ORCID Profile
0000-0002-9057-4416
Current Organisation
Australian National University
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Glaciology | Seismology and seismic exploration | Geophysics
Publisher: Authorea, Inc.
Date: 17-01-2023
DOI: 10.22541/ESSOAR.167397479.97900153/V1
Abstract: Determining the seismic moment tensors (MT) from the observed waveforms, known as full-waveform seismic MT inversion, remains challenging for small to moderate-size earthquakes at regional scales. Firstly, there is an intrinsic difficulty due to a tradeoff between the isotropic (ISO) and compensated linear vector dipole (CLVD) components of MT that impedes resolving shallow explosive sources, e.g., underground nuclear explosions. It is caused by the similarity of long-period waveforms radiated by ISO and CLVD at regional distances. Secondly, regional scales usually bear complex geologic structures thus, inaccurate knowledge of Earth’s structure should be considered a theoretical error in the MT inversion. However, this has been a challenging problem. So far, only the uncertainty of the 1D Earth model (1D structural error), apart from data errors, has been explored in the source studies. Here, we utilize a hierarchical Bayesian MT inversion to address the above problems. Our approach takes advantage of affine-invariant ensemble s lers to explore the ISO-CLVD tradeoff space thoroughly and effectively. Furthermore, we invert for station-specific time shifts to treat the structural errors along specific source-station paths (2D structural errors). We present synthetic experiments demonstrating the method’s advantage in resolving the ISO components. The application to nuclear explosions conducted by the Democratic People’s Republic of Korea (DPRK) shows highly similar source mechanisms, dominated by a high ISO, significant CLVD components, and a small DC component. The recovered station-specific time shifts from the nuclear explosions present a consistent pattern, which agrees well with the geological setting surrounding the event location.
Publisher: Springer Science and Business Media LLC
Date: 21-02-2023
DOI: 10.1038/S41467-023-36074-2
Abstract: Probing the Earth’s center is critical for understanding planetary formation and evolution. However, geophysical inferences have been challenging due to the lack of seismological probes sensitive to the Earth’s center. Here, by stacking waveforms recorded by a growing number of global seismic stations, we observe up-to-fivefold reverberating waves from selected earthquakes along the Earth’s diameter. Differential travel times of these exotic arrival pairs, hitherto unreported in seismological literature, complement and improve currently available information. The inferred transversely isotropic inner-core model contains a ~650-km thick innermost ball with P-wave speeds ~4% slower at ~50° from the Earth’s rotation axis. In contrast, the inner core’s outer shell displays much weaker anisotropy with the slowest direction in the equatorial plane. Our findings strengthen the evidence for an anisotropically-distinctive innermost inner core and its transition to a weakly anisotropic outer shell, which could be a fossilized record of a significant global event from the past.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 19-10-2018
Abstract: Earth's inner core is thought to be solid, which means it should support shear waves. However, the small size of the inner core makes detecting shear waves very difficult. Tkalčić and Phạm correlated different types of seismic phases to finally determine the speed of shear waves in Earth's inner core (see the Perspective by Irving). The detection of the waves closes an 80-year quest to find them and confirms a solid, but soft, inner core. Science , this issue p. 329 see also p. 294
Publisher: American Geophysical Union (AGU)
Date: 11-2021
DOI: 10.1029/2021JB022477
Abstract: To infer seismic moment tensor (MT) of moderate earthquakes at regional scales, seismologists typically simulate waveforms using available structural models of the Earth to match the observed seismograms. This procedure is known as waveform seismic MT inversion. However, the seismic data are noisy, and the Earth model is inevitably different from the actual structure of the Earth hence, there is a discrepancy between the predicted and the observed waveforms. This discrepancy arises from the noise in the data and imperfections in theoretical predictions, stemming most significantly from the Earth model. This study introduces structural uncertainty, estimated empirically, and referred to as “theory uncertainty,” as part of the combined covariance matrix. In the synthetic setting, we first show through a series of synthetic experiments that the structural uncertainty plays a critical role in retrieving MT solutions, especially for short‐period waveforms. The method is then benchmarked against the waveforms of non‐double‐couple earthquakes in Long Valley Caldera, California. We confirm the highly isotropic nature of the source in a pilot event but find a non‐negligible CLVD component that was overlooked in past studies ignoring the uncertainty in Earth model. Thus, careful consideration of the Earth model's uncertainty as part of the MT inversion schemes will be necessary for future applications to better understand the complicated physics of earthquake sources.
