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Showing papers in "Geophysical Journal International in 2015"


Journal ArticleDOI
TL;DR: Hongjian Fang, Huajian Yao, Haijiang Zhang, Yu-Chih Huang, and Robert D. van der Hilst as discussed by the authors proposed a method to detect the presence of volcanic activity.
Abstract: Hongjian Fang,1,2 Huajian Yao,1,2 Haijiang Zhang,1,2 Yu-Chih Huang3,4 and Robert D. van der Hilst5 1Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China. E-mail: hjyao@ustc.edu.cn 2National Geophysical Observatory at Mengcheng, Anhui, China 3Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 4Taiwan Volcano Observatory at Tatun, Ministry of Science and Technology, Taipei, Taiwan 5Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

176 citations


Journal ArticleDOI
TL;DR: In this paper, the authors use adjoint tomography to iteratively determine seismic models of the crust and upper mantle beneath the European continent and the North Atlantic Ocean, using three-component seismograms from 190 earthquakes recorded by 745 seismographic stations.
Abstract: We use adjoint tomography to iteratively determine seismic models of the crust and upper mantle beneath the European continent and the North Atlantic Ocean. Three-component seismograms from 190 earthquakes recorded by 745 seismographic stations are employed in the inversion. Crustal model EPcrust combined with mantle model S362ANI comprise the 3-D starting model, EU00. Before the structural inversion, earthquake source parameters, for example, centroid moment tensors and locations, are reinverted based on global 3-D Green's functions and Frechet derivatives. This study consists of three stages. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to constrain radially anisotropic wave speed variations. In stage two, frequency-dependent phase and amplitude measurements are combined to simultaneously constrain elastic wave speeds and anelastic attenuation. In these two stages, long-period surface waves and short-period body waves are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to simultaneously constrain radial and azimuthal anisotropy. After this three-stage inversion, we obtain a new seismic model of the European curst and upper mantle, named EU60. Improvements in misfits and histograms in both phase and amplitude help us to validate this three-stage inversion strategy. Long-wavelength elastic wave speed variations in model EU60 compare favourably with previous body- and surface wave tomographic models. Some hitherto unidentified features, such as the Adria microplate, naturally emerge from the smooth starting model. Subducting slabs, slab detachments, ancient suture zones, continental rifts and backarc basins are well resolved in model EU60. We find an anticorrelation between shear wave speed and anelastic attenuation at depths 1, indicating predominantly horizontal flow within the upper mantle. In addition, subduction zones, such as the Apennines and Hellenic arcs, are characterized by vertical flow with ξ < 1 at depths greater than 150 km. We find that the direction of the fast anisotropic axis is closely tied to the tectonic evolution of the region. Averaged radial peak-to-peak anisotropic strength profiles identify distinct brittle-ductile deformation in lithospheric strength beneath oceans and continents. Finally, we use the ‘point-spread function’ to assess image quality and analyse trade-offs between different model parameters.

164 citations


Journal ArticleDOI
TL;DR: A comprehensive magnetic field model named CM5 was derived from CHAMP, Orsted and SAC-C satellite and observatory hourly-means data from 2000 August to 2013 January using the Swarm Level-2 Comprehensive Inversion (CI) algorithm as mentioned in this paper.
Abstract: A comprehensive magnetic field model named CM5 has been derived from CHAMP, Orsted and SAC-C satellite and observatory hourly-means data from 2000 August to 2013 January using the Swarm Level-2 Comprehensive Inversion (CI) algorithm. Swarm is a recently launched constellation of three satellites to map the Earth's magnetic field. The CI technique includes several interesting features such as the bias mitigation scheme known as Selective Infinite Variance Weighting (SIVW), a new treatment for attitude error in satellite vector measurements, and the inclusion of 3-D conductivity for ionospheric induction. SIVW has allowed for a much improved lithospheric field recovery over CM4 by exploiting CHAMP along-track difference data yielding resolution levels up to spherical harmonic degree 107, and has allowed for the successful extraction of the oceanic M2 tidal magnetic field from quiet, nightside data. The 3-D induction now captures anomalous Solar-quiet features in coastal observatory daily records. CM5 provides a satisfactory, continuous description of the major magnetic fields in the near-Earth region over this time span, and its lithospheric, ionospheric and oceanic M2 tidal constituents may be used as validation tools for future Swarm Level-2 products coming from the CI algorithm and other dedicated product algorithms.

