Back Azimuth Research Articles

Observation and analysis of infrasound signals from the 1 January 2024 Japan earthquake

The 1 January 2024 Japan Earthquake occurred off the coast of Noto Peninsula, Japan. Three infrasound arrays s located more than 1400 km away from the epicenter received both local seismic and infrasound signals atmospherically propagating from the epicenter. The back azimuth, elevation angle and other parameters are calculated for the two types of infrasound signals received by the subarray through procedures of Progressive Multi-Channel Correlation (PMCC) and Frequency-Wavenumber (F-K) analysis. Source localization and other processing are also conducted. A recognition feature based on a low-frequency multi-scale entropy algorithm is proposed, and the received seismic signals of this event are correctly identified by using Support Vector Machine (SVM). This paper explores the calculation of earthquake parameters based on local infrasound wave and atmospherically propagated infrasound signals excited by distant earthquake events, and proposes a new method for infrasound signals event identification.

In-situ calibration for real-time infrasound station monitoring and improved wave parameter determination

Infrasound signals can be detected using a time-delay of arrival approach to derive the back azimuth and trace velocity of the coherent wave. For these calculations, it is necessary to have a calibrated measure of the pressure. Although the calibration of microbarometers can be performed in a laboratory setting and in the field with specific metrological means such as those developed by the CEA, it is much more difficult to determine the transfer function of the wind noise reduction systems (WNRS), designed to reduce the wind associated noise. In-situ calibration of these WNRS's can be performed using a co-located, calibrated reference sensor. System defects, such as flooded pipes or blocked inlets, have significant impacts on the response, which in turn would influence the calculated infrasonic wave parameters. These defects can be characterized using in-situ calibration measurements. Experiments were undertaken at the infrasound station IS26 as part of the EMPIR Infra-AUV project, using a temporary detector that was used to simulate defects on the WNRS. This allowed for the effects on real infrasound detections to be quantified and corrected using in-situ calibrations. Additionally, case studies at IMS stations were performed demonstrating the utility of these methods on operational infrasound stations.

On Monitoring of mining explosions conducted on the territory of the former Semipalatinsk Test Site

Several mineral deposits located on the territory of the former Semipalatinsk Test Site (STS) conduct active blasting activity. The largest number of explosions is carried out at the Karazhyra coal mine. These explosions are recorded by stations of the IGR NNC RK network, three of them are part of the International Monitoring System. These are seismic arrays Borovoye (AS057), Makanchi (PS23), Kurchatov-Cross (AS058). The distances to the mines from these stations are 668 km, 452 km, 68 km, respectively. In total, about 2800 explosions were recorded during 19 years of observations. The analysis of the results of routine processing on operational seismic bulletins of the IGR NNC RK Data Center has shown that the field of received epicenters of explosions exceeds the size of the quarry. A detailed processing of explosions waveforms was carried out to find out the reasons of this scatter. Geotool and DTK – GPMCC software included in the NDC-in-a Box package was used to process and analyze the waveforms. It was found that back azimuth values for different regional phases Pn, Pg, Sn, Lg differ from each other and have different dispersion. There are systematic deviations of back azimuths for some phases from the true one. A clear dependence of the epicenter determination accuracy on the energy (power) of the explosion was noted. Subjective factors in the work of analysts that affect the accuracy of estimates are also found. Processing recommendations to improve accuracy were given for analysts of Kazakhstan National Data Centre (KNDC).

