Anne Obermann

The Bhutan Himalaya, within the India‐Eurasia collision zone, is characterized by active deformation and significant seismic hazard. We image the Main Himalayan Thrust (MHT), the primary structure accommodating convergence, using surface wave dispersion derived from ambient noise and teleseismic earthquake recordings from the GANSSER network. Ambient noise data constrain shear wave velocities in the upper crust, whereas teleseismic Rayleigh waves extend resolution to ∼50 km depth. The resultant shear‐wave velocity model reveals pronounced lateral heterogeneity in the upper crust, including high‐velocity domains ( V _s > 3.6 km/s) consistent with quartzite‐dominated lithologies. The MHT is imaged as a laterally continuous low‐velocity layer (LVL) exhibiting a flat‐ramp geometry: in western Bhutan a ramp emerges at ∼10 km, whereas in eastern Bhutan, the structure dips more gently to ∼20 km. In central Bhutan, disruption of the LVL, together with a concave‐upward lower‐crustal anomaly, suggest possible tear or segmentation of the thrust system.

Improving our understanding of induced and natural earthquakes benefits from controlled experiments in insitu laboratories. To investigate the processes during an Mw∼0 earthquake, we performed the “ Mzero ” experiments in a densely instrumented testbed of the Bedretto Underground Laboratory. These multi‐day hydraulic stimulation experiments went the opposite direction to typical induced seismicity research in that they were designed to enhance seismic rupture. In the first experiment, MzeroA, the rock mass was preconditioned by injecting water for 4 days at a pressure just below fracture reactivation pressure followed by a hydraulic stimulation above reactivation pressure. This strategy aimed at increasing the fractures area near critical stress conditions for shear failure, thus facilitating larger ruptures. During MzeroA, an event with Mw–0.54 was induced, followed by a distinct aftershock sequence. This event was one magnitude unit larger than any preceding event. In the second experiment, MzeroB, stimulation was initiated directly, without preconditioning. Compared to MzeroA, MzeroB exhibited higher seismicity rates, a larger seismicity cloud, and a pronounced migrating seismicity front. Combining seismological and hydromechanical observations, we discuss mechanisms that may have influenced the contrasting seismicity responses. In addition to fluid preconditiong, stress transfer from the Mw–0.54 mainshock, and natural seasonal pressure changes, may have contributed to modulating the seismogenic response. However, the relative importance of these processes remains uncertain given experimental limitations. Our results highlight both the potential and challenge of designing hydraulic stimulations to enhance or suppress seismic rupture, with implications for earthquake physics and induced seismicity.

The Hengill volcanic system in Iceland is of exceptional geological interest and energetic potential. Hengill sits on the mid-Atlantic ridge, on a triple junction, and close to the Icelandic hotspot. It also hosts strong geothermal activity, as apparent at the surface through pools of boiling water scattered across the flanks of the mountain. This geothermal activity has been exploited for electricity production and heating. While Hengill is of great geological and energetic interest, the geological processes occurring beneath the surface remain only partially understood. Recently, the site has been increasingly instrumented, in particular with large deployments of seismic nodes and with distributed dynamic strain sensing (project DEEPEN and others). These give us an unprecedented opportunity to understand processes at work beneath this exceptional volcanic system and shed light on new geothermal energy reservoirs. Here we will give an update on ongoing work focused on very local estimates of structure of the geothermal fields, focusing on in situ vp/vs ratios within clusters of events and geothermal prospect scale tomography.

In situ CO ₂ mineral storage is gaining increasing attention as a carbon capture and storage (CCS) technology to reduce anthropogenic CO ₂ emissions. So far, in situ CO ₂ mineral storage projects have been monitored predominantly using fluid‐ and isotope geochemical methods. Here, we investigate the feasibility of crosshole seismic differential traveltime tomography as an additional, geophysical monitoring tool. First, we use a rock physics modeling approach to examine how secondary mineral precipitation influences the seismic velocity of the basaltic host rock. Then, we test two forward modeling approaches—an eikonal solver and spectral element modeling—to determine the sensitivity of a time‐lapse crosshole seismic survey toward seismic velocity anomalies of various spatial extents and strengths. We determine the minimal velocity increase and corresponding volumetric calcite precipitation required to successfully perform crosshole seismic time‐lapse monitoring.

