Highly efficient Bayesian joint inversion for receiver-based data and its application to lithospheric structure beneath the southern Korean Peninsula

With the deployment of extensive seismic arrays, systematic and efficient parameter and uncertainty estimation is of increasing importance and can provide reliable, regional models for crustal and upper-mantle structure.We present an efficient Bayesian method for the joint inversion of surface-wave dispersion and receiver-function data that combines trans-dimensional (trans-D) model selection in an optimization phase with subsequent rigorous parameter uncertainty estimation. Parameter and uncertainty estimation depend strongly on the chosen parametrization such that meaningful regional comparison requires quantitative model selection that can be carried out efficiently at several sites. While significant progress has been made for model selection (e.g. trans-D inference) at individual sites, the lack of efficiency can prohibit application to large data volumes or cause questionable results due to lack of convergence. Studies that address large numbers of data sets have mostly ignored model selection in favour of more efficient/simple estimation techniques (i.e. focusing on uncertainty estimation but employing ad-hoc model choices). Our approach consists of a two-phase inversion that combines trans-D optimization to select the most probable parametrization with subsequent Bayesian sampling for uncertainty estimation given that parametrization. The trans-D optimization is implemented here by replacing the likelihood function with the Bayesian information criterion (BIC). The BIC provides constraints on model complexity that facilitate the search for an optimal parametrization. Parallel tempering (PT) is applied as an optimization algorithm. After optimization, the optimal model choice is identified by the minimum BIC value from all PT chains. Uncertainty estimation is then carried out in fixed dimension. Data errors are estimated as part of the inference problem by a combination of empirical and hierarchical estimation. Data covariance matrices are estimated from data residuals (the difference between prediction and observation) and periodically updated. In addition, a scaling factor for the covariance matrix magnitude is estimated as part of the inversion. The inversion is applied to both simulated and observed data that consist of phase- and group-velocity dispersion curves (Rayleigh wave), and receiver functions. The simulation results show that model complexity and important features are well estimated by the fixed dimensional posterior probability density. Observed data for stations in different tectonic regions of the southern Korean Peninsula are considered. The results are consistent with published results, but important features are better constrained than in previous regularized inversions and are more consistent across the stations. For example, resolution of crustal and Moho interfaces, and absolute values and gradients of velocities in lower crust and upper mantle are better constrained

Intraplate volcanism controlled by back-arc and continental structures in NE Asia inferred from transdimensional Bayesian ambient noise tomography

Intraplate volcanism adjacent to active continental margins is not simply explained by plate tectonics or plume interaction. Recent volcanoes in northeast (NE) Asia, including NE China and the Korean Peninsula, are characterized by heterogeneous tectonic structures and geochemical compositions. Here we apply a transdimensional Bayesian tomography to estimate high-resolution images of group and phase velocity variations (with periods between 8 and 70 s). The method provides robust estimations of velocity maps, and the reliability of results is tested through carefully designed synthetic recovery experiments. Our maps reveal two sublithospheric low-velocity anomalies that connect back-arc regions (in Japan and Ryukyu Trench) with current margins of continental lithosphere where the volcanoes are distributed. Combined with evidences from previous geochemical and geophysical studies, we argue that the volcanoes are related to the low-velocity structures associated with back-arc processes and preexisting continental lithosphere

Seismic constraints on magma evolution beneath Mount Baekdu (Changbai) volcano from transdimensional Bayesian inversion of ambient noise data

The magmatic process of continental intraplate volcanism (CIV) is difficult to understand due to heterogeneous interactions with the crust and the lithospheric upper mantle. Mount Baekdu (Changbai) volcano (MBV) is one of the prominent CIVs in northeast Asia that has shown a complex history of eruptions and associated magmatic structures. In addition, the relationship between the crustal magmatic structures and upper mantle phenomena are enigmatic due to the lack of consistent seismic constraints for the lithospheric structure. To enhance comprehensive understanding of the MBV magma evolution, we image the lithospheric structure beneath the MBV and surrounding regions using ambient noise data and the following two approaches: (1) multiple measures of ambient noise dispersion are acquired through different methods and (2) a transdimensional Bayesian inversion method is utilized to obtain unbiased results in joint analysis of the multiple data sets. The estimated Earth structure shows a thick crust (~40 km) and a crustal anomaly with relatively high S wave velocity in the depth range 20–40 km. This type of structure extends to ~100 km north from the MBV and is accompanied by the shallow and rapid S wave velocity decrease beneath the mantle lid (~80 km). Through a comparison with previous P wave models, we interpret this structure as a consequence of compositional partitioning by mafic underplating and overlying cooled felsic layers as a result of fractional crystalization.This work was funded by the Korea Meteorological Administration under grant KIMPA2017-4020