Single-Station Back-Azimuth Determination with the Receiver Function Rotation Technique Validated by the Locations of Earthquakes, Impacts, and Explosions

Abstract The success of seismometer installations on the ocean floor, polar regions, remote continental areas, and even other planets’ surfaces has sparked renewed interest in determining the location via the azimuthal direction of a seismic event recorded by a single station, also known as the back azimuth (BAZ). However, classical algorithms for the BAZ estimate, like principal component and polarization analyses based on P-wave particle motions, are prone to ambiguities of 180°. Motivated by the sensor orientation correction techniques used for ocean-bottom seismometers and land stations for known event locations, we explore a receiver function rotation (RFR) method to determine the BAZ for events recorded by a single station. It is a parameter search over a range of horizontal component rotation angles from 0° to 360°. The fundamental feature of the method is that the direct P wave in the radial receiver function (RF) will have the maximum amplitude when the rotation from the ZNE system (vertical, north–south, and east–west) to ZRT (vertical, radial, and tangential) is aligned with the BAZ of the incoming P wave. Hence, the largest amplitude at zero time of the ensemble of RFs computed for different horizontal component rotations shows the optimal BAZ, which is consequently free of the 180° ambiguities. The technique’s performance is validated using the well-documented location of the 2017 Democratic People’s Republic of Korea nuclear explosion and over 1200 cataloged earthquakes on the two permanent stations in Australia. We further benchmark the RFR algorithm by the locations of two ground-truth Martian impact events documented by the orbital camera and recorded by InSight’s seismometer. Our method helps enhance the reliability of BAZ estimation as a complementary scheme to other methods. It can be used in remote areas on Earth and on the future missions to the Moon and other planets.

Lake Baikal Microseisms Based on Regional Seismic Network Data

Seismic ambient noise in the regional seismic network in the central part of the Baikal rift is studied. The probabilistic approach is used to thoroughly investigate the pattern of diurnal variations in microseisms and to analyze amplitude level and frequency content of spatial anomalies and temporal changes (seasonal and annual). Based on the 2020–2021 data, a regional probabilistic model of the microseismic noise is built in a wide range of periods. The study of microseisms in the frequency band of about 1 Hz revealed a seasonal anomaly against the level of the global minimum in the microseismic noise power spectrum. The anomaly is observed from May to December at seismic stations surrounding Lake Baikal except for the northern part of the lake. The direction of the back azimuth in the frequency range of about 1 Hz indicates the location of the lake, suggesting that these signals can be identified as lake microseisms. The high coherence values suggest a linear relationship between the wind speed and the occurrence of lake microseisms. The detailed analysis of the spectral and polarization parameters of seismic ambient noise revealed two types of lake microseisms with frequencies of 0.4–0.7 and 0.7–1.5 Hz. The first frequency interval is likely to correspond to single-frequency lake microseisms, while the second interval contains the frequency ranges of dual-frequency microseisms.

On-site infrasound calibration to correct wave parameter estimation.

The International Monitoring System (IMS) has been established as part of the Comprehensive Nuclear-Test-Ban Treaty to monitor nuclear testing and is comprised of infrasound, hydroacoustic, seismic, and radionuclide stations; it is also used more widely by the scientific community for scientific and civilian applications. For the infrasound stations, on-site calibration provides an accurate measure of the sensor (microbarometer + wind-noise reduction system) frequency response, used to monitor that the sensor response remains within tolerance of the baseline established when the station is certified. However, this on-site calibration can also be used when there are issues/defects with the sensors. As a result, the on-site calibration can be used to correct wave parameter estimations and increase the detection capability of the station. Examples using an experimental sensor at the IMS station IS26 (Germany) and IS47 (South Africa) demonstrate that errors of several degrees and tens of m/s can be introduced, under certain conditions, for the back azimuth and trace velocity, respectively. By using the on-site calibration, these errors are removed, and the correct back azimuth, trace velocity and amplitude are retrieved. This can be especially useful for the identification of infrasound signals, and the localization of their sources.

