SM5.2 | Ambient Seismic Noise and Seismic Interferometry
Ambient Seismic Noise and Seismic Interferometry
Convener: Sven SchippkusECSECS | Co-conveners: Yesim Cubuk SabuncuECSECS, Yang LuECSECS, Peter MakusECSECS, Qing-Yu Wang
Orals
| Wed, 17 Apr, 16:15–18:00 (CEST)
 
Room G2
Posters on site
| Attendance Thu, 18 Apr, 10:45–12:30 (CEST) | Display Thu, 18 Apr, 08:30–12:30
 
Hall X1
Posters virtual
| Attendance Thu, 18 Apr, 14:00–15:45 (CEST) | Display Thu, 18 Apr, 08:30–18:00
 
vHall X1
Orals |
Wed, 16:15
Thu, 10:45
Thu, 14:00
Interferometric techniques turn seismic networks into continuous observation devices for (time-varying) Earth structure, volcanic and hydrologic processes, ocean - solid Earth interactions and many other phenomena. The application of this technique has expanded to signals beyond ocean microseismic noise, capturing anthropogenic seismic signals as well.

Great strides have been taken in obtaining high-resolution images of seismic velocity and other properties, in observing and quantifying the sources of various ambient noise wave types, and in interpreting seismic property variations. Current challenges include the interpretation of signals from less-than-ideally situated sources, such as those in the context of traffic noise interferometry or ambient noise body waves from localized storms; the interpretation of ambient noise amplitudes for elastic effects and anelastic attenuation; the localization of seismic property changes; the implementation of spatial wavefield gradient measurements from fiber optic or rotational sensors.

This session offers a broad space for discussing recent advances in ambient noise seismology and seismic interferometry. We invite abstracts on theoretical and numerical developments as well as novel applications. Topics may include, but are not limited to, studies of ambient seismic sources; ocean wave quantification through ambient noise; urban seismic noise; interferometric imaging; monitoring subsurface properties and quantifying the response of seismic velocity to various stresses and strains; studies of the spatial sensitivity for imaging and monitoring under diverse source conditions; quantification of site effects, amplification and attenuation; improvements in processing and retrieval of high-quality interferometry observations, and interdisciplinary applications of seismic interferometry.

Orals: Wed, 17 Apr | Room G2

Chairpersons: Sven Schippkus, Yang Lu, Peter Makus
16:15–16:20
Interferometry/Sources
16:20–16:40
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EGU24-17146
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solicited
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Highlight
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On-site presentation
Diane Rivet, Gauthier Guerin, Clara Vernet, Alister Trabattoni, and Marie Baillet

Distributed acoustic sensing transforms fiber-optic cables into giant and very dense seismic networks. Although less sensitive to ground motion than traditional networks, they offer new possibilities for passive imaging and temporal monitoring, especially in hardly accessible locations such as the seafloor. From two case studies - in South of France, on a 42km long cable off-shore Toulon and in Central Chile on the northern leg of the Concón landing site of the GTD telecom cable - we explore the capability to perform passive imagery using ambient seismic noise and coda waves.

Despite a higher instrumental noise level and uneven ground coupling, underwater telecom cables can record the microseismic noise. This may be strong microseismic noise generated locally, or microseismic noise amplified by the resonance of the water column. The recorded microseismic noise at the seafloor allows a better understanding of its generation and provides high resolution images of shallow crustal structures.

From the observation of ocean gravity waves and microseismic noise, we highlight the strong localization of seismic noise sources near the coast, which can be highly variable over short time scales.  Due to the localized nature of the noise sources, and because it is not always possible to average the noise recorded over long periods of time (months, years), conventional methods for ambient noise imagery show significant discrepancies in velocity estimates, up to 30%, especially at greater depths. We present here a method that minimizes the errors due to highly localized sources by carefully correcting the apparent velocities from the azimuth of the sources.

In seismic areas, in addition to microseismic noise, it is possible to expand the frequency content toward higher frequencies using seismic coda. Coda waves are dominated by multi-diffracted surface waves on local heterogeneities. The spatial distribution of their energy is more isotropic. Using dispersion curves stacked over the coda of several earthquakes, we image the shallow crustal structure of the sediments. This innovative approach opens up new horizons for structural imaging and monitoring.In coastal environments, the distribution of noise sources must be systematically studied in order to obtain reliable results.

How to cite: Rivet, D., Guerin, G., Vernet, C., Trabattoni, A., and Baillet, M.: New perspectives on crustal imagery leveraging offshore submarine fiber optic cables, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17146, https://doi.org/10.5194/egusphere-egu24-17146, 2024.

16:40–16:50
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EGU24-2797
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On-site presentation
Jordi Diaz, Montserrat Torne, Martin Schimmel, Susana Rodríguez, David Martí, Mario Ruiz, Helena Seivane, Pilar Sánchez-Pastor, and Diego Davoise

We present the characterization of the seismic ambient noise wavefield in the open-pit Riotinto mine (southern Spain), in the mainframe of a collaborative research project aiming to use the ambient noise wave field to monitor structural subsurface changes in near real-time. Noise characterization is based on a dense seismic network of 30 stations located along a 1-km long segment of a tailings dam. We first describe the most frequent transient signals detected, including local and distant earthquakes, blasting, and vehicles. The time variations in the amplitude of the 10-40 Hz frequency band are then used to define three phases of activity during the recording period. The highest amplitudes are directly related to the regrowth of the dam wall carried out to properly store the constantly increasing amount of tailings. In the third phase, the seismic noise is dominated by the deposition of tailings into the deposit, allowing the use of seismic data to monitor in detail the evolution of the deposition process. The detailed knowledge of the sources of noise in the Riotinto mine provides the basis for developing ambient noise seismic interferometry methods to monitor the physical properties of the subsurface of this and other open-pit mining areas.

This research is part of the R+D+I project CPP2021 009072 funded by MCIN/AEI/10.13039/501100011033 (Ministry of Science, Innovation and Universities/State Innovation Agency) with funds from the European Union Next Generation/PRTR (Recovery, Transformation, and Resilience Plan).

How to cite: Diaz, J., Torne, M., Schimmel, M., Rodríguez, S., Martí, D., Ruiz, M., Seivane, H., Sánchez-Pastor, P., and Davoise, D.: Analysis of seismic noise sources in an open pit mining environment., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2797, https://doi.org/10.5194/egusphere-egu24-2797, 2024.

16:50–17:00
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EGU24-8004
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ECS
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On-site presentation
Giovanni Diaferia, Carlo Giunchi, Irene Molinari, Marco Olivieri, Fabio Di Felice, Andrea Contu, Domenico D'Urso, Luca Naticchioni, and Davide Rozza

The area in the municipalities of Lula, Bitti, and Onanì in Sardinia (Italy) is a candidate for hosting the “Einstein Telescope” (ET, the third-generation gravitational wave detector), given the extremely low level of natural and anthropogenic seismic noise at this site. For the same unique characteristics of this area, the multi-disciplinary geophysical far-field observatory “Faber” (PNRR-Meet project) will be set up.

However, the strength and persistence of wind make this area exceptionally favorable for the exploitation of wind energy, as testified by the nearby Buddusò wind park that, consisting of 69 turbines and about 130 MW of total installed power, is the largest in Italy.

It is well known that wind turbines are an important source of seismic noise between 1 and 10 Hz, posing a relevant concern for noise contamination of ET as it will operate in the same frequency range.

In the context of the seismic characterization of such a candidate site, the WINES experiment (Wind turbIne Noise assEsSment in the Italian site candidate for the Einstein Telescope) provided a two-month-long passive seismic recording of nine broad-band stations placed at increasing distances from the Buddusò wind park. The aim of the experiment was the evaluation of the noise generated by the wind park in terms of amplitude, spectral content, and decay with distance, in relation to the wind park operation.

Analyzing the frequency spectra at all stations, the spectral imprint of the wind park manifests through sharp, well-defined spectral peaks at 3.4, 5.0, 6.8, and 9.4 Hz, even in conditions of absent or moderate wind speed (0-3 m/s). With stronger winds (>20 m/s), all spectra increase their amplitude by an order of magnitude, and the sharpest and most persistent peaks are found at 3.5, 5.2, and 6.8 Hz. In both wind conditions, the amplitude of such peaks decreases with distance, being clearly distinguishable up to 5-6 km from the wind park. We use these spectral peaks to derive an empirical relationship for their amplitude vs. distance, highlighting a well-behaved exponential decay that translates into a two-orders-of-magnitude decrease within 10 km distance.