Publisher: Research Square Platform LLC
Date: 12-09-2022
DOI: 10.21203/RS.3.RS-2026936/V1
Abstract: Probing the Earth’s center is critical for understanding planetary formation and evolution. However, geophysical inferences have been challenging due to the lack of seismological probes sensitive to the Earth’s center. Here, by stacking waveforms recorded by a growing number of global seismic stations, we observe up-to-fivefold reverberating waves from selected earthquakes along the Earth's diameter. Differential travel times of these “exotic” arrival pairs, hitherto unreported in seismological literature, complement and improve currently available information. The inferred transversely isotropic inner-core model contains a ~ 650-km thick innermost ball with P-wave speeds ~ 4% slower at ~ 50° from the Earth’s rotation axis. In contrast, the inner core’s outer shell displays much weaker anisotropy with the slowest direction in the equatorial plane. Our findings strengthen the evidence for an anisotropically-distinctive innermost inner core and its transition to a weakly anisotropic outer shell, which could be a fossilized record of a significant global event from the past.
Publisher: Springer Science and Business Media LLC
Date: 27-09-2023
Publisher: Oxford University Press (OUP)
Date: 09-01-2019
DOI: 10.1093/GJI/GGZ002
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-10667
Abstract: A seismic moment tensor (MT, a 3x3 matrix) is a general source representation of various seismic events under the point source assumption, which is generally valid for small-to-medium size earthquakes. A full MT can be decomposed into isotropic (ISO), compensated linear vector dipole (CLVD), and double-couple (DC) components. The ISO represents the explosion/collapse source process that involves volumetric changes. Therefore, the relative significance of the ISO component, which can be learned from inverting seismic waveforms, is an essential indicator to discriminate between earthquakes and explosive events. However, an intrinsic ISO-CLVD tradeoff impedes resolving shallow explosive sources due to the high similarity of long-period waveforms at regional distances. Even though this tradeoff can be mitigated by extra constraints such as teleseismic P-waves, there is still an urgent need for advanced inversion algorithms to explore the solution space thoroughly. Apart from that, a rigorous uncertainty estimate is required to constrain the source better. Firstly, the inversion should consider the data noise. Secondly, the theory error primarily due to imperfect knowledge of Earth's structure is also significant but proven difficult to treat. Here, we propose a new Bayesian MT inversion scheme with affine-invariant ensemble s lers to explore the MT parameter space accounting for data and theory errors. Carefully designed synthetic experiments indicate the advantage of the newly developed method in resolving the isotropic components of a shallow seismic source. Our application to DPRK tests reveals a similar source mechanism dominated by a high ISO and significant CLVD components, including a small DC component. This study aims to characterize shallow explosive sources' physics better, thus helping verify compliance with the CTBT. &
Publisher: Elsevier BV
Date: 09-2018
Publisher: Authorea, Inc.
Date: 17-08-2023
DOI: 10.22541/ESSOAR.169230267.77488140/V1
Abstract: The misalignment of the observation and predicted waveforms in regional moment tensor inversion is mainly due to seismic models’ incomplete representation of the Earth’s heterogeneities. Current moment tensor inversion techniques, allowing station-specific time shifts to account for the model error, are computationally expensive. Here, we propose a lightweight method to jointly invert moment-tensor parameters and unknown station-specific time shifts utilizing the modern functionalities in deep learning frameworks. A $L_2^2$ misfit function between predicted synthetic and time-shifted observed seismograms is defined in the spectral domain, which is differentiable to all unknowns. The inverse problem is solved by minimizing the misfit function with a gradient descent algorithm. The method’s feasibility, robustness, and scalability are demonstrated on earthquakes in the Long Valley Caldera, California. This work presents an ex le of fresh opportunities to apply advanced computational infrastructures developed in deep learning to geophysical problems.
Publisher: The Australian National University
Date: 2019
Publisher: Oxford University Press (OUP)
Date: 05-08-2020
DOI: 10.1093/GJI/GGAA369
Abstract: Recorded globally, cross-correlated ground-motion time-series of the coda of large earthquakes enable the construction of a 2-D representation of correlation lapse time and inter-receiver distance—a global correlogram. A better understanding of how the features present in a correlogram are generated can revolutionize the characterization of planetary interiors. Here, we investigated correlograms based on in idual large earthquakes and identified 12 events from the past decade with a multitude of prominent and some ‘exotic’ features in the first 3 hr following correlation origin. We found that the type of the source mechanism and energy-release dynamics are the key influencers responsible for in idual correlograms equal in quality to a stack of hundreds of correlograms. A single event is sufficient in creating a correlogram resembling previous correlograms constructed from a large number of events, which reinforces the notion that the earthquake coda-correlation features are not ‘reconstructed’ body waves. Numerical simulations of the correlation wavefield can thus be based on exceptional-quality events, becoming more computationally affordable. Here, we explain more than 60 features of the global coda-correlogram, which presents the most extensive catalogue to date.