Estimating seismometer component orientation of the Brazilian seismographic network using teleseismic P-wave particle motion analysis and directional statistics

Seismometer component orientation is one of the most critical parameters for seismology, especially for methods that take wave field polarization effects into account, such as receiver functions, seismic anisotropy, surface wave dispersion and seismic tomography. Using P-wave particle motion combined with directional statistics, we estimated the orientation for 90 stations of the Brazilian Seismographic Network (RSBR), corresponding to 105 seismometers. The difference between the number of stations (90) and seismometers (105) is due to instrument replacement during station operation. We observed that 81 seismometers have an orientation of ±5° and 19 seismometers greater than ±5°. Only one seismometer has an orientation greater than 90° (BOAV station). ON and BR are the subnetworks of RSBR with the largest number of seismometers with orientation errors bigger than ±10° (6 for each one) and NB is the only subnetwork with no misoriented station. All the estimated orientations were close to the measured in the field, which is evidence that our approach is more accurate to detect misoriented stations than previous studies that analyzed the orientation of RSBR stations. The Watson's U2 statistical test indicated that the back azimuth deviations for Precambrian Basement and Phanerozoic Cover were significantly different, suggesting that the regional geology could affect back azimuth estimates.

Six‐Component Earthquake Synchronous Observations Across Taiwan Strait: Phase Velocity and Source Location

AbstractWith the development of rotational seismometers, especially the high‐sensitivity optical seismometers, rotational motions have shown increasing benefits for calculation of dispersion curves, estimation of back azimuth and many other applications. Here we present the six‐component observations of 2019 Hualien mww6.1 earthquake simultaneously on both sides of Taiwan Strait. Seismograms and time‐frequency spectrums show that the attenuations for rotations are more significant than translations. Calculations of the surface wave phase velocity are performed for both stations through single‐station records, indicating a distinct difference owning to the instrument installation conditions and site effect. Comparison of theoretical dispersion curves is consistently in trends with estimations, and reveals the influence of higher‐mode surface waves. For the first time, we locate the source of a earthquake through intersection of two stations' back azimuths estimated through rotational methods. Results of comparing to translational polarization verify that rotational observation can distinctly reduce the deviation of source location.

The optical, seismic, and infrasound signature of the March 5 2022, bolide over Central Italy

On March 5, 2022, a 12 kg meteoroid crossed the sky above Central Italy and was observed by three different observational systems: the PRISMA all-sky camera network (10 stations), the Italian national seismic network (61 stations), and a 4-element infrasound array. The corresponding datasets, each with its own resolution, provided three independent assessments of the trajectory, size and speed of the meteoroid. The bolide traveled across central Italy with an azimuth of 102 degrees, becoming visible at about 91 km above sea level with a velocity of about 15.4 km/s. Its visible trajectory lasted about 15 s. Reasonably, the residual portion of the ablated bolide terminated its path in the Adriatic Sea and could not be recovered. Seismic and infrasound data well match optical observations detecting the bolide Mach cone at 68 km above sea level with a back azimuth of 25 degrees with respect to the array. By comparing results from the three different systems, discrepancies are within the estimated uncertainties, thus confirming the mutual consistency of the adopted methodologies. Therefore, this study shows that different approaches can be integrated to improve the detection capability for bolide crossing the sky in monitored regions.

Crustal Anisotropy from the Birefringence of P-to-S Converted Waves: Bias Associated with P-Wave Anisotropy

Many researchers have used the birefringence of P‑to‑S converted waves from the Moho discontinuity to constrain the anisotropy of Earth’s crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, which adds to the splitting acquired during its propagation from Moho to the seismometer. We find that large variations in Ps birefringence estimates with back-azimuth occur theoretically in the presence of P‑wave anisotropy, which normally accompanies S‑wave anisotropy. The variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations, but is commonly found in Earth’s crust. We simulated globally-distributed P‑coda datasets for 36 distinct 4‑layer crustal models with combinations of elliptical shear anisotropy or compressional anisotropy, and also incorporated the higher-order anisotropic Backus parameter C. We tested both horizontal and tilted symmetry-axis geometries and tested the birefringence tradeoff associated with Ps converted phases at the top and bottom of a thin high‑ or low‑velocity basal layer. We computed composite receiver functions (RFs) with harmonic regression over back azimuth, using multipletaper correlation with moveout corrections for the epicentral distances of 471 events, to simulate a realistic data set. We estimate Ps birefringence from the radial and transverse RFs, a strategy that is similar to previous studies. We find that Ps splitting can be a useful indicator of bulk crustal anisotropy only under restricted circumstance, either in media with no compressional anisotropy, or if the symmetry axis is horizontal throughout. In other, more-realistic cases, the inferred fast polarization of Ps birefringence estimated from synthetic RFs tends either to drift with back-azimuth, form weak penalty-function minima, or return splitting times that depend on the thickness of an anisotropic layer, rather than the birefringence accumulated within it.

