Ambient Noise Analysis Research Articles

Shallow crustal structure of the northern Longmen Shan fault zone revealed by a dense seismic array with ambient noise analysis

The Longmen Shan (LMS) fault zone located in the southwest of China is not only the boundary tectonic zone of the Tibetan Plateau and South China block, but also an important part of the north–south seismic belt of China. To investigate the detailed crustal structure of the LMS fault zone, we deployed a dense seismic array of 151 short-period temporary stations in its northern section from March 16 to April 6, 2023. Continuous vertical component ambient noise waveforms recorded by these stations were cross-correlated for all station pairs, enabling the extraction of Rayleigh wave phase velocity dispersion curves across periods ranging from 0.5 s to 6 s. We then inverted these data to derive the crustal shear wave velocity using direct surface wave tomography. Meanwhile, a linear sub-array consisting of 54 stations and crossing the surface rupture of the 2008 Wenchuan earthquake was used to image the fault structure by applying the Transmitted Surface Wave Reverse Time Migration method (TSW-RTM). The ambient noise tomography results show that strong lateral heterogeneity exists in the shallow crust in the study area, with a high shear velocity in the northwest and a low shear velocity in the southeast. The low shear velocity is generally distributed between the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). The results of TSW-RTM indicate that the Wenchuan-Maoxian fault (WMF) and the YBF are both characterized with a steep dip to the northwest. Our results could serve as the important basis for the study of segmentation characteristics of the LMS fault and seismic hazard preparation and mitigation.

Assessment of seismic station performance in north Chhattisgarh, India: a comprehensive ambient noise analysis

A new network of 16 broadband seismic stations is operational in the northern Chhattisgarh region to monitor the seismicity arising in the otherwise seismically quiescent zone. Ambient noise is comprehensively analyzed to evaluate station performance and validate the data quality. Power spectral densities are computed for all stations, comparing ambient noise levels against global limits for data from November 2022 to December 2023. Results indicate that noise levels consistently fall within global noise limits. The study emphasizes the pronounced effect of instrumental tilt on the horizontal components of broadband seismometers. Spatial variation of ambient noise levels across various period bands of the noise spectrum highlights the contribution of diverse noise sources such as anthropogenic activities, surface waves, body waves, and barometric effects. Seasonal variations in short-period, microseism, and long-period bands reveal temporal variation of noise levels. Diurnal variation in the short-period band indicates a prominent effect of cultural noise. Furthermore, the proximity of seismic stations to major mining areas induces high noise levels in the very short-period band during blasting activities compared to non-blasting periods. Our study thus explores the variability of the ambient noise environment while validating the robustness and stability of the seismic installation across the study region.

Investigating Subglacial Water‐Filled Cavities by Spectral Analysis of Ambient Seismic Noise: Results on the Polythermal Tête‐Rousse Glacier (Mont Blanc, France)

AbstractPolythermal glaciers can trap considerable volumes of liquid water with the potential to generate devastating outburst floods. This study aims to identify water‐filled subglacial reservoirs from ambient seismic noise collected by moderate‐cost surveys. The horizontal‐to‐vertical spectral ratio technique is highly sensitive to impedance contrasts at interfaces, thus commonly used to estimate glacier thickness. Here, we focus on the inverse ratio, that is, the V/H spectral ratio (VHSR), whose high values indicate a low impedance volume beneath the surface, suggesting subglacial cavities. We analyze VHSR peaks from a seismic array of 60 nodes installed on the Tête‐Rousse Glacier (Mont Blanc massif, French Alps); data were gathered over 15 days. Mapping the VHSR amplitude over the free surface reveals the main cavity locations and the basal areas affected by melting within the glacier. Results obtained in the field are supported by a conceptual model based on 3D finite‐element simulations.