Publisher: Springer Science and Business Media LLC
Date: 29-07-2023
DOI: 10.1038/S41467-023-40307-9
Abstract: Observations of seismic body waves that traverse the Earth’s inner core (IC) as shear (J) waves are critical for understanding the IC shear properties, advancing our knowledge of the Earth’s internal structure and evolution. Here, we present several seismological observations of J phases detected in the earthquake late-coda correlation wavefield at periods of 15–50 s, notably via the correlation feature I-J, found to be independent of the Earth reference velocity model. Because I-J is unaffected by compressional wave speeds of the Earth’s inner core, outer core, and mantle, it represents an autonomous class of seismological measurements to benchmark the inner core properties. We estimate the absolute shear-wave speed in the IC to be 3.39 ± 0.02 km/s near the top and 3.54 ± 0.02 km/s in the center, lower than recently reported values. This is a 3.4 ± 0.5% reduction from the Preliminary Reference Earth Model (PREM), suggesting a less rigid IC than previously estimated from the normal mode data. Such a low shear-wave speed requires re-evaluating IC composition, including the abundance of light elements, the atomic properties and stable crystallographic phase of iron, and the IC solidification process.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-10662
Abstract: The Macquarie Ridge Complex, located at the boundary between Indo-Australian and Pacific plates in the southwest Pacific Ocean, hosts the largest sub-marine earthquakes in the 20th century, not associated with ongoing subduction. We deployed 27 ocean-bottom seismometers, of which 15 have been recovered successfully, to understand the origin of the sub-marine earthquakes and their potential earthquake and tsunami hazards to Australia and New Zealand. Additionally, we deployed five land-based seismometers on Macquarie Island.We explore state-of-the-art processing methods to analyze the new seismic dataset from the retrieved seismic stations. One of the goals is to image the tectonic settings beneath the MRC. Here, we present a first-order tomographic model and its relevant uncertainty estimate of the region constructed from ambient noise surface waves using a probabilistic inversion framework. The tomographic image will be complemented with receiver-based imaging results such as those from P-wave coda autocorrelations and receiver functions to confirm the existence of possible geometries. The results are expected to supply a fresh understanding of the tectonic settings under the MRC and unpuzzle the origin of the significant underwater earthquakes in the 20th century.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-10288
Abstract: Inferring the seismic source mechanisms of small-to-medium-size earthquakes from the observed waveforms via inverse methods remains challenging. Firstly, a more generalized source representation is required to include a broader range of seismic sources. A seismic moment tensor (MT) is widely used to parameterize a seismic point source by assuming no net torque. However, there are well-documented seismic sources for which net torques are significant, and single force (SF) components are necessary to describe the physics of the problem, e.g., landslides and volcanic and glacier earthquakes. Secondly, the inter-parameter correlation, e.g., the tradeoffs between the MT& #8217 s isotropic and compensated-linear-vector-dipole components for shallow explosive events and the MT and SF components at all depths, can be significant. Therefore, there is imperative for advanced s ling algorithms to explore the parameter space thoroughly and effectively. Thirdly, a complete uncertainty treatment should consider theory error primarily due to the imperfection of Earth's structure (referred to as structural error) apart from data noise. To date, the uncertainty of the 1D Earth model (1D structural error) has been investigated and proven indispensable in source studies. A rigorous uncertainty estimate can improve the resolvability of source parameters, but its implementation has been challenging.We propose a joint point-source MT and SF inversion within the hierarchical Bayesian framework to address the abovementioned set of challenges in treating the 2022 Hunga Tonga-Hunga Ha'apai event. MT and SF are combined to represent a broader range of sources in the waveform inversion. Our approach takes advantage of affine-invariant ensemble s lers to explore the parameter space thoroughly and effectively. Furthermore, we invert for station-specific time shifts to treat the structural errors along specific source-station paths (2D structural errors). After comprehensive synthetic experiments to demonstrate the feasibility of our approach, we focus on physics-based scenarios for the 2022 Hunga Tonga-Hunga Ha'apai volcanic earthquake. More specifically, we analyze the non-double-couple character and the role of SF in the source mechanism. Our approach provides further insights into this particular earthquake and a platform for future studies of seismic events in various geological environments.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-16047