Olivine Fabrics in the Oceanic Lithosphere Constrained by Pn Azimuthal Anisotropy

AbstractWe constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.

Contribution to Uncertainty Propagation Associated with On-Site Calibration of Infrasound Monitoring Systems

To improve the confidence and quality of measurements produced by regional and international infrasound monitoring networks, this work investigates a methodology for propagating uncertainty associated with on-site measurement systems. We focus on the propagation of sensor calibration uncertainties. The proposed approach is applied to synthetic infrasound signals with known back azimuth and trace velocity, recorded at the array elements. Relevant input uncertainties are investigated for propagation targeting the incoming signals (noise), instrumentation (microbarometers, calibration system, wind noise reduction system), and the time-delay-of-arrival (TDOA) model (frequency band). Uncertainty propagation is performed using the Monte Carlo method to obtain the corresponding uncertainties of the relevant output quantities of interest, namely back azimuth and trace velocity. The results indicate that, at high frequencies, large sensor uncertainties are acceptable. However, at low frequencies (<0.1 Hz), even a 2∘ sensor phase uncertainty can lead to errors in the back azimuth of up to 5∘ and errors in the trace velocity of 20 m/s.

Shear-Wave Anisotropy Measurements in the Crust from Receiver Functions: An Interplay of Lower and Upper Crustal Anisotropy

We report a study using teleseismic P-wave receiver functions to infer the orientation of the maximum horizontal principal stress from the direction of upper crustal shear-wave velocity anisotropy. We apply an inverse approach using the Neighborhood Algorithm to conduct a nonlinear search, attaining a best-fitting crustal model that includes shear velocity anisotropy. Unlike previous methods reported in the literature, this method is able to distinguish anisotropy in the upper, brittle crust from that in the lower, ductile crust in certain instances. We apply this method to teleseismically recorded earthquakes in the Central Valley of California, the Permian Basin, Texas, northern Oklahoma and sites near the San Andreas Fault in California. Of the forty-one stations to which we apply this method, twenty have a good apparent signal. A misfit calculation is performed by calculating a zero-lag cross-correlation coefficient for each modeled receiver function with the data for a given back azimuth range. While the fast polarization direction in the upper crust of some of these stations aligns with independent indicators of the direction of the maximum horizontal principal stress, the fast direction in the upper crust at other stations does not, apparently indicating that the anisotropy was resulting from a different mechanism.

Automatic Determination of Back Azimuth Based on a Single Mini Array for an Earthquake Early Warning System

ABSTRACT Earthquake early warning (EEW) systems systematically monitor ground motion and provide alerts prior to damaging shaking. However, these systems present certain challenges, which include avoiding alerts caused by false triggers or delayed alerts for earthquakes from outside the seismic network. Here, we show that an array processing approach, which is capable of identifying the propagation direction and type of seismic phase, can overcome these challenges. In this study, a new automatic array processing approach was implemented using a single mini array to determine the back azimuth and slowness of the phases for local events. Seismograms of local events with ML 2.0–4.6 that occurred in the Korean Peninsula were analyzed using three array processing methods: beam packing, frequency–wavenumber, and plane-wave fitting. In addition, a combined P-wave picking procedure was introduced, and its performance was evaluated. We developed the criteria associated with slowness range and consistency of back azimuths to determine a stable back azimuth from the three array processing methods, estimating the back azimuth of the seismic phase with more accuracy than any single method. It is expected that seismic mini arrays and the developed automatic array processing approach can be used for network EEW systems to discriminate alarms caused by false triggers. In addition, the developed approach can be used for rapid onsite and front-detection warnings using the array inside or on the outskirts of a network when combined with other estimations, such as an epicentral distance or another back azimuth.

