Sara Klaasen

Last Name

Klaasen

First Name

Sara

ORCID

0000-0003-4094-7891

Organisational unit

03971 - Fichtner, Andreas / Fichtner, Andreas

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Publications 1 - 10 of 11

Challenges in Submarine Fiber-Optic Earthquake Monitoring
Igel, Jonas K.H.; Klaasen, Sara; Noe, Sebastian; et al. (2024)
Journal of Geophysical Research: Solid Earth
Utilizing existing telecommunication cables for Distributed Acoustic Sensing (DAS) experiments has eased the collection of seismological data in previously difficult-to-access areas such as the ocean bottom. To assess the potential of submarine DAS for monitoring seismic activity, we conducted an experiment from mid-October to mid-December 2021 using a 45 km long dark fiber extending from the Greek island of Santorini along the ocean bottom to the neighboring island of Ios. This region is of great geophysical and public interest because of its historical and recent seismic and volcanic activity, especially along the Kolumbo volcanic chain. Besides recording anthropogenic noise and around 1,000 seismic events, we observe the primary and secondary microseisms in the submarine section, the latter inducing Scholte waves in a sediment layer where the cable is well-coupled. By using the spectral element wave propagation solver Salvus, we compute synthetic strains for earthquakes with varying degrees of model complexity. Despite including topography, a water layer, and a heterogeneous velocity model, we are unable to reproduce the lack of coherence in our observed earthquake waveforms. Backpropagation simulations for four observed earthquakes indicate that clear convergence of the wavefield, and thus the ability to constrain a source region, is only possible when all model complexities are considered. We conclude that, despite the promising emergence of DAS, monitoring capabilities are limited by often unfavorable cable geometries, cable coupling, and the complexity of the medium. Interrogating multiple cables simultaneously or jointly analyzing DAS and seismometer data could help improve future monitoring experiments.
Complex spatial distribution of onset amplitude and waveform correlation: case studies from different DAS experiments
Bozzi, Emanuele; Saccorotti, Gilberto; Piana Agostinetn, Nicola; et al. (2024)
Bulletin of Geophysics and Oceanograph
Distributed Acoustic Sensing (DAS) technology repurposes fiber optic cables (FOCs) into seismic arrays, offering unprecedented dense strain/strain-rate measurements. The metre -scale virtual sensor spacing is typically unattainable with standard seismological equipment. Consequently, DAS provides an extraordinary amount of suitable data for seismic monitoring applications. However, intrinsic characteristics of this technology, such as signal axial polarisation, coupling inhomogeneities, or sensitivity to site conditions, can affect seismic phase amplitudes and their coherence, potentially reducing the number of useful measurement points. To gain a deeper understanding on the relative importance of these phenomena, this study analyses 'real data' from various seismic events recorded by shallow -horizontal DAS deployments. Thus, we take advantage of the pool of different array dimensions and geometries to avoid biased observations. We focus on the spatial variability of P -wave amplitudes, signal-to-noise ratios and waveform correlation, ideally mimicking the usage of absolute and differential arrival times for seismological monitoring purposes. We observed significant amplitude variations, which cannot be fully explained by signal polarisation along the FOC. Additionally, waveform correlation often exhibits a complex and faster decay with interchannel distance. These findings suggest the importance of avoiding 'blind' usage of shallow -horizontal DAS arrays and emphasise the need for case -dependent data selection/weighting procedures.
Seismic Wave Detectability on Venus Using Ground Deformation Sensors, Infrasound Sensors on Balloons and Airglow Imagers
Garcia, Raphael F.; van Zeist, Iris; Kawamura, Taichi; et al. (2024)
Earth and Space Science