A Seismic Monitoring Tool for Tidal-Forced Aquifer Level Changes in the Río de la Plata Coastal Plain, Argentina

Ambient seismic noise has gained extensive applications in seismology and plays a pivotal role in environmental seismic studies. This study focuses on the Río de la Plata Coastal Plain, employing the horizontal-to-vertical spectral ratio (HVSR) method on ambient seismic noise records to analyze subsurface dynamics. The region’s hydrogeology is complex, featuring partially interconnected coastal aquifers. The HVSR analysis reveals two peaks, with P0 associated with the sediment-basement interface and P1 linked to a shallower stratigraphic discontinuity. Temporal analysis of P1 highlights cyclical patterns correlated with estuarine levels, suggesting a relationship between variations in seismic velocities and tidal dynamics. Comparisons with aquifer data support the hypothesis that tidal variations influence subsurface mechanical properties, impacting the HVSR function. The study hints at the potential of ambient seismic noise analysis as a non-invasive and cost-effective method for studying coastal aquifers and understanding groundwater dynamics. Ongoing research aims to further explore these relationships for enhanced groundwater resource management.

Sensor Misorientation of CHINArray-II in Northeastern Tibetan Plateau from P- and Rayleigh-Wave Polarization Analysis

Abstract Accurate sensor orientation is important in providing reliable data used for seismological analysis such as P-wave receiver function analysis, shear-wave splitting, and ambient noise analysis. In this study, we used three distinct P-wave and Rayleigh-wave polarization analysis methods to estimate actual sensor orientation of 660 stations from CHINArray-II in northeastern margin of Tibetan plateau. We found that ∼42.12%–45.76% of the stations are well oriented with the absolute misorientation angle <3°, 42.42%–45.45% of the stations are fairly oriented with the absolute misorientation angle ranging from 3° to 10°, and 8.94%–11.82% of the stations are oriented with the absolute misorientation angle >10°. We further compared the results of some seismological analyses before and after sensor misorientation correction, such as Rayleigh-wave ellipticity (horizontal-to-vertical ratio) and P-wave receiver functions analysis. We found that when the sensor misorientation angle is large, it may lead to incorrect seismological results. With the same sensor misorientation, its influence on different seismological analysis is also different.

Characterizing the Background Noise Level of Rotational Ground Motions on Earth

Abstract The development of high-sensitive ground-motion instrumentation for Earth and planetary exploration is governed by so-called low-noise models, which characterize the minimum level of physical ground motions, observed across a very broad frequency range (0.1 mHz–100 Hz). For decades, broadband instruments for seismic translational ground-motion sensing allowed for observations down to the Earth’s low-noise model. Knowing the lowermost noise level distribution across frequencies enabled not only to infer characteristics of Earth such as the ocean microseismic noise (microseisms) and seismic hum, but also to develop highly successful ambient seismic noise analysis techniques in seismology. Such a low-noise model currently does not exist for rotational ground motions. In the absence of a substantial observational database, we propose a preliminary rotational low-noise model (RLNM) for transverse rotations based on two main wavefield assumptions: the frequency range under investigation is dominated by surface-wave energy, and the employed phase velocity models for surface waves are representative. These assumptions hold, in particular, for a period range of about 2–50 s and lose validity towards long periods when constituents produced by atmospheric pressure dominate. Because noise levels of vertical and horizontal accelerations differ, we expect also different noise levels for transverse and vertical rotations. However, at this moment, we propose a common model for both types of rotations based on the transverse RLNM. We test our RLNM against available direct observations provided by two large-scale ring lasers (G-ring and ROMY) and array-derived rotations (Piñon Flats Observatory array, Gräfenberg array, and ROMY array). We propose this RLNM to be useful as guidance for the development of high-performance rotation instrumentation for seismic applications in a range of 2–50 s. Achieving broadband sensitivity below such a RLNM remains a challenging task, but one that has to be achieved.

Potential geothermal reservoir systems in the Kenyan Great Rift Valley and volcanic region assessed by ambient noise analysis

To identify potential geothermal reservoirs in the Kenyan Great Rift, we analyzed ambient noise data to reveal seismic velocities and map azimuth-based Rayleigh-wave phase velocities. We extracted phase velocity dispersion curves from the ambient noise cross-correlation using the zero-crossing method and constructed S-wave velocity models using 3D surface-wave tomography. S-wave velocities were lower under Silali and Paka volcanoes (i.e., in magmatic regions) and in fractured areas (i.e., geothermal reservoirs) normal to the rift axis. We detected magmas at depths of >3 km beneath the volcanoes. Low-velocity anomalies may be related to fluid migration pathways. Most potential geothermal reservoirs are located above magma bodies and below the rift axis and are associated with intensive fracturing and porous rocks. Our approach used a limited number of seismometers, making it useful for application in areas with restricted access and challenging deployment; therefore, it could be used for the assessment of preliminary results in other magmatically active regions.