Lastly, considering the assumption that the generated seismic noise propagates as Rayleigh waves, the continuous recordings along the array have been used for the estimation of the direction of noise arrival at each station. Signal coherence allows the recovery of this information for stations within 5 km from the wind park, showing a dominant back-azimuth of the incoming signal that is fully compatible with the position of the wind park with respect to each station.

How to cite: Diaferia, G., Giunchi, C., Molinari, I., Olivieri, M., Di Felice, F., Contu, A., D'Urso, D., Naticchioni, L., and Rozza, D.: Results from WINES: Wind turbIne Noise assEsSment in the Italian site candidate for “Einstein Telescope”, the third-generation gravitational wave detector., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8004, https://doi.org/10.5194/egusphere-egu24-8004, 2024.

Monitoring
17:00–17:10
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EGU24-5425
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ECS
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On-site presentation
Pilar Sánchez-Pastor, Sin-Mei Wu, Ketil Hokstad, Bjarni Kristjánsson, Vincent Drouin, Cécile Ducrocq, Gunnar Gunnarsson, Antonio Rinaldi, Anne Obermann, and Stefan Wiemer

Harvesting geothermal energy often leads to a pressure drop in reservoirs that promotes the formation of steam. In some reservoirs, steam coexists with liquid water forming two-phase fluids, as happens in the Hengill geothermal field. This field is located in a triple junction of three large tectonic features in Iceland, 30 km east of the capital Reykjavik. The accumulation of steam in the top part of the reservoir forms a so-called steam cap. While steam caps are valuable energy resources, they also alter the reservoir thermodynamics and entail diverse risks such as land subsidence. Therefore, monitoring the steam content in reservoirs is essential for both operational and economic perspectives. However, this is an inherently challenging task and quantifying the steam content from indirect and surface-based measurements is still an unsolved matter.

Here, we present a new method for indirectly sampling the steam content in the subsurface using the ever-present seismic background noise. We analyse the seismic velocity changes in the area, estimate the land subsidence via Interferometric Synthetic Aperture Radar (InSAR) and work with in situ borehole data. We observe a consistent annual velocity drop in the Hengill geothermal field and establish a correlation between the velocity drop and steam buildup. This study introduces seismic noise interferometry as a powerful tool for monitoring two-phase fluids in the crust with minimal infrastructure, only one seismic station. Beyond geothermal sites, the methodology could extend to diverse geological settings, such as volcanoes, CO2 storage sites, hydrocarbon reservoirs, among others.

How to cite: Sánchez-Pastor, P., Wu, S.-M., Hokstad, K., Kristjánsson, B., Drouin, V., Ducrocq, C., Gunnarsson, G., Rinaldi, A., Obermann, A., and Wiemer, S.: Monitoring two-phase fluids in geothermal fields using seismic noise interferometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5425, https://doi.org/10.5194/egusphere-egu24-5425, 2024.

17:10–17:20
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EGU24-16014
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ECS
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On-site presentation
Laura Ermert, Anne Obermann, and Lapo Boschi

Ambient seismic noise is a useful tool to monitor the crust and shallow subsurface, with application cases that vary widely, including for example fault zone, volcano, and groundwater monitoring. Classic ambient-noise monitoring applications are based on observing changes in seismic velocity, or changes in cross-correlation waveforms. Here, we explore the possibility of monitoring crustal attenuation (both viscous and scattering) with ambient seismic noise.

Attenuation monitoring is envisioned to be an immensely useful complement to velocity monitoring, because it is sensitive to the material properties of the subsurface, and can be used together with seismic velocities to monitor the crust, for example to track crustal fluids. However, short-term measurements of attenuation from ambient seismic noise may suffer biases due to the variability of natural ambient seismic noise sources. This is of particular concern when working with the ocean-generated primary and secondary microseismic noise, which provide energy to monitor the crust at several kilometer depth, but have sources that vary strongly and rapidly.

To examine the limitations imposed by oceanic seismic noise source variability quantitatively, we first investigate the temporal behavior of Rayleigh wave attenuation coefficient α, as well as Coda-Q of ambient noise cross-correlations, at broadband seismic stations in Switzerland and in the Hengill region of Iceland, over 12 and 2.5 years, respectively. These parameters have previously been used to study crustal attenuation with ambient noise and have been shown to yield geologically meaningful information as long as long-term and array averaging is performed, which makes the observations more robust with respect to noise source variability (Soergel et al., 2020, Magrini et al., 2021).

Second, we simulate ambient noise cross-correlations with secondary microseism source models based on ocean wave hindcasts. To generate the synthetic ambient noise cross-correlations, we consider the spatiotemporal variation of the noise source spectra as well as realistic seismic wave propagation computed using the spectral element technique.

Based on the simulated and observed time series of α and Coda-Q, we evaluate the effect of noise source variability on the attenuation parameters. In this way, we intend to estimate the presence and severity of noise source bias. We consider this as a necessary step towards regional ambient noise-based attenuation monitoring.

 

 

Soergel, D., Pedersen, H. A., Stehly, L., Margerin, L., Paul, A., & AlpArray Working Group. (2020). Coda-Q in the 2.5–20 s period band from seismic noise: Application to the greater Alpine area. Geophysical Journal International, 220(1), 202–217. https://doi.org/10.1093/gji/ggz443

 

Magrini, F., Boschi, L., Gualtieri, L., Lekić, V., & Cammarano, F. (2021). Rayleigh-wave attenuation across the conterminous United States in the microseism frequency band. Scientific Reports, 11(1), Article 1. https://doi.org/10.1038/s41598-021-89497-6

 

How to cite: Ermert, L., Obermann, A., and Boschi, L.: Towards ambient noise attenuation monitoring: Time-varying noise source effects on noise-based attenuation estimates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16014, https://doi.org/10.5194/egusphere-egu24-16014, 2024.

17:20–17:30
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EGU24-10427
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On-site presentation
Nicola D'Agostino, Stefania Tarantino, Piero Poli, Maurizio Vassallo, Gerardo Ventafridda, Gaetano Festa, and Aldo Zollo

The conventional picture of the earthquake cycle implies that rupture is reached by progressive stress buildup until reaching fault’s failure strength. Alternatively, the failure strength may be altered by changes in pore pressure and/or properties of fault rocks. This last scenario may be associated with significant modifications of the elastic properties of the crust potentially detectable with seismological tools. Natural oscillatory stress sources (tides, seasonal and multiannual) can thus be      exploited to probe the time-dependent response of active fault zones to stress variations at various temporal and spatial scales and investigate time-dependent variations of its elastic properties (Delorey et al., 2021). A multidisciplinary (seismology, geodesy, geochemistry) study is carried out along the Irpinia Fault System (IFS, Southern Apennines) to investigate the response of the crust to hydrological forcing associated with      phases of recharge/discharge of karst aquifers in terms of time-dependent variations of its elastic and hydraulic properties. Charge/discharge phases of the karst aquifers in the Apennines cause significant seasonal and multi-annual strain transients (Silverii et al, 2019), that modulate the secular, tectonic deformation (~3 mm/yr extension across the Apennines). It has been previously observed that these seasonal and multi-annual transients correlate with the seismicity rate (D’Agostino et al, 2018) and seismic velocity variations (Poli et al., 2020). Recent studies (Silverii et al., 2016; D’Agostino et al., 2018) have shown the high sensitivity of the IFS to hydrological stresses reflected in a complex, time-dependent response of deformation and seismicity. We performed a natural pump-probe experiment to assess the non-linear behavior of the seismogenic volumes in response to non-tectonic deformations. Seasonal horizontal strains associated with the discharge and recharge of karst aquifers are used as the “pump”. Coda wave interferometry demonstrates to be a powerful tool to probe time-dependent crustal elastic properties. We computed seismic velocity variations using empirical Green's functions reconstructed by autocorrelation on continuous 14-year-long time series of ambient noise. We analyzed two different sites (co-located GPS and seismic stations), near and afar the IFS. We found that velocity variations are significant (∼0.2%) near IFS and not significant farther away from IFS. We compared the velocity variations near IFS with the time series of Caposele spring discharge, temperature, horizontal deformation and seismicity rate. Our observations are coherent at seasonal and multi-annual scales and can be explained by the same mechanism. At the time of the maximum peak of the discharge spring, representing a proxy of the hydraulic head, the seismic wave velocity is minimum, the dilation of crust is maximum and related to the opening of pre-existing cracks’ system. The background microseismicity occurrence is favored by the hydrologically-related dilatation, superimposed on the ongoing tectonic extension. From the comparison between hydrological strain variations and velocity changes, we estimate a strain sensitivity of velocity change of ~-10^3 typical of worn crustal material and in good agreement with laboratory experiments.  This nonlinear elasticity regime suggests the presence of a multi-fractured and damaged crust subject to periodic seasonal phases of weakening/healing, potentially affecting earthquake nucleation processes.