The relatively unconstrained internal structure of Venus is a missing piece in our understanding of the formation and evolution of the Solar System. Detection of seismic waves generated by venusquakes is crucial to determine the seismic structure of Venus' interior, as recently shown by the new seismic and geodetic constraints on Mars' interior obtained by the InSight mission. In the next decade multiple missions will fly to Venus to explore its tectonic and volcanic activity, but they will not be able to conclusively detect seismic waves, despite their potential to detect fault movements. Looking toward the next fleet of Venus missions after the ones already decided, various concepts to measure seismic waves have been proposed. These detection methods include typical geophysical ground sensors already deployed on Earth, the Moon, and Mars; pressure sensors on balloons; and imagers of high altitude emissions (airglow) on orbiters. The latter two methods target the detection of the infrasound signals generated by seismic waves and amplified during their upward propagation. Here, we provide a first comparison between the detection capabilities of these different measurement techniques and recent estimates of Venus' seismic activity. In addition, we discuss the performance requirements and measurement durations required to detect seismic waves with the various detection methods. Our study clearly presents the advantages and limitations of the different seismic wave detection techniques and can be used to drive the design of future mission concepts aiming to study the seismicity of Venus.
Distributed Acoustic Sensing in Volcano-Glacial Environments - Mount Meager, British Columbia
Klaasen, Sara; Paitz, Patrick; Lindner, Nadja; et al. (2021)
Journal of Geophysical Research: Solid Earth
We demonstrate the logistic feasibility and scientific potential of distributed acoustic sensing (DAS) in alpine volcano-glacial environments that are subject to a broad range of natural hazards. Our work considers the Mount Meager massif, an active volcanic complex in British Columbia, estimated to have the largest geothermal potential in Canada, and home of Canada's largest recorded landslide in 2010. From September to October 2019, we acquired continuous strain data, using a 3-km long fiber-optic cable, deployed on a ridge of Mount Meager and on the uppermost part of a glacier above 2,000 m altitude. The data analysis detected a broad range of unexpectedly intense, low-magnitude, local seismicity. The most prominent events include long-lasting, intermediate-frequency (0.01–1 Hz) tremor, and high-frequency (5–45 Hz) earthquakes that form distinct spatial clusters and often repeat with nearly identical waveforms. We conservatively estimate that the number of detectable high-frequency events varied between several tens and nearly 400 per day. We also develop a beamforming algorithm that uses the signal-to-noise ratio (SNR) of individual channels, and implicitly takes the direction-dependent sensitivity of DAS into account. Both the tremor and the high-frequency earthquakes are most likely related to fluid movement within Mount Meager's geothermal reservoir. Our work illustrates that DAS carries the potential to reveal previously undiscovered seismicity in challenging environments, where comparably dense arrays of conventional seismometers are difficult to install. We hope that the logistics and deployment details provided here may serve as a starting point for future DAS experiments.
Towards a widely applicable earthquake detection algorithm for fibreoptic and hybrid fibreoptic-seismometer networks
Hudson, Thomas; Klaasen, Sara; Fontaine, Olivier; et al. (2025)
Geophysical Journal International
Distributed acoustic sensing (DAS) is a promising technology for providing dense (metre-scale) sampling of the seismic wavefield. However, harnessing this potential for earthquake detection with accurate phase picking and associated localization remains challenging. Single-channel algorithms are limited by individual channel noise, while machine learning and semblance methods are typically imited to specific geological settings, have no physically constrained phase association and/or require specific fibre geometries. Here, we present a method that seeks to detect seismicity for any geological setting, applicable for any fibre geometry, and combining both fibreoptic and conventional seismometer data to maximize the information used for detection and source localization. This method adapts a proven back-migration detection method to also include DAS observations, migrating energy from many receivers back in time to search for localized peaks in energy, corresponding to seismic sources. The strengths of this method are capitalizing on coherency over many channels to enhance detection sensitivity even in high-noise environments compared to single-channel algorithms, applicability to arbitrary fibre geometries, as well as built-in, physics-informed phase association and source localization. We explore the performance of the method using three geologically and geometrically diverse settings: a glacier, a volcanic eruption and a geothermal borehole. Our results evidence the effect of spatial-sampling extent and non-optimal fibreoptic geometries, accounting for P- and S-wave sensitivity, coupling effects and how the sensitivity of native fibreoptic strain measurements to shallow subsurface heterogeneities can affect detection. Finally, we attempt to also present a method-ambivalent overview of key challenges facing fibreoptic earthquake detection and possible avenues of future work to address them.
