Seismica

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

Sara Klaasen — 2026-01-16

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.

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

Brigitte Knapmeyer-Endrun — 2026-02-03

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.

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

Jack Williams — 2026-01-19

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.

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

Alessandro Vuan — 2026-02-09

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.

A compilation of elastic anisotropy measurements from metamorphic rocks

Nikolas Christensen — 2026-02-08

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.

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

Douglas Schmitt — 2026-02-08

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.