How to cite: D'Agostino, N., Tarantino, S., Poli, P., Vassallo, M., Ventafridda, G., Festa, G., and Zollo, A.: A natural pump-probe experiment reveals nonlinear elastic properties along the Irpinia Fault, Southern Apennines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10427, https://doi.org/10.5194/egusphere-egu24-10427, 2024.

Imaging
17:30–17:40
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EGU24-17324
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On-site presentation
Trond Ryberg, Corinna Roy, Christian Haberland, Kacie Wellington, and Conor Moynihan

Europe set out its goals for decarbonization in the EU Green Deal, which includes reducing net greenhouse gas emissions by relying more on renewable energy and green technologies. One goal of the EU project VECTOR (https://vectorproject.eu) is to test and develop passive, non-disruptive exploration methods to investigate Europe‘s raw material potential.

We test the application of passive seismic imaging in the Irish midlands, which contain potential areas for zinc mineralization, one of the multiple raw materials needed for green energy technologies. More specifically, we apply ambient noise tomography to image the Earth’s subsurface and assess the utility of this technique for mineral exploration at depth:

Thus, 210 temporary, continuously running digital seismic stations were deployed in the Irish midlands (north of Collinstown) in an area of ~8 x ~6 km, and recorded ambient noise data for ~6 weeks. We then extracted Rayleigh wave group velocities in the frequency range 0.625 – 10 Hz by cross-correlating the data (~42517 time series in total) and using the FTAN method. In the first step we used 1% of the data (long offsets) in a stochastic, transdimensional, hierarchical Monte Carlo search with Markov Chains to derive a three-dimensional shear wave velocity model. In the second step, we added shorter offsets, which did not lead to any significant changes in the 3D model.

The velocity model shows distinct velocity anomalies down to approximately 1.6 km depth that correspond to features also seen in reflection seismic profiles provided by Teck Ireland Ltd, a subsidiary of Teck Resources Limited, that owns the project area and is an Associated Partner of VECTOR. This demonstrates the potential of low-cost passive seismic methods to investigate the Earth’s subsurface compared to expensive active seismic methods. We used synthetic 3D Checkerboard tests to assess which areas of the model are well resolved and we will further compare our models with other data sets, for example, petrophysical borehole data available in this area.

How to cite: Ryberg, T., Roy, C., Haberland, C., Wellington, K., and Moynihan, C.: Ambient seismic noise tomography for mineral exploration in the Irish Midlands , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17324, https://doi.org/10.5194/egusphere-egu24-17324, 2024.

17:40–17:50
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EGU24-7746
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ECS
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On-site presentation
Ya Liu and Jianghai Xia

Karst disasters pose a substantial threat to geological environmental stability, resulting in ground collapse and groundwater contamination. To determine the spatial distribution of karst terrain, high-resolution underground imaging is essential. However, traditional synchronous observation systems struggle to achieve high-density spatial sampling due to limitations in instrument quantity. In this study, we conducted short-term synchronous and asynchronous ambient noise observations in a deserted parking lot in Hangzhou, China, to overcome the limitations of insufficient sampling density. We performed the first-round observation on one half of the parking lot for around 24 hours, followed by an immediate relocation of the stations to the other half for the second-round ~24-hour observation. Additionally, 29 fixed stations were placed outside the parking lot to continuously record ambient noise.

The imaging results of noise source distribution indicate that high-frequency noise sources exhibit significant non-uniform distribution during the daytime, which could affect the accuracy of the retrieved surface waves. To address this, we propose using the similarity of cross-component cross-correlation functions to select only data segments with stronger in-line noise sources, thereby enhancing the reliability of synchronous cross-correlation functions. Furthermore, we utilized the cross-component cross-correlation stacking method to suppress higher-mode surface waves and reduce their impact on the accuracy of the computation of asynchronous cross-correlation functions. Applying the ambient noise source-receiver interferometry method to the synchronous cross-correlation functions, we successfully retrieved the surface waves between asynchronous stations. In total, we obtained 66,472 pairs of cross-correlation functions, comprising 38,416 synchronous pairs and 28,056 asynchronous pairs. We extracted phase velocity dispersion curves of the fundamental mode surface wave for station pairs within the parking lot and utilized the direct surface wave tomography method to obtain the subsurface 3D shear-wave velocity structure.

The inversion results revealed the presence of two distinct low-velocity anomalies in the northeastern and southwestern parts of the site at depths around 40 m, which align with the location and depth of karst caves obtained from drilling data, confirming the reliability of the inversion results. Furthermore, we uniformly subsampled half of the data to simulate the case of insufficient station quantity, and the inversion model exhibited less apparent responses to the low-velocity anomalies, emphasizing the necessity of dense array observations. This study demonstrates that the combined observations of synchronous and asynchronous ambient noise can be utilized for high-resolution imaging of karst characteristics.

How to cite: Liu, Y. and Xia, J.: Short-term synchronous and asynchronous ambient noise tomography in urban areas: Application to Karst investigation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7746, https://doi.org/10.5194/egusphere-egu24-7746, 2024.

17:50–18:00
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EGU24-10645
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ECS
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On-site presentation
Chaoqiang Xi, Ya Liu, and Hao Zhang

The determination of shear (S) wave velocities within near-surface earth layers holds paramount significance in the realms of hazard assessment and geotechnical applications. In the fields of geophysics and civil engineering, ambient noise surface wave methods have garnered considerable attention due to their effectiveness in determining shear wave velocities within near-surface layers, particularly in densely populated urban areas. The spatial autocorrelation (SPAC) method, introduced in 1957 for the analysis of ambient noise dispersion, has maintained enduring relevance and widespread utilization within the realm of engineering geophysics in recent years[Aki 1957; Hayashi et al.,2022].

However, the dispersion energy generated using the SPAC (Spatial Autocorrelation) method is susceptible to contamination, especially at high frequencies, resulting from the occurrence of 'crossed' artifacts[Cheng et al.,2023]. The presence of these 'crossed' artifacts leads to the intersection and distortion of dispersion energy within the frequency–velocity domain[Xi et al.,2021]. These artifacts emerge from the simultaneous fitting of both inward and outward propagating cylindrical wavefields, encapsulated within the Bessel function. To mitigate the impact of these artifacts, we advocate for the exclusive fitting of the outward propagating cylindrical wavefield. To achieve this, a combination of the spatial autocorrelation coefficient and its Hilbert transform is employed, facilitating the construction of the outward propagating cylindrical wavefield.

In our proposed improved SPAC method, we replace the Bessel function with the Hankel function for fitting the constructed outward propagating cylindrical wave. Both synthetic and real-world field examples substantiate the efficacy of the proposed method in enhancing the accuracy of surface wave multimode dispersion measurements. This modification not only eliminates the 'crossed' artifacts but also underscores the robustness of our approach in refining the precision of dispersion analysis, particularly in scenarios involving complex wavefields and varying geological conditions.

Aki, K., 1957. Space and time spectra of stationary stochastic waves, with special reference to microtremors. Bulletin of the Earthquake Research Institute, 35(3), 415–456.

Cheng F, Xia J, Xi C. Artifacts in High-Frequency Passive Surface Wave Dispersion Imaging: Toward the Linear Receiver Array. Surveys in Geophysics, 2023: 1-31.

Hayashi K, Asten M W, Stephenson W J, et al. Microtremor array method using spatial autocorrelation analysis of Rayleigh-wave data. Journal of Seismology, 2022, 26(4): 601-627.

Xi C, Xia J, Mi B, et al. Modified frequency–Bessel transform method for dispersion imaging of Rayleigh waves from ambient seismic noise. Geophysical Journal International, 2021, 225(2): 1271-1280.

How to cite: Xi, C., Liu, Y., and Zhang, H.: Improved Spatial Autocorrelation Method for Dispersion Imaging of Ambient Seismic Noise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10645, https://doi.org/10.5194/egusphere-egu24-10645, 2024.