Ambient seismic noise tomography of the southern East Sea (Japan Sea) and the Korea Strait

Group velocity maps were derived for the southern East Sea (Japan Sea) and the Korea Strait (Tsushima Strait) for the 5–36 s period range, which is sensitive to shear wave velocities of the crust and the uppermost mantle. Images produced in our study enhance our understanding of the tectonic evolution of a continental margin affected by subducting oceanic slabs and a colliding continental plate. The seismic structure of the study area has not been described well because seismic data for the region are scarce. In this study, we applied the ambient noise tomography technique that does not rely on earthquake data. We calculated ambient noise cross-correlations recorded at station pairs of dense seismic networks located in the regions surrounding the study area, such as the southern Korean Peninsula and southwestern part of the Japanese Islands. We then measured the group velocity dispersion curves of the fundamental mode Rayleigh waves from cross-correlograms and constructed 2-D group velocity maps reflecting group velocity structure from the upper crust to uppermost mantle. The results show that three distinct anomalies with different characteristics exist. Anomalies are located under the Ulleung Basin (UB), the boundary of the Basin, and the area between Tsushima Island and the UB. 1-D velocity models were obtained by inversion of dispersion curves that represent vertical variations of shear wave velocity at locations of three different anomalies. The 1-D velocity models and 2-D group velocity maps of lateral variations in shear wave group velocities show that the high velocity anomaly beneath the UB originates from crustal thinning and mantle uplift. Confirming the exact causes of two low velocity anomalies observed under the UB boundary and between Tsushima Island and the UB is difficult because additional information is unavailable. However, complex fault systems, small basins formed by faulting, and deep mantle flow can be possible causes of the existence of low velocity anomalies in the region.This work was funded by the Korean Meteorological Administration and Development Program under Grant CATER 2012-5051

Enhanced hypocenter determination of the 2017 Pohang earthquake sequence, South Korea, using a 3-D velocity model

© 2022, The Association of Korean Geoscience Societies and Springer.As a fundamental task in observational seismology, accurate hypocenter determination is crucial for seismic hazard analysis, delineating faults, and elucidating seismic source characteristics. However, hypocenters determined using an inaccurate velocity model can exhibit significant deviations from the actual hypocenter. In this study, we investigated how a 3-D velocity model results in a better constraint than a 1-D model for the hypocenter determination problem associated with the 2017 MW 5.5 Pohang earthquake. This study determined the hypocenter of the Pohang earthquake sequence using a 3-D velocity model of 32 events including the mainshock that occurred on November {vn15}, 2017, in South Korea. The S wave velocity model, based on an ambient noise tomography, was combined with the average Vp/Vs ratio of the crust of the Korean Peninsula to construct a 3-D velocity model; additional 1-D velocity model was used to compare the results. The hypocenters were determined via a nonlinear method, which allowed the calculation of the posterior probability density of the source via a direct search method, confirming that the accuracy improved when using the 3-D model compared with the 1-D model. We observed that our 3-D velocity model enables hypocenters to be consistently determined, less affected by station configuration, or a lack of adjacent seismic stations. Further numerical investigation showed that complex basin geometry and the heterogeneity of the crustal thickness, which cannot be considered in 1-D model, are critical for hypocenter determination.N

