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• Oxford University Press  (156,631)
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• 1
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Oxford University Press
In:  Climate Justice: Integrating Economics and Philosophy
Publication Date: 2019-02-13
Type: inbook
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• 2
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Oxford University Press
Publication Date: 2019-04-03
Description: Seed dispersal enables plants to reach hospitable germination sites and escape natural enemies. Understanding when and how much seed dispersal matters to plant fitness is critical for understanding plant population and community dynamics. At the same time, the complexity of factors that determine if a seed will be successfully dispersed and subsequently develop into a reproductive plant is daunting. Quantifying all factors that may influence seed dispersal effectiveness for any potential seed-vector relationship would require an unrealistically large amount of time, materials and financial resources. On the other hand, being able to make dispersal predictions is critical for predicting whether single species and entire ecosystems will be resilient to global change. Building on current frameworks, we here posit that seed dispersal ecology should adopt plant functional groups as analytical units to reduce this complexity to manageable levels. Functional groups can be used to distinguish, for their constituent species, whether it matters (i) if seeds are dispersed, (ii) into what context they are dispersed and (iii) what vectors disperse them. To avoid overgeneralization, we propose that the utility of these functional groups may be assessed by generating predictions based on the groups and then testing those predictions against species-specific data. We suggest that data collection and analysis can then be guided by robust functional group definitions. Generalizing across similar species in this way could help us to better understand the population and community dynamics of plants and tackle the complexity of seed dispersal as well as its disruption.
Description: Ideas for this manuscript initiated during the Seed Dispersal Workshop held in May 2016 at the Socio-Environmental Synthesis Center in Annapolis, MD and supported by the US National Science Foundation Grant DEB-1548194 to N.G.B. and the National Socio‐Environmental Synthesis Center under the US National Science Foundation Grant DBI‐1052875. D.Z. received funding from the Swiss National Science Foundation (SNF, grant: PZ00P3_168136/1) and from the German Science Foundation (DFG, grant: ZU 361/1- 1). Contributions by the authors C.A. led the development of the concepts, writing, and revising of the manuscript with input from N.G.B. and H.S.R. All authors contributed to the development of concepts and are listed in order of contribution and alphabetical order within each level of contribution.
Keywords: dependency ; directed dispersal ; dispersal vectors ; generalization ; mutualism ; seed dispersal effectiveness
Repository Name: Woods Hole Open Access Server
Type: Article
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• 3
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Oxford University Press
In:  Journal of travel medicine
Publication Date: 2019-06-05
Description: Humans have a long history of mobility on a spectrum from voluntary migration to forced displacement in response to social, political and environmental change. While many migration drivers exist, climate change is likely to amplify the environmental drivers of migration. At least 1.5?C of warming above pre-industrial levels between 2030 and 2052 are projected if global warming continues to increase at the current rate. The associated impacts are diverse and include temperature and precipitation extremes in most inhabited regions and increased probability of drought and flood. Migration can be an important and useful adaptive response to climate impacts when it increases household resilience and reduces socio-economic vulnerabilities, and yet can also have negative health consequences. The climate?migration?health nexus entails complex interactions including the following: first, climate-related risks to health faced by migrants at all stages of the migration journey. Second, the impacts of migration itself on health with possible specific health implications of climate-related migration. This article provides a brief overview of climate-related migration, identifies climate hotspots where substantial migration and displacement are anticipated and explores the health implications of climate-related migration.
Language: English
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• 4
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Oxford University Press
In:  National Science Review
Publication Date: 2019-06-21
Description: The Tibetan Plateau and its surroundings are known as the Third Pole (TP). This region is noted for its high rates of glacier melt and the associated hydrological shifts that affect water supplies in Asia. Atmospheric pollutants contribute to climatic and cryospheric changes through their effects on solar radiation and the albedos of snow and ice surfaces; moreover, the behavior and fates within the cryosphere and environmental impacts of environmental pollutants are topics of increasing concern. In this review, we introduce a coordinated monitoring and research framework and network to link atmospheric pollution and cryospheric changes (APCC) within the TP region. We then provide an up-to-date summary of progress and achievements related to the APCC research framework, including aspects of atmospheric pollution's composition and concentration, spatial and temporal variations, trans-boundary transport pathways and mechanisms, and effects on the warming of atmosphere and changing in Indian monsoon, as well as melting of glacier and snow cover. We highlight that exogenous air pollutants can enter into the TP?s environments and cause great impacts on regional climatic and environmental changes. At last, we propose future research priorities and map out an extended program at the global scale. The ongoing monitoring activities and research facilitate comprehensive studies of atmosphere?cryosphere interactions, represent one of China's key research expeditions to the TP and the polar regions and contribute to the global perspective of earth system science.
Language: English
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• 5
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Oxford University Press
In:  FEMS Microbiology Letters, 366 (11).
Publication Date: 2019-07-10
Description: Metabolites give us a window into the chemistry of microbes and are split into two subclasses: primary and secondary. Primary metabolites are required for life whereas secondary metabolites have historically been classified as those appearing after exponential growth and are not necessarily needed for survival. Many microbial species are estimated to produce hundreds of metabolites and can be affected by differing nutrients. Using various analytical techniques, metabolites can be directly detected in order to elucidate their biological significance. Currently, a single experiment can produce anywhere from megabytes to terabytes of data. This big data has motivated scientists to develop informatics tools to help target specific metabolites or sets of metabolites. Broadly, it is imperative to identify clear biological questions before embarking on a study of metabolites (metabolomics). For instance, studying the effect of a transposon insertion on phenazine biosynthesis in Pseudomonas is a very different from asking what molecules are present in a specific banana-derived strain of Pseudomonas. This review is meant to serve as a primer for a ‘choose your own adventure’ approach for microbiologists with limited mass spectrometry expertise, with a strong focus on liquid chromatography mass spectrometry based workflows developed or optimized within the past five years.
Type: Article , PeerReviewed
Format: text
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• 6
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Oxford University Press
In:  Zoological Journal of the Linnean Society, 185 (3). pp. 555-635.
Publication Date: 2019-08-06
Description: Polynoidae contains ~900 species within 18 subfamilies, some of them restricted to the deep sea. Macellicephalinae is the most diverse among these deep-sea subfamilies. In the abyssal Equatorial Pacific Ocean, the biodiversity of benthic communities is at stake in the Clarion-Clipperton Fracture Zone (CCFZ) owing to increased industrial interest in polymetallic nodules. The records of polychaetes in this region are scarce. Data gathered during the JPI Oceans cruise SO239 made a significant contribution to fill this gap, with five different localities sampled between 4000 and 5000 m depth. Benthic samples collected using an epibenthic sledge or a remotely operated vehicle resulted in a large collection of polynoids. The aims of this study are as follows: (1) to describe new species of deep-sea polynoids using morphology and molecular data (COI, 16S and 18S); and (2) to evaluate the monophyly of Macellicephalinae. Based on molecular and morphological phylogenetic analyses, ten subfamilies are synonymized with Macellicephalinae in order to create a homogeneous clade determined by the absence of lateral antennae. Within this clade, the Anantennata clade was well supported, being determined by the absence of a median antenna. Furthermore, 17 new species and four new genera are described, highlighting the high diversity hidden in the deep. A taxonomic key for the 37 valid genera of the subfamily Macellicephalinae is provided.
Type: Article , PeerReviewed
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• 7
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Oxford University Press
Publication Date: 2019-09-05
Description: DNA sequencing technology has revolutionized the field of biology, shifting biology from a data-limited to data-rich state. Central to the interpretation of sequencing data are the computational tools and approaches that convert raw data into biologically meaningful information. Both the tools and the generation of data are actively evolving, yet the practice of re-analysis of previously generated data with new tools is not commonplace. Re-analysis of existing data provides an affordable means of generating new information and will likely become more routine within biology, yet necessitates a new set of considerations for best practices and resource development. Here, we discuss several practices that we believe to be broadly applicable when re-analyzing data, especially when done by small research groups.
Description: Funding was provided by the Gordon and Betty Moore Foundation (award GBMF4551 to C.T.B.).
Keywords: reproducibility ; data reuse ; open data
Repository Name: Woods Hole Open Access Server
Type: Article
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• 8
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Oxford University Press
Publication Date: 2019-09-05
Description: Background: De novo transcriptome assemblies are required prior to analyzing RNA sequencing data from a species without an existing reference genome or transcriptome. Despite the prevalence of transcriptomic studies, the effects of using different workflows, or “pipelines,” on the resulting assemblies are poorly understood. Here, a pipeline was programmatically automated and used to assemble and annotate raw transcriptomic short-read data collected as part of the Marine Microbial Eukaryotic Transcriptome Sequencing Project. The resulting transcriptome assemblies were evaluated and compared against assemblies that were previously generated with a different pipeline developed by the National Center for Genome Research. Results: New transcriptome assemblies contained the majority of previous contigs as well as new content. On average, 7.8% of the annotated contigs in the new assemblies were novel gene names not found in the previous assemblies. Taxonomic trends were observed in the assembly metrics. Assemblies from the Dinoflagellata showed a higher number of contigs and unique k-mers than transcriptomes from other phyla, while assemblies from Ciliophora had a lower percentage of open reading frames compared to other phyla. Conclusions: Given current bioinformatics approaches, there is no single “best” reference transcriptome for a particular set of raw data. As the optimum transcriptome is a moving target, improving (or not) with new tools and approaches, automated and programmable pipelines are invaluable for managing the computationally intensive tasks required for re-processing large sets of samples with revised pipelines and ensuring a common evaluation workflow is applied to all samples. Thus, re-assembling existing data with new tools using automated and programmable pipelines may yield more accurate identification of taxon-specific trends across samples in addition to novel and useful products for the community.
Description: Funding was provided by the Gordon and Betty Moore Foundation under award number GBMF4551 to C.T.B. Jetstream cloud platform was used with XSEDE allocation TG-BIO160028 [66, 67].
Keywords: marine microbial eukaryote ; transcriptome assembly ; automated pipeline ; re-analysis
Repository Name: Woods Hole Open Access Server
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• 9
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Oxford University Press
In:  Genome Biology and Evolution, 11 (8). pp. 2344-2359.
Publication Date: 2019-10-01
Description: Repeated and independent emergence of trait divergence that matches habitat differences is a sign of parallel evolution by natural selection. Yet, the molecular underpinnings that are targeted by adaptive evolution often remain elusive. We investigate this question by combining genome-wide analyses of copy number variants (CNVs), single nucleotide polymorphisms (SNPs), and gene expression across four pairs of lake and river populations of the three-spined stickleback (Gasterosteus aculeatus). We tested whether CNVs that span entire genes and SNPs occurring in putative cis-regulatory regions contribute to gene expression differences between sticklebacks from lake and river origins. We found 135 gene CNVs that showed a significant positive association between gene copy number and gene expression, suggesting that CNVs result in dosage effects that can fuel phenotypic variation and serve as substrates for habitat-specific selection. Copy number differentiation between lake and river sticklebacks also contributed to expression differences of two immune-related genes in immune tissues, cathepsin A and GIMAP7. In addition, we identified SNPs in cis-regulatory regions (eSNPs) associated with the expression of 1,865 genes, including one eSNP upstream of a carboxypeptidase gene where both the SNP alleles differentiated and the gene was differentially expressed between lake and river populations. Our study highlights two types of mutations as important sources of genetic variation involved in the evolution of gene expression and in potentially facilitating repeated adaptation to novel environments.