Abstract: Seismological observations of J-phases, the seismic waves traversing the Earth& #8217 s inner core (IC) as shear waves, are critical to understanding the inner core shear properties. That, in turn, will shed light on the solidification process and the evolution of the inner core and our planet. Most body-wave detections of the J waves have been controversial due to their small litudes, which involve energy conversion from P- to S- and vice versa at the inner core boundary. Recent advances in understanding the nature of the late coda correlation offer a new way to s le the deep Earth, including the shear properties of the Earth& #8217 s inner core. The correlation-based features provide the sensitivity of the periods between 15 and 50 s, placing it between the body waves and normal mode data. Therefore, the observations of late coda correlation are vital in refining the shear properties of the IC, such as velocity, anisotropy, and attenuation.This study employs several uninvestigated J-wave correlation features detected in the global coda-correlation wavefield building on the study of Tkal& #269 i& #263 and Pham (2018) that determined the shear wave speed reduction of 2.5% relative to PREM. The correlation features observed in the coda-correlation wavefield arise from similar seismic phases in which one contains a shear-wave leg in the IC. Improved data selection process and knowledge acquired from recent theoretical and observational developments in understanding the anatomy of coda correlation wavefield enable significant improvements in the data quality. We benchmark the waveforms of observed correlation features using numerical modeling, confirm the observations of J waves and inner core solidity and update its shear properties& #8217 values, including shear-wave speed and Poisson& #8217 s ratio.
Publisher: Annual Reviews
Date: 31-05-2022
DOI: 10.1146/ANNUREV-EARTH-071521-063942
Abstract: Understanding how Earth's inner core (IC) develops and evolves, including fine details of its structure and energy exchange across the boundary with the liquid outer core, helps us constrain its age, relationship with the planetary differentiation, and other significant global events throughout Earth's history, as well as the changing magnetic field. Since its discovery in 1936 and the solidity hypothesis in 1940, Earth's IC has never ceased to inspire geoscientists. However, while there are many seismological observations of compressional waves and normal modes sensitive to the IC's compressional and shear structure, the shear waves that provide direct evidence for the IC's solidity have remained elusive and have been reported in only a few publications. Further advances in the emerging correlation-wavefield paradigm, which explores waveform similarities, may hold the keys to refined measurements of all IC shear properties, informing dynamical models and strengthening interpretations of the IC's anisotropic structure and viscosity. ▪ What are the shear properties of the IC, such as the shear-wave speed, shear modulus, shear attenuation, and shear-wave anisotropy? ▪ Can the shear properties be measured seismologically and confirmed experimentally?
Publisher: Wiley
Date: 23-06-2023
Publisher: American Geophysical Union (AGU)
Date: 04-2022
DOI: 10.1029/2021JB023456
Abstract: The Jaz Murian Basin in Southeast Iran is a forearc basin that sits on top of the western Makran subduction zone. A recent seismic deployment inside the western Jaz Murian Basin documents extremely strong reverberations between the free surface and the top of the basement. These strong multiples, which h er studies of deeper structures, carry precious information on the structure of the basin. Here we combine three independent measurements, including teleseismic P/S‐wave coda autocorrelations, P‐wave polarization analysis and receiver functions, to image the architecture of the western Jaz Murian Basin. The S‐wave velocity structure beneath each station is constrained by teleseismic P‐wave polarization analysis and a simple grid search method. In addition, both P‐ and S‐wave reflections are extracted to measure the Poisson's ratio and the thickness of sediments, in addition to P‐ and/or S‐wave velocities. We find that the sedimentary cover has an average thickness of ∼3 km with very low S‐wave velocities around 1.2 km/s. The estimated Vp/Vs ratios are relatively high between 2.23 and 2.52, suggesting that the western Jaz Murian Basin contains unconsolidated recently deposited sediments.
Publisher: The Royal Society
Date: 06-2018
Abstract: The seismic correlation wavefield constructed from the stacked cross-correlograms of the late coda of earthquake signals at stations across the globe provides a wealth of observed pulses as a function of inter-station distance. The interval from 3 to 10 h after the onset of major earthquakes is employed for the period range from 15 to 50 s. The observations can be well matched by synthetic seismograms for a radially stratified Earth. Many of the correlation phases have similar time behaviour to those in the regular wavefield, but others have no correspondence. All such correlation phases can be explained by the interaction of arrivals with a common slowness at the each of the stations being correlated. Using a generalized ray description of the seismic wavefield, the time-distance behaviour of these correlation phases arises from differences in accumulated phase on different propagation paths through the Earth. Distinct arrivals emerge from the correlation field when there are many ways in which combinations of seismic phases can arise with the same difference in propagation legs. The constituents of the late coda are dominated by steeply travelling waves, and in consequence features associated with multiple passages through the whole Earth emerge distinctly, such as high-order multiples of PKIKP .
