Sources of uncertainties and artefacts in back-projection results

Back-projecting high-frequency (HF) waves is a common procedure for imaging rupture processes of large earthquakes (i.e. M_w > 7.0). However, obtained back-projection (BP) results could suffer from large uncertainties since high-frequency seismic waveforms are strongly affected by factors like source depth, focal mechanisms, and the Earth's 3-D velocity structures. So far, these uncertainties have not been thoroughly investigated. Here, we use synthetic tests to investigate the influencing factors for which scenarios with various source and/or velocity set-ups are designed, using either Tohoku-Oki (Japan), Kaikoura (New Zealand), Java/Wharton Basin (Indonesia) as test areas. For the scenarios, we generate either 1-D or 3-D teleseismic synthetic data, which are then back-projected using a representative BP method, MUltiple SIgnal Classification (MUSIC). We also analyse corresponding real cases to verify the synthetic test results. The Tohoku-Oki scenario shows that depth phases of a point source can be back-projected as artefacts at their bounce points on the earth's surface, with these artefacts located far away from the epicentre if earthquakes occur at large depths, which could significantly contaminate BP images of large intermediate-depth earthquakes. The Kaikoura scenario shows that for complicated earthquakes, composed of multiple subevents with varying focal mechanisms, BP tends to image subevents emanating large amplitude coherent waveforms, while missing subevents whose P nodal directions point to the arrays, leading to discrepancies either between BP images from different arrays, or between BP images and other source models. Using the Java event, we investigate the impact of 3-D source-side velocity structures. The 3-D bathymetry together with a water layer can generate strong and long-lasting coda waves, which are mirrored as artefacts far from the true source location. Finally, we use a Wharton Basin outer-rise event to show that the wavefields generated by 3-D near trench structures contain frequency-dependent coda waves, leading to frequency-dependent BP results. In summary, our analyses indicate that depth phases, focal mechanism variations and 3-D source-side structures can affect various aspects of BP results. Thus, we suggest that target-oriented synthetic tests, for example, synthetic tests for subduction earthquakes using more realistic 3-D source-side velocity structures, should be conducted to understand the uncertainties and artefacts before we interpret detailed BP images to infer earthquake rupture kinematics and dynamics

Sources of uncertainties and artefacts in back-projection results

Back-projecting high-frequency (HF) waves is a common procedure for imaging rupture processes of large earthquakes (i.e. M_w > 7.0). However, obtained back-projection (BP) results could suffer from large uncertainties since high-frequency seismic waveforms are strongly affected by factors like source depth, focal mechanisms, and the Earth's 3-D velocity structures. So far, these uncertainties have not been thoroughly investigated. Here, we use synthetic tests to investigate the influencing factors for which scenarios with various source and/or velocity set-ups are designed, using either Tohoku-Oki (Japan), Kaikoura (New Zealand), Java/Wharton Basin (Indonesia) as test areas. For the scenarios, we generate either 1-D or 3-D teleseismic synthetic data, which are then back-projected using a representative BP method, MUltiple SIgnal Classification (MUSIC). We also analyse corresponding real cases to verify the synthetic test results. The Tohoku-Oki scenario shows that depth phases of a point source can be back-projected as artefacts at their bounce points on the earth's surface, with these artefacts located far away from the epicentre if earthquakes occur at large depths, which could significantly contaminate BP images of large intermediate-depth earthquakes. The Kaikoura scenario shows that for complicated earthquakes, composed of multiple subevents with varying focal mechanisms, BP tends to image subevents emanating large amplitude coherent waveforms, while missing subevents whose P nodal directions point to the arrays, leading to discrepancies either between BP images from different arrays, or between BP images and other source models. Using the Java event, we investigate the impact of 3-D source-side velocity structures. The 3-D bathymetry together with a water layer can generate strong and long-lasting coda waves, which are mirrored as artefacts far from the true source location. Finally, we use a Wharton Basin outer-rise event to show that the wavefields generated by 3-D near trench structures contain frequency-dependent coda waves, leading to frequency-dependent BP results. In summary, our analyses indicate that depth phases, focal mechanism variations and 3-D source-side structures can affect various aspects of BP results. Thus, we suggest that target-oriented synthetic tests, for example, synthetic tests for subduction earthquakes using more realistic 3-D source-side velocity structures, should be conducted to understand the uncertainties and artefacts before we interpret detailed BP images to infer earthquake rupture kinematics and dynamics

An SEM-DSM three-dimensional hybrid method for modelling teleseismic waves with complicated source-side structures

Despite recent advances in High Performance Computing (HPC), numerical simulation of high frequency (e.g. 1 Hz or higher) seismic wave propagation at the global scale is still prohibitive. To overcome this difficulty, we propose a hybrid method to efficiently compute teleseismic waveforms with 3-D source-side structures. By coupling the Spectral Element Method (SEM) with the Direct Solution Method (DSM) based on the representation theorem, we are able to limit the costly SEM simulation to a small source-side region and avoid computation over the entire space of the Earth. Our hybrid method is benchmarked against 1-D DSM synthetics and 3-D SEM synthetics. We also discuss numerical difficulties in the implementation, including slow DSM convergence near source depth, discretization error, Green’s function interpolation and local 3-D wavefield approximations. As a case study, we apply our hybrid method to two subduction earthquakes and show its advantage in understanding 3-D source-side effects on teleseismic P-waves. Our hybrid method reduces computational cost by more than two orders of magnitude when only source-side 3-D complexities are of concern. Thus our hybrid method is useful for a series of problems in seismology, such as imaging 3-D structures of a subducting slab or a mid-ocean ridge and studying source parameters with 3-D source-side complexities using teleseismic waveforms