Type: Article , PeerReviewed
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• 10
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Oxford University Press
In:  Molecular Biology and Evolution .
Publication Date: 2019-10-01
Description: Cichlid fishes provide textbook examples of explosive phenotypic diversification and sympatric speciation, thereby making them ideal systems for studying the molecular mechanisms underlying rapid lineage divergence. Despite the fact that gene regulation provides a critical link between diversification in gene function and speciation, many genomic regulatory mechanisms such as microRNAs (miRNAs) have received little attention in these rapidly diversifying groups. Therefore, we investigated the posttranscriptional regulatory role of miRNAs in the repeated sympatric divergence of Midas cichlids (Amphilophus spp.) from Nicaraguan crater lakes. Using miRNA and mRNA sequencing of embryos from five Midas species, we first identified miRNA binding sites in mRNAs and highlighted the presences of a surprising number of novel miRNAs in these adaptively radiating species. Then, through analyses of expression levels, we identified putative miRNA/gene target pairs with negatively correlated expression level that were consistent with the role of miRNA in downregulating mRNA. Furthermore, we determined that several miRNA/gene pairs show convergent expression patterns associated with the repeated benthic/limnetic sympatric species divergence implicating these miRNAs as potential molecular mechanisms underlying replicated sympatric divergence. Finally, as these candidate miRNA/gene pairs may play a central role in phenotypic diversification in these cichlids, we characterized the expression domains of selected miRNAs and their target genes via in situ hybridization, providing further evidence that miRNA regulation likely plays a role in the Midas cichlid adaptive radiation. These results provide support for the hypothesis that extremely quickly evolving miRNA regulation can contribute to rapid evolutionary divergence even in the presence of gene flow.
Type: Article , PeerReviewed
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Format: archive
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• 11
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Oxford University Press
In:  Geophysical Journal International, 219 (3). pp. 1876-1884.
Publication Date: 2019-10-23
Description: Standard seismic acquisition and processing require appropriate source-receiver offsets. P-cable technology represents the opposite, namely, very short source-receiver offsets at the price of increased spatial and lateral resolution with a high-frequency source. To use this advantage, a processing flow excluding offset information is required. This aim can be achieved with a processing tuned to diffractions because point diffractions scatter the same information in offset and midpoint direction. Usually, diffractions are small amplitude events and a careful diffraction separation is required as a first step. We suggest the strategy to use a multiparameter stacking operator, e.g, common-reflection surface, and stack along the midpoint direction. The obtained kinematic wavefront attributes are used to calculate time-migration velocities. A diffractivity map serves as filter to refine the velocities. This strategy is applied to a 3D P-cable data set to obtain a time-migrated image.
Type: Article , PeerReviewed
Format: text
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• 12
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉North Korea conducted sixth underground nuclear test on September, 3〈sup〉rd〈/sup〉, 2017. Unlike its previous tests, a rare subsequent collapse event occurred after about 8.5 minutes. As two types of distinctive shallow seismic events, accurate inversion of their focal mechanisms is important for event identification for CTBT. In this paper, we carry out moment tensor inversion of the nuclear test and the collapse event with gCAP using waveform data from dense regional seismic stations. And their focal mechanisms are further constrained with surface wave amplitude ratio. The results show that the surface wave amplitude ratio has further constraints for screening the waveform inversion results. The resolution of the focal mechanism inversion for the nuclear test is high, which is close to a Crack source. However, the resolution for the collapse event inversion is not so high and the source type is difficult to be accurately determined. One reason of the poor resolution for the collapse event may be due to the limited availability of high quality data, and complexity of the source process might be another factor.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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• 13
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉The Greater Geneva Basin is one of the key targets for geothermal exploration in Switzerland. Until recently, information about the subsurface structure of this region was mostly composed of well-logs, seismic reflection lines, and gravity measurements. As part of the current effort to further reduce subsurface uncertainty, and to test passive seismic methods for exploration purposes, we performed an ambient-noise tomography of the Greater Geneva Basin. We used ∼1.5 years of continuous data collected on a temporary seismic network composed of 28 broadband stations deployed within and around the basin. From the vertical component of the continuous noise recordings, we computed cross-correlation functions and retrieved Rayleigh-wave group-velocity dispersion curves. We then inverted the dispersion curves to obtain 2D group-velocity maps and proceeded to a subsequent inversion step to retrieve a large-scale 3D shear-wave velocity model of the basin. We discuss the retrieved features of the basin in the light of local geology, previously acquired geophysical datasets, and ongoing geothermal exploration. The Greater Geneva Basin is an ideal natural laboratory to test innovative geothermal exploration methods because of the substantial geophysical datasets available for comparison. While we point out the limits of ambient-noise exploration with sparse networks and current methodology, we also discuss possible ways to develop ambient-noise tomography as an affordable and efficient subsurface exploration method.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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• 14
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Marchenko methods are a suite of geophysical techniques that convert seismograms of energy created by surface sources and measured by surface receivers into seismograms that would have been recorded by a virtual receiver at an arbitrary point inside the subsurface – an operation called redatuming. In principle these redatumed seismograms contain all contributions from direct, primary (singly-reflected) and multiply-reflected waves that would have been recorded by a real subsurface receiver, without requiring prior information about interfaces that generated the reflections. The potential of these methods for seismic imaging and redatuming has been demonstrated extensively in previous literature, but only using one- and two-dimensional Marchenko methods. There remain aspects of the methods that are poorly understood when applied in a three-dimensional world, so we investigate the application of Marchenko methods to three-dimensional data, subsurface structures and wavefields. We first show that for waves propagating in three dimensions, Marchenko methods can be applied to seismic data collected using both linear (so-called 2D-seismic) and areal (3D-seismic) acquisition arrays. However, for 2D acquisition arrays the Marchenko workflow requires additional dimensionality correction factors to obtain accurate solutions, even in a subsurface that only varies with depth. Without these correction factors phase errors occur in redatumed Marchenko estimates; these errors propagate through the Marchenko algorithm and create depth errors in the Marchenko images. Furthermore, applying Marchenko methods to fully three-dimensional seismic wavefields recorded by linear (2D-seismic) arrays that contain out-of-plane reflections deteriorates surface-to-subsurface Green’s function estimates with spurious energy and resulting images are less accurate than those created using ‘conventional’ imaging methods. The application of fully three-dimensional Marchenko methods using data recorded on areal arrays solves both of the above problems, creating accurately redatumed wavefields and images with reduced artifact contamination. However, it appears that source/receiver spacing at most of $\lambda _A\Big /4$ is required for accurate results using existing Marchenko methods, where λ〈sub〉〈span〉A〈/span〉〈/sub〉 is the dominant wavelength and in many real 3D seismic acquisition scenarios this is impractical.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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• 15
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Oxford University Press
Publication Date: 2019
Description: 〈span〉In the original version of this paper, there was an error in the name of author Qiang Guo. This has now been corrected and the publisher apologises for the error.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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• 16
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Land seismic multiparameter full waveform inversion in anisotropic media is challenging because of high medium contrasts and surface waves. With a data-residual least-squares objective function, the surface wave energy usually masks the body waves and the gradient of the objective function exhibits high values in the very shallow depths preventing from recovering the deeper part of the earth model parameters. The optimal transport objective function, coupled with a Gaussian time-windowing strategy, allows to overcome this issue by more focusing on phase shifts and by balancing the contributions of the different events in the adjoint-source and the gradients. We first illustrate the advantages of the optimal transport function with respect to the least-squares one, with two realistic examples. We then discuss a vertical transverse isotropic (VTI) example starting from a quasi 1-D isotropic initial model. Despite some cycle-skipping issues in the initial model, the inversion based on the windowed optimal transport approach converges. Both the near-surface complexities and the variations at depth are recovered.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
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• 17
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Over the past few decades, seismic studies have revealed complex structural anomalies in the Earth’s deep interior at various scales, such as large low-shear-velocity provinces (LLSVPs) and ultra-low velocity zones (ULVZs) in the lowermost mantle, and small-scale scatterers in the mid-mantle. These structures which are critical for better understanding of the geodynamics and evolution of the deep Earth, need to be further resolved by high-resolution imaging techniques. The spectral-element method (SEM) can be used to accurately simulate seismic wave propagation in heterogeneous Earth models, and its application in full-waveform inversion (FWI) provides a promising high-resolution and high-fidelity imaging technique. But it can be computationally prohibitive when used to model small scale structures in the deep Earth based upon high-frequency seismic waves. The heavy computational cost can be circumvented by using hybrid methods, which restrict the main computation by SEM solver to only a small target region (e.g. above the CMB) encompassing possible 2-D/3-D anomalies, and apply efficient analytical or numerical methods to calculate the wavefield for 1-D background models. These forward modelling tools based on hybrid methods can be then used in the so-called ‘box tomography’ approach to resolve fine-structures in the deep Earth.In this study, we outline the theory of a hybrid method used to model small scale structures in the deep Earth and present its implementation based on SEM solvers in a three-step workflow. First, the wavefield generated by the source is computed for the 1-D background model with traction and velocity saved for the virtual boundary of the target region, which are then used as boundary inputs to simulate the wavefield in the target region based on absorbing boundary condition in SEM. In the final step, the total wavefield at receivers is reconstructed based upon the total wavefield on the virtual boundary computed in the previous step. As a proof-of-concept study, we demonstrate the workflow of the hybrid method based on a 2-D SEM solver. Examples of the hybrid method applied to a coupled fluid–solid model show that our workflow can accurately recover the scattered waves back to the surface. Furthermore, we benchmark the hybrid method on a realistic heterogeneous Earth model built from 〈span〉AK135-F〈/span〉 and show how teleseismic scattered waves can be used to model deep Earth structures. By documenting the theory and SEM implementation of the hybrid method, our study lays the foundation for future two-way coupling of 3-D SEM solver with other efficient analytic or numerical 1-D solvers.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
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• 18
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Since 2013 to date more than 1000 seismic events have been recorded by the Colombian Geological Survey (GSC) in the municipality of Puerto Gaitán (Colombia). A total of fourteen earthquakes are moment magnitude M〈sub〉W〈/sub〉 greater than 4.0. The largest event ever recorded in the area occurred in November 2015 with M〈sub〉W〈/sub〉 4.8. It seems like the case of Puerto Gaitán is associated with the deep injection of co-produced wastewater from oil and gas extraction. The data presented in this work suggests a close relationship in space and time between injection operations and seismicity. An analysis of temporality between both datasets resulted in a time lag equivalent to about 218 days. For this paper, we computed the input and output energy during injection operations from 2013 to 2015 in order to estimate the fraction of total input energy that is radiated as seismic waves. Our results suggest that the seismic energy is only a small fraction of the total energy into the system. Although Puerto Gaitan is one of the places with the most significant volume of wastewater injected among the ones reported in the literature, the energy efficiency of the system is the lowest reported to date in comparison with other applied technologies. The low efficiency seems to be associated to the aseismic deformation of the reservoir rocks. The observed clustering of earthquakes is delimited by the basement crystalline depth. From an operational point of view, we determine that, like most cases associated with fluid injection, volume of fluid is the variable that determines change in the seismic Moment released. Furthermore, the sequence of events in Puerto Gaitán may not fit into a well-known correlation between the volume of fluid injected and the maximum expected magnitude. The observed magnitudes in Puerto Gaitan are well bellow compared to those reported in the literature for similar volumes of injected fluid.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