The Åknes rockslide in Western Norway represents one of the country’s most hazardous unstable slopes, with potential failure capable of generating destructive fjord tsunamis. Therefore, reliable monitoring of its internal deformation processes is crucial for hazard assessment and early warning. This study investigates the feasibility of using passive seismic interferometry (PSI) to monitor internal mechanical changes within the Åknes slope by analyzing relative seismic velocity variations (dv/v) derived from continuous recordings of a vertical borehole geophone array (15–50 m depth). Cross-correlation analyses in the 35–70 Hz frequency band reveal distinct seasonal patterns, with pronounced dv/v decreases during the spring snowmelt periods of 2023 and 2024. These velocity drops coincide with rapid groundwater level increases measured in a nearby borehole, suggesting that elevated pore pressure from meltwater infiltration temporarily reduces rock mass stiffness. Additional correlations with air temperature and snow depth indicate sensitivity to freeze–thaw and hydrological cycles, whereas wind-induced variations at shallow depth highlight the need for careful data processing. Beyond surface related forcings, a large dv/v decrease is observed in 2024 and correlates with an acceleration of displacement near the lower landslide shear zone, indicating sensitivity to localized changes in elastic properties. The results demonstrate that PSI is sensitive to relevant hydromechanical processes and allows distinguishing environmental effects from irreversible structural changes related to the slope creep.

Distributed Acoustic Sensing (DAS) enables the acquisition of seismic data with unrivalled spatio-temporal resolution over very large distances. Railway fiber-optic networks, originally deployed for telecommunications, offer cost-effective opportunities to monitor and characterize the subsurface at multiple scales. Here, we present a project conducted with the Swiss Federal Railways (SBB) involving the interrogation of dark fibers running along two perpendicular railway tracks, each approximately 40 km long. Data were acquired over three months using a dual-channel Sintela Onyx interrogator, with variable acquisition setups (spatial sampling, gauge length, and sampling frequency) tailored to different scientific objectives described below.The primary objective was to assess the feasibility of using pre-existing telecommunications fibers for structural track-bed monitoring, specifically shallow subsurface Vs characterization through inversion of Rayleigh-wave dispersion curves (MASW). This requires high spatial sampling and short gauge length (3 m and 6 m, respectively) to capture short wavelengths. Several ambient noise interferometry strategies were tested, including stacking (1) all available time windows with various preprocessing schemes, (2) only time windows exhibiting strong directional wavefields, and (3) a coherent-source subsampling approach based on a Symmetric Variational Autoencoder to identify time windows contributing the most useful seismic energy. Unsurprisingly, trains constitute the most energetic and reliable seismic sources, from which dense Vs profiles can be derived, demonstrating the effectiveness of both the processing and inversion workflows.Beyond shallow characterization, the experiment also yielded valuable data to complement dense nodal arrays deployed near Lavey-les-Bains, a site of significant geothermal interest and complex geological structure. The main objectives in this context are to (1) help characterizing the subsurface over the first kilometers, (2) investigate its relationship to geothermal circulation, (3) evaluate the joint use of dense nodal and DAS data for imaging, and (4) establish a high-quality, open-access dataset to support the development of next-generation passive imaging methodologies.Finally, at an even larger scale, the experiment provided the opportunity to explore how DAS data can be leveraged within the operational Swiss Seismological Service (SED) network and to assess whether DAS can augment standard seismicity catalogues. Lower-resolution data (100 m spatial sampling, 200 Hz sampling frequency) were streamed and converted in real time into standard seismic formats (miniSEED and StationXML), demonstrating the feasibility of integrating DAS data into SeisComP for both automatic and manual processing.We will present the dataset along with key results relevant to the three purposes outlined above.We acknowledge Allianz Fahrbahn (grant agreement No. 100 072 202) for enabling this study.

