Earth Noise Research Articles

Robust estimates of the ratio between S- and P-wave velocity anomalies in the Earth's mantle using normal modes

Seismic tomography allows us to image the interior of the Earth. In general, to determine the nature of seismic anomalies, constraints on more than one seismic parameter are required. For example, the ratio R between perturbations in vs and vp (dlnvs and dlnvp, respectively) is studied extensively in the lowermost mantle and interpreted in terms of thermal and/or chemical anomalies. However, to jointly interpret tomographic models of variations in vs and vp or their ratio R, it is essential for them to share the same local resolution. Most existing models do not provide resolution information, and thus cannot guarantee to honour this condition. In addition, uncertainties are typically not provided, making it difficult to robustly interpret the ratio R=dlnvs/dlnvp. To overcome these issues, we utilise the recently developed SOLA tomographic method, a variant of the linear Backus–Gilbert inversion scheme. SOLA retrieves local-average model estimates, together with information on their uncertainties, whilst it also provides direct control on model resolution through target kernels. In this contribution, we apply SOLA to normal-mode data with sensitivity to both vs and vp, as well as density throughout the mantle. Specifically, we aim to develop models of both vs and vp with the same local resolution. We test our methodology and approach using synthetic tests for various noise cases (random noise, data noise or also additional 3D Earth noise due to variations in other physical parameters than the one of interest). We find that the addition of the 3D noise increases the uncertainties in our model estimates significantly, only allowing us to find model estimates in six or four layers for vs and vp, respectively. While the synthetic tests indicate that no satisfactory density models can be obtained, we easily manage to construct models of dlnvs and dlnvp with almost identical resolution, from which the ratio R can be robustly inferred. The obtained values of R in our synthetic experiments significantly depend on the noise case considered and the method used to calculate it, with the addition of 3D noise always leading to an overestimate of R. When applying our approach to real data, we obtain values of R in the range of 2.5–4.0 in the lowest 600 km of the mantle, which are consistent with previous studies. Our model estimates with related resolving kernels and uncertainties can be used to test geodynamic model predictions to provide further insights into the temperature and composition of the mantle.

Seismic Background Noise Analysis from the Seismic Station Located at the University of Abuja, Northcentral Nigeria

Abstract: In October 2018, the Federal Government of Nigeria established several seismic stations in different places in the country. Three of these stations are located in the Abuja region (the Federal Capital Territory, or FCT) and are managed by the Nigerian Geological Survey Agency. Over the past few years, several low to intermediate earthquakes have been recorded within and around the Abuja region. These occurrences have provided valuable opportunities to study seismic earth noise using the dataset collected from the VolksMeter II seismometer installed at the University of Abuja. The data was processed using the Win QUAKE software to analyze the seismic background noise by identifying its sources. This analysis aimed to examine the seasonal and/or daily noise levels and check the seismometer’s stability through spectral display. The physical examination and/or quantitative observation of the spectrum revealed that there was a distinct area with densely high ranges between 0.08 and 10 s, corresponding to a frequency range of 0.1Hz–12.53Hz. This assessment involved a full Fast Fourier Transform (FFT) calculation. The spectral analysis indicated the presence of noise burbles, primarily attributable to instrumental noise. These noise burbles were prominent in the east-west direction during the months of March, April, May, and June 2019, with frequencies of 0.03Hz, 0.04Hz, 0.03Hz, and 0.03Hz, respectively. A similar pattern was observed in the north-south direction, with noise peaks of 0.07Hz in the months of January, April, and July 2019. The findings from this study of seismic noise amplitude demonstrate a clear correlation with the global seismic noise models. The sources contributing to the station’s noise levels encompass factors such as wind, geological influences, instrumental noise, and human activities including machinery and airport operations as well as vehicular traffic. These observations highlight the potential of using background seismic noise to quantify tremors and monitor their spatial variations, a task that remains difficult with traditional approaches. Keywords: VolksMeter II, Seismic station, Background noise, Spectrum, University of Abuja.