Posters on site: Thu, 18 Apr, 10:45–12:30 | Hall X1

Display time: Thu, 18 Apr, 08:30–Thu, 18 Apr, 12:30
Chairpersons: Yesim Cubuk Sabuncu, Qing-Yu Wang
Interferometry/Sources/Amplitudes
X1.125
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EGU24-14919
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ECS
Bo Guan and Jianghai Xia

Retrieving surface waves using linear arrays is becoming gradually popular in urban areas with abundant anthropogenic noise [Mi et al., 2020]. Dispersion measurements can be problematic due to the presence of off-line noise sources. We use the polarization analysis of three-component noise recordings to estimate the back-azimuth and intensity of sources for linear arrays. The noise segment where the source locates in the stationary-phase zones (SPZs) is retained and the noise cross-correlation function (NCF) is weighted according to the source intensity. In this way, we obtain accurate virtual shot gathers and dispersion images.

A single three-component seismic station can simultaneously record vertical, north, and east displacements. The back-azimuth and intensity of a noise source within a time segment can be estimated by the relationship between the vertical-horizontal cross-spectra [Takagi et al., 2018]. In practical applications, we remove the mean and trend of the raw three-component noise recordings and divide them into multiple segments. We use the polarization analysis for each segment to locate the orientations and intensities of the noise sources. We then average the results obtained at multiple stations in a linear array to obtain more robust results. We retain the noise segments where the noise sources are distributed in the SPZs and perform weighted stacking of their NCFs according to the intensities of the noise sources in these segments to obtain the final NCF and perform the subsequent dispersion measurement. We use a synthetic experiment and two field examples to demonstrate the superiority of our proposed method. After using the proposed method, the NCFs become more accurate with a higher signal-to-noise ratio, and the trend of the dispersion energy is more continuous.

 

Takagi, R., Nishida, K., Maeda, T. & Obara, K., 2018. Ambient seismic noise wavefield in Japan characterized by polarization analysis of Hi-net records. Geophysical Journal International, 215, 1682–1699. doi:10.1093/gji/ggy334

Mi, B., Xia, J., Bradford, J.H. & Shen, C., 2020. Estimating near-surface shear-wave-velocity structures via multichannel analysis of Rayleigh and Love waves: an experiment at the Boise hydrogeophysical research site. Surveys in Geophysics, 41, 323–341. doi:10.1007/s10712-019-09582-4

How to cite: Guan, B. and Xia, J.: Improving dispersion measurement using weighted stacking based on polarization analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14919, https://doi.org/10.5194/egusphere-egu24-14919, 2024.

X1.126
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EGU24-18405
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ECS
Théo Rebert, Thomas Bardainne, Caifang Cai, Thibaut Allemand, and Hervé Chauris

Railways are exposed to geotechnical hazards such as sinkholes or subsidence because they encounter many geological settings. Railway subsurface imaging is thus important in order to detect small anomalies in the elastic properties of the upper 50 meters of soils. Seismic interferometry applied on train induced signals is a promising technique, and previous works have shown clear Rayleigh waves and reflections in the retrieved Green’s functions. However, extracting the dispersion curves of the reconstructed Rayleigh waves in a automated and robust way with high-resolution is challenging.

We study a continuously acquired dataset consisting of a dense array of five lines of accelerometers deployed parallel to 120 m of track. We use Matched Field Processing (MFP) to recover a high-resolution S-wave velocity model of the subsurface. The workflow begins by correlating signals in time windows when the train is outside the array, to ensure nearly planar wavefronts before the interferometry step. However, using only trains far from the array discards the measurements associated to the train crossing the array which have a very high signal-to-noise ratio but are difficult to model. We observe experimentally that train crossings the array generate correlations compatible with the isotropic and uncorrelated source distribution hypothesis used in passive seismology. Under this assumption, we correlate signals when the train is directly next to the sensors. Combined with correlations for the train in the far-field, this allows to track the Rayleigh dispersion curve in the very high frequencies (> 50 Hz). This broadband dispersion curve extraction, along with the balanced azimuthal coverage of our image due to the source diversity, is helpful for reliable imaging of shallow structures ranging from the bottom of the ballast to the bedrock. Since array methods are robust, and trains are repeatable sources, this paves the way for reliable monitoring of the subsurface with unprecedented temporal and spatial resolution.

How to cite: Rebert, T., Bardainne, T., Cai, C., Allemand, T., and Chauris, H.: Matched Field Processing of train vibrations for opportunistic surface wave tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18405, https://doi.org/10.5194/egusphere-egu24-18405, 2024.

X1.127
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EGU24-16682
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ECS
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Clara Vernet, Trabattoni Alister, Diane Rivet, and Marie Baillet

Distributed acoustic sensing (DAS) provides an attractive solution for ocean-bottom seismological instrumentation by providing a dense and long-distance measurement of the deformation of the ground along offshore submarine fiber-optic cables. This study reports analyses made on records acquired with a network located along the Chilean margin. We focus onto the analysis of the structure of the shallow crust, in particular, the sedimentary layer of the overlying crust, whose lateral variations suggest strong contrasts of the sedimentary recharge of the slab.

The POST experiment was carried out from October 27 to December 3, 2021 on a fiber-optic cable connecting the city of Concón (100km northwest of Santiago) to La Serena. Using strain-rate recordings for twenty local and regional earthquakes, we estimated both the thickness and shear wave velocity of sediments. We used jointly (1) travel time delays between the direct P-wave and converted Ps at the bedrock/sediment interface that were estimated from manual picks and (2) coda wave interferometry. This later was done by identifying the phase velocities of the fundamental Rayleigh wave mode on frequency-wavenumber (FK) diagrams over 2km linear arrays along the fiber in the 0.3 to 7Hz frequency band. Each dispersive curve and travel time delays between the direct and converted wave were then jointly inverted to create a 2D S-wave velocity (Vs) structure of the sedimentary layer under the fiber.

Our results show significant differences in thickness and in Vs along the cable. Two basins are observed, including the Valparaiso Forearc Basin separated by the Punta Salinas Ridge and another basin limited by a thin sedimentary layer with Vs of a few hundred m/s. In the extreme northern part of the cable, a thin layer of unconsolidated Quaternary sediments is on top of a deeper compacted sediments with faster Vs. The developed methodology comforts the potential of DAS for subsurface imaging purposes. Moreover, accurate modeling of the subsurface could be used to correct the location of earthquakes on the \iber sensors.

How to cite: Vernet, C., Alister, T., Rivet, D., and Baillet, M.: Determination of the shallow S-wave velocity structure and sedimentary thickness offshore central Chile using distributed acoustic sensing., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16682, https://doi.org/10.5194/egusphere-egu24-16682, 2024.

X1.128
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EGU24-18041
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ECS
Yajian Gao, Andreas Rietbrock, Michael Frietsch, Hans Agurto-Detzel, Sofia-Katerina Kufner, Edmond Dushi, Besian Rama, Damiano Koxhaj, Bernd Schurr, and Frederik Tilmann

Dense seismic networks are ideally suited to detect daily or even hourly variations of the global secondary microseism via ambient noise cross-correlation beamforming (CCBF) and backprojection (BP) in the slowness-backazimuth domain. We combine the seismic recordings from Hi-net in Kyushu (HINET) network, Southern California Seismic Network (SCSN), and the Large-N AlbaNian TectonIcs of Continental Subduction (ANTICS) network to capture 3-hourly and daily northern hemisphere secondary microseism variations during 2022-2023. We calculate stable ambient noise CC with 300 s lag and 24 substacks per day. In the secondary microseism period band, 1-10s, we detect clear and vigorous high apparent velocity P phase (> 8 km/s) arrivals in 3-hourly and daily stacks for these networks. Both the 3-hourly and daily stacks show clear temporal amplitude and delay time changes in station-pair-distance and symmetry changes of causal and acausal branches, indicating the active evolution of ambient noise source location and strength. For ANTICS, the strongest energy patch emerges with back-azimuth (BAZ) 280°-330° and slowness around 8-10 s/deg. Further two energy patches appear with BAZ 90°-135° and slowness of 4-6 s/deg as well as 0° BAZ and slowness of 5-7.5 s/deg . We back-project the energy from the beamforming to the source location based on IASP91 velocity model assuming the propagation of teleseismic energy as direct P wave (including Pdiff, PKiKP and PKIKP). The back-projection results reveal that the strongest energy comes from the North Atlantic covering a broad arc-shape area (from the northeast coast of the US to the west coast of the UK, and from the south of Greenland and Iceland down to 45°N). The two other energy patches with much higher apparent velocities originate from the south Indian Ocean and the north Pacific near the Aleutian Islands. The 3-hourly and daily changes are tracked and recovered by the CCBF and BP approach for all three networks. The secondary microseism variations in the north Pacific could be improved by the SCSN and HINET whereas the north Atlantic is constrained by ANTICS and SCSN. Some small-scale autumn storms near the Japanese trench are also detected and tracked. Our results are consistent with existing wave height maps and provide a new and cheap observation for hindcasting of the state of the coupling of oceans and the solid earth. 