146 citations


Journal ArticleDOI
TL;DR: Cheng et al. as discussed by the authors investigated the behavioural regimes of rapidly rotating convection in high-latitude planetary core-style settings and found that coherent, axial columns have a relatively narrow range of stability.
Abstract: Author(s): Cheng, JS; Stellmach, S; Ribeiro, A; Grannan, A; King, EM; Aurnou, JM | Abstract: We present laboratory and numerical models investigating the behavioural regimes of rapidly rotating convection in high-latitude planetary core-style settings. Our combined laboratorynumerical approach, utilizing simplified geometries, can access more extreme parameters (e.g. Rayleigh numbers Ra ≲ 1013; Nusselt numbers Nu ≲ 103; Ekman numbers E ≳ 3 × 10-8) than current global-scale dynamo simulations. Using flow visualizations and heat transfer measurements, we study the axialized flows that exist near the onset of rotating convection, as well as the 3-D flows that develop with stronger forcing. With water as the working fluid (Prandtl number Pr ≲ 7), we find a steep scaling trend for rapidly rotating convective heat transfer, Nu~(Ra/RaC)3.6, that is associated with the existence of coherent, axialized columns. This rapidly rotating trend is steeper than the trends found at moderate values of the Ekman number, and continues a trend of ever-steepening scalings as the rotation rate of the system is increased. In contrast, in more strongly forced or lower rotation rate cases, the heat transfer scaling consistently follows a shallower slope equivalent to that of non-rotating convection systems. The steep heat transfer scaling in the columnar convection regime, corroborated by our laboratory flow visualizations, imply that coherent, axial columns have a relatively narrow range of stability. Thus, we hypothesize that coherent convection columns are not stable in planetary core settings,where the Ekman number is estimated to be~10-15. As a consequence, convective motions in the core may not be related to the columnar motions found in presentday global-scale models. Instead, we hypothesize that turbulent rotating convection cascades energy upwards from 3-D motions to large-scale quasi-2-D flow structures that are capable of efficiently generating planetary-scale magnetic fields. We argue that the turbulent regimes of rapidly rotating convection are essential aspects of core dynamics and will be necessary components of robust, next-generation and multiscale dynamo models.

135 citations


Journal ArticleDOI
TL;DR: In this article, the authors propose an iterative substitution of the coupled Marchenko equations to retrieve the Green's functions from a source or receiver array at an acquisition surface to an arbitrary location in an acoustic medium.
Abstract: Iterative substitution of the coupled Marchenko equations is a novel methodology to retrieve the Green's functions from a source or receiver array at an acquisition surface to an arbitrary location in an acoustic medium. The methodology requires as input the single-sided reflection response at the acquisition surface and an initial focusing function, being the time-reversed direct wavefield from the acquisition surface to a specified location in the subsurface. We express the iterative scheme that is applied by this methodology explicitly as the successive actions of various linear operators, acting on an initial focusing function. These operators involve multidimensional crosscorrelations with the reflection data and truncations in time. We offer physical interpretations of the multidimensional crosscorrelations by subtracting traveltimes along common ray paths at the stationary points of the underlying integrals. This provides a clear understanding of how individual events are retrieved by the scheme. Our interpretation also exposes some of the scheme's limitations in terms of what can be retrieved in case of a finite recording aperture. Green's function retrieval is only successful if the relevant stationary points are sampled. As a consequence, internal multiples can only be retrieved at a subsurface location with a particular ray parameter if this location is illuminated by the direct wavefield with this specific ray parameter. Several assumptions are required to solve the Marchenko equations. We show that these assumptions are not always satisfied in arbitrary heterogeneous media, which can result in incomplete Green's function retrieval and the emergence of artefacts. Despite these limitations, accurate Green's functions can often be retrieved by the iterative scheme, which is highly relevant for seismic imaging and inversion of internal multiple reflections.