Feature Extraction of a Non-Stationary Seismic–Acoustic Signal Using a High-Resolution Dyadic Spectrogram

Using a novel mathematical tool called the Te-gram, researchers analyzed the energy distribution of frequency components in the scale–frequency plane. Through this analysis, a frequency band of approximately 12 Hz is identified, which can be isolated without distorting its constituent frequencies. This band, along with others, remained inseparable through conventional time–frequency analysis methods. The Te-gram successfully addresses this knowledge gap, providing multi-sensitivity in the frequency domain and effectively attenuating cross-term energy. The Daubechies 45 wavelet function was employed due to its exceptional 150 dB attenuation in the rejection band. The validation process encompassed three stages: pre-, during-, and post-seismic activity. The utilized signal corresponds to the 19 September 2017 earthquake, occurring between the states of Morelos and Puebla, Mexico. The results showcased the impressive ability of the Te-gram to surpass expectations in terms of sensitivity and energy distribution within the frequency domain. The Te-gram outperformed the procedures documented in the existing literature. On the other hand, the results show a frequency band between 0.7 Hz and 1.75 Hz, which is named the planet Earth noise.

Synthetic Modeling of Ambient Seismic Noise Tomography Data

Ambient seismic noise tomography is one of the most widely used methods in seismological studies today, especially after a comprehensive Earth noise model was published and noise analysis was performed on the IRIS Global Seismographic Network. Furthermore, the Power Spectral Density technique was introduced to identify background seismic noise in the United States. Many studies have been carried out using the ambient seismic noise tomography method which can be broadly grouped into several groups based on the objectives and research targets, such as to determine the structure of the earth’s crust and the upper mantle, to know the thickness of the sedimentary basins, to know the tectonic settings and geological structures, to know volcanic systems and geothermal systems, knowing near-surface geological features and as a monitoring effort the Ambient Noise Tomography method carried out by repeated measurements or time lapse. In this study, we investigate the characteristics of the ambient noise seismic tomography method, both its advantages and limitations of the method by utilizing synthetic data modeling using a simple geological model. Synthetic data is generated based on 1D dispersion curve forward modelling and the forward modeling of surface waves travel time for each period, which is then convoluted with the wavelets of each periods, then doing reverse correlation using a reference signal to produce synthetic recording data. We found that the estimate target depth and vertical resolution depend on the recorded data periods and the synthetic data modeling can be used as a basis in determining the acquisition design.

Establishing High-Frequency Noise Baselines to 100 Hz Based on Millions of Power Spectra from IRIS MUSTANG

ABSTRACTAdvances in seismic instrumentation have enabled data to be recorded at increasing sample rates. This has in turn created a need to establish higher frequency baselines for assessing data quality, as the widely used new high-noise model (NHNM) and new low-noise model (NLNM) of Peterson (1993) do not extend to frequencies above 10 Hz. To provide a baseline for higher frequencies (10–100 Hz), we examine power spectral density probability density functions (PSDPDFs) for high-sample rate stations available from the Incorporated Research Institutions for Seismology Data Services Modular Utility for STAtistical kNowledge Gathering (IRIS MUSTANG) quality control system. We compute high-frequency high- and low-noise baselines by matching the appropriate composite PSDPDF percentile points to NHNM and NLNM power levels at overlapping frequencies (1–10 Hz) and then extending to higher frequencies (10–100 Hz) with piecewise linear fits to the matching PSDPDF percentile.We find that the Peterson NLNM remains an accurate representation of the lower bound of global ambient Earth noise because it is lower than 99.9% of Global Seismographic Network power spectral densities. We present high-frequency high- and low-noise baselines intended primarily for use by temporary networks targeting high-frequency signals (e.g., monitoring of aftershocks or induced seismicity) based on statistics of PSDPDFs from all publicly available high-sample rate data.Most publicly available high-sample rate data are recorded by temporary deployments, and the experiment design and scientific targets of these deployments strongly influence the observed statistical distribution of high-frequency noise. We anticipate that the noise baselines presented here will be useful in automated quality control of high-sample rate seismic data. However, we note that establishing a low-noise model that accurately represents the lowest possible ambient Earth noise at frequencies up to 100 Hz will require additional continuous high-sample rate data from high-quality permanent stations in low-noise environments.