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-10875
Abstract: The Macquarie Ridge Complex (MRC) constitutes the boundary between the Indo-Australian and Pacific plates in the southwest Pacific Ocean. It accommodates the world& #8217 s most potent sub-marine earthquakes that are not associated with ongoing subduction. To better understand the nature of MRC and its associated earthquakes, we aim to explore the crustal structures using recordings from island-based stations and ocean bottom seismometers (OBS). In particular, these OBSs, which are deployed in the surroundings of Macquarie Island from October 2020 to November 2021, enable us to image the refined oceanic structures beneath the study area. In this study, we obtain the body-wave reflections by computing phase coherence autocorrelations of both ambient noise and earthquake data. Our preliminary reflection profiles by both methods reveal coherent reflected P waves that may be related to Moho and additional structures within the crust and upper mantle.
Publisher: Elsevier BV
Date: 09-2020
Publisher: American Geophysical Union (AGU)
Date: 25-11-2020
DOI: 10.1029/2020JB020270
Abstract: Due to a sharp contrast in elastic properties across the basement rocks of sedimentary basins (SBs), strong reverberations are generated during the passage of seismic waves. Traditional receiver function methods become inadequate for imaging crustal structure due to the existence of these strong reverberations. We investigate the feasibility of an autocorrelation technique to extract vertical component receiver functions from teleseismic earthquake data and the efficiency of the method to image the crustal architecture in presence of a SB. The method involves spectral whitening followed by autocorrelation and stacking in the depth domain. We show promising results when using temporary seismic networks in the eastern United States. Using synthetic and field‐data ex les, we demonstrate that vertical autocorrelations are more efficient than classical radial receiver functions for interpretation purposes in an SB context. We also perform a joint analysis of the litudes on radial and vertical receiver functions for characterizing the thickness of the Mohorovičić discontinuity (Moho). We find that the Moho in the eastern United States is a transitional layer (up to 5‐km thick) instead of a sharp boundary. Further, we point out that it is challenging to unambiguously pick and interpret reflected phases on autocorrelations because of the effects of reverberations, cross‐mode contaminations, and a narrow frequency band limiting the resolution of velocity gradients. We therefore send a message of caution for future interpretations based on this technique.
Publisher: American Geophysical Union (AGU)
Date: 04-2021
DOI: 10.1029/2020JB021082
Abstract: The recent deployment of a broadband seismic array on the floating ice shelf in the Antarctica's Ross sea presents a great opportunity to study the shelf structure using broadband seismic data. In this study, we develop a further improvement of the P‐wave coda autocorrelation method, which proved capable of characterizing grounded ice‐cap structures. Ice shelves are floating ice sheets connected to a landmass, and in order to decipher their structures, a water layer has to be added to the problem. We construct the power spectrum stacks of P‐wave coda data, waveform records that immediately follow P arrivals, in the spectral domain, which are equivalent, via a Fourier transform, to widely used autocorrelograms in the time domain. At half of temporary seismic stations under consideration, we report prominent resonant peaks in the spectral autocorrelograms, associated with the ice‐water configuration of the ice shelf. The lack of clear resonant pattern for the rest of the stations is suspected due to a high noise level in the icy environment and significant lateral heterogeneity at the local scale. Subsequently, we develop a formalism to explain the observed resonance and devise a grid‐search scheme to estimate ice‐ and water‐thicknesses underneath the stations. Our water‐thickness estimates agree well with the previously documented measurements, but there is a discrepancy in the ice thickness results. Therefore, the method has a great potential to complement the existing ice‐shelf model, to be used in future monitoring applications of ice shelves, or near‐future space exploration to icy planets.
Publisher: Oxford University Press (OUP)
Date: 07-09-2023
DOI: 10.1093/GJI/GGAD348
Publisher: American Geophysical Union (AGU)
Date: 09-2018
DOI: 10.1029/2018JB016115
Publisher: American Geophysical Union (AGU)
Date: 05-2017
DOI: 10.1002/2017JB013975
Publisher: American Geophysical Union (AGU)
Date: 13-04-2018
DOI: 10.1002/2018GL077244
Location: No location found
Location: Italy
Start Date: 04-2023
End Date: 05-2026
Amount: $425,143.00
Funder: Australian Research Council
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