133 citations


Journal ArticleDOI
TL;DR: In this paper, a 3D frequency-domain full waveform inversion (FWI) was applied to ocean-bottom cable data from the Valhall oil field in the visco-acoustic vertical transverse isotropic (VTI) approximation.
Abstract: Computationally efficient 3-D frequency-domain full waveform inversion (FWI) is applied to ocean-bottom cable data from the Valhall oil field in the visco-acoustic vertical transverse isotropic (VTI) approximation. Frequency-domain seismic modelling is performed with a parallel sparse direct solver on a limited number of computer nodes. A multiscale imaging is performed by successive inversions of single frequencies in the 3.5–10 Hz frequency band. The vertical wave speed is updated during FWI while density, quality factor QP and anisotropic Thomsen's parameters δ and ϵ are kept fixed to their initial values. The final FWI model shows the resolution improvement that was achieved compared to the initial model that was built by reflection traveltime tomography. This FWI model shows a glacial channel system at 175 m depth, the footprint of drifting icebergs on the palaeo-seafloor at 500 m depth, a detailed view of a gas cloud at 1 km depth and the base cretaceous reflector at 3.5 km depth. The relevance of the FWI model is assessed by frequency-domain and time-domain seismic modelling and source wavelet estimation. The agreement between the modelled and recorded data in the frequency domain is excellent up to 10 Hz although amplitudes of modelled wavefields propagating across the gas cloud are overestimated. This might highlight the footprint of attenuation, whose absorption effects are underestimated by the homogeneous background QP model (QP = 200). The match between recorded and modelled time-domain seismograms suggests that the inversion was not significantly hampered by cycle skipping. However, late arrivals in the synthetic seismograms, computed without attenuation and with a source wavelet estimated from short-offset early arrivals, arrive 40 ms earlier than the recorded seismograms. This might result from dispersion effects related to attenuation. The repeatability of the source wavelets inferred from data that are weighted by a linear gain with offset is dramatically improved when they are estimated in the FWI model rather than in the smooth initial model. The two source wavelets, estimated in the FWI model from data with and without offset gain, show a 40 ms time-shift, which is consistent with the previous analysis of the time-domain seismograms. The computational efficiency of our frequency-domain approach is assessed against a recent time-domain FWI case study performed in a similar geological environment. This analysis highlights the efficiency of the frequency-domain approach to process a large number of sources and receivers with limited computational resources, thanks to the efficiency of the substitution step performed by the direct solver. This efficiency can be further improved by using a block-low rank version of the multifrontal solver and by exploiting the sparsity of the source vectors during the substitution step. Future work will aim to update attenuation and density at the same time of the vertical wave speed