An illustrated guide to: Distributed and integrated fibre-optic sensing in seismology
Fichtner, Andreas; Walter, Fabian Thomas; Paitz, Patrick; et al. (2025)
Earthquake Science
The properties of laser signals are affected by deformation of the optical fibre through which they are transmitted. While this deformation dependence is undesirable in telecommunication, it can be exploited for the construction of novel seismic sensors that fill a niche in data acquisition where traditional seismometer arrays would be difficult to deploy. This includes densely populated urban centers, the oceans, volcanoes and the Earth’s polar regions. These notes complement a presentation on recent methodological developments and applications in fibre-optic seismology. The first part is focused on the use of distributed fibre-optic sensing in cryosphere research, and specifically the investigation of the internal structure and seismicity of glaciers and ice sheets. The second part is dedicated to recent advances in integrated fibre-optic sensing, with emphasis on novel measurement principles and sensitivity.
The Seismogenic Thickness of Venus
Maia, Julia S.; Plesa, Ana-Catalina; van Zelst, Iris; et al. (2025)
Journal of Geophysical Research: Planets
Growing evidence that volcanism is currently ongoing on Venus suggests that the sister planet of the Earth may also be seismically active. Given the success of seismic measurements on Mars and the Moon to reveal the interior structure of these bodies, seismic investigations on Venus are a natural next step. The potential for seismic activity is closely linked to the thickness of the so-called seismogenic layer, that is, the region where rocks behave in a brittle manner and quakes can nucleate. On Earth, the seismogenic thickness is correlated with the thermal structure of the lithosphere, and is typically associated with the depth of the 600 degrees C isotherm. Here, we combine geophysical constraints with thermal evolution models to estimate the thermal structure of Venus' lithosphere and determine the corresponding seismogenic thickness. Taking all estimates into account, our results show that the seismogenic thickness overall varies from 2 to 35 km. The lowest values are associated with areas that probably correspond to local thermal anomalies associated with magmatic processes. This interpretation is corroborated by geodynamic models, which show that intrusive magmatism can largely increase the temperature within the lithosphere at local scales. The seismogenic layer is thickest at volcanic plains which are commonly associated with regions of mantle downwellings. In these regions, the seismogenic layer likely reaches Venus' mantle, while in areas with a thick crust or anomalously high thermal gradients, quakes might be limited to the crust. Our study provides evidence that Venus has a substantial seismic potential.
Modelling uncertainty in P-wave arrival-times retrieved from DAS data: case-studies from 15 fibre optic cables
Bozzi, Emanuele; Agostinetti, Nicola Piana; Fichtner, Andreas; et al. (2024)
Geophysical Journal International
Distributed acoustic sensing (DAS) technology enables the detection of waves generated by seismic events, generally as uniaxial strain/strain rate time-series observed for dense, subsequent, portions of a Fibre Optic Cable (FOC). Despite the advantages in measurement density, data quality is often affected by uniaxial signal polarization, site effects and cable coupling, beyond the physical energy decay with distance. To better understand the relative importance of these factors for data inversion, we attempt a first modelling of noise patterns affecting DAS arrival times for a set of seismic events. The focus is on assessing the impact of noise statistics, together with the geometry of the problem, on epicentral location uncertainties. For this goal, we consider 15 ‘real-world’ cases of DAS arrays with different geometry, each associated with a seismic event of known location. We compute synthetic P-wave arrival times and contaminate them with four statistical distributions of the noise. We also estimate P-wave arrival times on real waveforms using a standard seismological picker. Eventually, these five data sets are inverted using a Markov chain Monte Carlo method, which offers the evaluation of the relative event location differences in terms of posterior probability density (PPD). Results highlight how cable geometry influences the shape, extent and directionality