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• 19
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉We present 2-D attenuation images of the Mt. Etna volcanic region on the basis of separation of intrinsic and scattering effects. The analysis presented here exploits a large active seismic database that fully covers the area under study. We observe that scattering effects dominate over intrinsic attenuation, suggesting that the region is very heterogeneous. Comparison with analyses conducted at other volcanoes reveals that the Mt. Etna region is characterised by high intrinsic attenuation, resulting from the presence of large volcanoclastic deposits at shallow depth. The 2-D distributions of intrinsic and scattering anomalies show the presence of regions characterised by high and low attenuation effects, corresponding to several tectonic and volcanic features. In particular, we identify a high attenuation region in the SW sector of the Mt. Etna volcanic complex, which is correlated with high seismicity rates and volcanism. This work supports the hypothesis of a link between the dynamics of the SW flank and the recharge of the volcano in the last decades, occurring under the summit crater and, secondarily, the upper South rift zone.〈/span〉
Print ISSN: 2051-1965
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Topics: Geosciences
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• 20
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Earthquake focal mechanisms put primary control on the distribution of ground motion, and also bear on the stress state of the crust. Most routine focal mechanism catalogs still use 1D velocity models in inversions, which may introduce large uncertainties in regions with strong lateral velocity heterogeneities. In this study, we develop an automated waveform-based inversion approach to determine the moment tensors of small-to-medium-sized earthquakes using 3D velocity models. We apply our approach in the Los Angeles region to produce a new moment tensor catalog with a completeness of M〈sub〉L 〈/sub〉≥ 3.5. The inversions using the Southern California Earthquake Center Community Velocity Model (3D CVM-S4.26) significantly reduces the moment tensor uncertainties, mainly owing to the accuracy of the 3D velocity model in predicting both the phases and the amplitudes of the observed seismograms. By comparing the full moment tensor solutions obtained using 1D and 3D velocity models, we show that the percentages of non-double-couple components decrease dramatically with the usage of 3D velocity model, suggesting that large fractions of non-double-couple components from 1D inversions are artifacts caused by unmodeled 3D velocity structures. The new catalog also features more accurate focal depths and moment magnitudes. Our highly accurate, efficient, and automatic inversion approach can be expanded in other regions, and can be easily implemented in near real-time system.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
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• 21
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Salt structures are high potential targets for oil and gas exploration. However, large-scale salt domes with irregular surfaces pose significant challenges for velocity model building. For full waveform inversion, in the absence of a high-fidelity initial model, the success of the inversion depends on low-frequency seismic data, which are scarce in the exploration data sets. This paper presents a new idea to solve the problem of salt structure velocity modeling. Firstly, we propose an envelope-based full-band seismic data reconstruction algorithm. The smoothness of envelope is used to segment the events in seismic data, and the phase independence of envelope is used for the identification of the seismic event's arrival-time to obtain the apparent reflection sequences of the subsurface. Full-band seismic data are obtained by convolving the apparent reflection sequence with full-band source. Window averaging function and threshold strategy are used to ensure the accuracy of seismic event segmentation and the stability of the algorithm when dealing with noisy data. Then the multiscale reflection waveform inversion based on reconstructed data is proposed for salt structure velocity building. The numerical experiment results of the Sigbee2A model demonstrate the performance of the inversion algorithm in the case where the seismic data lack low-frequency components and contain noise. The limitations of the algorithm have also been analyzed and studied.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
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• 22
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉It has long been recognized that the effects of superficial geological layers, or site effects, can play a major role on the seismic ground motion at the free surface. In this study, we compute wave propagation in a 2-D asymmetrical basin considering both soil non-linearity and pore-pressure effects. Equations of elastodynamics of wave propagation are solved using the spectral element method (SEM). The geometry of the basin gives rise to basin-edge generated waves, that are different for in-plane (P-SV) and out-of-plane (SH) wave propagation and resulting in different non-linear response. Moreover, the excess-pore pressure development in superficial liquefiable layers (effective stress analysis) brings larger deformation and loss of strength than the analysis without pore-pressure effects (total stress analysis). The coupling of vertically propagating waves and the waves specifically generated in 1-D model leads to waves whose amplitude and duration are higher than the 1-D case. This multidimensional effect increases material non-linearity. Such complex wavefield provokes larger deformation and higher pore-pressure rise that cannot be predicted by 1-D modelling. Therefore, our paper suggests the use of multidimensional modelling while studying seismic wave propagation in both linear and non-linear complex media.〈/span〉
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• 23
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Understanding the fluid dependence of poroelastic constants of a layered porous package is important for various aspects of applied and fundamental geosciences. To decouple the effects of fluid substitutions and anisotropy in a layered package on vertical stiffness constants, a set of approximations to anisotropic fluid substitution theory is introduced in conjunction with Thomsen's anisotropy parameters. Validation of the approximations is performed by physical modelling and theoretical examples. In physical modelling, synthetic porous layers are used and interbedded by Plexiglas sheets to build a layered transversely isotropic symmetry with a vertical symmetry axis package. Seismic acquisitions over the physical model saturated with air, oil and water are carefully conducted, respectively. The reflection amplitudes are properly corrected and inverted by a specific seismic inversion scheme to recover 〈span〉P〈/span〉- and 〈span〉S〈/span〉-wave impedances. Poroelastic constants of the thin package then are deduced from the inverted results. Applying the approximations to the physical modelling results, a good match between the estimated vertical stiffness constant values of the physical model and the theoretical predictions is observed. Results of both physical modelling and theoretical analysis demonstrate that fluid substitutions when going from drained to undrained behaviour will enhance or reduce the degree of anisotropy of the medium, depending on the sign and magnitude of Thomsen's anisotropy parameter δ. Results show that the shear modulus of the individual layer plays a key role in controlling the degree of the initial anisotropy of the thin package, which directly dominates the effect of pore fluids on poroelastic constants of the upscaled medium.〈/span〉
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• 24
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Trend drift is an annoying background interference in induced polarization (IP) exploration, which has great influence on the final calculation of apparent complex resistivity spectrum at low frequency (〈0.1 Hz). This paper proposed a modified empirical mode decomposition (EMD) detrending technique for multiperiod IP data. The method uses local extreme values of the rising edges and the falling edges to form multiple envelopes and then to fit and eliminate the trend term. Through comparing with the traditional EMD methods using IP data with simulated trend drift, we find that the modified method can be used to obtain a more accurate fitting trend and the computational cost is only a fraction of that of the conventional one. Additionally, this detrending is little affected by other strong noise. We also used IP data with and without trend interferences to analyse this method, respectively. The results show that, for data without trend drift, the signals remain almost unchanged; however, for data with strong trend drift, the data quality is greatly improved and the calculation error is reduced. This technique is also applied to a large-scale multiperiod full-waveform IP data acquired in Zhegu Zn-Sb-Ag polymetallic deposit in southern Tibet, China. The apparent complex resistivity and phase of a survey line, a planar contour map and a pseudo-section with and without using the modified EMD were compared, respectively. Overall, before EMD detrending, the apparent phase results are rough and full of outliers. After detrending, the profiles are smooth and reasonable, and the outliers disappear. Both the results demonstrated that our proposed method can be adopted to effectively suppress trend drift interference without additional deviation in distributed full-waveform IP exploration.〈/span〉
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• 25
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Oxford University Press
Publication Date: 2019
Description: 〈span〉The title of my recent article (Flament 〈a href="http://academic.oup.com/gji#bib1"〉2019〈/a〉) should be ‘Present-day dynamic topography and lower-mantle structure from palaeogeographically constrained mantle flow models’, not ‘Present-dayd dynamic topography and lower-mantle structure from palaeogeographically constrained mantle flow models’. This spelling mistake was introduced at proof stage and was present in the published version of the article.〈/span〉
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• 26
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉The gravity gradient tensor has been increasingly used in practical applications. Among them, how to extract information contained in different gravity gradient components is a challenging problem. Gravity gradient joint inversion is one effective method to solve this problem. We integrate different gravity gradient components in a matrix and then apply them in inversion directly. In this paper, we modify the method to get a new gravity gradient joint inversion (NGGJI). The method is based on the reweighted regularized inversion. We choose one component, for example, 〈span〉g〈/span〉〈sub〉zz〈/sub〉, and use the other components to build a weighting matrix. Then we apply the weighting matrix in 〈span〉g〈/span〉〈sub〉zz〈/sub〉 inversion. We present the method to construct the weighting matrix based on a single component and multiple components. We analyse the characteristics of different weighting matrices and the noise effects on weighting matrices. We compare the inversion results obtained from the conventional gravity gradient joint inversion (CGGJI) with the inversion results obtained from the NGGJI. We conclude that the NGGJI's requirement for memory storage is lower and the resolution of the NGGJI inversion results is higher. We apply the method to survey data from Vinton Salt Dome, Louisiana, USA. The results have proved to be consistent with known geological information.〈/span〉
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• 27
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉In this study, a straightforward and rapid methodology is proposed and tested to determine the seismic moment, the earthquake rupture length/duration and the static stress drop. To this purpose, three ground motion parameters, that is, 〈span〉P〈/span〉-wave peak acceleration (${P_a}$), velocity (${P_v}$) and displacement (${P_d}$) are evaluated as a function of time from the first 〈span〉P〈/span〉 arrival. The average of the logarithm of the 〈span〉P〈/span〉-wave amplitude (LPDT curves), corrected for the distance-attenuation effect, is calculated using all the available stations in expanded 〈span〉P〈/span〉-wave time windows. The LPDT curves show an exponential growth shape and increase with time until they reach a constant value (plateau), which is related to the magnitude of the earthquake. From the obtained observations, we demonstrate that the corner time of the plateau level on the weighted-fit curve to the LPDT curves is related to the half-duration of the rupture. Thus, using the theoretical scaling, the source radius and stress drop can be obtained from the measured half-duration of the source. This method has been applied and tested to the records of the 2016–2017 Central Italy seismic sequence, with moment magnitude ranging between 3.4 and 6.5. Our study shows that source parameters match a self-similar, constant-stress-drop scaling with a relatively low average stress drop of about $1.1 \pm 0.5\ \mathrm{ MPa}$, except for the largest event of the sequence showing a relatively higher stress release, which is associated with the dominant radiation from a localized high slip patch on the fracture surface. The proposed approach based on a simple time domain signal analysis is innovative and may complement longer spectral technique for fast estimating earthquake source properties.〈/span〉
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• 28
