Global Seismology Research Articles

Suppressing Coda Events with a Bayesian Model of Global Scale Seismology

AbstractLarge seismic events often trigger a wave train of slow decaying energy known as the coda that can mislead signal detectors into forming coda detections that appear to look like regular phase detections. These coda detections can confuse event formation algorithms into building false events known as coda events. Naive solutions to this problem by dropping any detection that looks like a coda detection can have the negative consequence of missing real events. We propose to address this issue by extending an existing Bayesian approach, NET-VISA that has been designed to build event bulletins using a generative model of global-scale seismology. Our extensions significantly boost the existing work by reducing the total number of false events by nearly half and virtually eliminating coda events without changing the number of real events.

DRRGlobal: Uncovering the weak phases from global seismograms using the damped rank-reduction method

Some target seismic signals in the earthquake data can be very weak compared with interfering phases, and are thus difficult to detect, which further hinders the effective usage of these weak phases for subsequent high-resolution imaging of earth interiors. The strong ambient noise makes this situation even more troublesome since the weak signals can be mostly buried in the noise. Here, we present an open-source package for uncovering the weak phases from global seismograms. We adopt a two-step scheme to reconstruct and denoise array data. The first step is weighted average interpolation which puts the data into irregular grids. The second step adopts the weighted projection-onto-convex sets based on damped rank-reduction to further interpolate and denoise for the binned data. Taking the complexity of the weak signal into consideration, we adopt the automatic strategy to select an appropriate rank in different localized windows. We conduct several synthetic tests to carefully investigate the performance regarding effectiveness, robustness, and efficiency, and compare the algorithm with the frequency–wavenumber-domain projection onto convex sets method that is already used in the global seismology literature. Finally, the proposed framework is validated via a recorded array data set of the 1995 May 5 Philippines earthquake.

DASCore: a Python Library for Distributed Fiber Optic Sensing.

In the past decade, distributed acoustic sensing (DAS) has enabled many new monitoring applications in diverse fields including hydrocarbon exploration and extraction; induced, local, regional, and global seismology; infrastructure and urban monitoring; and several others. However, to date, the open-source software ecosystem for handling DAS data is relatively immature. Here we introduce DASCore, a Python library for analyzing, visualizing, and managing DAS data. DASCore implements an object-oriented interface for performing common data processing and transformations, reading and writing various DAS file types, creating simple visualizations, and managing file system-based DAS archives. DASCore also integrates with other Python-based tools which enable the processing of massive data sets in cloud environments. DASCore is the foundational package for the broader DAS data analysis ecosystem (DASDAE), and as such its main goal is to facilitate the development of other DAS libraries and applications.

Preface to Focus Section on New Frontiers and Advances in Global Seismology

Preface to Focus Section on New Frontiers and Advances in Global Seismology

2-D seismic wave propagation using the distributional finite-difference method: further developments and potential for global seismology

SUMMARY We present a time-domain distributional finite-difference scheme based on the Lebedev staggered grid for the numerical simulation of wave propagation in acoustic and elastic media. The central aspect of the proposed method is the representation of the stresses and displacements with different sets of B-splines functions organized according to the staggered grid. The distributional finite-difference approach allows domain-decomposition, heterogeneity of the medium, curvilinear mesh, anisotropy, non-conformal interfaces, discontinuous grid and fluid–solid interfaces. Numerical examples show that the proposed scheme is suitable to model wave propagation through the Earth, where sharp interfaces separate large, relatively homogeneous layers. A few domains or elements are sufficient to represent the Earth’s internal structure without relying on advanced meshing techniques. We compare seismograms obtained with the proposed scheme and the spectral element method, and we show that our approach offers superior accuracy, reduced memory usage, and comparable efficiency.