Study TOMAS Cyclone Using Seismic Array and Single Station

The continuous data from the YL array and four AU stations during the lifetime of the cyclone TOMAS in March 2010 were downloaded from IRIS. By performing frequency-wave number (F-K) analysis on the array data, it was found that the orientation of the maximum energy of the secondary microseisms (0.1~0.5 Hz) was consistent with the movement of TOMAS when the central wind speed reached the typhoon level. The high wind speed of the cyclone could generate secondary microseisms as well as the high swell. However, the large global earthquake can affect the microseismic observations using F-K. The AU stations have a better microseism observation than the YL array, which might be due to the vibrating and tilting of the hydrophone caused by the turbulence. The F-K analysis on microseisms can produce better slowness and back azimuth observations than polarization.

Megameter propagation and correlation of T-waves from Kermadec Trench and Islands

On 18 June 2020 and 4 March 2021, very energetic low-frequency underwater T-wave signals (2 to 25 Hz) were recorded at the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydrophone stations in the Pacific Ocean (Stations HA11 and HA03) and the South Atlantic Ocean (Station HA10). This work investigates the long-range (megameters) propagation of these T-waves. Their sources were three powerful submarine earthquakes in the Kermadec Trench and Islands, located at approximately 6000, 8800, and 15100 km from Stations HA11, HA03, and HA10, respectively. Arrival time and back azimuth of the recorded T-waves were estimated using the Progressive Multi-Channel Correlation algorithm installed on the CTBT Organization (CTBTO) virtual Data Exploitation Centre (vDEC). Different arrivals within the duration of the earthquake signals were identified, and their correlations were also analyzed. The data analysis at HA03 and HA10 revealed intriguing T-wave propagation paths reflecting, refracting, or even transmitting through continents, as well as T-wave excitation along a chain of seamounts. The analysis also showed much higher transmission loss (TL) in the propagation paths to HA11 than to HA03 and HA10. Moreover, strong discrepancies between expected and measured back azimuths were observed for HA11, and a three-dimensional (3D) parabolic equation model was utilized to identify the cause of these differences. Numerical results revealed the importance of 3D effects induced by the Kermadec Ridge, Fiji archipelago, and Marshall Islands on T-wave propagation to HA11. This analysis can guide future improvements in underwater event localization using the CTBT-IMS hydroacoustic sensor network.

Multi-array analysis of volcano-seismic signals at Fogo and Brava, Cape Verde

Abstract. Seismic arrays provide tools for the localization of events without clear phases or events outside the network, where the station coverage prohibits classical localization techniques. Beam forming allows the determination of the direction (back azimuth) and horizontal (apparent) velocity of an incoming wavefront. Here we combine multiple arrays to retrieve event epicentres from the area of intersecting beams without the need to specify a velocity model. The analysis is performed in the time domain, which allows selecting a relatively narrow time window around the phase of interest while preserving frequency bandwidth. This technique is applied to earthquakes and hybrid events in the region of Fogo and Brava, two islands of the southern chain of the Cape Verde archipelago. The results show that the earthquakes mainly originate near Brava, whereas the hybrid events are located on Fogo. By multiple-event beam stacking we are able to further constrain the epicentral locations of the hybrid events in the northwestern part of the collapse scar of Fogo. In previous studies, these events were attributed to shallow hydrothermal processes. However, we obtain relatively high apparent velocities at the arrays, pointing to either deeper sources or complex ray paths. For a better understanding of possible errors of the multi-array analysis, we also compare slowness values obtained from the array analysis with those derived from earthquake locations from classical (local network) localizations. In general, the results agree well. Nevertheless, some systematic deviations of the array-derived back-azimuth and slowness values occur that can be quantified for certain event locations.