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉When performing a cooperative inversion using a structural constraint, extracting an accurate average direction from the high-resolution model is important because two models with different resolutions should be used in the procedure. However, when the average direction of the high-resolution model, which indicates the major change direction of the model at each inversion block location, is calculated, the conventional gradient methods such as cross-gradient have a limitation on components having opposite directions. Therefore, in this study, an effective average-direction extraction algorithm was developed by introducing the concept of the structure tensor to accurately calculate average-direction information. And finally, based on the extracted average-direction information, structure-tensor-constrained cooperative inversion algorithm was proposed. To verify the effectiveness of the proposed inversion method, the method was tested using data obtained from the synthetic model containing an anomalous body with a complicated shape and the result compared with results of individual EM inversion and the conventional cross-gradient-constrained cooperative inversion. Lastly, to evaluate the performance with a realistic mode, the proposed cooperative inversion was applied to the data acquired using the complex SEG Advanced Modelling Program model. In all experiments, the cooperative inversion with the structure-tensor constraint provided better location estimation results as well as better estimations of the shape of the anomaly. In addition, the resistivity distribution of the anomaly was estimated to be closer to the truth in the inversion result.〈/span〉
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• 29
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉The goal of next generation gravity missions (NGGM) is to improve the monitoring of mass transport in the Earth system by an increased space–time sampling capability as well as higher accuracies of a new generation of instrumentation. They should be able to fulfil the scientific and societal needs of providing high-resolution short-time gravity field solutions for geophysical applications like for for example service applications such as flood and drought monitoring and forecast or applications in water management. To facilitate this need a near-real time (NRT) processing scheme based on a coparametrization of low-resolution daily and longer-term gravity field solution, combined with a sliding window averaging, was set up. In contrast to other strategies that are usually based on Kalman filtering, the proposed NRT concept is independent of any prior information about the temporal gravity field, and does not require any regularization. The enhanced spatial-temporal resolution opens the possibility to self-dealias high-frequency atmospheric and oceanic signals, and additionally provides gravity field solutions on short timescales. In order to quantify the capabilities of the proposed NRT approach, a numerical closed-loop simulation of a low-low satellite-to-satellite tracking (ll-sst) mission for a two-pair Bender-type constellation with realistic noise assumptions was performed. While for the daily parametrization a spherical harmonics degree and order of 15 turns out to be a favourable choice, by applying the sliding window NRT approach stable daily gravity field estimates up to degree/order 50 with latencies of down to 1 d could be achieved.〈/span〉
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• 30
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉From a suite of 56 chemically-driven dynamo simulations with aspect ratio χ (inner to outer core radii) ranging from 0.10 to 0.44, we conduct the first systematic investigation of the impact of inner-core size on the reversing behaviour of dynamos. We show that the growth of the inner core leads to a transition between a “small inner-core” regime (χ ≤ 0.18), when the field produced is intermediately strong and dipolar, and a “large inner-core” regime (χ 〉 0.26), when the field is stronger and more dipolar. During that transition the field is weaker and slightly less dipolar. For aspect ratios 0.20 ≤ χ ≤ 0.22, reversal frequencies may be more sensitive to changes in the vigour of the convection, allowing high frequencies to be reached much more easily. Although other factors than the size of the inner core likely contribute to controlling the reversal frequency of the Earth’s dynamo, we hypothesise that the occurrence of such a transition for the Earth’s core between the end of the Precambrian and the end of the Devonian could possibly account for the manifestation of an unusual long-lasting episode of predominantly reversal hyperactivity and complex low intensity fields during that still poorly documented period of time.〈/span〉
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• 31
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉A promising way to perform seismological studies in the Arctic region is deploying seismic stations on ice floes. The pioneering works by the Alfred Wegener Institute (AWI) Bremerhaven (Schlindwein 〈span〉et al.〈/span〉 2007; Laderach and Schlindwein, 2011) have demonstrated the efficiency of such floating networks to explore local and regional seismicity and to build 3D seismic models. However, problems remain, related to the identification of different types of seismic waves, particularly S-waves. Here, we perform 2D and 3D numerical simulations of seismic waves emitted by an earthquake to explore the possibility of recording different phases on the sea surface. We use different types of simple shear source models, namely, strike-slip, vertical displacement and normal faults. In the calculated wave field, we obtain three major types of seismic waves recorded on the sea surface: 〈span〉Pw, Sw〈/span〉 and 〈span〉SPw〈/span〉 (〈span〉w〈/span〉 denotes an acoustic wave in the water layer) and numerous multiple waves. The clarity of the recorded phases strongly depends on the type of wave, source mechanism, epicentral distance, thickness of the water layer and depth of the source. For example, the 〈span〉Pw〈/span〉 phase is clearest for the strike-slip mechanism, less clear for the normal fault and almost invisible for the vertical displacement. The 〈span〉Sw〈/span〉 phase is observable in all of these cases; however, it can be confused with the 〈span〉SPw〈/span〉 phase that arrives earlier. In addition, at some distances, the 〈span〉Sw〈/span〉 wave interferes with the multiple 〈span〉Pw2〈/span〉 wave and therefore is hardly detectable. In summary, the numerical simulations in a model with a water layer have demonstrated several non-obvious features of wave propagation that should be taken into account when analysing experimental data recorded on ice floes.〈/span〉
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• 32
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉The formation of fold-thrust belts at convergent margins is a dynamic process. Accretion of weak sediments to the front of the overriding plate results in crustal thickening and continued flexural subsidence of the underthrusting plate. Fold-thrust belts are often treated as a Coulomb wedge having self-similar geometries with a critical taper, and either a rigid or isostatically compensated base. In this paper we build upon this work by developing a new dynamic model to investigate both the role of the thickness and material properties of the incoming sediment, and the flexure in the underthrusting plate in controlling the behaviour and evolution of fold-thrust belts. Our analysis shows that the evolution of fold-thrust belts can be dominated by either gravitational spreading or vertical thickening, depending on the relative importance of sediment flux, material properties and flexure. We apply our model to the Makran accretionary prism and the Indo-Burman Ranges, and show that for the Makran flexure must be considered in order to explain the dip of the sediment-basement interface from seismic reflection profiles. In the Indo-Burman Ranges, we show that incoming sediment thickness has a first-order control on the variations in the characteristics of the topography from north to south of the Shillong Plateau.〈/span〉
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• 33
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Developing a model for anthropogenic seismic hazard remains an open challenge whatever the geo-resource production. We analyze the (M〈sub〉max〈/sub〉) largest reported magnitude on each site where (RTS) Reservoir Triggered Seismicity in documented, (37 events, 1933–2008), for aftershocks of reservoir impoundment loading. We relate each reservoir impoundment to its magnitude-equivalent M*〈sub〉reservoir〈/sub〉 = M*(L〈sub〉r〈/sub〉). We use (L〈sub〉r〈/sub〉) the reservoir length as a proxy for a rupture length of the reservoir mainshock-equivallent. This latter is derived from the empirical relationship that exists for tectonic earthquake among magnitude and rupture length. We resolve (i) M〈sub〉max〈/sub〉 for RTS are bounded by M*〈sub〉reservoir〈/sub〉 at a 95 per cent confidence level; (ii) in average M〈sub〉max〈/sub〉 are smaller than M*〈sub〉reservoir〈/sub〉 by 2.2 units (iii) 50 per cent of the M〈sub〉max〈/sub〉 occurrence is within 2 + /- 1 years from the reservoir impoundment. These triggering patterns support the signature of fluid driven seismicity during the slow reservoir impoundment emerges as a weaker efficiency (larger ΔM = M*〈sub〉reservoir〈/sub〉—M〈sub〉max〈/sub〉) to trigger M〈sub〉max〈/sub〉 events than from earthquake interactions.〈/span〉
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• 34
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉The 20 July 2017, M〈sub〉w〈/sub〉6.6 Bodrum-Kos Earthquake occurred in the Gulf of Gökova in the SE Aegean, a region characterized by N-S extension in the back-arc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, mainshock modeling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR), and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E-W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3–15 km depth, and the largest slip exceeding 2 m between depths of 4–10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gökova. While all of the larger aftershocks are consistent with N-S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trench-ward extension and anticlockwise rotation within the southeastern Aegean.〈/span〉
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• 35
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Electrical conductivity is one of the most commonly used geophysical method for reservoir and environmental studies. Its main interest lies in its sensitivity to key properties of storage and transport in porous media. Its quantitative use therefore depends on the efficiency of the petrophysical relationship to link them. In this work, we develop a new physically based model for estimating electrical conductivity of saturated porous media. The model is derived assuming that the porous media is represented by a bundle of tortuous capillary tubes with a fractal pore-size distribution. The model is expressed in terms of the porosity, electrical conductivity of the pore liquid and the microstructural parameters of porous media. It takes into account the interface properties between minerals and pore water by introducing a surface conductivity. Expressions for the formation factor and hydraulic tortuosity are also obtained from the model derivation. The model is then successfully compared with published data and performs better than previous models. The proposed approach also permits to relate the electrical conductivity to other transport properties such as the hydraulic conductivity.〈/span〉
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• 36
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Static and quasi-static Coulomb stress changes produced by large earthquakes can modify the probability of occurrence of subsequent events on neighboring faults. This approach is based on physical (Coulomb stress changes) and statistical (probability calculations) models, which are influenced by the quality and quantity of data available in the study region. Here, we focus on the Wasatch Fault Zone (WFZ), a well-studied active normal fault system having abundant geologic and paleoseismological data. Paleoseismological trench investigations of the WFZ indicate that at least 24 large, surface-faulting earthquakes have ruptured the fault's five central, 35–59-km long segments since ∼7 ka. Our goal is to determine if the stress changes due to the youngest paleoevents have significantly modified the present-day probability of occurrence of large earthquakes on each of the segments. For each segment, we modeled the cumulative (coseismic + postseismic) Coulomb stress changes (∆CFS〈sub〉cum〈/sub〉) due to earthquakes younger than the most recent event on the segment in question and applied the resulting values to the time-dependent probability calculations. Results from the Coulomb stress modeling suggest that the Brigham City, Salt Lake City, and Provo segments have accumulated ∆CFS〈sub〉cum〈/sub〉 larger than 10 bars, whereas the Weber segment has experienced a stress decrease of 5 bars, in the scenario of recent rupture of the Great Salt Lake fault to the west. Probability calculations predict high probability of occurrence for the Brigham City and Salt Lake City segments, due to their long elapsed times (〉 1–2 ka) when compared to the Weber, Provo, and Nephi segments (〈 1 ka). The range of calculated coefficients of variation (CV) has a large influence on the final probabilities, mostly in the case of the Brigham City segment. Finally, when the Coulomb stress and the probability models are combined, our results indicate that the ∆CFS〈sub〉cum〈/sub〉 resulting from earthquakes postdating the youngest events on each of the five segments substantially affects the probability calculations for three of the segments: Brigham City, Salt Lake City, and Provo. The probability of occurrence of a large earthquake in the next 50 years on these three segments may therefore be underestimated if a time-independent approach, or a time-dependent approach that does not consider ∆CFS, is adopted.〈/span〉
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• 37