131 citations


Journal ArticleDOI
TL;DR: In this article, a unified formalism of full waveform inversion (FWI) is presented, named as Joint FWI, whose aim is to efficiently combine the diving and reflected waves for velocity model building.
Abstract: Full waveform inversion (FWI) aims to reconstruct high-resolution subsurface models from the full wavefield, which includes diving waves, post-critical reflections and short-spread reflections. Most successful applications of FWI are driven by the information carried by diving waves and post-critical reflections to build the long-to-intermediate wavelengths of the velocity structure. Alternative approaches, referred to as reflection waveform inversion (RWI), have been recently revisited to retrieve these long-to-intermediate wavelengths from short-spread reflections by using some prior knowledge of the reflectivity and a scale separation between the velocity macromodel and the reflectivity. This study presents a unified formalism of FWI, named as Joint FWI, whose aim is to efficiently combine the diving and reflected waves for velocity model building. The two key ingredients of Joint FWI are, on the data side, the explicit separation between the short-spread reflections and the wide-angle arrivals and, on the model side, the scale separation between the velocity macromodel and the short-scale impedance model. The velocity model and the impedance model are updated in an alternate way by Joint FWI and waveform inversion of the reflection data (least-squares migration), respectively. Starting from a crude velocity model, Joint FWI is applied to the streamer seismic data computed in the synthetic Valhall model. While the conventional FWI is stuck into a local minimum due to cycle skipping, Joint FWI succeeds in building a reliable velocity macromodel. Compared with RWI, the use of diving waves in Joint FWI improves the reconstruction of shallow velocities, which translates into an improved imaging at deeper depths. The smooth velocity model built by Joint FWI can be subsequently used as a reliable initial model for conventional FWI to increase the high-wavenumber content of the velocity model.

131 citations


Journal ArticleDOI
TL;DR: Ben-Zion et al. as discussed by the authors proposed a method to detect nodal seismic activity in the Earth Sciences at the University of Southern California, Los Angeles, CA 90089-0740, USA.
Abstract: Yehuda Ben-Zion,1 Frank L. Vernon,2 Yaman Ozakin,1 Dimitri Zigone,1 Zachary E. Ross,1 Haoran Meng,1 Malcolm White,2 Juan Reyes,2 Dan Hollis3 and Mitchell Barklage3 1Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA. E-mail: benzion@usc.edu 2Scripps Institute of Oceanography, University of California San Diego, La Jolla, CA 92093, USA 3Nodal Seismic, Signal Hill, CA 90755, USA

125 citations


Journal ArticleDOI
TL;DR: In this paper, a series of triaxial cyclic loading tests and X-ray micro-computed tomography (micro-CT) observations were conducted to analyse the mechanical damage characteristics of sandstone with respect to different confining pressures.
Abstract: The mechanical damage characteristics of sandstone subjected to cyclic loading is very significant to evaluate the stability and safety of deep excavation damage zones. However to date, there are very few triaxial experimental studies of sandstone under cyclic loading. Moreover, few X-ray micro-computed tomography (micro-CT) observations have been adopted to reveal the damage mechanism of sandstone under triaxial cyclic loading. Therefore, in this research, a series of triaxial cyclic loading tests and X-ray micro-CT observations were conducted to analyse the mechanical damage characteristics of sandstone with respect to different confining pressures. The results indicated that at lower confining pressures, the triaxial strength of sandstone specimens under cyclic loading is higher than that under monotonic loading; whereas at confining pressures above 20 MPa, the triaxial strength of sandstone under cyclic loading is approximately equal to that under monotonic loading. With the increase of cycle number, the crack damage threshold of sandstone first increases, and then significantly decreases and finally remains constant. Based on the damage evolution of irreversible deformation, it appears that the axial damage value of sandstone is all higher than the radial damage value before the peak strength; whereas the radial damage value is higher than the axial damage value after the peak strength. The evolution of Young's modulus and Poisson's ratio of sandstone can be characterized as having four stages: (i) Stage I: material strengthening; (ii) Stage II: material degradation; (iii) Stage III: material failure and (iv) Stage IV: structure slippage. X-ray micro-CT observations demonstrated that the CT scanning surface images of sandstone specimens are consistent with actual surface crack photographs. The analysis of the cross-sections of sandstone supports that the system of crack planes under triaxial cyclic loading is much more complicated than that under triaxial monotonic loading. More axial and lateral tensile cracks were observed in the specimens under cyclic loading than under monotonic loading.

