wave splitting. A detailed experimental investigation of the anisotropic properties of metamorphic rocks has been carried out to hydrostatic pressures of 1GPa. Each measurement averages the orientations and correct elastic properties of hundreds of thousands of grains, as well as takes into account the important effects of grain shape and grain boundaries on velocities. Common metamorphic rocks, especially those with pelagic protoliths, often have axial symmetries with slow P-wave velocities normal to cleavage, schistosity, and banding. For slates, phyllites, and quartz mica schists, S-wave singularities occur at angles averaging 42° from their symmetry axes, as well as parallel to symmetry axes. Many axial symmetry amphibolites also have slow P velocities and elastic properties similar to crystals with hexagonal symmetry, but unlike metapelitic rocks do not possess off axis S-wave singularities. Rocks with fast axis P-waves and axial symmetry include blueschists, marbles, and dunites. S-wave singularities for these rocks appear to be limited to propagation parallel to symmetry axes. Of importance, maximum S-wave splitting does not always coincide with propagation normal to symmetry axes, and fast vibration directions can be normal as well as parallel to the strike of foliation. Rocks with well-developed foliations and lineations have, as expected, seismic properties similar to those of orthorhombic single crystals. P-wave velocities are fast parallel to lineations originating from foliation crenulations and mineral elongations. Orthorhombic rock S-wave singularities are rare for propagation in mirror planes, but, when present, occur in symmetry planes defined by the maximum and minimum P-wave velocities. Crustal regions most likely to show strong seismic anisotropy include accretionary prisms containing abundant slate and phyllite and crustal regions rich in quartz mica schist and amphibolite.

ISOLA-BaBoo, a code for full moment tensor inversion with uncertainty estimation; application to the 2025 Anydros earthquakes

2025-11-25T07:40:08-05:00

Full moment tensors are important for revealing non-shear mechanisms of earthquake faulting. Nevertheless, non-double-couple components represent sensitive source parameters, less robust than strike, dip, rake, and moment. Thus, inversion of full moment tensors must be accompanied by uncertainty analyses. Here, we present a new version of traditional ISOLA software for inverting complete waveforms, in which uncertainty is analyzed with Bayesian bootstrap. This approach is particularly useful for obtaining uncertainty of model parameters without assuming a specific (e.g., Gaussian) distribution of data error and evaluating its covariance matrix. We assume that the set of recorded waveforms is representative, noise is low relative to the signal, and the velocity model is free of systematic error. The method is applied to 25 earthquakes of the 2025 Anydros crisis, Aegean Sea, Greece. Most of the analyzed events are M_w>4.5, and they consistently indicate a shear-tensile process, i.e., crack opening on mostly normal faults. The stress field calculated from these and previously published focal mechanisms is transtensional. The fault friction coefficient is low (~0.15). We preliminarily interpret the observed moment tensors as pointing to a fluid-assisted rupture process in a complex network of tectonic faults, likely triggered by a dike emplacement.

Commentary on ‘A compilation of elastic anisotropy measurements from metamorphic rocks’ by N.I. Christensen

2025-11-01T12:14:39-04:00

My purpose in this commentary is to provide further context to Christensen (2026) in this issue of Seismica in order to fill some gaps so that readers better understand how the measurements are made, know what the analyses are based on, and know where the pitfalls in employing these results might lie. My experiences in reviewing and editing have revealed that many papers that focus on elastic or seismic anisotropy err as they are written often without understanding the basic principles. This problem is aggravated because some modern methods, such as Electron Backscatter Diffraction (EBSD), have made calculation of the anisotropy of metamorphic rocks readily accessible and production of figures perhaps too easy. I hope to both amplify and clarify the results archived in Christensen (2026) so they are not similarly misused and to provide readers with some tutorial background and more in-depth sources in order that they can avoid overinterpretation of anisotropy results more generally.