New Seismic Noise Models Obtained Using Very Broadband Stations

It has been two decades since the last comprehensive standard model of ambient earth noise was published Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The PETERSON model was updated by analyzing the absolute quietest conditions for stations within the GSN (Berger et al. in J Geophys Res 109, 2005; Mcnamara and Buland in Bull Seism Soc Am 94:1517–1527, 2004; Ringler et al. in Seismol Res Lett 81(4) doi: 10.1785/gssrl.81.4.605 , 2010). Unfortunately, both the original model and the updated models did not include any deployed station in North Africa and Middle East, which reflects the noise levels within the desert environment of those regions. In this study, a survey was conducted to create a new seismic noise model from very broadband stations which recently deployed in North Africa. For this purpose, 1 year of continuous recording of seismic noise data of the Egyptian National Seismic Network (ENSN) was analyzed in order to create a new noise model. Seasonal and diurnal variations in noise spectra were recorded at each station. Moreover, we constructed a new noise model for each individual station. Finally, we obtained a new cumulative noise model for all the stations. We compared the new high-noise model (EHNM) and new low-noise model (ELNM) with both the high-noise model (NHNM) and low-noise model (NLNM) of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The obtained noise levels are considerably lower than low-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993) at ultra long period band (ULP band), but they are still below the high-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The results of this study could be considered as a first step to create permanent seismic noise models for North Africa and Middle East regions.

Moments, magnitudes, and radiated energies of non-volcanic tremor near Cholame, CA, from ground motion spectra at UPSAR

[1] By averaging the spectra of events within two episodes of tremor (on Jan. 21 and 24, 2005) across the 12 stations of UPSAR, we improved the S/N sufficiently to define source spectra. Analysis of eleven impulsive events revealed attenuation-corrected spectra of displacement similar to those of earthquakes, with a low-frequency plateau, a corner frequency, and a high frequency decay proportional to f−2. Seismic moments, M0, estimated from these spectra range from about 3 to 10 × 1011 N-m or moment magnitudes in the range 1.6 to 1.9. The corner frequencies range from 2.6 to 7.2 Hz and, if interpreted in the same way as for earthquakes, indicate low stress drops that vary from 0.001 to 0.04 MPa. Seismic energies, estimated from the ground motion spectra, vary from 0.2 × 105 to 4.4 × 105 J, or apparent stresses in the range 0.002 to 0.02 MPa. The low stress parameters are consistent with a weak fault zone in the lower crust at the depth of tremor. In contrast, the same analysis on a micro-earthquake, located near Cholame (depth = 10.3 km), revealed a stress drop of 0.5 MPa and an apparent stress of 0.02 MPa. Residual spectra from ω−2 model fits to the displacement spectra of the non-volcanic tremor events show peaks near 4 Hz that are not apparent in the spectra for the microearthquake nor for the spectrum of earth noise. These spectral peaks may indicate that tremor entails more than shear failure reminiscent of mechanisms, possibly entailing fluid flow, associated with volcanic tremor or deep volcanic earthquakes.

Self-Noise Models of Seismic Instruments

Based on coherence analysis methods we develop a method for computing low self-noise models of seismic sensors. We calculate self-noise models for 11 different production seismometers. This collection contains the majority of sensors currently in use at Global Seismographic Network Stations. By developing these noise models, with a standard estimation method, we are able to make absolute comparisons between different models of seismic sensors. This also provides a method of identifying quality variations between two or more of the same model sensor. Studying Earth's free oscillations requires a large amount of seismic data with a high signal-to-noise ratio at long periods (Laske 2004). Recent tomographic studies using ambient seismic noise (Shapiro et al. 2005) also require the self-noise of seismic instruments to be below that of the Earth's ambient background noise, because as they use Earth noise as the seismic signal. It is also important when making temporary sensor deployments that the instrument's noise levels are below that of the signals being used in the study (Wilson et al. 2002). In order to verify that seismic instruments meet the above demands and other user requirements it is important from a testing standpoint, that one be able to measure the self-noise of seismic sensors and develop baselines for different models of seismic instruments. The different methods used to estimate self-noise of seismic sensors have made it difficult to do side-by-side comparisons of their performance (Hutt et al. 2009). This lack of a self-noise estimate standard makes it difficult to assess when a sensor's self-noise is above the manufacturers' specifications, indicating a possible problem with the sensor or noisy site conditions. In sensor development it is important to be able to compare a prototype sensor's self-noise to that of known self-noise levels of a reference sensor. On top of these complications some …