116 citations


Journal ArticleDOI
TL;DR: A new weighted POCS method is derived through the Iterative Hard Threshold (IHT) view, and a new adaptive method is proposed to achieve simultaneous seismic data interpolation and denoising based on dreamlet transform to demonstrate the validity of the proposed method.
Abstract: S U M M A R Y Interpolation and random noise removal is a pre-requisite for multichannel techniques because the irregularity and random noise in observed data can affect their performances. Projection Onto Convex Sets (POCS) method can better handle seismic data interpolation if the data’s signal-to-noise ratio (SNR) is high, while it has difficulty in noisy situations because it inserts the noisy observed seismic data in each iteration. Weighted POCS method can weaken the noise effects, while the performance is affected by the choice of weight factors and is still unsatisfactory. Thus, a new weighted POCS method is derived through the Iterative Hard Threshold (IHT) view, and in order to eliminate random noise, a new adaptive method is proposed to achieve simultaneous seismic data interpolation and denoising based on dreamlet transform. Performances of the POCS method, the weighted POCS method and the proposed method are compared in simultaneous seismic data interpolation and denoising which demonstrate the validity of the proposed method. The recovered SNRs confirm that the proposed adaptive method is the most effective among the three methods. Numerical examples on synthetic and real data demonstrate the validity of the proposed adaptive method.

115 citations


Journal ArticleDOI
TL;DR: In this article, Luo et al. presented the results of a multi-scale imaging lab at the China University of Geosciences in Wuhan, Hubei, China.
Abstract: Yinhe Luo,1,2 Yingjie Yang,3 Yixian Xu,1,2 Hongrui Xu,1 Kaifeng Zhao1 and Kai Wang1 1Hubei Subsurface Multi-scale Imaging Lab (SMIL), Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan, Hubei 430074, China. E-mail: luoyinhe@cug.edu.cn 2State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, China 3ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS)/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2109, Australia

Journal ArticleDOI
TL;DR: Adhikari et al. as discussed by the authors proposed a method to solve the problem of Nepal's earthquake and proposed an approach to solve it with the help of the National Seismological Centre (NSC).
Abstract: L.B. Adhikari,1 U.P. Gautam,1 B.P. Koirala,1 M. Bhattarai,1 T. Kandel,2 R.M. Gupta,1 C. Timsina,1 N. Maharjan,1 K. Maharjan,2 T. Dahal,2 R. Hoste-Colomer,3,4 Y. Cano,3 M. Dandine,3 A. Guilhem,3 S. Merrer,3 P. Roudil3 and L. Bollinger3 1National Seismological Centre, Department of Mines and Geology, Lainchaur, Kathmandu, Nepal. E-mail: adhikari.research@gmail.com 2Regional Seismological Centre, Department of Mines and Geology, Surkhet-Birendranagar, Nepal 3CEA, DAM, DIF, F-91297 Arpajon, France 4Laboratoire de Geologie, Ecole Normale Superieure, CNRS, 24 rue Lhomond, F-75004 Paris, France


Journal ArticleDOI
TL;DR: In this paper, the match and locate (M&L) method was proposed to detect small events that have large distance separations from the template by stacking cross-correlograms between template events and potential small event signals in the continuous waveforms.
Abstract: S U M M A R Y Detection of low magnitude event is critical and challenging in seismology. We develop a new method, named the match and locate (M&L) method, for small event detection. The M&L method employs some template events and detects small events through stacking crosscorrelograms between waveforms of the template events and potential small event signals in the continuous waveforms over multiple stations and components, but the stacking is performed after making relative traveltime corrections based on the relative locations of the template event and potential small event scanning through a 3-D region around the template. Compared to the current methods of small event detection, the M&L method places event detection to a lower magnitude level and extends the capability of detecting small events that have large distance separations from the template. The method has little dependence on the accuracy of the velocity models used, and, at the same time, provides high-precision location information of the detected small-magnitude events. We demonstrate the effectiveness of the M&L method and its advantage over the matched filter method using examples of scaled-down earthquakes occurring in the Japan Island and foreshock detection before the 2011 Mw 9.0 Tohoku earthquake. In the foreshock detection, the M&L method detects four times more events (1427) than the templates and 9 per cent (134) more than the matched filter under the same detection threshold. Up to 41 per cent (580) of the detected events are not located at the template locations with the largest separation of 9.4 km. Based on the identified foreshocks, we observe five sequences of foreshock migration along the trench-parallel direction toward the epicentre of the Mw 9.0 main shock.