GEOSCOPE Network: 40 Yr of Global Broadband Seismic Data

Abstract The GEOSCOPE observatory (Institut de physique du globe de Paris [IPGP] and École et Observatoire des Sciences de la Terre de Strasbourg, 1982) provides more than four decades of high-quality continuous broadband data to the scientific community. Started in 1982 with only two stations, the network has grown over the years thanks to numerous international partnerships. At present, 34 stations operate in 18 countries across all continents and on islands throughout the oceans, filling important gaps in global Earth coverage. Most of the first installed stations are still running today, allowing for long-term observations, and new sites are being prospected to further improve global coverage. Over the years, GEOSCOPE has contributed to defining today’s global seismology standards (data format, data quality level, instrumentation requirements), being the French contribution to the international effort for global seismic observations. The stations are instrumented with the best quality seismometers (from the very first STS-1 in the early 80s to the last STS-6A and Trillium T360 today) and digitizers (Q330HR and Centaur) to record with high fidelity the ground motions generated by all types of seismic sources. Real-time data are sent to the tsunami warning centers and both validated and real-time data are available at the IPGP, Epos-France and Earthscope data centers. The quality of GEOSCOPE data and metadata is ensured by daily and yearly validation that enables issue detection and mitigation. GEOSCOPE, in collaboration with the other global networks, has played and continues to play a crucial role in the study of Earth’s structure and global dynamics and the characterization of all types of seismic sources.

Kindling the “Geo”-Scientific Spirit amid COVID-19 Pandemic: Second International Virtual Workshop on Global Seismology and Tectonics ()

Abstract The second International Virtual Workshop on Global Seismology and Tectonics (IVWGST, 2021) was hosted online from 20 to 30 September 2021 by the Geoscience and Technology Division, GSTD, of the Council of Scientific and Industrial Research-North East Institute of Science and Technology (CSIR-NEIST) in Jorhat, India. The main goal of IVWGST (2021) akin to the previous edition was to encourage the participating students who were distressed by the unusual circumstances created by the COVID-19 pandemic to continue engagement in seismology and tectonics via remote access to high-quality lectures and interaction with the international community of seismologists. The lectures presented through Microsoft Teams were delivered by 21 speakers from a variety of geoscience organizations in 10 countries and were attended by over 1750 people from 42 different countries, the majority of whom were students. This article presents a summary of a virtual scientific workshop hosted in the midst of a pandemic, reflections on the experiences gained from it, and exploration of possible improvements for conducting online scientific meetings. We hope this summary of our experience will contribute to larger global impact through including a broader international audience and making such events more diverse, equitable, and inclusive.

William “Willie” Hung Kang Lee (1940–2022)

Obituary| January 12, 2023 William “Willie” Hung Kang Lee (1940–2022) Bill Ellsworth; Bill Ellsworth 1U.S. Geological Survey (retired), Stanford University, Stanford, California, U.S.A. https://orcid.org/0000-0001-8378-4979 Search for other works by this author on: GSW Google Scholar Steve Kirby Steve Kirby * 2U.S. Geological Survey, California, U.S.A. *Corresponding author: stevenlyle@icloud.com Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2023) https://doi.org/10.1785/0220220389 Article history first online: 12 Jan 2023 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bill Ellsworth, Steve Kirby; William “Willie” Hung Kang Lee (1940–2022). Seismological Research Letters 2023; doi: https://doi.org/10.1785/0220220389 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The seismological community has lost one of its leaders and pioneers who led the creation of modern seismic networks while maintaining a sharp focus on the importance and value of the early instrumental and historical record. William “Willie” Hung Kang Lee was also a pioneer in such global seismology fields as seismic tomography. The Earth science literature abounds with his papers on diverse subjects under his frequently used moniker, W. H. K. Lee. Willie was born in wartime Guangxie, southern China on October 6, 1940, the sixth of seven children. His family fled to Macao in 1950 and then to... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Big Data Seismology

AbstractThe discipline of seismology is based on observations of ground motion that are inherently undersampled in space and time. Our basic understanding of earthquake processes and our ability to resolve 4D Earth structure are fundamentally limited by data volume. Today, Big Data Seismology is an emergent revolution involving the use of large, data‐dense inquiries that is providing new opportunities to make fundamental advances in these areas. This article reviews recent scientific advances enabled by Big Data Seismology through the context of three major drivers: the development of new data‐dense sensor systems, improvements in computing, and the development of new types of techniques and algorithms. Each driver is explored in the context of both global and exploration seismology, alongside collaborative opportunities that combine the features of long‐duration data collections (common to global seismology) with dense networks of sensors (common to exploration seismology). The review explores some of the unique challenges and opportunities that Big Data Seismology presents, drawing on parallels from other fields facing similar issues. Finally, recent scientific findings enabled by dense seismic data sets are discussed, and we assess the opportunities for significant advances made possible with Big Data Seismology. This review is designed to be a primer for seismologists who are interested in getting up‐to‐speed with how the Big Data revolution is advancing the field of seismology.