Ambient Earth noise: A survey of the Global Seismographic Network

It has been a decade since the last comprehensive model of ambient Earth noise was published (Peterson, 1993). Since then, observations of ambient Earth noise from the Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network (GSN) of widely distributed, similarly equipped, and well‐calibrated stations have become available. The broad geographic sampling of this large data set and the ease of access to waveform data provided by the IRIS Data Management System facilitate analysis of global noise samples. We have analyzed data from the 118 GSN stations operating during the year July 2001 through June 2002. On the basis of over 738,000 hourly spectral estimates computed from these stations' data, we have developed a robust noise model that exhibits significant differences from previous models both in the normal mode and body wave bands. Our analysis technique has the advantage that we do not need to search for quiet periods but can include all data where the instruments are operating correctly.

Ambient Noise Levels in the Continental United States

We present a new approach to characterize the background seismic noise across the continental United States. Using this approach, power spectral den- sity (PSD) is estimated at broadband seismic stations for frequencies ranging from 0.01 to 16 Hz. We selected a large number of 1-hr waveform segments during a 3-yr period, from 2001 to 2003, from continuous data collected by the U.S. National Seismograph Network and the Advanced National Seismic System (ANSS). For each segment of continuous data, the PSD is estimated and smoothed in full- octave averages at 1/8 octave intervals. Powers for each 1/8 period interval were then accumulated in 1-dB power bins. A statistical analysis of power bins yields probability density functions (PDFs) as a function of noise power for each of the octave bands at each station and component. There is no need to account for earth- quakes since they map into a background probability level. A comparison of day and night PDFs and an examination of artifacts related to station operation and episodic cultural noise allow us to estimate both the overall station quality and the level of earth noise at each site. Percentage points of the PDFs have been derived to form the basis for noise maps of the contiguous United States at body-wave frequencies. The results of our noise analysis are useful for characterizing the performance of existing broadband stations and for detecting operational problems and should be relevant to the future siting of ANSS backbone stations. The noise maps at body- wave frequencies should be useful for estimating the magnitude threshold for the ANSS backbone and regional networks or conversely for optimizing the distribution of regional network stations.

Seismic transient deconvolution with model‐based signal processing

Short duration seismic disturbances, obscured by earth noise and distorted by the seismometers used to measure them, can be reconstructed using model‐based signal processing. “Model based” means that mathematical models of the seismic transient, earth noise, and seismometer dynamics are infused into the signal processor that estimates the disturbance. The processor imposes no predetermined structure on the transient and the earth noise need not be white. Model‐based processors produce good quality estimates for a broad class of transient waveforms.

Rock friction and its implications for earthquake prediction examined via models of Parkfield earthquakes.

The friction of rocks in the laboratory is a function of time, velocity of sliding, and displacement. Although the processes responsible for these dependencies are unknown, constitutive equations have been developed that do a reasonable job of describing the laboratory behavior. These constitutive laws have been used to create a model of earthquakes at Parkfield, CA, by using boundary conditions appropriate for the section of the fault that slips in magnitude 6 earthquakes every 20-30 years. The behavior of this model prior to the earthquakes is investigated to determine whether or not the model earthquakes could be predicted in the real world by using realistic instruments and instrument locations. Premonitory slip does occur in the model, but it is relatively restricted in time and space and detecting it from the surface may be difficult. The magnitude of the strain rate at the earth's surface due to this accelerating slip seems lower than the detectability limit of instruments in the presence of earth noise. Although not specifically modeled, microseismicity related to the accelerating creep and to creep events in the model should be detectable. In fact the logarithm of the moment rate on the hypocentral cell of the fault due to slip increases linearly with minus the logarithm of the time to the earthquake. This could conceivably be used to determine when the earthquake was going to occur. An unresolved question is whether this pattern of accelerating slip could be recognized from the microseismicity, given the discrete nature of seismic events. Nevertheless, the model results suggest that the most likely solution to earthquake prediction is to look for a pattern of acceleration in microseismicity and thereby identify the microearthquakes as foreshocks.