Journal ArticleDOI
TL;DR: In this article, the role of the subducting plate viscosity, thickness, and density of the overriding plate (OP) has been investigated in numerical models of self-consistent subduction in order to investigate how the mechanical properties of the OP modify the slab rollback rate and the state of stress.
Abstract: S U M M A R Y On Earth, oceanic plates subduct beneath a variety of overriding plate (OP) styles, from relatively thin and negatively buoyant oceanic OPs to thick and neutrally/positively buoyant continental OPs. The inclusion of an OP in numerical models of self-consistent subduction has been shown to reduce the rate that subducting slabs roll back relative to the equivalent single plate models. We use dynamic, 2-D subduction models to investigate how the mechanical properties, namely viscosity, thickness, and density, of the OP modify the slab rollback rate and the state of stress of the OP. In addition, we examine the role of the subducting plate (SP) viscosity. Because OP deformation accommodates the difference between the slab rollback rate and the far-field OP velocity, we find that the temporal variations in the rollback rate results in temporal variations in OP stress. The slabs in our models roll back rapidly until they reach the lower mantle viscosity increase, at which point the rollback velocity decreases. Concurrent with this reduction in rollback rate is a switch from an OP dominated by extensional stresses to a compressional OP. As in single plate models, the viscosity of the SP exerts a strong control on subducting slab kinematics; weaker slabs exhibit elevated sinking velocities and rollback rates. The SP viscosity also exerts a strong control on the OP stress regime. Weak slabs, either due to reduced bulk viscosity or stress-dependent weakening, have compressional OPs, while strong slabs have dominantly extensional OPs. While varying the viscosity of the OP alone does not substantially affect the OP stress state, we find that the OP thickness and buoyancy plays a substantial role in dictating the rate of slab rollback and OP stress state. Models with thick and/or negatively buoyant OPs have reduced rollback rates, and increased slab dip angles, relative to slabs with thin and/or positively buoyant OPs. Such elevated trench rollback for models with positively buoyant OPs induces extensional stresses in the OP, while OPs that are strongly negatively buoyant are under compression. While rollback is driven by the negative buoyancy of the subducting slab in such models of free subduction, we conclude that the physical properties of the OP potentially play a significant role in modulating both rollback rates and OP deformation style on Earth.

Journal ArticleDOI
TL;DR: In this paper, the authors present a new method that is free of artefact singularities and numerical instabilities in analytical solutions for triangular dislocations in both full-space and half-space.
Abstract: S U M M A R Y Displacements and stress-field changes associated with earthquakes, volcanoes, landslides and human activity are often simulated using numerical models in an attempt to understand the underlying processes and their governing physics. The application of elastic dislocation theory to these problems, however, may be biased because of numerical instabilities in the calculations. Here, we present a new method that is free of artefact singularities and numerical instabilities in analytical solutions for triangular dislocations (TDs) in both full-space and half-space. We apply the method to both the displacement and the stress fields. The entire 3-D Euclidean space R3 is divided into two complementary subspaces, in the sense that in each one, a particular analytical formulation fulfils the requirements for the ideal, artefact-free solution for a TD. The primary advantage of the presented method is that the development of our solutions involves neither numerical approximations nor series expansion methods. As a result, the final outputs are independent of the scale of the input parameters, including the size and position of the dislocation as well as its corresponding slip vector components. Our solutions are therefore well suited for application at various scales in geoscience, physics and engineering. We validate the solutions through comparison to other well-known analytical methods and provide the MATLAB codes.