Comparing Sensitivities of Geodetic Processing Methods for Rapid Earthquake Magnitude Estimation

Abstract Rapid earthquake magnitude estimation from real-time space-based geodetic observation streams provides an opportunity to mitigate the impact of large and potentially damaging earthquakes by issuing low-latency warnings prior to any significant and destructive shaking. Geodetic contributions to earthquake characterization and rapid magnitude estimation have evolved in the last 20 yr, from post-processed seismic waveforms to, more recently, improved capacity of regional geodetic networks enabled real-time Global Navigation Satellite System seismology using precise point positioning (PPP) displacement estimates. In addition, empirical scaling laws relating earthquake magnitude to peak ground displacement (PGD) at a given hypocentral distance have proven effective in rapid earthquake magnitude estimation, with an emphasis on performance in earthquakes larger than ∼Mw 6.5 in which near-field seismometers generally saturate. Although the primary geodetic contributions to date in earthquake early warning have focused on the use of 3D position estimates and displacements, concurrent efforts in time-differenced carrier phase (TDCP)-derived velocity estimates also have demonstrated that this methodology has utility, including similarly derived empirical scaling relationships. This study builds upon previous efforts in quantifying the ambient noise of three-component ground-displacement and ground-velocity estimates. We relate these noise thresholds to expected signals based on published scaling laws. Finally, we compare the performance of PPP-derived PGD to TDCP-derived peak ground velocity (PGV), given several rich event datasets. Our results indicate that TDCP-PGV is more likely than PPP-PGD to detect intermediate magnitude (∼Mw 5.0–6.0) earthquakes, albeit with greater magnitude estimate uncertainty and across smaller epicentral distances. We conclude that the computationally lightweight TDCP-derived PGV magnitude estimation is complementary to PPP-derived PGD magnitude estimates, which could be produced at the network edge at high rates and with increased sensitivity to ground motion than current PPP estimates.

SphGLLTools: A toolbox for visualization of large seismic model files based on 3D spectral-element meshes

Adjoint tomography, a full-waveform inversion technique based on 3D wave simulations, has become a commonly used tool in passive-source seismology, drawing on advances in computational power and numerical methods. From global to reservoir scales, seismic models can iteratively be updated in adjoint inversions by extracting information from full seismic waveforms. Seismic models are typically constructed on the numerical mesh used for wave simulations. Thus the size of model files depends on the minimum resolvable period achieved in simulations controlled by the numerical mesh (the higher the mesh resolution, the larger the model and mesh files). This is specifically a concern for recent global-scale adjoint tomographic models where the size and format of numerical meshes pose challenges for model visualization, analysis, interpretation, and sharing model files. Here, we present SphGLLTools, an open-source toolbox that intends to diminish these challenges by expanding global adjoint models onto spherical harmonic functions, which are widely used in global seismology. Our tools are initially designed for spectral-element meshes used in recent global adjoint tomography studies. SphGLLTools facilitate many commonplace tasks for model visualization and analysis, including spherical harmonic expansion of models sampled on spectral-element meshes together with associated tools for easy sharing, visualization, and interpretation of large-scale seismic model files. All the developed routines are accompanied by user instructions and are available through GitHub. For transparency, reproducibility, and educational purposes, we also include Colab notebooks, which provide an intuitive and comprehensive review of the principles and methods for spectral-element meshes, spherical harmonic expansion, and other model analysis tools.

CCREM: New Reference Earth Model From the Global Coda‐Correlation Wavefield

AbstractThe existing reference Earth models have provided an excellent one‐dimensional representation of Earth's properties as a function of its radius and explained many seismic observations in a broad frequency band. However, some discrepancies still exist among these models near the first‐order discontinuities (e.g., the core‐mantle and the inner‐core boundaries) due to different data sets and approaches. As a new paradigm in global seismology, the analysis of coda‐correlation wavefield is fundamentally different from interpreting direct observations of seismic phases or free oscillations of the Earth. The correlation features exist in global correlograms due to the similarity of body waves reverberating through the Earth's interior. As such, there is a great potential to utilize the information stored in the coda‐correlation wavefield in constraining the Earth's internal structure. Here, we deploy the global earthquake‐coda correlation wavefield as an independent data source in the 15–50 s period interval to increase the Earth's radial structure constraints. We assemble a data set of multiple pronounced correlation features and fit both their travel times and waveforms by computing synthetic correlograms through a series of candidate models. Misfit measurements for correlation features are then computed to search for the best‐fitting model. The model that provides an optimal representation of the correlation features in the coda‐correlation wavefield is CCREM. It displays differences in radial seismic velocities, especially near the first‐order discontinuities, relative to previously proposed Earth‐reference models. This is the first application of the earthquake‐coda correlation wavefield in constraining the whole Earth's radial velocity structure.