of the PPDs. However, synthetic tests demonstrate how noise assumptions on arrival times often have important effects on location uncertainties. Moreover, for half of the analysed case studies, the observed and synthetic locations are more similar when considering noise sources that are independent of the geometrical characteristics of the arrays. Thus, the results indicate that axial polarization, site conditions and cable coupling, beyond other intrinsic features (e.g. optical noise), are likely responsible for the complex distribution of DAS arrival times. Overall, the noise sensitivity of DAS suggests caution when applying geometry-only-based approaches for the a priori evaluation of novel monitoring systems.
Distributed acoustic sensing for volcano monitoring
Klaasen, Sara (2025)
This thesis explores the potential of distributed acoustic sensing (DAS) for volcano monitoring. The last decade has seen a rapid rise in the use of fibre-optic technologies for seismic sensing, of which DAS has been one of the most well-established. DAS interrogates an optical fibre, resulting in a seismic network with a high spatiotemporal resolution. Dense sampling and flexible deployments make DAS particularly well suited to the study of volcanoes. Volcanoes pose hazards from local to global scales, and flexible deployments facilitate real-time monitoring to identify eruptive precursors, while dense sampling captures details in the seismic signature of a volcano. This thesis investigates the research question "What are the advantages and challenges to using DAS in volcanic settings?" through a series of case studies at different volcanoes, from submarine volcanic cones to glacially covered calderas. In each experiment, I highlight different aspects concerning the potential of DAS on volcanoes, such as logistical constraints, the range of signals that DAS detects, and how to optimise algorithms for DAS. Specifically, I focus on the detection of events through image-processing techniques, their location through probabilistic sampling and grid search approaches, and their focal mechanism with full-waveform inversion (FWI). In general, I find that the spatial density of DAS leads to a higher sensitivity to detect seismicity, and that DAS has a consistent detection threshold given the magnitude and distance of events. The data also support the discovery of previously-unknown environmental signals over a broad range of frequencies, such as geothermal tremor and ice sheet resonance. Through simulating DAS data in complex media, we can identify and physically interpret the origin of local seismic events. Yet, I discuss in several experiments that the design of the fibre layout and coupling of the cable remain essential to facilitate the analysis. Through the combination of DAS with a range of data processing algorithms of varying complexity, as appropriate to the data quality and desired outcomes, I can maximise the extraction of information from volcanoes. From initial estimates that indicate clusters of seismicity, to high-resolution modelling to constrain the focal mechanism of a source within a complex landscape: DAS has the potential to transform volcano monitoring.
Seismic constraints on glacier density
Lanteri, Ariane; Keating, Scott; Gebraad, Lars; et al. (2025)
Scientific Reports
Terrestrial ice bodies are important regulators of climate and sea level variations. They influence the water cycle, provide fresh water and energy for human society, and contribute to the living basis of numerous ecosystems. Understanding the structure and dynamics of land ice requires knowledge of its mass density, which is essential for ice core climatology and estimates of mass balance components, such as mass loss, ice discharge and surface melt. We combine densely sampled fiber-optic sensing data from strong serendipitous anthropogenic sources with Hamiltonian Monte Carlo sampling to extract direct seismic constraints on firn density (i.e. the transitional layer between fresh snow and glacial ice). Our approach avoids biases introduced by subjective regularization choices, does not require empirical scaling relations from seismic wave speeds to density, and provides reliable uncertainty estimates. We demonstrate that high-quality surface-wave overtone data can directly constrain density to around 100 m depth. Commonly used scaling relations from seismic wave speeds to density, however, fail to reproduce resolvable details of glacial density structure, and they tend to deviate from direct constraints on the order of ±10 %. Consequently, ice mass inferred from seismic wave speed may be incorrect by a similar amount.

Publications 1 - 10 of 11

Sara Klaasen

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