How to cite: Gao, Y., Rietbrock, A., Frietsch, M., Agurto-Detzel, H., Kufner, S.-K., Dushi, E., Rama, B., Koxhaj, D., Schurr, B., and Tilmann, F.: Teleseismic body wave phase extracted from ambient noise interferometry constrains the secondary microseism sources of Northern Hemisphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18041, https://doi.org/10.5194/egusphere-egu24-18041, 2024.

X1.129
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EGU24-7251
Yong-seok Jang and Woo-dong Lee

The understanding of microseism-source characteristics has become increasingly important, particularly in the context of retrieving Green's functions, which play a crucial role in various fields of seismology. This study aims to elucidate the characteristics of microseismic sources, encompassing seasonal variations in the activity of primary and secondary microseisms, along with periodic changes observed in microseismic peaks. The analysis involved data from seven stations carefully selected from the Korean Meteorological Administration seismic network, each with over 10 years of continuous data. Employing cross-correlation techniques, we calculated Empirical Green's Functions (EGFs) between 17 station pairs. The averaged spectra of the calculated EGFs revealed two primary peaks, concentrating energy distribution around 18 seconds and in the period range of 2-5 seconds, aligning with the peaks associated with Primary and Secondary microseism. We then categorized spectral energy distribution data into less than and more than 10 seconds, aiming to discern distinct characteristics associated with the two microseismic peaks. Subsequently, we examined average temporal energy variations for each microseism, generating, we say, spectral-time series data by summing and averaging separated energy in the period direction, and calculating energy variations for each period using a multi-filtering technique (MFT). Observing prominent dominant changes with a 1-year period in both Primary and Secondary microseisms, we noted additional periodic variations with 6 months, 3 months, and 2 months in Secondary microseism. Specifically focusing on 1-year period changes, Primary microseism displayed dominance during the summer, with lower energy levels in the winter across the entire area. For Secondary microseism, 1-year period changes often showed the lowest values in the summer and the highest values in the winter. Additionally, in Secondary microseism, maximum or minimum values were observed in the spring and autumn, resembling patterns observed in Primary microseism. Simultaneously, we reconstructed the dominant period of ocean gravity waves from Wave Watch III to explore the effect on microseisms around the stations used in this study. Comparing this data with calculated the spectral time series data of Primary and Secondary microseisms revealed not only a match in the 1-year period but also in detailed phases below 1 year. This implies a close relationship between microseisms around the Korean Peninsula and ocean activities, prompting future research to delve into detailed periodic changes, correlate them with ocean activities, and identify their underlying causes.

How to cite: Jang, Y. and Lee, W.: Microseismic Characteristics: Seasonal Variations, Periodic Changes, and Oceanic Influences in the Korean Peninsula, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7251, https://doi.org/10.5194/egusphere-egu24-7251, 2024.

X1.130
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EGU24-17499
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ECS
Ting-Yu Lin, Ying-Nien Chen, and Ruey-Juin Rau

    Under a uniform scattered wave field, the continuous recording of Cross-Correlation Functions (CCF) between monitoring stations can be approximated by the Green's function between these stations. Therefore, passive seismic noise interference techniques can be employed to monitor changes in the structural properties of the Earth. The Zhongliao Tunnel in the southern section of National Highway No. 3 cuts through two significant active structures, the chishan Fault and the Chekualin Fault. In order to investigate the impact of fault activity on the tunnel's structure, our laboratory deployed a dense array composed of 33 portable seismometers around the Zhongliao Tunnel starting in 2020, conducting seismic observations for an entire year. With an average station spacing of less than 1 kilometer in this dense array, there is a chance to obtain high-frequency Green's functions, enabling monitoring of shallow structures. However, non-uniform distribution of noise energy may cause differences between interference waveforms and real Green's functions. Therefore, this study focuses on the spatiotemporal characteristics of background high-frequency signals, aiming to clarify their sources as a foundation for future research. Through Power Spectral Density (PSD) analysis of the stations, we observed energy drops at night in the range of 1-12Hz , possibly attributed to body waves or surface waves generated by vehicular traffic on the highway. To identify the distribution of noise sources, the research area was subdivided into 121 grid points as potential signal sources. Surface wave and body wave energy decay characteristics were fitted separately to the spatial distribution of that energy, revealing that the predominant seismic mode is surface waves, and the most likely noise source is located at the tunnel entrance, unevenly distributed on the highway. Furthermore, an analysis of the amplitude asymmetry in the cross-correlation functions between stations indicated that the high-frequency signals originate from the tunnel entrance. As there are no specific conditions near the tunnel entrance that can autonomously generate high-frequency signals, we speculate that these signals are still caused by vehicles on the highway. When seismic waves propagate around the tunnel, the velocity structure causes energy to focus at the tunnel entrance, radiating outward. In the future, we will use Eikonal tomography to analyze the velocity structure beneath the array and conduct waveform simulations to test this hypothesis.

How to cite: Lin, T.-Y., Chen, Y.-N., and Rau, R.-J.: Ambient noise characteristics of the Chung-Liao Tunnel area in National Highway No. 3, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17499, https://doi.org/10.5194/egusphere-egu24-17499, 2024.

X1.131
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EGU24-8529
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ECS
Mahsa Safarkhani, Sven Schippkus, and Céline Hadziioannou

Previous research suggests that continuous seismic noise records can effectively extract information about the properties of the Earth's subsurface. The coda of the correlation wavefield between station pairs shows sensitivity to crustal heterogeneity and has been described as a multiple scattering signal. These signals allow to monitor variations in dv/v to detect weak changes in the medium at depth. Oceanic regions, which are highly effective in generating microseisms, play a crucial role in the distribution of seismic energy sources. In Green's function estimates from cross correlations, highly asymmetric correlation wavefields are common due to non-homogeneous source distributions.

This study focuses on the impact of oceanic noise sources on the coda of the correlation wavefield between station pairs. We utilized ambient seismic noise interferometry to retrieve the correlation wavefields between some master stations throughout Europe and the Gräfenberg array located in Germany, in the microseism frequency range. We then applied cross-correlation beamforming to these correlation wavefields. This identifies the source direction for correlation wavefields over a three-year period, allowing us to compare variations in source direction and seasonality with results from raw data beamforming. We find dominant source directions towards the north-northwest of Gräfenberg in winter (with slowness expected for surface waves) and towards the south in summer (with slowness expected for body waves) in the raw data and throughout the coda of the correlation wavefields up to lapse times of one hour. This is in contrast to the diffuse wavefield expected from classical seismic interferometry and demonstrates that higher-order correlations, which are computed during the correlation beamforming of correlation functions, do not improve the degree of scattering in the correlation wavefield coda when persistent, isolated noise sources are present. Additionally, the findings demonstrate notable correlation between the seasonal incidence of microseisms and the very late coda of the correlation wavefields, raising questions about the current understanding of the correlation wavefield coda.

How to cite: Safarkhani, M., Schippkus, S., and Hadziioannou, C.: Sensitivity of coda correlation wavefields to spatio-temporal variations of microseism noise sources, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8529, https://doi.org/10.5194/egusphere-egu24-8529, 2024.