A 3D complexity-adaptive approach to explore sparsity in elastic wave propagation

We have developed an adaptive approach for seismic modeling by which the computational cost of a 3D simulation can be reduced while retaining the resolution and accuracy. This azimuthal complexity adaptation (ACA) approach relies upon the inherent smoothness of wavefields around the azimuth of a source-centered cylindrical coordinate system. Azimuthal oversampling is thereby detected and eliminated. The ACA method has recently been introduced as part of AxiSEM3D, an open-source solver for global seismology. We use a generalization of this solver that can handle local-scale Cartesian models and that features a combination of an absorbing boundary condition and a sponge boundary with automated parameter tuning. The ACA method is benchmarked against an established 3D method using a model featuring bathymetry and a salt body. We obtain a close fit when the models are implemented equally in both solvers and an expectedly poor fit otherwise, with the ACA method running an order of magnitude faster than the classic 3D method. Furthermore, we evaluate maps of maximum azimuthal wavenumbers that are created to facilitate ACA. We determine how these maps can be interpreted in terms of the 3D complexity of the wavefield and in terms of seismic resolution. The expected performance limits of the ACA method for complex 3D structures are tested on the SEG/EAGE salt model. In this case, ACA still reduces the overall degrees of freedom by 92% compared with a complexity-blind AxiSEM3D simulation. In comparison to the reference 3D method, we again find a close fit and a speedup of a factor of 7. We explore how the performance of ACA is affected by model smoothness by subjecting the SEG/EAGE salt model to Gaussian smoothing. This results in a doubling of the speedup. Thus, ACA represents a convergent, versatile, and efficient method for a variety of complex settings and scales.

An International Virtual Workshop on Global Seismology and Tectonics (IVWGST-2020)

Abstract An International Virtual Workshop on Global Seismology and Tectonics (IVWGST-2020) was organized by the Geoscience and Technology Division of Council of Scientific and Industrial Research—North East Institute of Science and Technology, Jorhat, India from 14 to 25 September 2020. This workshop predominantly catered to undergraduate, postgraduate, and Ph.D. students, scientists, and academicians from across the globe. The primary motive of IVWGST-2020 was to inspire the participating students, perturbed by the unprecedented situation brought about by the COVID-19 pandemic, with quality lecture sessions, so as to lift their spirits. The virtual workshop served as a conduit for the students and researchers to directly interact with several pioneers and prominent geoscience researchers from around the world. Lectures, via Microsoft Teams, were given by 15 eminent speakers from diverse geoscience forums and institutions, and were attended by more than 1000 participants, mostly students and researchers, from 30 different countries. This report briefly summarizes the agenda, describes our experiences hosting the virtual workshop, and documents the challenges faced.

Automatic Detection and Location of Microseismic Events from Sparse Network and Its Application to Post-mining Monitoring

In post-mining regions with seismic hazard, timely decision making for risk management faces the challenge of quick and reliable detection and location of seismic events. As a response to the increasing density of monitoring stations, generating large volumes of seismic data, automatic, full waveform-based methods have been developed in recent years in global seismology. Such methods often cannot be directly applied to post-mining monitoring with a limited station coverage, as it is the case when temporarily networks are installed as an emergency response. In this paper we propose a new methodology that bridges this gap and enables the application of a full waveform, backprojection based method (BackTrackBB) to data of sparse network. The methodology was successfully tested on an abandoned and flooded underground coalmine in South-eastern France. Steps preceding BackTrackBB application were implemented in order to remove the coherent noise that otherwise results in numerous false detections. First results indicate that seismic activity in the study area is controlled by water level variation within former room-and-pillar mine works and fault segments (re)activation below them.

Global seismology and tectonics

The article offers information on the ‘International Virtual Workshop on Global Seismology and Tectonics' was organized to bring national and international experts together and boost the morale of the students and researchers during the COVID- 19 pandemic Topic include providing opportunities to interact with eminent scientists in the domain of seismology and tectonics

Stochastic Inversion of Gaussian Random Media Using Transverse Coherence Functions for Reflected Waves: Theory and Method

AbstractThe transverse coherence functions (TCFs) of phase and amplitude fluctuations of a seismic wave are powerful to estimate the spatial distribution, length scales, and strength of random heterogeneities. However, TCFs have been formulated for transmitted waves only, not for reflected waves. In this paper, we derive reflection TCFs for Gaussian random media using the mirror reflection principle. Furthermore, we propose to invert for Gaussian random media using the reflection TCFs based on a grid search. We validate the new reflection TCF formulas using 2‐D elastic finite‐difference numerically modeled seismic data. We also show the feasibility and efficiency of the inverse problem. The stochastic inversion using reflected waves can be used in both exploration and global seismology.