Sensing Human Activity of the Guangdong–Hong Kong–Macao Greater Bay Area by Ambient Seismic Noise

Effective monitoring of human activity in urban areas is essential for social stability and urban development. Traditional monitoring methods include wearable devices, survey sensor networks, and satellite remote sensing, which may be affected by privacy and weather conditions. Ambient seismic noise recorded by seismometers contains rich information about human activity and exhibits significant temporal and spatial variations, which provides valuable insights into social mobility. In this study, we investigated the correlation between human activity and ambient seismic noise in the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) using the data recorded by 138 seismometers. Our results indicate that ambient seismic noise produced by human activity in the GBA is mainly concentrated between 2 and 20 Hz. The spatial distribution of ambient seismic noise exhibits a strong correlation with population and economy. Our results show that the analysis of ambient seismic noise can reveal the spatial and temporal impacts of different factors on human activity in the GBA, such as day and night, holidays, weather changes, national policies, and the coronavirus disease 2019 (COVID-19) pandemic. Furthermore, the analysis of 12-year-long ambient seismic noise at the Hong Kong seismic station shows a close connection between long-term changes in ambient seismic noise and local social development. This study suggests that the analysis of ambient seismic noise represents a novel method to gather critical information about human activity. Seismometers, which are widely deployed worldwide, have great potential as innovative tools for sensing human activity.

Crustal and uppermost mantle structure near the Gloria Fault, North Atlantic, from ocean bottom seismometer surface wave observations

SUMMARY In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (< 14 s) and teleseismic surface waves at long periods (> 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.

Seismic attenuation tomography of Eastern Europe from ambient seismic noise analysis

SUMMARY The Eastern-Europe region (EER), is a complex geotectonic area that captures part of the Alpine-Himalayan Orogen, the subduction of multiple NeoTethys Branches and part of the East European Craton. It is one of the most exciting geological areas in Europe due to a diversity of tectonic processes acting within it: extensional basin evolution, oceanic subduction, post-collisional volcanism, as well as active crustal deformation associated with the push of the Adria plate or the pull of the actively detaching Vrancea slab. This makes EER an excellent natural laboratory to study the behaviour of the lithosphere–asthenosphere system in a heterogeneous tectonic setting. To investigate the lateral heterogeneity and physical properties of the crust in the EER, we use ambient seismic noise data recorded by the vertical components of broad-band stations that have been operational between 1999 and 2020 in Eastern Europe and surrounding regions. We used this significant amount of data and the latest processing techniques of the ambient seismic noise field based on the continuous wavelet transform to compute cross-correlations between various station pairs, turning every available seismic station into a virtual source. The coda of the interstation cross-correlograms were used to determine coda quality factors (Qc) of Rayleigh waves in four different period ranges (3.0–5.0, 5.0–10.0, 10.0–20.0 and 20.0–30.0 s) and to invert them in the 2-D space, constructing the highest resolution attenuation tomography of the region. Our results reveal high attenuation features throughout the northeast Pannonian region, the Bohemian Massif, the East Carpathians and the Moesian Platform. Nevertheless, our findings do not emphasize a close correlation between the depth of sedimentary basins and attenuation features identified at longer periods. In addition, Qc variations are larger at short periods, indicating higher heterogeneity in the uppermost crust of Eastern Europe. Our findings demonstrate the higher efficiency of noise correlation approaches relative to earthquake data analyses investigating Qc at low frequencies.