Method for Estimating the Tripartite Array Back-Azimuth Error Caused by the Far-Field Approximation

Abstract In recent decades, tripartite arrays (i.e., three-element array) have become an important tool for various seismoacoustic applications, mainly due to their superior back-azimuth estimation. However, the back azimuth is estimated assuming the far-field approximation. Lately, tripartite arrays have been used to monitor microseismicity and aftershocks at distances comparable with the size of the array in which the far-field assumption might not hold. In this work, we determined the validity of the far-field assumption by analyzing the plane-wave errors, that is, the errors of the back azimuth and slowness computations caused by the plane-wave assumption. Computational formulas for estimating the absolute errors, due to the plane-wave assumption, were developed. Several case studies demonstrated that the plane-wave errors are not theoretical issues only and taking them into account can improve the results of field measurements. Proposed practical methods to account for plane-wave errors can improve the performance of arrays aimed to measure low magnitude events such as in induced microseismicity monitoring or on-site inspection.

Updated Global Reference Models of Broadband Coherent Infrasound Signals for Atmospheric Studies and Civilian Applications

AbstractThe International Monitoring System (IMS) infrasound network has been established to detect nuclear explosions and other signals of interest embedded in the station‐specific ambient noise. The ambient noise can be separated into coherent infrasound (e.g., real infrasonic signals) and incoherent noise (such as that caused by wind turbulence). Previous work statistically and systematically characterized coherent infrasound recorded by the IMS. This paper expands on this analysis of the coherent ambient infrasound by including updated IMS data sets with data up to the end of 2020 for all 53 of the currently certified IMS infrasound stations using an updated configuration of the Progressive Multi‐Channel Correlation (PMCC) method. This paper presents monthly station‐dependent reference curves for the back azimuth, trace velocity, and root mean squared amplitude, which provides a means to determine the deviation from the nominal monthly behavior. In addition, a daily Ambient Noise Stationarity (ANS) factor based on deviations from the reference curves is determined for a quick reference to the coherent signal quality compared to the nominal situations. Newly presented histograms provide a higher resolution spectrum, including the observations of the microbarom peak, as well as additional peaks reflecting station‐dependent environmental noise. The aim of these reference curves is to identify periods of suboptimal operation (e.g., nonoperational sensor) or instances of strong abnormal signals of interest.

Performance of a Rotational Sensor to Decipher Volcano Seismic Signals on Etna, Italy

AbstractVolcano‐seismic signals such as long‐period events and tremor are important indicators for volcanic activity and unrest. However, their wavefield is complex and characterization and location using traditional seismological instrumentation is often difficult. In 2019 we recorded the full seismic wavefield using a newly developed 3C rotational sensor co‐located with a 3C traditional seismometer on Etna, Italy. We compare the performance of the rotational sensor, the seismometer and the Istituto Nazionale di Geofisica e Vulcanologia‐Osservatorio Etneo (INGV‐OE) seismic network with respect to the analysis of complex volcano‐seismic signals. We create event catalogs for volcano‐tectonic (VT) and long‐period (LP) events combining a STA/LTA algorithm and cross‐correlations. The event detection based on the rotational sensor is as reliable as the seismometer‐based detection. The LP events are dominated by SH‐type waves. Derived SH phase velocities range from 500 to 1,000 m/s for LP events and 300–400 m/s for volcanic tremor. SH‐waves compose the tremor during weak volcanic activity and SH‐ and SV‐waves during sustained strombolian activity. We derive back azimuths using (a) horizontal rotational components and (b) vertical rotation rate and transverse acceleration. The estimated back azimuths are consistent with the INGV‐OE event location for (a) VT events with an epicentral distance larger than 3 km and some closer events, (b) LP events and tremor in the main crater area. Measuring the full wavefield we can reliably analyze the back azimuths, phase velocities and wavefield composition for VT, LP events and tremor in regions that are difficult to access such as volcanoes.