Regionally heterogeneous uppermost inner core observed with Hi-net array

Studies of velocity structure in the Earth's inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition. Here we use an extensive number of seismograms recorded by the Hi-net array to construct complete empirical curves of PKPBC-PKPDF differential traveltimes. The nature of these curves implies that significant variations in traveltimes are accumulated during the passage of PKPDF waves through the uppermost inner core and rules out outer core structure effects. Uniformcylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow. The configuration and strength of inhomogeneities froma recent tomographicmodel of the lowermostmantle cannot account for the observed traveltime variations. Therefore, we infer that either variations of P wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains, and both of these features may be superimposed on long-wavelength hemispherical structure. If the former holds true, the absolute magnitude of required P wave velocity perturbations from referent values is 0.60 ± 0.10% in the quasi-eastern and 1.55 ± 0.15% in the quasi-western hemisphere (0.85 ± 0.05% and 1.10 ± 0.10%, respectively, with the lowermost mantle correction). The existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification

Moment Tensor Solutions for Earthquakes in the Southern Korean Peninsula Using Three-Dimensional Seismic Waveform Simulations

Precise estimates of earthquake source properties are crucial for understanding earthquake processes and assessing seismic hazards. Seismic waveforms can be affected not only by individual event properties, but from the Earth's interior heterogeneity. Therefore, for accurate constraints on earthquake source parameters, the effects of three-dimensional (3D) velocity heterogeneity on seismic wave propagation need evaluation. In this study, regional moment tensor solutions for earthquakes around the southern Korean Peninsula were constrained based on the spectral-element moment tensor inversion method using a recently developed high-resolution regional 3D velocity model with accurate high-frequency waveform simulations. Located at the eastern margin of the Eurasian plate, the Korean Peninsula consists of complex geological units surrounded by thick sedimentary basins in oceanic areas. It exhibits large lateral variations in crustal thickness (> 10 km) and seismic velocity (>10% dlnVs) at its margins in the 3D model. Seismic waveforms were analyzed from regional earthquakes with local magnitudes > 3.4 that occurred within and around the peninsula recorded by local broadband arrays. Moment tensor components were inverted together with event locations using the numerically calculated Frechet derivatives of each parameter at periods >= 6 s. The newly determined solutions were compared with the results calculated from the one-dimensional (1D) regional velocity model, revealing a significant increase in a double-couple component of > 20% for earthquakes off of the coastal margins. Further, compared to initial solutions, <= 5 km change in depth was observed for earthquakes near the continental margin and sedimentary basins. The combination of a detailed 3D crustal model and accurate waveform simulations led to an improved fit between data and synthetic seismograms. Accordingly, the present results provide the first confirmation of the effectiveness of using 3D velocity structures for accurately constraining earthquake source parameters and the resulting seismic wave propagation in this region. We suggest that accurate 3D wave simulations, together with improved source mechanisms, can contribute a reliable assessment of seismic hazards in regions with complex continental margin structures and sedimentary basins from offshore earthquakes whose seismic waveforms can be largely affected by 3D velocity structures.N

Measurement of seismometer misorientation based on P-wave polarization: application to dense temporary broadband seismic array in the epicentral region of 2016 Gyeongju earthquake, South Korea

© 2022, The Association of Korean Geoscience Societies and Springer.In this study, we probe the misalignment of 200 temporary broadband seismometers based on the polarization of P waves from regional and teleseismic earthquakes. The seismometers were deployed in the epicentral region of 2016 ML 5.8 Gyeongju earthquake, South Korea, and this unprecedented dense array provided a unique opportunity for investigating fault structures from microseismicity. For the full use of three-component seismic records, we estimate and provide time-dependent misorientation angles of the 200 seismometers from June 2018 to March 2021 with uncertainty assessments. Two methods based on the principal component analysis and the minimization of transverse P-wave energy are applied. Our estimates are characterized by small uncertainty (average median absolute deviation of 3.14°). Moreover, periods of suspected temporal changes in misorientation angles mostly coincide with periods of reported technical operations, which demonstrates reliability of our methods to precisely detect the temporal variation of misorientation angles. We expect our misorientation angles to serve as an essential metadata for further seismological researches utilizing the dense array data.N

Traveltime data used in “Imaging of Lithospheric Structure Beneath Jeju Volcanic Island by Teleseismic Traveltime Tomography”

This file contains seismic waveform data that were used in our study, location of sources and receivers, travel times residuals, and final velocity models retrieved by tomographic inversion. Please refer to README.txt file for general description for the individual files