Towards the Monitoring of Underground Caves Using Geomatics and Geophysical Techniques: 3D Analyses and Seismic Response

Analyses of climate change, due to its impact not only on the weather and the environment but also on human health and life, are one of the most important study activities made in recent years. There is relatively high confidence that glacial melt and heavy rainfall events will continue to increase. These climate-related events carry a microseismic signature that can guide monitoring activities. In the last decade, there have been growing applications of long-term continuous ambient seismic noise systems to monitor landslides and potentially unstable rock sites. This work reports some of the activities made during a project performed under the Department of Excellence on Climate Change (2018–2022), funded by the Italian Ministry for University and Research (MUR), in order to improve environmental seismic analyses. The selected test site is the Bossea Cave (NW Italy), where two seismic stations were installed. The goals were to use these stations to understand and study climate change events above the Bossea Cave, analyzing the data from a geophysical and geomatics point of view. Starting with UAV flights and photogrammetric processing to obtain a 3D model of the cave, both ambient seismic noise and microseismicity analyses highlighted an important effect of air temperature and precipitation on the seismic response of the monitored rock mass overlying the Bossea Cave. In particular, a clear effect on the ambient seismic noise spectral content and the peak frequency of the microseismic events driven by temperature and precipitation was found during the warmer monitoring months, with almost zero delays in the seismic response. This is a preliminary but important study, even if longer monitoring data and thermal modeling efforts are needed to fully understand this seasonal variation.

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Soil liquefaction potential assessment using ambient noise: A case study in Pohang, Korea

An MW 5.5 earthquake occurred in Pohang, Korea, on 15 November 2017 and caused large damage. In and around the epicentral area the earthquake also remained numerous sand boils, which is a surface representation of soil liquefaction at shallow depth during the earthquake. Soil liquefaction is one of the most dangerous consequences of an earthquake. Here we show that the spatial distribution of the ground vulnerability index (Kg), which we estimate via ambient noise analysis, correlates very well with the distribution of sand boils. Our Kg model based on the dense microtremor surveys at differing geological conditions and urbanizations indicates that only 28.4% of the study area is vulnerable to ground shaking (Kg > 20), with 91.1% of the observed sand boils occurring in these vulnerable areas. We also observed that Kg values estimated in the study correlate well with both VS30 values and geologic units in the study area. This case report confirmed that the Kg can be an affordable supplement to traditional, but expensive and time-consuming, geotechnical/geophysical techniques for the initial screening and regional evaluations. Such a Kg map can assist stakeholders in earthquake-prone regions in identifying areas more susceptible to liquefaction and bring a recommendation to consider seismic hazard mitigation.

A Temporary Broadband Seismic Array in the Largest Desert of China: TASTE

Abstract In early Permian, a mantle plume heated up the Tarim block and formed the Tarim large igneous province. It is an interesting phenomenon to explore the interaction between mantle plume and a thick continent lithosphere, because mantle plume has been proposed to be an important route of material and energy transportation in the earth. Currently, the Tarim block is surrounded by Tibetan plateau to its south edge and Tian Shan orogeny to its north edge. The combined effects of both mantle plume activity and plate tectonics should have significantly changed the structure and composition of the Tarim block because of its amalgamations in Neoproterozoic. Seismic imaging plays a key role on revealing the deep structure of the Earth, which could help unravel the questions mentioned before. However, the harsh natural environment in the central part of the Tarim basin, the Taklimakan desert—the largest desert in China—has seriously hampered the broadband seismological observation. From July 2017 to November 2019, the Institute of Geology and Geophysics, Chinese Academy of Sciences deployed a 2D broadband seismic array named the Tarim seismic Array for lithoSpheric signaTure of mantle plumE (TASTE) over the inner part of the Tarim basin with an averaged spacing of 60–70 km. The primary target of this project is to obtain a detailed lithospheric structure to resolve the possible lithospheric signatures of Permian mantle plume activity. Here we introduce the basic information of the TASTE network and the performance of these instruments by analyzing the background noise level. Preliminary results of receiver function and ambient noise analyses are also shown, which may suggest a thick sedimentary layer, as well as complicated crust and lithospheric feature.