X1.132
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EGU24-10482
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ECS
Alfio Marco Borzì, Vittorio Minio, Raphael De Plaen, Thomas Lecocq, Flavio Cannavò, Giuseppe Ciarolo, Sebastiano D'Amico, Carlo Lo Re, Carmelo Monaco, Marco Picone, Giovanni Scardino, Giovanni Scicchitano, and Andrea Cannata

Microseism, the most continuous seismic signal on the Earth generated by the interaction between the hydrosphere, the atmosphere, and the solid Earth, is a useful tool for acquiring information about climate change. Indeed, several authors dealt with the relationship microseism-sea state and microseism-cyclonic activity, considering in particular tropical cyclones, hurricanes, typhoons, and recently Medicanes (small-scale tropical cyclones that occur in the Mediterranean Sea). In this study, we analyze, from a seismic point of view, several meteorological events that occurred in the Mediterranean Sea during the period November 2011 - February 2023. In particular, we consider 9 Medicanes and 4 more common storms. Despite the marked differences between them, each of these events caused heavy rainfall, strong wind gusts, violent storm surges with significant wave heights usually greater than 3 meters, and damage along the exposed coast. Occasionally, these events caused deaths and injuries. In this work, we analyzed the seismic signal recorded by 104 seismic stations, installed along the Italian, Maltese, Greek, and France coastal areas, and 15 seismic stations, installed in the Etnean area used only to perform array analysis. We deal with the relationships between the considered meteorological events and the features of microseism in terms of spectral content, space-time variation of the amplitude, and source locations tracked using two different methods (a grid search approach based on seismic amplitude decay and array techniques). By comparing the positions of the microseism sources, obtained from our analysis, with the areas of significant storm surges, retrieved from hindcast data, we observe that the microseism locations are in agreement with the actual locations of the storm surges for 10 out of 12 events analyzed (two Medicanes present very low intensity in terms of meteorological parameters and the microseism amplitude does not show significant variations during these two events). In addition, we also carried out two analyses that allowed us to obtain both the seismic signature of these events, by using a method that exploits the coherence of continuous seismic noise, and their strength from a seismic point of view, called Microseism Reduced Amplitude. By integrating the results obtained from these two methods, we can “seismically” distinguish Medicanes and common storms. Consequently, we demonstrate the possibility of creating a novel monitoring system for Mediterranean meteorological events by incorporating microseism information alongside with other techniques (e.g. wave buoy, wave gauge, and High-Frequency coastal radar) commonly used for studying and monitoring meteorological phenomena. In addition, since the seismometers were among the first geophysical instruments installed, it is possible to digitize old seismograms and examine historical data shedding new light on extreme weather events in a climate change scenario.

How to cite: Borzì, A. M., Minio, V., De Plaen, R., Lecocq, T., Cannavò, F., Ciarolo, G., D'Amico, S., Lo Re, C., Monaco, C., Picone, M., Scardino, G., Scicchitano, G., and Cannata, A.: Distinguishing between Medicanes and common seasonal storms using microseism, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10482, https://doi.org/10.5194/egusphere-egu24-10482, 2024.

X1.133
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EGU24-3756
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ECS
Microseism source estimated with three-component broadband seismometer array in Fujian and its adjacent areas
(withdrawn after no-show)
Lina Zhang and Xianglong Liu
Monitoring
X1.134
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EGU24-16974
Graça Silveira, Joana Carvalho, Martin Schimmel, Virgílio Mendes, Nuno Dias, Susana Custódio, João Fontiela, Stephen P. Hicks, and Ana Ferreira

In March 2022, a seismic crisis was declared in São Jorge Island. Despite the regular seismotectonic activity observed in the Azores Central Group, São Jorge has not exhibited significant activity since the crisis associated with an eruption in 1964. Between the fall of 2021 and the end of 2022, approximately 12,000 earthquakes (magnitudes up to ML 3.8) have been recorded, with the seismicity and geodetic modelling pointing to a magmatic intrusion. Intrusions cause gas release, fluid circulation, and pressure perturbations in the subsurface volcanic system that often induce changes in seismic velocity. Here, we probe spatial-temporal changes in the seismic velocity structure beneath São Jorge using ambient noise interferometry.

In this study, we analyzed data continuously recorded between January 2021 and December 2022 by two permanent stations (PMAN and ROSA) operated by the Instituto Português do Mar e da Atmosfera (IPMA) to investigate the presence of subsurface structural changes in response to the seismic crisis. Data were cut into 1-hr length files and filtered between 1 and 3 Hz for autocorrelation, and between 0.1 and 1.0 Hz for cross-correlation. We applied the Phase Auto- and Cross-Correlation (PAC and PCC) method to the filtered data. This method is based on phase coherence and is amplitude-unbiased. PAC and PCC functions were then linearly stacked over three days to achieve a stable noise response. To infer changes in the velocity structure, we analyzed the waveform similarity values for different time lag windows. We compared the waveform similarity results with meteorological data and ground deformation inferred from GPS. Additionally, relative velocity changes have been estimated. 

The two analyzed stations exhibit different waveform-similarity results. Preliminary interpretation of PMAN results (closer to the island center) show, in the second half of 2022, a very slight recovery of the waveform similarity at shorter lag times (shallower depths) that decreases again in the fall of the same year. Globally, data from this station exhibits a more systematic decorrelation when the crisis was declared, most likely due to perturbations in the seismic structure between < 8 - 10 km and deeper than 15 km. 

This work is a contribution to RESTLESS (DOI:10.54499/PTDC/CTA-GEF/6674/2020) and GEMMA (DOI:10.54499/PTDC/CTA-GEO/2083/2021). It was also funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).



How to cite: Silveira, G., Carvalho, J., Schimmel, M., Mendes, V., Dias, N., Custódio, S., Fontiela, J., Hicks, S. P., and Ferreira, A.: Ambient noise interferometry to investigate temporal changes in the São Jorge Island (Azores) subsurface structure associated with the 2022 seismic crisis. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16974, https://doi.org/10.5194/egusphere-egu24-16974, 2024.

X1.135
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EGU24-15768
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ECS
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Richard Kramer, Yang Lu, Qingyu Wang, and Götz Bokelmann

We use an adapted approach for long-distance high temporal resolution monitoring to investigate the daily and sub-daily behavior of seismic velocity changes. We analyze four years of continuous data from AlpArray and other local networks throughout the Central-Southern Europe. Focusing on the 1 Hz frequency we calculate seismic velocity changes based on coda wave interferometry. Our results show that we can observe a consistent periodic behavior with periods of 24 h and 12 h, with a focus primarily on the latter. We attribute these changes predominantly to variations in atmospheric pressure. These changes manifest through loading effects on the unsaturated zone and alterations in the water bodies below that.  By analyzing the spatial variations of this two-cycle-per-day behavior we found a strong correlation with extensively karstified water-bearing formations. This connection may contribute to the hydrological characterization of the near-subsurface in central Europe identifying large water reservoirs.

How to cite: Kramer, R., Lu, Y., Wang, Q., and Bokelmann, G.: Sub-daily seismic velocity changes as indicator for large vulnerable groundwater reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15768, https://doi.org/10.5194/egusphere-egu24-15768, 2024.

X1.136
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EGU24-8922
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ECS
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Highlight
Marcel van Laaten, Jozef Müller, and Ulrich Wegler

Campi Flegrei is a volcanic field located in the immediate vicinity of the densely populated area of Naples, Italy. Since 2005, the area has been experiencing a new bradyseismic crisis, i.e., a slow uplift of the subsurface caused by rising fluids in the subsurface. The uplift is accompanied by earthquake activity that has been steadily increasing for years, culminating in the strongest earthquake (ML 4.2) in the last 40 years on September 27, 2023. Such uplift and earthquakes can cause changes in seismic velocity and are often succeeded by a volcanic eruption. In this study, we utilize seismic noise to calculate velocity changes at different levels/frequencies over a 7-year period using passive image interferometry. The observed long-term velocity decrease of 1.39 % near the surface over the period from 2016 to 2023 can be explained by a volume increase of the hydrothermal system at the depth of 3 km. In 2023, the Campi Flegrei underwent several phases of velocity change. After a period of minor velocity changes, there was a gradual 0.7 % increase in velocity starting in May. Following the onset of the earthquake swarm in August, the velocity slowly decreased once again.

How to cite: van Laaten, M., Müller, J., and Wegler, U.: Monitoring seismic velocity changes at Campi Flegrei (Naples) using seismic noise interferometry - Do we see precursors of the future volcanic activity?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8922, https://doi.org/10.5194/egusphere-egu24-8922, 2024.

X1.137
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EGU24-4190
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ECS
Yang Li, Mathieu Perton, Francisco J. Sánchez-Sesma, and Zack Spica

Mexico City, the most populated city in the Americas, undergoes significant seismic hazards. Conventional seismometers often suffer from limited spatial density, restricting detailed observations in urban areas. In contrast, Distributed Acoustic Sensing (DAS) can convert standard telecommunication fiber-optic cables into dense seismic arrays, providing great potential for high-resolution spatiotemporal monitoring. Therefore, we installed a DAS interrogator in Mexico City in May 2022 in a long-term fashion to collect data for observational studies in the region. The fiber crosses the city from south to north along a 29-kilometer path following the subway track. The dataset comprises 2266 channels with a 12.8-m spacing and a 200-Hz sampling rate. 