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉The paper focuses on the propagation of low-frequency pseudo-Rayleigh and pseudo-Scholte waves at the liquid/soft porous sediment interface with an underlying hard porous sediment half-space. The overlying liquid is assumed to be ideal compressible medium and the porous sediments are modeled by Biot theory. Based on the boundary conditions, the closed-form dispersion equations of far-field interface waves are deduced using 2-D Helmholtz decomposition theorem and Fourier transform. The velocity and attenuation of pseudo-Rayleigh and pseudo-Scholte waves are determined by Newton iteration in a reasonable rooting interval. The analytical expressions of the displacement field and liquid pressure distribution caused by interface waves are also derived. Then, the dispersion equations for four degenerate systems are derived as special cases by assuming the thickness of the liquid layer or the sandwiched porous soft sediment layer to be zero or infinite. Lastly, numerical examples are used to verify the degeneracy of the system and to analyze the propagation characteristics of pseudo-Rayleigh and pseudo-Scholte waves. They show the dependences of the velocity and displacement field on dimensionless modulus and dimensionless wavelength. When the dimensionless wavelength is small or very large, the phase velocity and displacement field calculated by the present system is the same as the special cases, thus proving the validating of the new system.〈/span〉
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• 38
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉The homogenous slip finite fault model is commonly used in tsunami hazards for a variety of applications. These include early warning and short-term forecasts of tsunami amplitudes, scenario ruptures for risk assessments, and probabilistic tsunami hazard analysis (PTHA). Over the last decade, however, it has become feasible to calculate stochastic slip models which reflect the expected spatial statistics of slip observed in real events. In this paper we examine the impacts of the homogenous slip model when compared to stochastic slip distributions and ask whether, in light of these technical advancements, the homogenous slip assumption remains a reasonable one. We employ a simplified subduction zone geometry, free of complex path and site effects, and study simulated tsunamis from earthquakes in the magnitude 7 to magnitude 9 range. We find that homogenous slip models have lower tsunami potential energies and frequentlyunder-predict the peak tsunami amplitudes and the resulting tsunami hazard, particularly at low probabilities of exceedance. This finding has important implications for all tsunami hazards applications. Calculating a suite of realistic stochastic slip distributions is now within reach of tsunami scientists, thus, we conclude that use of heterogeneous slip models for tsunami hazards applications is preferable〈/span〉
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• 39
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Thinning of the lithosphere under continental collisional orogens is often attributed to delamination or convective thinning. Both processes remove part or all of the mantle lithosphere that has become denser and gravitationally unstable. Previous studies mostly focused on the different thermo-magmatic consequences of these two processes; the dynamic links between them, and the critical conditions for one or the other process to dominate lithosphere thinning, remain uncertain. Here we used high-resolution thermo-mechanical models with various rheology (linear viscous, power-law viscous and/or the extended Drucker-Prager plasticity) to systematically investigate the dynamics of delamination and convective thinning under collisional orogens. Our results show that convective thinning is favored in models of linear (Newtonian) viscous rheology and low viscosity $( {{{10}^{19}} - {{10}^{20}}{\rm{Pa}}\,{\rm{s}}} )$. Power-law viscous rheology promotes strain localization, which reduces the effective viscosity and may lead to localized rising of the asthenosphere to the crustal base, thus triggering delamination. Further strain localization and stronger delamination are predicted with inclusion of plastic rheology in the model. These results indicate that convective thinning and delamination are dynamically linked and can occur in the same orogeny. Their relative dominance during orogenesis may be distinguished by the resulting spatiotemporal evolutions of thermal perturbation, magmatism, and elevation changes. We applied the models to show that the evolution of the Central Anatolian Plateau is consistent with the dominance of convective thinning, whereas delamination played a major role in thinning the mantle lithosphere under central-northern Tibetan Plateau.〈/span〉
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• 40
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉To elucidate the nature and extent of the lithospheric modification in the central and western North China Craton (NCC) and adjacent regions, we used the wave equation–based migration technique of S-receiver function on teleseismic data collected from 314 broadband stations in this region to image the lithospheric structure. Incorporating data from previous lithospheric structure studies, we obtained unprecedented high-resolution depth maps of the lithosphere–asthenosphere boundary (LAB) and mid-lithospheric discontinuity (MLD) in the NCC. Our results show more detailed variations of the lithospheric thickness in the central and western NCC and adjacent regions, which ranges from 100 to 〉 170 km, in marked contrast to the thinned lithosphere (60–100 km) in the eastern NCC. Despite its generally thick lithosphere (〉 130 km), the Ordos Block shows a concordant N–S difference from the surface to deep lithosphere with a boundary at the latitude of 37–38° N. The central NCC is laterally heterogeneous in the lithospheric structure, and the thick lithosphere (∼160 km) in the south is interpreted as a remnant cratonic mantle root. The central Qinling Orogenic Belt preserves a thick lithosphere (∼150 km), which may block the asthenospheric flow driven by extrusion of the Tibetan Plateau to the west of the NCC. Moreover, a negative MLD is widely identified at the depth of 80–110 km within the thick lithosphere, which corroborates the global existence of the MLD in continental regions. The consistence in the depth of the MLD and the shallow LAB in the eastern NCC supports the conjecture that the MLD may have played an important role in the lithospheric modification of the eastern NCC.〈/span〉
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• 41
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Moment-tensor inversion of induced microseismic events can provide valuable information for tracking CO〈sub〉2〈/sub〉 plumes at geological carbon storage sites, and study the physical mechanism of induced microseismicity. Accurate moment-tensor inversion requires a wide-azimuthal coverage of geophones. Cost-effective microseismic monitoring for geological carbon storage often uses only one geophone array within a borehole, leading to a large uncertainty in moment-tensor inversion. We develop a new adaptive moment-tensor joint inversion method to reduce the inversion uncertainty, when using limited but typical geophone receiver geometries. We first jointly invert a number of clustered microseismic events using a uniform focal mechanism to minimize the waveform misfit between observed and predicted 〈span〉P〈/span〉 and 〈span〉S〈/span〉 waveforms. We then invert the moment tensor for each event within a limited searching range around the joint inversion result. We apply our adaptive joint inversion method to microseismic data acquired using a single borehole geophone array at the CO〈sub〉2〈/sub〉-Enhanced Oil Recovery field at Aneth, Utah. We demonstrate that our inversion method is capable of reducing the inversion uncertainty caused by the limited azimuthal coverage of geophones. Our inverted strikes of focal mechanisms of microseismic events are consistent with the event spatial distribution in subparallel pre-existing fractures or geological imperfections. The large values up to 40 per cent of the CLVD components might indicate crack opening induced by CO〈sub〉2〈/sub〉/wastewater injection or rupture complexity.〈/span〉
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• 42
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉The active tectonic processes in convergent margins confer a high degree of complexity to the crust. Determining the thermal structure is, therefore, key to better elucidate the nature of those processes. In order to reconstruct the thermal structure of the crust beneath the Italian peninsula, we combine the most recent and accurate shear-wave velocity model that is currently available with thermodynamic modelling, assuming a global average crustal composition with no lateral variations. Our model, presented in terms of Moho temperature and crustal thermal gradients, shows a very good agreement with the known thermal anomalies associated with the backarc spreading related to the Apennine subduction. Importantly, we envisage a new anomalous region of high Moho temperatures in NW Italy (〈span〉T〈/span〉 〉 800 °C at 30 km), at the transition between the Alps and Apennine orogens. The lowest temperatures of our model, corresponding to geothermal gradients 〈19 °C km〈sup〉−1〈/sup〉, are obtained in the still active but slow-convergent portion of the northern Apennine. Moho temperatures increase moving southwards along the Apennine chain, an observation that is coherent with the evidence of ceasing subduction and consequent rebalancing of the depressed isotherms along the slab. Our results suggest that a thermal structure in different tectonic settings can be inferred with acceptable uncertainties based on absolute seismic velocity models. In this sense, our approach can be extended to any other region.〈/span〉
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• 43
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉In fully fluid-saturated rocks, two common phenomena are documented both experimentally and theoretically for frequency-dependent elastic moduli and attenuation, that is, the drained/undrained transition and the relaxed/unrelaxed transition. When investigating these transitions with the forced oscillation method in the laboratory, it is crucial to consider the boundary differences between the laboratory and the underground. A 1-D poroelastic numerical model was previously established to describe these differences and their effects; however, the boundary conditions used in the model are actually different from the real experiment case, thus leading to inaccurate predication of the measurement results in a laboratory. In this paper, we established a 3-D poroelastic numerical model with a new set of boundary conditions that better represent the experiment conditions. Furthermore, the 3-D poroelastic modelling results were compared with laboratory measurements under the same boundary conditions, showing a much better fit than the 1-D model. Therefore, the 3-D model provides a more accurate and reliable approach to understand the regimes and transitions of elastic modulus dispersion and attenuation, and thus has great importance in interpreting the measurements of frequency-dependent properties of rocks in the laboratory.〈/span〉
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• 44
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Magnetite is an abundant magnetic mineral that commonly records the ancient magnetic field in a wide variety of rock types. When cooled below ≈124 K, magnetite undergoes a phase transition, called the Verwey transition, whose characteristics are highly sensitive to grain size and stoichiometry. Studying the Verwey transition thus yields information on the formation conditions and compositions of rocks. The transition is also stress sensitive, thereby opening an avenue to understanding a rock’s strain history; however, the reason for the stress sensitivity is poorly understood. In particular, the temperature of the transition decreases when measured under pressure, yet mostly increases upon pressure release. Moreover, the stress sensitivity of the transition as a function of dopant concentration, especially after pressure cycling, was never systematically tested. We addressed these issues in order to further develop magnetite as a pressure gauge. Multidomain magnetite samples were pressure cycled up to maximum pressures of ∼5 GPa at room temperature to measure the influence of strain on the Verwey transition temperature as a function of dopant concentration after full decompression. The transition temperature measured via changes in magnetic remanence ($T_{\rm V}^{M}$) systematically increased with respect to pressure (〈span〉P〈/span〉) in more doped samples, where domain wall pinning from impurities dominates $\mathrm{d}T_{\rm V} ^{\rm M}/\mathrm{d}P$. In less doped samples, no to only moderate pressure cycling dependence on $T_{\rm V}^{\rm M}$ was observed. Bulk coercive force (〈span〉B〈/span〉〈sub〉c〈/sub〉) and magnetic remanence after saturation (〈span〉M〈/span〉〈sub〉rs〈/sub〉) measured above or below the transition also increased with respect to pressure, but here effects related to permanent strain of the lattice structure prevail, and 〈span〉B〈/span〉〈sub〉c〈/sub〉 versus 〈span〉P〈/span〉 is steeper for less doped samples. 〈span〉B〈/span〉〈sub〉c〈/sub〉 versus 〈span〉P〈/span〉 increases in all cases, with a difference in slope dictated by dopant concentrations segregating the first to second-order nature of the transition. Thus, strain developed during pressure cycling controls $T_{\rm V}^{\rm M}$ and coercivity by a mechanism based on pinning of magnetic domains by both interstitial cations and structural lattice distortions. The combined observables, $T_{\rm V}^{\rm M}$ and 〈span〉B〈/span〉〈sub〉c〈/sub〉−〈span〉M〈/span〉〈sub〉rs〈/sub〉, reflect both the dopant level and strain state of magnetite, which can quantify the pressure multidomain magnetite has experienced, especially in the range between 1 and 5 GPa. Based on these new results, we present a model that distinguishes between electronic versus defect-driven processes explaining the strain-related influences on the transition. Magnetite’s use as a geobarometer is thus a measure of its defect state, which is expressed through two somewhat independent mechanisms when sensed by magnetic observations.〈/span〉
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• 45
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The 〈span〉CAYSEIS〈/span〉 experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north.Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s〈sup〉−1〈/sup〉 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle.Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher 〈span〉P〈/span〉-wave velocity is observed at shallow crustal depths. 〈span〉S〈/span〉-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a 〈span〉Vp〈/span〉/〈span〉Vs〈/span〉 ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated 〈span〉Vp〈/span〉/〈span〉Vs〈/span〉 ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho.The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at 〉2 km subseabed, by a ∼20-km-wide region of density 〉3000 kg m〈sup〉−3〈/sup〉 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre.Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.〈/span〉