An iterative algorithm for separation of S and ScS waves of great earthquakes

Teleseismic SH waves are essential for imaging the rupture processes of large earthquakes. However, for great earthquakes (M8+) such as the 2004 Sumatra earthquake, the 2008 Wenchuan earthquake and the recent Tohoku-Oki earthquake, the source duration is very long (>100 s). Thus the direct SH waves are overlapped with ScS waves for epicentral distances larger than 60°, leaving contaminated S waves for source processes modelling. Therefore artefacts in finite fault models of large earthquake could be produced with such contaminated body waves. We propose an iterative algorithm based on the slowness information of S and ScS waves and stacking technique, to separate S and ScS waves with records from a regional seismic network. Tests on various synthetic data sets show that the algorithm is effective in retrieving teleseismic SH waveforms from complicated wave trains containing both S and ScS. Separation of waveforms for the 2008 Wenchuan earthquake with our algorithm clearly demonstrates the influence of ScS energy, suggesting necessity of recovering S waves

Pervasive Crustal Volcanic Mush in the Highly Stretched Sunda Plate Margin of Northern Sumatra

Arc volcanism, crustal deformation, and their interplay are poorly understood in northwestern Sumatra. Traditional receiver function H-κ stacking studies constrain the variations in crustal thickness and Vp/Vs ratio in volcanic zones but rarely estimate the melt fractions. Here, we propose a H-Φ stacking method, a variant of the H-κ stacking method, and apply it to the dense nodal array data from Aceh, northern Sumatra, to estimate crustal thickness, Vp/Vs ratio, and melt fraction. Most results show considerably high Vp/Vs ratios (∼1.98) and melt fractions (up to 19%), indicating pervasive crustal magmatic mush. The northwestern edge of the Aceh crust is much thinner (∼22 km) than extended crust globally, reflecting a highly stretched crust due to tectonic processes governing the opening of the Andaman Sea. This thin crust and high melt fractions explain the Bouguer gravity anomaly, and partly explain the northward migration of Quaternary volcanics.Ministry of Education (MOE)Published versionThis research is jointly supported by the National Natural Science Foundation of China (42288201) and the Ministry of Education, Singapore, under its MOE Academic Research Fund Tier 3 (Award MOE-MOET32021-0002)

An SEM-DSM three-dimensional hybrid method for modelling teleseismic waves with complicated source-side structures

Despite recent advances in High Performance Computing (HPC), numerical simulation of high frequency (e.g. 1 Hz or higher) seismic wave propagation at the global scale is still prohibitive. To overcome this difficulty, we propose a hybrid method to efficiently compute teleseismic waveforms with 3-D source-side structures. By coupling the Spectral Element Method (SEM) with the Direct Solution Method (DSM) based on the representation theorem, we are able to limit the costly SEM simulation to a small source-side region and avoid computation over the entire space of the Earth. Our hybrid method is benchmarked against 1-D DSM synthetics and 3-D SEM synthetics. We also discuss numerical difficulties in the implementation, including slow DSM convergence near source depth, discretization error, Green’s function interpolation and local 3-D wavefield approximations. As a case study, we apply our hybrid method to two subduction earthquakes and show its advantage in understanding 3-D source-side effects on teleseismic P-waves. Our hybrid method reduces computational cost by more than two orders of magnitude when only source-side 3-D complexities are of concern. Thus our hybrid method is useful for a series of problems in seismology, such as imaging 3-D structures of a subducting slab or a mid-ocean ridge and studying source parameters with 3-D source-side complexities using teleseismic waveforms

Ventilation simulation in an underground ant nest structure of Camponotus japonicus Mayr

Ants are ancient animals on the earth and are known as excellent architects in the animal kingdom. The structure and performance of their nests are full of remarkable mysteries. At present, there are only a limited number of studies on the ventilation performance of underground ant nest structures. In this study, the nests of Camponotus japonicus Mayr were collected manually, and a three-dimensional digital model of the ant nest structure was obtained by the method of industrial CT scanning. The ventilation performance of the Camponotus japonicus Mayr nest structure was numerically simulated using the finite element analysis software, FLUENT. By changing the air inlet and outlet of the nest, the pressure changes inside the nest and the trajectory of the air flow inside the nest could be calculated and analysed, in order to explore the ventilation characteristics of the underground nest structure during natural ventilation. It was found that the ventilation environment inside the nest was stable, and that the external air flow had little effect on the life of the ants inside the nest

Amphiphilic PMMA/PEI core-shell nanoparticles as polymeric adsorbents to remove heavy metal pollutants

Amphiphilic nanoparticles with poly(methyl methacrylate) (PMMA) cores and poly(ethyleneimine) (PEI) shells were successfully synthesized through a one-step emulsifier-free polymerization method. The morphologies of PMMA/PEI nanoparticles were spherical in shape with uniform size distribution and well-defined core-shell nanostructure illustrated by field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). PMMA/PEI core-shell nanoparticles were applied as novel polymeric adsorbents to remove heavy metal pollutants. In present work, copper (II) ions were selected as the target pollutants to evaluate these nanoparticles adsorption capability. It was investigated by varying solution pH, weight ratio of nanoparticles to copper (II) ions, adsorption time and adsorption temperature, respectively. The maximum copper (II) ions adsorbed onto PMMA/PEI core-shell nanoparticles was 14 mg/g obtained under simple and fast experimental conditions, indicating these nanoparticles can be used as effective and practical polymeric adsorbents. (C) 2011 Elsevier B.V. All rights reserved