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir System in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir System in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Characterization of El Kherba landslide triggered by the August 07, 2020, Mw = 4.9 Mila earthquake (Algeria) based on post-event field observations and ambient noise analysis

Characterization of El Kherba landslide triggered by the August 07, 2020, Mw = 4.9 Mila earthquake (Algeria) based on post-event field observations and ambient noise analysis

Common-midpoint spatial autocorrelation analysis of seismic ambient noise obtained from a spatially unaliased sensor distribution

We have introduced a passive surface-wave method using seismic ambient noise obtained from dozens of receivers forming spatially unaliased 2D arrays. The method delineates 2D or 3D S-wave velocity ([Formula: see text]) models to depths of several hundreds of meters, without using any sources. Typical data acquisition uses 50–100 vertical-component 2 Hz geophones on the surface with 5–30 m receiver spacing. Cableless seismographs with GPS record 20–60 min of ambient noise. We establish a 2D grid covering the investigation area and use a common-midpoint spatial autocorrelation (CMP-SPAC) method to calculate phase velocities, resulting in a dispersion curve for each grid point. The method provides dozens of ispersion curves in the investigation area. We use a 1D nonlinear inversion to estimate a 1D [Formula: see text] profile for each grid point, and then we construct pseudo-2D or pseudo-3D [Formula: see text] models from the 1D [Formula: see text] profiles. The precision and accuracy of the CMP-SPAC method were tested with a numerical simulation using a 3D finite-difference method. The results of the simulation demonstrated the applicability of the method to complex velocity structures. We applied the method to an active fault investigation in China. Sixty-four cableless seismographs were deployed in an investigation area of 330 × 660 m (217,800 m2) with 5 and 30 m receiver spacings for dense and sparse grids, respectively. A 3D [Formula: see text] model was obtained to a depth of 150 m from CMP-SPAC analysis. The resultant 3D [Formula: see text] model indicates approximately 50 m of vertical displacement on a known fault.

Ocean-Bottom Seismographs Based on Broadband MET Sensors: Architecture and Deployment Case Study in the Arctic.

The Arctic seas are now of particular interest due to their prospects in terms of hydrocarbon extraction, development of marine transport routes, etc. Thus, various geohazards, including those related to seismicity, require detailed studies, especially by instrumental methods. This paper is devoted to the ocean-bottom seismographs (OBS) based on broadband molecular–electronic transfer (MET) sensors and a deployment case study in the Laptev Sea. The purpose of the study is to introduce the architecture of several modifications of OBS and to demonstrate their applicability in solving different tasks in the framework of seismic hazard assessment for the Arctic seas. To do this, we used the first results of several pilot deployments of the OBS developed by Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) and IP Ilyinskiy A.D. in the Laptev Sea that took place in 2018–2020. We highlighted various seismological applications of OBS based on broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s), including the analysis of ambient seismic noise, registering the signals of large remote earthquakes and weak local microearthquakes, and the instrumental approach of the site response assessment. The main characteristics of the broadband MET sensors and OBS architectures turned out to be suitable for obtaining high-quality OBS records under the Arctic conditions to solve seismological problems. In addition, the obtained case study results showed the prospects in a broader context, such as the possible influence of the seismotectonic factor on the bottom-up thawing of subsea permafrost and massive methane release, probably from decaying hydrates and deep geological sources. The described OBS will be actively used in further Arctic expeditions.

Ambient Seismic Noise and Microseismicity Monitoring of a Prone-To-Fall Quartzite Tower (Ormea, NW Italy)

Remote sensing techniques are leading methodologies for landslide characterization and monitoring. However, they may be limited in highly vegetated areas and do not allow for continuously tracking the evolution to failure in an early warning perspective. Alternative or complementary methods should be designed for potentially unstable sites in these environments. The results of a six-month passive seismic monitoring experiment on a prone-to-fall quartzite tower are here presented. Ambient seismic noise and microseismicity analyses were carried out on the continuously recorded seismic traces to characterize site stability and monitor its possible irreversible and reversible modifications driven by meteorological factors, in comparison with displacement measured on site. No irreversible modifications in the measured seismic parameters (i.e., natural resonance frequencies of the tower, seismic velocity changes, rupture-related microseismic signals) were detected in the monitored period, and no permanent displacement was observed at the tower top. Results highlighted, however, a strong temperature control on these parameters and unusual preferential vibration directions with respect to the literature case studies on nearly 2D rock columns, likely due the tower geometric constraints, as confirmed by 3D numerical modeling. A clear correlation with the tower displacement rate was found in the results, supporting the suitability of passive seismic monitoring systems for site characterization and early waning purposes.