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• 46
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉This work presents a 3-D resistivity model of the Séchilienne unstable slope acquired with a network of portable resistivimeters in summer 2017. The instrumentation consisted in distributed measuring systems (IRIS Instruments FullWaver) to measure the spatial variations of electrical potential. 23 V-FullWaver receivers with two 50 m dipoles have been deployed over an area of circa 2 km〈sup〉2〈/sup〉; the current was injected between a fixed remote electrode and a mobile electrode grounded successively at 30 locations. The data uncertainty has been evaluated in relation to the accuracy of electrodes positioning. The software package BERT (Boundless Electrical Resistivity Tomography) is used to invert the apparent resistivity and model the complex data set providing the first 3-D resistivity model of the slope. Stability tests and synthetic tests are realized to assess the interpretability of the inverted models. The 3-D resistivity model is interpreted up to a depth of 500 m; it allows identifying resistive and conductive anomalies related to the main geological and hydrogeological structures shaping the slope. The high fracturation of the rock in the most active zone of the landslide appears as a resistive anomaly where the highest resistivity values are located close to the faults. A major drain formed by a fault in the unaltered micaschist is identified through the discharge of a perched aquifer along the conductive zone producing an important conductive anomaly contrasting with the unaltered micaschist.〈/span〉
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• 47
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Modelling the porous flow of melt through a viscously deforming solid rock matrix is a useful tool for interpreting observations from the Earth’s surface, and advances our understanding of the dynamics of the Earth’s interior. However, the system of equations describing this process becomes mathematically degenerate in the limit of vanishing melt fraction. Numerical methods that do not consider this degeneracy or avoid it solely by regularizing specific material properties generally become computationally expensive as soon as the melt fraction approaches zero in some part of the domain.Here, we present a new formulation of the equations for coupled magma/mantle dynamics that addresses this problem, and allows it to accurately compute large-scale 3-D magma/mantle dynamics simulations with extensive regions of zero melt fraction. We achieve this by rescaling one of the solution variables, the compaction pressure, which ensures that for vanishing melt fraction, the equation causing the degeneracy becomes an identity and the other two equations revert to the Stokes system. This allows us to split the domain into two parts: in mesh cells where melt is present, we solve the coupled system of magma/mantle dynamics. In cells without melt, we solve the Stokes system as it is done for mantle convection without melt transport and constrain the remaining degrees of freedom.We have implemented this formulation in the open source geodynamic modelling code 〈span〉Aspect〈/span〉 and illustrate the improved performance compared to the previous three-field formulation, showing numerically that the new formulation is robust in terms of problem size and only slightly sensitive to model parameters. Beyond that, we demonstrate the applicability to realistic problems by showing large-scale 2-D and 3-D models of mid-ocean ridges with complex rheology. Hence, we believe that our new formulation and its implementation in 〈span〉Aspect〈/span〉 will prove a valuable tool for studying the interaction of melt segregating through and interacting with a solid host rock in the Earth and other planetary bodies using high-resolution, 3-D simulations.〈/span〉
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• 48
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Oxford University Press
Publication Date: 2019
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• 49
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Iceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone (ULVZ) in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S-ScS and SKS-SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small dataset, particularly for S-ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying 〈span〉VSH〈/span〉 〉 〈span〉VSV〈/span〉) to nearly vertical for a set of paths that sample directly beneath Iceland (implying 〈span〉VSV〈/span〉 〉 〈span〉VSH〈/span〉). We also find evidence for sporadic SKS-SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modeling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modeling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modeling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.〈/span〉
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• 50
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉The harmonic electromagnetic noise produced by anthropic electrical structures is a critical component of the global noise affecting geophysical signals and increasing data uncertainty. It is composed of a series of harmonic signals whose frequencies are multiple integers of the fundamental frequency specific to the electrical noise source. To date, most model-based noise removal strategies assume that the fundamental frequency constraining the harmonic noise is single and constant over the duration of the geophysical record. In this paper, we demonstrate that classical harmonic processing methods lose efficacy when these assumptions are not valid. We present several surface nuclear magnetic resonance field data sets, which testify of the increasing probability of recording harmonic noise with such multiple or unstable frequency content. For each case (multiple frequencies or unstable frequency) we propose new processing strategies, namely the 〈span〉2D grid-search〈/span〉 and the 〈span〉segmentation〈/span〉 approach, respectively, which efficiently manage to remove harmonic noise in these difficult conditions. In the process, we also apply a fast frequency estimator called the Nyman, Gaiser and Saucier estimation method (NGSE), which shows equivalent performance as classical estimators while allowing a reduction of the computing time by a factor of 2 to 5.〈/span〉
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• 51
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ〈sub〉1〈/sub〉, σ〈sub〉2〈/sub〉, and σ〈sub〉3〈/sub〉) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of maximum compressive horizontal stress (S〈sub〉H〈/sub〉) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate, and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of S〈sub〉H〈/sub〉 from north to south across the study area, with the direction of S〈sub〉H〈/sub〉 remaining roughly sub-parallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counterclockwise. The direction of horizontal maximum compression strain rate (ε〈sub〉H〈/sub〉) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the S〈sub〉H〈/sub〉, ε〈sub〉H〈/sub〉, and the dextral maximum shear planes directions, together with that of the horizontal displacement field to be related to the reactivation of east-northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastward, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. S〈sub〉H〈/sub〉 in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counterclockwise rotation westward across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.〈/span〉
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• 52
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Published laboratory elastic-wave velocity versus porosity data in carbonate rocks exhibit significant scatter even at a fixed mineralogy. This scatter is usually attributed to the strong variability in the rock-frame or pore-space geometry, which, in turn, is driven by the richness and complexity of diagenetic alteration in these very reactive sediments. Yet, by examining wireline data from oil-bearing high-to-medium porosity chalk deposits, we find surprisingly tight velocity-porosity trends. Moreover, these trends are continued into the low-porosity domain by data from a location thousands of miles away from the chalk field. This congruence implies a universality of diagenetic trends, at least in the massive deposits under examination. We also find that the elastic bulk and shear moduli of the pure-calcite end member are somewhat smaller than such values reported in the literature. Using the end-member elastic constants relevant to the data under examination, we establish a theoretical rock physics model to match and generalize these data.〈/span〉
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• 53
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Gravimetry is a technique widely used to image the structure of the Earth. However, inversions are ill-posed and the imaging power of the technique rapidly decreases with depth. To overcome this limitation, muography, a new imaging technique relying on high energy atmospheric muons, has recently been developed. Because muography only provides integrated densities above the detector from a limited number of observation points, inversions are also ill-posed. Previous studies have shown that joint muographic and gravimetric inversions better reconstruct the 3D density structure of volcanic edifices than independent density inversions. These studies address the ill-posedness of the joint problem by regularizing the solution with respect to a 〈span〉prior〈/span〉 density model. However, the obtained solutions depend on some hyperparameters, which are either determined relative to a single test case or rely on 〈span〉ad-hoc〈/span〉 parameters. This can lead to inaccurate retrieved models, sometimes associated with artefacts linked to the muon data acquisition. In this study, we use a synthetic example based on the Puy de Dôme volcano to determine a robust method to obtain the resulting model closest to the synthetic model and devoid of acquisition artefacts. We choose a Bayesian approach to include an 〈span〉a priori〈/span〉 density model and a smoothing by a Gaussian spatial correlation function relying on two hyperparameters: an 〈span〉a priori〈/span〉 density standard-deviation and an isotropic spatial correlation length. This approach has the advantage to provide 〈span〉a posteriori〈/span〉 standard-deviations on the resulting densities. Using our synthetic volcano, we investigate the most reliable criterion to determine the hyperparameters. Our results suggest that 〈span〉k〈/span〉-fold Cross-Validation Sum of Squares and the Leave One Out methods are more robust criteria than the classically used L-curves. The determined hyperparameters allow to overcome the artefacts linked to the data acquisition geometry, even when only a limited number of muon telescope is available. We also illustrate the behaviour of the inversion in case of offsets in the 〈span〉a priori〈/span〉 density or in the data and show that they lead to recognizable structures that help identify them.〈/span〉
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• 54
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉We present a new methodology to compute the gravitational fields generated by tesseroids (spherical prisms) whose density varies with depth according to an arbitrary continuous function. It approximates the gravitational fields through the Gauss–Legendre Quadrature along with two discretization algorithms that automatically control its accuracy by adaptively dividing the tesseroid into smaller ones. The first one is a preexisting 2-D adaptive discretization algorithm that reduces the errors due to the distance between the tesseroid and the computation point. The second is a new density-based discretization algorithm that decreases the errors introduced by the variation of the density function with depth. The amount of divisions made by each algorithm is indirectly controlled by two parameters: the distance-size ratio and the delta ratio. We have obtained analytical solutions for a spherical shell with radially variable density and compared them to the results of the numerical model for linear, exponential, and sinusoidal density functions. The heavily oscillating density functions are intended only to test the algorithm to its limits and not to emulate a real world case. These comparisons allowed us to obtain optimal values for the distance-size and delta ratios that yield an accuracy of 0.1 per cent of the analytical solutions. The resulting optimal values of distance-size ratio for the gravitational potential and its gradient are 1 and 2.5, respectively. The density-based discretization algorithm produces no discretizations in the linear density case, but a delta ratio of 0.1 is needed for the exponential and most sinusoidal density functions. These values can be extrapolated to cover most common use cases, which are simpler than oscillating density profiles. However, the distance-size and delta ratios can be configured by the user to increase the accuracy of the results at the expense of computational speed. Finally, we apply this new methodology to model the Neuquén Basin, a foreland basin in Argentina with a maximum depth of over 5000 m, using an exponential density function.〈/span〉
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• 55
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉This study presents the first detailed analysis of ambient noise tomography in an area of the continental upper crust in the Cantabrian Mountains (NW Spain), where a confluence of crustal scale faults occurs at depth. Ambient noise data from two different seismic networks have been analyzed. In one side, a 10 short-period station network was set recording continuously for 19 months. A second set of data from 13 broadband stations was used to extend at depth the models. The phase cross-correlation processing technique was used to compute in total more than 34,000 cross-correlations from 123 station pairs. The empirical Green's functions were obtained by applying the time-frequency, phase-weighted stacking methodology and provided the emergence of Rayleigh waves. After measuring group velocities, Rayleigh wave group velocity tomographic maps were computed at different periods and then they were inverted in order to calculate S-wave velocities as a function of depth, reaching the first 12 km of the crust.The results show that shallow velocity patterns are dominated by geological features that can be observed at the surface, particularly bedding and/or lithology and fracturing associated to faults. In contrast, velocity patterns below 4 km depth seem to be segmented by large structures, which show a velocity reduction along fault zones. The best example is the visualization in the tomography of the frontal thrust of the Cantabrian Mountains at depth, which places higher velocity Paleozoic rocks over Cenozoic sediments of the foreland Duero basin. One of the major findings in the tomographic images is the reduction of seismic velocities above the area in the crust where one seismicity cluster is nucleated within the otherwise quiet seismic area of the range. The noise tomography reveals itself as a valuable technique to identify shear zones associated to crustal scale fractures and hence, lower strain areas favorable to seismicity.〈/span〉