On Sep. 19, 2022, a Mw7.6 earthquake occurred in Michoacán, approximately 450 km away from the City. Exactly 37 years after the great 1985 Mw8.1 event. The DAS system provided high-quality, ultra-dense, yet unique data for this earthquake in particular. One of the goals of this study is to assess the earthquake-induced changes in the sedimentary basin material properties. For this endeavor, we employ seismic interferometry on the ambient noise field of DAS data and the stretching method to monitor seismic velocity variations in Mexico City. Our analysis reveals a velocity drop following the 2022 Mw7.6 earthquake in some city areas. The results indicate that DAS can effectively monitor the velocity variations in urban environments, offering valuable insights for urban hazard assessment.

How to cite: Li, Y., Perton, M., Sánchez-Sesma, F. J., and Spica, Z.: Monitoring Spatiotemporal Seismic Velocity Changes Using Seismic Interferometry and Distributed Acoustic Sensing in Mexico City, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4190, https://doi.org/10.5194/egusphere-egu24-4190, 2024.

X1.138
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EGU24-10391
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ECS
Yang Lu, Qing-Yu Wang, and Götz Bokelmann

Soil moisture is a key metric to assess soil health. Water held in the shallow subsurface between soil particles enables various biogeochemical and hydrological processes indispensable to soil functions. Potential soil moisture deficit may raise the irrigation demands, which further exacerbates the stress on the water supply. The changes in soil moisture can impact climate, further amplifying the climatic anomalies and intensifying extreme weather events. Thus, understanding soil moisture and its dynamics over time are of broad scientific interest and practical implications.

Despite the vital importance of soil moisture, it still lacks sufficient means to properly assess the parameter at a regional scale, which is an essential research dimension for addressing practical issues in the agricultural and environmental sectors. 

Ambient noise seismology provides new possibilities to infer subsurface changes in a real-time, non-intrusive, and costless manner.
In this study, we map the temporal variations in soil moisture for Central-Southern Europe with ambient seismic noise. It is the first time that the seismic method has been applied to map soil moisture at a regional scale using an ordinary seismic network setup. The method helps in bridging the resolution gap between current pointwise (e.g., tensio-, electrical- and neutron-meter) and global (e.g., satellite-based remote sensing) investigations, providing complementary information for both scientific research and public decision-making. 

How to cite: Lu, Y., Wang, Q.-Y., and Bokelmann, G.: Mapping large-scale deep soil moisture variation using ambient seismic noise, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10391, https://doi.org/10.5194/egusphere-egu24-10391, 2024.

X1.139
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EGU24-19994
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ECS
Alexander Yates, Corentin Caudron, Philippe Lesage, Aurélien Mordret, Thomas Lecocq, and Jean Soubestre

Passive seismic interferometry has become a popular technique towards monitoring subsurface activities in a variety of settings. This includes active volcanoes and geothermal fields spanning a large range of temperatures (25C to 250C) in Belgium and Iceland. The method depends on the relative stability of background seismic sources in order to make repeatable measurements of subsurface properties. Such stability is typically assessed by examining the similarity of cross-correlation functions through time. Thus, techniques that can better assess the temporal similarity of cross-correlation functions may aid in discriminating between real subsurface processes and artificial changes related variable seismic sources.

In this work, we apply agglomerative hierarchical clustering to cross-correlation functions computed using seismic networks at volcanoes. These include Piton de la Fournaise volcano (La Réunion island) and Mt Ruapehu volcano (New Zealand). Clustering is then used to form groups of cross-correlation functions that share similar characteristics and also, unlike common similarity measures, the method does not require a defined reference period. At Piton de la Fournaise, we resolve distinct clusters that relate both to changes in the seismic source (volcanic tremor onset) and changes in the medium following volcanic eruptions. At Mt Ruapehu, we observe a consistency to cross-correlation functions computed in the frequency band of volcanic tremor, suggesting tremor could be useful as a repeatable seismic source. 

Our results demonstrate the potential of hierarchical clustering as a similarity measure for cross-correlation functions, suggesting it could be a useful step towards recognizing structure, or complex patterns, in seismic interferometry datasets. This can benefit both decisions in processing and interpretations of observed subsurface changes.

How to cite: Yates, A., Caudron, C., Lesage, P., Mordret, A., Lecocq, T., and Soubestre, J.: Assessing similarity in continuous seismic cross-correlation functions using hierarchical clustering, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19994, https://doi.org/10.5194/egusphere-egu24-19994, 2024.

Imaging and other applications
X1.140
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EGU24-8313
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ECS
Yongki Andita Aiman, Yang Lu, and Götz Bokelmann

Understanding the nature of the Mantle Transition Zone (MTZ) can foster our knowledge about the dynamics of the Earth, especially related to the vertical heat and mass exchange between the upper and the lower mantle. The MTZ, characterized by seismic-velocity discontinuities at depths of 410 km and 660 km, is conventionally studied using seismic waves emitted by earthquakes. However, this approach suffers from a typically uneven distribution of earthquakes, biases in earthquake location, and the complexity of earthquake processes.

In this study, we used body waves retrieved from ambient noise correlations to map the mantle transition zone beneath the US. We analyzed cross-correlation functions from more than 3500 seismic stations, including the EarthScope USArray stations during its deployment time frame between 2004 and 2013. We obtained clear short period (<10 s) P410P and P660P reflection phases by using a stacking strategy that considers global noise wave field data selection. This allows us to image the MTZ at an unprecedented high resolution, providing new constraints that can shed light on the tectonic history and the mantle dynamics in this area.

How to cite: Aiman, Y. A., Lu, Y., and Bokelmann, G.: Mantle Transition Zone (MTZ) beneath the contiguous US revealed by ambient noise cross-correlations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8313, https://doi.org/10.5194/egusphere-egu24-8313, 2024.

X1.141
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EGU24-4644
Hsiao-Ming Chang, Yuan-cheng Gung, Wu‐Yu Liao, Kai-Xun Chen, Chun-Fu Liao, Ban-Yuan Kuo, Ying-Nien Chen, and En‐Jui Lee

In this study, we aim to explore the depth range of stress-aligned anisotropy (SAA) in Taiwan. Our recent works have shown that the near-surface SAA is consistent with shear-wave splitting studies employing local earthquakes. However, it contrasts with SWS studies using deep phase (SKS) and the shallow crustal Vs anisotropy model derived from noise-derived broad-band (4-20 s) Rayleigh waves. This suggests that SAA is likely confined to the uppermost crust. Despite micro-cracks assumed to be fully closed with increasing ambient stress at depths, the depth range of the SAA mechanism remains unclear.

Our approach involves noise tomography using short-period (1-10 s) Rayleigh waves enhanced by the multicomponent stacking technique. To measure the dispersion of the isolated fundamental mode Rayleigh waves accurately and effectively, we employ a modified machine learning algorithm based on the algorithm proposed by Yang et al.(2022) We employ the Recurrent-Residual U-Net (R2U-Net) developed by Liao et al. (2021) for training. The model training data consists of dispersion diagrams from CCFs derived in various regions, including Taiwan, Japan, and the South Island of New Zealand. Approximately six thousand data are included in the training stage.

With the obtained dispersion data, we apply the wavelet-based multi-scale inversion technique to derive 3D models of Vs and Vs anisotropy. In this inversion process, the results from prior studies by Lee et al. (2023) serve as a priori constraint for the uppermost section of the model.

 

 

How to cite: Chang, H.-M., Gung, Y., Liao, W., Chen, K.-X., Liao, C.-F., Kuo, B.-Y., Chen, Y.-N., and Lee, E.: Probing the SAA Depth Range using ML-measured short-period dispersion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4644, https://doi.org/10.5194/egusphere-egu24-4644, 2024.