Benchmarking wave equation solvers using interface conditions: the case of porous media

SUMMARYThe correct implementation of the continuity conditions between different media is fundamental for the accuracy of any wave equation solver used in applications from seismic exploration to global seismology. Ideally, we would like to benchmark a code against an analytical Green’s function. The latter, however, is rarely available for more complex media. Here, we provide a general framework through which wave equation solvers can be benchmarked by comparing plane wave simulations to transmission/reflection (R/T) coefficients from plane-wave analysis with exact boundary conditions (BCs). We show that this works well for a large range of incidence angles, but requires a lot of computational resources to simulate the plane waves. We further show that the accuracy of a numerical Green’s function resulting from a point-source spherical-wave simulation can also be used for benchmarking. The data processing in that case is more involved than for the plane wave simulations and appears to be sufficiently accurate only below critical angles. Our approach applies to any wave equation solver, but we chose the poroelastic wave equation for illustration, mainly due to the difficulty of benchmarking poroelastic solvers, but also due to the growing interest in imaging in poroelastic media. Although we only use 2-D examples, our exact R/T approach can be extended to 3-D and various cases with different interface configurations in arbitrarily complex media, incorporating, for example, anisotropy, viscoelasticity, double porosities, partial saturation, two-phase fluids, the Biot/squirt flow and so on.

Seismic event coda-correlation's formation: implications for global seismology

SUMMARY The seismic-event-coda correlograms are characterized by many prominent features, which, if understood thoroughly, could supply valuable information on the internal structure of the Earth. To further refine our knowledge and be able to utilize that information, all-embracing comprehension of coda-correlation's formation apart from a conjecture, is a pre-requisite. Here, we conduct a comprehensive analysis that aims at a quantitative ‘dissection’ of the formation mechanism of coda correlation. Our analysis presents relevant implications for global seismology. We demonstrate that coda correlation is dominated by a few contributions, most of which arise from the late-coda time window, 3 hr after the earthquake origin time. Our identification analysis confirms that the contributions are cross-terms between body waves. That represents an observational proof of the conjecture that coda-correlation features are formed due to body waves arriving at a pair of receivers with the same slowness. We further quantify the relationship between body-wave cross-terms and event-receiver geometries and Earth structure, which has significant practical implications. Our analysis demonstrates that body-wave cross-terms that contribute to the same coda-correlation feature sample the Earth along fundamentally different paths. They are significantly different depending on event locations, although the resulting time variation is quite small if the late coda (e.g. 3–9 hr after event origin time) is used. That explains why the late coda is more effective than an earlier time window in producing relatively stable features, as empirically suggested by previous studies. Our study enables quantitative and practical understanding of coda-correlation features in terms of their formation progress, and this opens a way to distill valuable information about Earth structure from coda correlation.

An Extreme Ultraviolet Wave Associated with a Solar Filament Activation

Abstract Extreme ultraviolet (EUV) waves are impressive coronal propagating disturbances. They are closely associated with various eruptions and can be used for global coronal seismology and the acceleration of solar energetic particles. Hence, the study of EUV waves plays an important role in solar eruptions and space weather. Here we present an EUV wave associated with a filament activation that did not evolve into any eruption. Due to the continuous magnetic flux emergence and cancellation around its one end, the filament rose with untwisting motion, and the filament mass flowed toward another end along the rising fields. Intriguingly, following the filament activation, an EUV wave formed with a fast constant speed (∼500 km s−1) ahead of the mass flow, and the overlying coronal loops expanded in both lateral and radial directions. Excluding the possibility of a remote flare and an absent coronal mass ejection, we suggest that the EUV wave was only closely associated with the filament activation. Furthermore, their intimate spatial and temporal relationship indicates that the EUV wave was likely directly triggered by the lateral expansion of overlying loops. We propose that the EUV wave can be interpreted as a linear fast-mode wave, and the most vital key for the successful generation of the EUV wave is the impulsive early phase lateral expansion of overlying loops that was driven by the activated filament mass flow without any eruption.