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• 56
Unknown
Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Receiver functions are sensitive to sharp seismic velocity variations with depth and are commonly used to constrain crustal thickness. The H-κ stacking method of Zhu and Kanamori (〈span〉2000〈/span〉) is often employed to constrain both the crustal thickness (H) and ${V_P}$/${V_S}$ ratio ($\kappa$) beneath a seismic station using P-to-s converted waves (Ps). However, traditional H-κ stacks require an assumption of average crustal velocity (usually ${V_P}$). Additionally, large amplitude reverberations from low velocity shallow layers, such as sedimentary basins, can overprint sought-after crustal signals, rendering traditional H-$\ \kappa$ stacking uninterpretable. We overcome these difficulties in two ways. When S-wave reverberations from sediment are present, they are removed by applying a resonance removal filter allowing crustal signals to be clarified and interpreted. We also combine complementary Ps receiver functions, Sp receiver functions, and the post-critical P wave reflection from the Moho (SP〈sub〉m〈/sub〉p) to remove the dependence on an assumed average crustal ${V_P}$. By correcting for sediment and combining multiple data sets, the crustal thickness, average crustal P-wave velocity, and crustal ${V_P}$/${V_S}$ ratio is constrained in geologic regions where traditional H-$\ \kappa$ stacking fails, without making an initial P-wave velocity assumption or suffering from contamination by sedimentary reverberations.〈/span〉
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• 57
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉Simulation of acoustic wave propagation in the Laplace-Fourier (LF) domain, with a spatially uniform mesh, can be computationally demanding especially in areas with large velocity contrasts. To improve efficiency and convergence, we use 3D second- and fourth-order velocity-pressure finite difference (FD) discontinuous meshes (DM). Our DM algorithm can use any spatial discretization ratio between meshes. We evaluate direct and iterative parallel solvers for computational speed, memory requirements and convergence. Benchmarks in realistic 3D models and topographies show more efficient and stable results for DM with direct solvers than uniform mesh results with iterative solvers.〈/span〉
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• 58
Unknown
Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Building geomechanical models for induced seismicity in complex reservoirs poses a major challenge, in particular if many faults need to be included. We developed a novel way of calculating induced stress changes and associated seismic moment response for structurally complex reservoirs with tens to hundreds of faults. Our specific target was to improve the predictive capability of stress evolution along multiple faults, and to use the calculations to enhance physics-based understanding of the reservoir seismicity. Our methodology deploys a mesh-free numerical and analytical approach for both the stress calculation and the seismic moment calculation. We introduce a high-performance computational method for high-resolution induced Coulomb stress changes along faults, based on a Green's function for the stress response to a nucleus of strain. One key ingredient is the deployment of an octree representation and calculation scheme for the nuclei of strain, based on the topology and spatial variability of the mesh of the reservoir flow model. Once the induced stress changes are evaluated along multiple faults, we calculate potential seismic moment release in a fault system supposing an initial stress field. The capability of the approach, dubbed as MACRIS (〈strong〉M〈/strong〉echanical 〈strong〉A〈/strong〉nalysis of 〈strong〉C〈/strong〉omplex 〈strong〉R〈/strong〉eservoirs for 〈strong〉I〈/strong〉nduced 〈strong〉S〈/strong〉eismicity) is proven through comparisons with finite element models. Computational performance and suitability for probabilistic assessment of seismic hazards are demonstrated though the use of the complex, heavily faulted Gullfaks field.〈/span〉
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• 59
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Oxford University Press
Publication Date: 2019
Description: 〈span〉In the original version of this article the author, Adrian Flores Orozco, was incorrectly listed. This has now been corrected and the publisher apologises for the error.〈/span〉
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• 60
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉We test the feasibility of GPS-based rapid centroid moment tensor (GPS CMT) methods for Taiwan, one of the most earthquake prone areas in the world. In recent years, Taiwan has become a leading developer of seismometer-based earthquake early warning systems, which have successfully been applied to several large events. The rapid determination of earthquake magnitude and focal mechanism, important for a number of rapid response applications, including tsunami warning, is still challenging because of the limitations of near-field inertial recordings. This instrumental issue can be solved by an entirely different observation system: a GPS network. Taiwan is well posed to take advantage of GPS because in the last decade it has developed a very dense network. Thus, in this research, we explore the suitability of the GPS CMT inversion for Taiwan. We retrospectively investigate six moderate to large (〈span〉M〈/span〉〈sub〉w〈/sub〉6.0 ∼ 7.0) earthquakes and propose a resolution test for our model, we find that the minimum resolvable earthquake magnitude of this system is ∼〈span〉M〈/span〉〈sub〉w〈/sub〉5.5 (at 5 km depth). Our tests also suggest that the finite fault complexity, often challenging for the near-field methodology, can be ignored under such good station coverage and thus, can provide a fast and robust solution for large earthquake directly from the near field. Our findings help to understand and quantify how the proposed methodology could be implemented in real time and what its contributions could be to the overall earthquake monitoring system.〈/span〉
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• 61
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Quantifying landslide activity in remote regions is difficult because of the numerous complications that prevent direct landslide observations. However, building exhaustive landslide catalogues is critical to document and assess the impacts of climate change on landslide activity such as increasing precipitation, glacial retreat and permafrost thawing, which are thought to be strong drivers of the destabilization of large parts of the high-latitude/altitude regions of the Earth. In this study, we take advantage of the capability offered by seismological observations to continuously and remotely record landslide occurrences at regional scales. We developed a new automated machine learning processing chain, based on the Random Forest classifier, able to automatically detect and identify landslide seismic signals in continuous seismic records. We processed two decades of continuous seismological observations acquired by the Alaskan seismic networks. This allowed detection of 5087 potential landslides over a period of 22 yr (1995–2017). We observe an increase in the number of landslides for the period and discuss the possible causes.〈/span〉
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• 62
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Measurements of seismo-acoustic events by collocated seismic and infrasound arrays allow for studying the two wavefields that were produced by the same event. However, some of the scientific and technical constraints on the building of the two technologies are different and may be contradicting. For the case of a new station, an optimal design that will satisfy the constraints of the two technologies can be found. However, in the case of upgrading an existing array by adding the complementing technology, the situation is different. The site location, the array configuration and physical constraints are fixed and may not be optimal for the complementing technology, which may lead to rejection of the upgrade. The International Monitoring System (IMS) for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes 37 seismic arrays and 51 infrasound arrays. Although the CTBT verification regime is fixed in the treaty, an upgrade of the existing arrays by adding more technologies is possible.The Mount Meron seismic array (MMAI), which is part of the IMS, is composed of 16 sites. Microbarometers were installed at five MMAI sites to form the Mount Meron infrasound array. Due to regulation and physical constraints, it was not possible to relocate the sites nor to install analogue noise reduction filters (i.e. a pipe array). In this study, it is demonstrated that the installation of the MMAI infrasound array is beneficial despite the non-optimal conditions. It is shown that the noise levels of the individual array sites are between the high and median global noise levels. However, we claim that the more indicative measures are the noise levels of the beams of interest, as demonstrated by analysing the microbaroms originated from the Mediterranean Sea. Moreover, the ability to detect events relevant to the CTBT is demonstrated by analysing man-made events during 2011 from the Libya region.〈/span〉
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• 63
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉In the profile analysis of faults, the distribution of GNSS sites directly affects the accuracy of the results of slip rate and locking depth. This paper discusses strategies for designing the layout of GNSS stations perpendicular to strike-slip faults in terms of site spacing and the Minimum Effective Distance, which is 20 times the locking depth of the fault. Three layout models are proposed considering the complexity of strike-slip faults: (1) Equal spacing layout, in which many stations are deployed in the far field, only a few are deployed in the near field. (2) Equal deformation layout, in which stations are densely arranged in the near field and sparsely arranged in the far field according to the frequency of deformation curve. (3) Equal slope spacing layout, in which stations are arranged according to the nonlinear degree of the deformation curve, with dense distribution in regions with high nonlinearity and sparse distribution in approximately linear regions. The three models were used to redistribute the sites in the Qiaojia to Dongchuan segment of the Xiaojiang fault profile, and their performances were compared with that of the current sites distribution of the segment. The results showed that model 1 is optimal for fitting the accuracy of slip rate and model 3 is optimal for the accuracy of locking depth. Overall, model 3 appears to be the best choice, considering that the accuracy of the locking depth is more difficult to control. One of the main purposes of deployment is to identify the seismogenic depth of the fault. With the locking depth of the fault gradually approaching the depth of the seismogenic layer during an interseismic period, the accuracy of observations of sites deployed at a preset value of historical seismogenic depth of the fault would improve.〈/span〉
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• 64
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉To describe the energy transport in the seismic coda, we introduce a system of radiative transfer equations for coupled surface and body waves in a scalar approximation. Our model is based on the Helmholtz equation in a half-space geometry with mixed boundary conditions. In this model, Green’s function can be represented as a sum of body waves and surface waves, which mimics the situation on Earth. In a first step, we study the single-scattering problem for point-like objects in the Born approximation. Using the assumption that the phase of body waves is randomized by surface reflection or by interaction with the scatterers, we show that it becomes possible to define, in the usual manner, the cross-sections for surface-to-body and body-to-surface scattering. Adopting the independent scattering approximation, we then define the scattering mean free paths of body and surface waves including the coupling between the two types of waves. Using a phenomenological approach, we then derive a set of coupled transport equations satisfied by the specific energy density of surface and body waves in a medium containing a homogeneous distribution of point scatterers. In our model, the scattering mean free path of body waves is depth dependent as a consequence of the body-to-surface coupling. We demonstrate that an equipartition between surface and body waves is established at long lapse-time, with a ratio which is predicted by usual mode counting arguments. We derive a diffusion approximation from the set of transport equations and show that the diffusivity is both anisotropic and depth dependent. The physical origin of the two properties is discussed. Finally, we present Monte Carlo solutions of the transport equations which illustrate the convergence towards equipartition at long lapse-time as well as the importance of the coupling between surface and body waves in the generation of coda waves.〈/span〉
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• 65