Understanding how faults are activated and earthquakes are triggered is still a central challenge in seismology and seismic hazard assessment. Controlled hydraulic stimulation experiments offer a valuable opportunity to study these processes under well-constrained conditions and at spatial and temporal resolutions that are rarely achievable in natural settings. In this study, we present the results of three hydraulic stimulations experiments conducted at the Bedretto Underground Laboratory and monitored by a dense, high-sensitivity seismic network.These experiments revealed a complex spatio-temporal evolution of induced seismicity, characterized by the activation of a multi-segment fault network. Two dominant seismic clusters were activated early on and show a clear spatial connection to the injection borehole, suggesting that pore pressure is the main driver of seismicity within these clusters. At later stages, a third cluster with a different orientation was activated, despite showing no obvious direct hydraulic connection to the injection interval. Seismicity within this cluster occurred with a temporal delay compared to the other two clusters. This suggests that the fault activation was likely driven by indirect processes such as aseismic deformation, stress transfer, and delayed fluid migration.The observed fault network activation closely resembles patterns commonly reported in natural earthquake sequences. These findings suggest that the physical mechanisms controlling fault reactivation and earthquake triggering are largely independent of scale, linking controlled field experiments and natural earthquakes. Our results emphasize the importance of fault network geometry and stress interactions in understanding induced and natural seismicity.

Imaging the Earth’s subsurface is fundamental to a wide range of geophysical applications, including natural hazard assessment and mitigation, geothermal and mineral exploration, and crustal characterization. However, achieving reliable seismic images in strongly heterogeneous media remains a significant challenge. In such environments, conventional seismic imaging approaches, including tomography and migration, often perform poorly due to the prevalence of multiple scattering and high attenuation, which obscures primary reflections and degrades image quality.While multiple scattering has traditionally been regarded as a major impediment to seismic imaging, recent advances have demonstrated that this scattered energy can instead be exploited to extract valuable information. One such approach is Reflection Matrix Imaging (RMI). RMI involves using seismic interferometry to construct a reflection matrix that contains the full wavefield response between virtual source–receiver pairs, allowing for the analysis of reflected energy generated by subsurface heterogeneities. From this, the distortions undergone by the incident and reflected waves can be isolated and compensated for even with a rough estimate of the background seismic velocity. RMI has been shown to enhance imaging in complex geological settings, including volcanic environments, and has also been seen to be effective in 3D imaging applications in fields such as optical microscopy and medical ultrasound.In this study, RMI is adapted to data from a dense seismic array deployed in the Hengill Geothermal Field, Iceland. The subset of the array considered here comprises 267 stations distributed over a rectangular approximately 5X10km2 , with continuous recordings spanning 2.5 months. Reflection matrices are constructed, and the applicability and performance of RMI in this highly heterogeneous geothermal setting are systematically evaluated.

We explore the application of passive seismic interferometry across multiple spatial and temporal scales to investigate hydro-mechanical processes in mountainous terrain. At the larger scale, we analyze continuous seismic data from multiple stations in the Swiss Seismological Network spanning over two decades to investigate long-term trends and seasonal patterns in seismic velocity across the Swiss Alps. Our results reveal clear seasonal cycles for all investigated stations, likely due to environmental influences, such as temperature and precipitation, as well as coupled mechanisms that potentially influence subsurface water systems. These observations are part of an ongoing effort to establish an understanding of mass-balance changes driven by glacier retreat and altered precipitation patterns, which are directly affecting slope stability and groundwater recharge dynamics in a rapidly changing Alpine environment.Complementing the insights of this large-scale study across Switzerland, we present a detailed case study from the Åknes rockslide in Western Norway, to highlight the potential of passive seismic interferometry (Bogner et al. 2026) to monitor rapid groundwater level rises and pore pressure induced reductions in rock mass stiffness. We show that a significant decrease in seismic velocity in 2024 correlates with accelerated displacement in the landslide shear zone, demonstrating the method’s sensitivity to both reversible environmental effects and irreversible structural changes.Together, the studies are showing the versatility of passive seismic monitoring for hydro-mechanical processes from smaller scale site-specific hazard assessment to large-scale regional characterization of climate driven subsurface changes in the Alpes. Bogner, L., Bruland, C., Hadziioannou, C., Obermann, A. and Langet, N. (2026). Seismic noise interferometry to disentangle environmental effects from irreversible subsurface changes at the Åknes rockslide in Western Norway. Submitted to Seismological Research Letters. Under review.