Geodetic Predictions of a strike‐slip fault model: Implications for intermediate‐ and short‐term earthquake prediction

Mechanical models of faulting during an earthquake cycle now exist that are based on laboratory‐derived constitutive laws for rock friction. Such models allow calculation of geodetic changes as a function of depth, distance from the fault, and time and are useful for gaining insight on the magnitude and timing of geodetic changes that might occur prior to an earthquake. We have used the Tse and Rice (1986) model of a crustal strike‐slip fault to study the sensitivity of surface deformation to changes in the values of the friction constitutive parameters and the amount of dynamic overshoot during an earthquake. Models were examined with values of the characteristic decay distance Dc in the constitutive law ranging from 5 to 80 mm, with near‐surface values of the steady state velocity dependence a−b of −0.0015 and −0.003, and with either zero or 30% dynamic overshoot. The time variations of the predicted displacement, velocity, strain, and strain rate distributions are strongly affected by changes in Dc and a−b, and only slightly affected by the amount of dynamic overshoot. The larger values of Dc and the less negative value of a−b (i.e., large ratios of Dc to b−a) produce larger and earlier precursory signals. The most useful signals for predicting the model earthquakes are strain changes near the fault, because for nearly all model parameters studied, the predicted strain changes are considerably larger than the detectability limits of existing borehole strain meters in the presence of earth noise. Detectable changes occur a few minutes to a month prior to the earthquake, depending on parameter values. Premonitory increases in velocity also occur; a pattern of larger velocities at distances a few kilometers away from the fault trace is a diagnostic indication of accelerating slip at depth that would be useful for predictive purposes. Unfortunately, these velocity changes are detectable with creep meters and two‐color laser electronic distance measuring devices only for the largest ratios of Dc to b−a. In agreement with recent measurements for several earthquakes, the predicted amount of preseismic moment release is <0.5% of the earthquake moment for most simulations, and the postseismic moment release is a significant fraction of the earthquake moment. This two‐dimensional model suggests that earthquakes of magnitude 6 or greater on strike‐slip faults that cut the Earth's surface could be predicted using existing instrumentation, unless the ratio of Dc to b−a for natural faults is equal to or smaller than the smallest values investigated by Tse and Rice (1986). Study of three‐dimensional models is needed, since premonitory signals will be smaller if along‐strike variation is considered.

Use of a signal averager with minicomputer facilities for the control, data recording and evaluation with an ODMR spectrometer system

With simple interfacing the following features could be achieved: accumulation of weak luminescence spectra by scanning several times over the region of interest; digital data acquisition from a photon counting system, thereby eliminating problems with earth loops and noise; closed loop control system of averaging; and immediate data evaluation during the experiments.

The specification of wide-band recorders

Working models for signal, path and earth-noise characteristics are developed which collectively define the input expected to a seismic recorder. These are used to elucidate the problems of frequency and dynamic ranges of wide-band recorders designed to record the whole body-wave signal received above earth noise. An optimised recorder characteristic is derived which minimises dynamic range.

The sensitivity and dynamic range of inertial seismographs

Optimizing the sensitivity and dynamic range of inertial seismographs can best be accomplished by giving consideration to the ambient earth noise spectrum, as related to the periods of the waves to be observed and, by inference, the response band of the seismograph. Seismometer sensitivity is limited by thermal agitation, related to the damping mechanisms, some of which provide the necessary output of information. Earth noise spectrums and seismometer thermal agitation are presented as accelerations per unit bandwith to show their common nature and inseparable evidence in practice. Feedback systems do not alter the fundamental relations but may be advantageous for controlling the instrumental bandwith or where the physical size is restricted.

The sensitivity and dynamic range of inertial seismographs

The literature on earth noise over the spectrum of periods from 0.1 s to 12 h is reviewed and presented as accelerations which, when expressed as spectral‐noise density, allow all seismographs to be considered on a common basis of sensitivity. Their ultimate sensitivity is limited by thermal agitation of their suspended masses and is calculable by considering the energy flow to and from their lossy elements in accordance with principles elucidated by Nyquist. Practical instruments, including those with feedback loops, are considered as examples. Seismometers and accelerometers are treated here as resonant systems having one degree of freedom. The electrical outputs of their transducers are classified as ‘velocity’ or ‘displacement.’ The dynamic range required to record these outputs depends on the range of accelerations imposed on the instrument and upon the combined filtering effects of the resonant system and transducer. Gravitational variation, tilt, and tidal accelerations are considered as they affect the instrumental problem.