X1.142
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EGU24-18648
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ECS
Michael Frietsch, Thomas Forbriger, Carlo Giunchi, Matteo Di Giovanni, Luca Naticchioni, and Andreas Rietbrock

The next generation gravitational wave detector Einstein Telescope (ET) is planned to be built at a depth of about 200 m to 300 m to significantly reduce the influence of ambient seismic noise with respect to current surface detectors. Three candidate sites for ET are currently under investigation: Sardinia (Italy), Lausitz (Germany), and the Euregio Meuse-Rhine (EMR, Netherlands, Belgium, Germany). Broadband downhole and surface seismometers have been installed at all three sites over the last couple of years which now allows the comparison of seismic noise levels and reduction with depth. Furthermore, we include the Sos Enattos mine in Sardinia, as an additional reference location. We see a significant reduction in seismic noise with depth over a broad frequency range above 1 Hz and below 0.1 Hz. The most significant noise reduction is observed in the frequency band between 3 to 30 Hz for which all sites reach a noise level below 10-7 m2 s-4 Hz-1 at depth. For the Lausitz and EMR sites we measure a reduction of seismic noise with depth of up to 40 dB while Sardinia shows an exceptionally low seismic noise above 2 Hz even below the NLNM but shows the smallest improvement with depth because noise levels are remarkably low at the surface. The noise can be attributed to various sources such as anthropogenic and ocean generated microseisms. The EMR and Lausitz sites show a clear reduction of seismic noise during nights and weekends. The day/night and week/weekend dynamic of cultural noise is not very pronounced for the Sardinia site. Our favoured explanation for the extremely low noise level in Sardinia is therefore the low level of anthropogenic noise. However, the ocean generated microseism is strongest at the Sardinia site due to the Mediterranean Sea that is located only a few tens of kilometers from the candidate site location. The exceptionally low ambient noise level in Sardinia at above 2Hz exposes the self-noise of the Trilium Slimline borehole seismometer and the noise floor of the CENTAUR digitizer as the limiting factor at frequencies above 4 Hz. 

How to cite: Frietsch, M., Forbriger, T., Giunchi, C., Di Giovanni, M., Naticchioni, L., and Rietbrock, A.: Reduction of seismic noise with depth - Characterisation of ambient seismic noise for surface and borehole stations at three candidate sites of the Einstein Telescope, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18648, https://doi.org/10.5194/egusphere-egu24-18648, 2024.

X1.143
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EGU24-11436
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ECS
Martina Raggiunti, Douglas Stumpp, Paola Baccheschi, Fabio Villani, Matteo Lupi, Geneviève Savard, Vincenzo Sapia, Marina Pastori, Alessandra Smedile, Alessandra Sciarra, Sara Lovati, Marco Massa, Andrea Antonioli, Pamela Roselli, and Roberto Tonini

The intermediate-depth earthquakes usually occur at depths between 40-300 km, and are commonly related to deformation along and within the subducting plate. Promising but contrasting mechanisms of their seismic failure are proposed to model their generation and associated deformation processes, including ductile shear instability, dehydration embrittlement and failure of dry rocks. This work exploits one of the best examples worldwide of exposed sources of intermediate-depth earthquakes to better understand their nucleation environment. The study area consists of the Moncuni ultramafic massif (Southern Lanzo Massif, Western Italian Alps), a peridotite and gabbro section considered as a dry remnant of the Tethyan oceanic lithosphere subducted, during the Alpine orogeny, and then exhumed without experiencing ductile deformation and metamorphism. Moncuni geological units are extensively crossed by a network of pseudotachylytes (geological product of seismic slip associated to earthquakes), that locally preserve high-pressure minerals, suggesting an intermediate-depth seismic environment origin.

In this study, we want to better understand the nucleation environment of intermediate-depth earthquakes by peeking into the deeper structure of the ophiolitic peridotite and gabbro of the Moncuni area. We are performing a Nodal Ambient Noise Tomography (NANT), which allows crustal imaging based on the measurement of short-period surface wave dispersion curves between pairs of seismic stations. The used dataset was acquired by installing a temporary seismic network with 197 three-component nodal geophones over 250 km2 area surronunding the Moncuni massif and operating for about one month.

To perform the ambient noise data processing, we followed the procedure of Bensen et al. (2007). Before using the data, we accomplished a careful data quality analysis by checking the possible occurrence of some perturbations, monitoring several parameters like recording time, and sensor absolute position and stability. We also computed power spectral density curves for each node to investigate the occurrence of anthropogenic noise, and to select the optimal frequency band to use for the NANT. NANT is being performed extracting the empirical Green's functions (EGFs) cross-correlating time series of noise recorded at pairs of stations, so using the frequency-time analysis (FTAN) as proposed in Bensen et al., (2007), we have produced a huge amount of dispersion curves, and we applied a machine learning approach, deep convolutional neural networks, to perform automatic picking and to attribute a quality picking score. We evaluate as reliable picks with a score > 0.7. Conversely, the picks with a score < 0.7 were checked and manually corrected. The dispersion curves will be used to construct a shear-wave velocity model of the study area, allowing us to obtain a detailed image of the deep structure of the Moncuni massif with the goal of understanding whether these earthquakes originate in presence of fluids or in dry oceanic slab.

How to cite: Raggiunti, M., Stumpp, D., Baccheschi, P., Villani, F., Lupi, M., Savard, G., Sapia, V., Pastori, M., Smedile, A., Sciarra, A., Lovati, S., Massa, M., Antonioli, A., Roselli, P., and Tonini, R.: Three-dimensional deep crustal structure of the Moncuni ultramafic massif (Western Italian Alps) imaged by ambient noise tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11436, https://doi.org/10.5194/egusphere-egu24-11436, 2024.

X1.144
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EGU24-8272
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ECS
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Highlight
Ugo Nanni, Philippe Roux, and Florent Gimbert
Understanding glaciers structural heterogeneity is crucial for assessing their fate. Yet, places where structure changes are strong are often inaccessible for direct instrumentation, such as in crevasses fields. To overcome this limitation, we introduce an innovative technique that transforms seismic sources, here generated by crevasses, into virtual receivers using source-to-receiver spatial reciprocity. We demonstrate that phase interference patterns between well-localized seismic events can be leveraged to retrieve phase velocity maps using seismic Michelson interferometry. The obtained phase velocity exhibit sensitivity to changes in glacier structure, offering valuable insights into the origins of mechanical properties changes, with spatial resolution surpassing traditional methods by a factor of four. In particular, we observe sharp variations in phase velocity related to strongly-damaged subsurface areas and indicative of a complex 3-D medium. Applying this method more systematically and in other contexts will enhance our understanding of the structure of glaciers and other seismogenic environments.

 

How to cite: Nanni, U., Roux, P., and Gimbert, F.: Mapping glacier structure in inaccessible areas from turning seismic sources into a dense seismic array, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8272, https://doi.org/10.5194/egusphere-egu24-8272, 2024.

Posters virtual: Thu, 18 Apr, 14:00–15:45 | vHall X1

Display time: Thu, 18 Apr, 08:30–Thu, 18 Apr, 18:00
Chairperson: Yesim Cubuk Sabuncu
vX1.14
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EGU24-586
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ECS
Gyanasmita Pradhan, Ramakrushna Reddy, and Paresh Nath Singha Roy

Microseisms are the continuous oscillations of the earth originated from the interaction of ocean waves with the solid earth. It is divided into two types, primary microseism and secondary microseism. Primary microseisms are generated by the interaction of ocean waves in the shallow coastal part and secondary microseisms are generated due to the interaction of two waves traveling opposite towards each other in the deeper or shallower part. Secondary microseism is also known as double frequency microseism band which is divided into two parts, long period double frequency microseism band and short period double frequency microseism band.

We have taken data from IRIS DMC for ten seismic stations of the year 2018. Our study area is on the Indian Ocean. Indian ocean is considered as one of the global sources of microseism noise. In comparison to the North Indian Ocean, the Southern Ocean generates very strong amplitudes of microseism noise. Storm activity of Southern Ocean is very hazardous and destructive. In addition to that, the Antarctic circumpolar current brings warm water to the Ocean. Therefore, every year it experiences multiple cyclones which play a major role in the generation of microseism noise.

In this study, we are using the frequency-dependent polarization analysis method. Our aim is to understand the spatial variation of noise and their possible sources. Power spectral density (PSD) is calculated using the spectral covariance matrix. Diagonal elements of the matrix represent the power spectra of each component (EW, NS, and Z). For analysing the spatial variation of PSD, we have used the vertical component (Z). We have observed higher PSD in the stations that are present close to the Southern Ocean and comparatively lower amplitudes are observed in the stations far away from the Southern Ocean. Back azimuth is used to determine the dominant source direction of the noise. From our results, major source direction of noise is from the Southern Ocean while minor sources are from Bay of Bengal and Arabian Sea. Clear seasonal variation in the source direction is not observed but seasonal variation in the number of polarized signals is observed indicating maximum polarized signal in the winter season and minimum polarized signals in the summer season. Combined results of spatial variation of PSD and back azimuth analysis help us to better understand the noise sources.  

How to cite: Pradhan, G., Reddy, R., and Singha Roy, P. N.: Delineation of microseism noise sources in the Indian Ocean., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-586, https://doi.org/10.5194/egusphere-egu24-586, 2024.