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Time-domain processing of seismic reflection data has always been an important engine that is routinely utilized to produce seismic images and to expeditiously construct subsurface models. The conventional procedure involves analysing parameters related to the derivatives of reflection traveltime with respect to offset including normal moveout (NMO) velocities (second-order derivatives) and quartic coefficients (fourth-order derivatives). In this study, we propose to go beyond the typical assumption of 1-D laterally homogeneous medium when relating those ‘processing’ parameters to the subsurface medium parameters and take into account the additional influences from lateral heterogeneity including curved interfaces and smoothly variable velocities. We fill in the theoretical gap from previous studies and develop a general framework for such connection in layered anisotropic media. We show that in general, the influences of lateral heterogeneity get accumulated from all layers via a recursive relationship according to the Fermat’s principle and can be approximately quantified in terms of the lateral derivatives of the layer interface surfaces and velocities. Based on the same general principle, we show that our approach can also be used to study the lateral heterogeneity effects on diffraction traveltime and its second-order derivative related to time-migration velocity. In this paper, we explicitly specify expressions for NMO and time-migration velocities with the influences from both types of heterogeneity suitable for 2-D data sets and also discuss possible extensions of the proposed theory to 3-D data sets and to parameters related to higher-order traveltime derivatives. Using numerical examples, we demonstrate that the proposed theory can lead to more accurate reflection and diffraction traveltime predictions in comparison with those obtained based on the 1-D assumption. Both the proposed theoretical framework and its numerical testing for forward traveltime computation presented in this study aid in understanding the effects from lateral heterogeneity on time-processing parameters and also serve as an important basis for designing an efficient technique to separate those influences in important processes such as Dix inversion for a more accurate subsurface model in the future.〈/span〉
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• 66
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉Summary〈/div〉We estimated the maximum magnitude of earthquakes in the Japan-Kuril-Kamchatka trench subduction zone with a method based on the conservation of seismic moment and the record of interplate seismicity from 1977 to 2017. The key point of this method is to base calculations on the tectonic moment rate instead of the total seismic moment rate. We modeled a seismic-moment-frequency distribution for the Japan-Kuril-Kamchatka trench on the basis of the truncated Gutenberg-Richter (G-R) law, the formula published by Utsu in 1974, the gamma distribution, and the tapered G-R law. We estimated the maximum magnitude along the Japan-Kuril-Kamchatka trench as ∼10 under the truncated G-R law and ∼11 under Utsu's formula, although the latter may be an overestimate. Therefore, the 2011 Tohoku earthquake, of moment magnitude 9.2, may not be the largest possible event in this area. The recurrence interval for magnitude 10 events based on the truncated G-R law is 4000 years. Although these two models perform equally well in terms of Akaike Information Criterion, the range of the 95 per cent confidence level is consistently narrower for the truncated G-R law than for Utsu's formula. The estimated maximum magnitude depends not only on the model used, but also on the parameters that constitute the tectonic moment. It is essential to accumulate more seismic data and achieve more precise estimates of tectonic moment to improve estimates of maximum magnitude.〈/span〉
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• 67
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉The evolution of the Philippine Sea Plate (PSP) since Jurassic is one of the key issues in the dynamics of lithosphere and mantle. The related studies benefited mostly from seismic tomography which provides velocity structures in the upper mantle. However, the upper-mantle structure is not well resolved compared to the continental areas due to the lack of seismic data in the Philippine Sea. We employ a 3-D gravity inversion constrained by an initial model based on the 〈span〉S〈/span〉-wave tomography (SL2013sv; Schaeffer & Lebedev 2013) to image the density structure of the upper mantle of the PSP and adjacent region. The resulting model shows a three-layer pattern of vertical high-low-high density variation in the upper mantle under the PSP. The thin high-density layer evidences for strong oceanic lithosphere in the West Philippine Sea. The relatively low dense mantle located below the PSP possibly originates from the asthenosphere. The PSP differs from the Pacific and the Indian-Australian plates in the whole depth range, while its structure is similar to the eastern Eurasian and Sunda plates. In the depth range, 200–300 km, the relative high-density zone beneath PSP extends to the Sunda Plate and to the eastern Eurasian Plate. We further estimated the conversion factor of our density model and the velocity model (SL2013sv; Schaeffer & Lebedev 2013) in order to locate the changes of compositional effects in the upper mantle. The negative conversion factor indicates that the compositional changes primarily affect the density anomalies beneath the PSP. We, therefore, describe the layered density structures as ‘sandwich’ pattern, which is unique and different from adjacent regions.〈/span〉
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• 68
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉Low-velocity layers within the crust can indicate the presence of melt and lithologic differences with implications for crustal composition and formation. Seismic wave conversions and reverberations across the base of the crust or intracrustal discontinuities, analysed using the receiver function method, can be used to constrain crustal layering. This is commonly accomplished by inverting receiver functions jointly with surface wave dispersion. Recently, the proliferation of model-space search approaches has made this technique a workhorse of crustal seismology. We show that reverberations from shallow layers such as sedimentary basins produce spurious low-velocity zones when inverted for crustal structure with surface wave data of insufficiently high frequency. Therefore, reports of such layers in the literature based on inversions using receiver function data should be re-evaluated. We demonstrate that a simple resonance-removal filter can suppress these effects and yield reliable estimates of crustal structure, and advocate for its use in receiver-function based inversions.〈/span〉
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• 69
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉The localization of passive seismic sources in form of microseismic tremors as well as large-scale earthquakes is a key issue in seismology. While most previous studies are assuming fairly good knowledge of the underlying velocity model, we propose an automatic spatial localization and joint velocity model building scheme that is independent of detailed 〈span〉a priori〈/span〉 information. The first step is a coherence analysis, estimating so-called wavefront attributes to locally describe the wavefield in terms of slopes and curvatures. In a similar fashion, we also obtain an initial guess of the source excitation times of the recorded events. The wavefront attributes constitute the input for wavefront tomography which represents the next step of the workflow and allows for a refinement of the previously evaluated source excitation times while simultaneously approximating the velocity distribution. In a last step, we use the final estimate of the velocity distribution and compute the respective image function by reverse time modelling to gain the source locations. This paper introduces the theoretical concept of our proposed approach for the general 3-D case. We analyse the feasibility of our strategy and the influences of different acquisition settings by means of a synthetic 2-D data example. In a final 3-D field data example we use the workflow to localize a deep earthquake without relying on a given velocity model. The approach can deal with high levels of noise and low signal amplitudes, respectively, as well as sparse geophone sampling. The workflow generally delivers good approximations of the long-wavelength velocity variations along with accurate source locations.〈/span〉
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• 70
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉In the case of long-range propagation of forward scattering, due to the accumulation of phase changes caused by the velocity perturbations, the validity of the Born approximation will be violated. In contrast, the phase-change accumulation can be handled by the Rytov approximation, which has been widely used for long-distance propagation with only forward scattering or small-angle scattering involved. However, the weak scattering assumption (i.e. small velocity perturbation) in the Rytov approximation limits its scope of application. To address this problem, we analyse the integral kernel of the Rytov transform using the Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) approximation and we demonstrate that the integral kernel is a function of velocity perturbation and scattering angle. By applying a small scattering angle approximation, we show that the phase variation has a linear relationship with the slowness perturbation, no matter how strong the magnitude of perturbation is. Therefore, the new integral equation is then referred to as the generalized Rytov approximation (GRA) because it overcomes the weak scattering assumption of the Rytov approximation. To show the limitations of the Rytov approximation and the advantages of the proposed GRA method, first we design a two-layer model and we analytically calculate the errors introduced by the small scattering angle assumption using plane wave incidence. We show that the phase (traveltime) variations predicted by the GRA are always more accurate than the Rytov approximation. Particularly, the GRA produces accurate phase variations for the normal incident plane wave regardless of the magnitude of velocity perturbation. Numerical examples using Gaussian anomaly models demonstrate that the scattering angle has a crucial impact on the accuracy of the GRA. If the small scattering angle assumption holds, the GRA can produce an accurate phase approximation even if the velocity perturbation is very strong. On the contrary, both the first-order Rytov approximation and the GRA fail to get satisfying results when the scattering angle is large enough. The proposed GRA method has the potential to be used for traveltime modelling and inversion for large-scale strong perturbation media.〈/span〉
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• 71
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉We present a numerically exact method for calculating the internal and external gravitational potential of aspherical and heterogeneous planets. Our approach is based on the transformation of Poisson’s equation into an equivalent equation posed on a spherical computational domain. This new problem is solved in an efficient iterative manner based on a hybrid pseudospectral/spectral element discretization. The main advantage of our method is that its computational cost reflects the planet’s geometric and structural complexity, being in many situations only marginally more expensive than boundary perturbation theory. Several numerical examples are presented to illustrate the method’s efficacy and potential range of applications.〈/span〉
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• 72
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉We use seismic noise cross-correlations to obtain a 3-D tomography model of 〈span〉SV〈/span〉-wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km〈sup〉2〈/sup〉 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of 〉2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s〈sup〉−1〈/sup〉 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement.〈/span〉
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• 73
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Oxford University Press
Publication Date: 2019
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• 74
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉We investigate the possibility of passive monitoring of a salt-water disposal well in British Columbia, Canada, using continuously recorded ambient seismic noise. We find seismic velocity variations induced by a reduction of injection pressure in an effort to mitigate an elevated level of seismicity, most likely associated with the disposal of salt water. The relative velocity variations are derived from time-shifts measured between consecutive cross-correlation functions for each station pair in a surface array composed of five broad-band seismometers. The probable driving mechanisms responsible for the velocity changes are reduced pore pressures and/or lowered poroelastic stresses beyond the injection wellbore, respectively. Hydrologic data (e.g. snow and rainfall), noise energy trends and fluctuations in the incident direction of dominant noise sources do not correlate with the estimated relative velocity variations. Velocity variations are detected ahead of the zone of induced seismicity, thus indicating that seismic interferometry may aid in mitigation efforts to reduce the risk of induced seismicity by (1) providing verifiable and repeatable measurements of physical changes within the surrounding area and (2) providing hard constraints for modelling efforts to constrain how and where pore-pressure fronts change.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences
Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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Oxford University Press
Publication Date: 2019
Description: 〈span〉〈div〉SUMMARY〈/div〉We present a numerical method for simulating both single-event dynamic ruptures and earthquake sequences with full inertial effects in antiplane shear with rate-and-state fault friction. We use the second-order form of the wave equation, expressed in terms of displacements, discretized with high-order-accurate finite difference operators in space. Advantages of this method over other methods include reduced computational memory usage and reduced spurious high frequency oscillations. Our method handles complex geometries, such as non-planar fault interfaces and free surface topography. Boundary conditions are imposed weakly using penalties. We prove time stability by constructing discrete energy estimates. We present numerical experiments demonstrating the stability and convergence of the method, and showcasing applications of the method, including the transition in rupture style from crack-like ruptures to slip pulses for strongly rate-weakening friction and the simulation of earthquake sequences in a viscoelastic solid with a fully dynamic coseismic phase.〈/span〉
Print ISSN: 2051-1965
Electronic ISSN: 1365-246X
Topics: Geosciences