A comprehensive study of long-period (20 to 200 seconds) earth noise at the high-gain worldwide seismograph stations

abstract The characteristics of long-period (20 to 200 sec) earth noise have been investigated at the 11 high-gain, long-period (HGLP) seismograph stations using as many as 3 years of analog seismograms and digital data from these stations. A major contribution of this study is the significant increase in the number of long-period earth-noise spectra that are available from areas of vastly differing geological and tectonic settings outside the contiguous United States. This information is important to the study of the long-period portion of the spectra of earthquakes and to the discrimination of earthquakes and underground explosions. A significant feature of earth-noise spectra based on data from all the HGLP stations is a minimum between about 25 and 45 sec. This minimum spectra level is most pronounced at sites with more than 200 m of overburden. The level of earth noise generally recorded by vertical-component seismographs at both the shallow and deep stations is independent of local meteorological conditions if the seismographs are located on competent bedrock and are subject to rigid environmental controls. The level of earth noise recorded by horizontal component seismographs at HGLP stations, however, is dependent on local meteorological conditions. This dependency decreases with increasing depth of overburden. The decay rate with depth of this horizontal earth noise is such that approximately 150 m of competent overbuiden is usually sufficient to attenuate about 90 per cent of the earth noise caused by local atmospheric conditions.

A preliminary report on infrasonic waves as a source of long-period seismic noise

Earth motion in response to atmospheric pressure changes is a major contributor to the ambient seismic background in the 20- to 100-s period range. During windy intervals at the surface the quasi-static deformations caused by wind-generated pressure changes are often quite large in comparison to ambient vibrations from other sources. Recent studies [Sorrells et al., 1971; Ziolkowski, 1973] have demonstrated that the wind-related noise can be eliminated by installing the sensors at depths of the order of several hundred meters. When such action is taken, it is found that the noise levels obtained approximate those observed at the surface during calm intervals. The calm interval noise level thus constitutes the current minimum threshold. The calm interval noise level in the 20- to 100-s period range is largely the result of earth motion from atmospheric pressure sources of undetermined origin [Savino et al., 1972]. If noise levels are to be reduced below the currently realizable minimums, it will be necessary to establish the origin of the atmospheric pressure variations with some precision. In this paper, evidence is presented that supports the hypothesis that much of the earth noise observed during calm intervals in the 20- to 100-s period range is the result of atmospheric infrasonic waves. If this hypothesis is borne out by additional studies, then efforts to reduce further the current long-period noise threshold by increasing installation depths will not yield satisfactory results unless depths of the order of 1–3 km are reached. The relatively high costs of installing systems at such depths would rule out this particular method for obtaining further reductions in the long-period seismic noise threshold. Thus if the current noise threshold is largely the result of infrasonic waves, then the practical limits of using depth of installation for the suppression of seismic noise have already been reached.

Teleseismic and Earth Noise Monitoring with the Block-Moore Quartz Accelerometer

A study has been made of some recent data recorded through the seismic channel of a broad-band vertical quartz accelerometer operating near San Diego, California. Such a study has provided information concerning instrument capabilities for detection of surface waves from small teleseisms and Earth noise levels, including temporal variations. The data suggest that detection thresholds of the same order as those of the high-gain long-period installation at Ogdensburg are attainable at a surface site with the quartz instrument. Indications are that an approximate 10 db gain in surface wave detection would in general provide high probability of detection (c. 0.85) of mb = 4 earthquakes at distances of ∼40°. In addition, results suggest that noise levels in the range 5–200s recorded at this site with the accelerometer are comparable to those obtained at other surface sites, and somewhat higher than those obtained at depth for periods longer than about 30s. A study of number of events recorded has demonstrated that nearly half (∼47 per cent) of all events reported by the NOAA PDE program were detected.