Journal ArticleDOI
TL;DR: In this paper, the authors observed seasonal seismic wave speed changes (dv/v) in the San Jacinto fault area and investigated several likely source mechanisms, including temperature variations, wind speed, rain, ground water level, barometric pressure and atmospheric temperature.
Abstract: S U M M A R Y We observe seasonal seismic wave speed changes (dv/v) in the San Jacinto fault area and investigate several likely source mechanisms. Velocity variations are obtained from analysis of 6 yr data of vertical component seismic noise recorded by 10 surface and six borehole stations. We study the interrelation between dv/v records, frequency-dependent seismic noise properties, and nearby environmental data of wind speed, rain, ground water level, barometric pressure and atmospheric temperature. The results indicate peak-to-peak seasonal velocity variations of ∼0.2 per cent in the 0.5–2 Hz frequency range, likely associated with genuine changes of rock properties rather than changes in the noise field. Phase measurements between dv/v and the various environmental data imply that the dominant source mechanism in the arid study area is thermoelastic strain induced by atmospheric temperature variations. The other considered environmental effects produce secondary variations that are superimposed on the thermal-based changes. More detailed work with longer data on the response of rocks to various known external loadings can help tracking the evolving stress and effective rheology at depth.

Journal ArticleDOI
TL;DR: In this article, the performance of two microseismic event detectors, namely the STA/LTA and PSD methods, was compared and the PSD method has the advantage that no prior bandpass filtering is required to enhance the SNR and also permits detection of signals with characteristically different frequency contents if the background noise spectrum is stationary.
Abstract: Summary We have compared the performance of two microseismic event detectors, namely the STA/LTA and PSD methods. We conclude that the PSD technique outperforms the STA/LTA method by detecting a higher number of weak microseismic events that are obscured by background noise. The PSD method has the advantage over STA/LTA method that no prior bandpass filtering is required to enhance the SNR and also permits for detection of signals with characteristically different frequency contents if the background noise spectrum is stationary.

Journal ArticleDOI
TL;DR: In this paper, the authors studied ionospheric responses to the 2012 Mw8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System (GNSS) network.
Abstract: We studied ionospheric responses to the 2012 April 11 Mw8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System (GNSS) network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CID) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (~1 km/s). Resonant atmospheric oscillation with a frequency ~4 mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes worldwide with moment magnitudes (Mw) 6.6-9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for the Mw increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, i.e. small vertical crustal movements for their magnitudes.

Journal ArticleDOI
TL;DR: In this paper, the authors exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects.
Abstract: Between 2011 and 2013, two dense transects were deployed across the central and western Pyrenees to get better constraints on the deep lithospheric architecture and discriminate the competing models of the structure and formation of the Pyrenees. Each transect recorded the regional and global seismicity during a period of approximately 1 yr. Here, we exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects. The migrated sections, obtained by performing common conversion point stacks, are in remarkable agreement with the results of the ECORS-Pyrenees and ECORS-Arzacq deep seismic surveys. However, the migrations of converted waves reveal new details of the deep lithospheric architecture that could not be seen with the active source experiments. The new images provide clear and definite evidence for the subduction of a thinned Iberian crust down to at least ∼70 km depth, a result that has important implications for the formation of the Pyrenees. The subduction of the Iberian lithosphere leads to reconsider the amount of convergence between Iberia and Europe during the Cenozoic. A recent regional P-wave tomography, relying on the data of the PYROPE and IBERARRAY temporary experiments, revealed the segmentation of lithospheric structures by inherited Hercynian NE-SW transfer faults that were reactivated during the Albian rifting. Our migration images are consistent with this model, and give further support to the idea that the Pyrenees were produced by the tectonic inversion of a segmented hyperextended rift that was buried by subduction beneath the European Plate.

Journal ArticleDOI
TL;DR: This work presents a novel framework using transdimensional sampling over tree structures that offers greater flexibility, performance and efficiency for geophysical imaging problems than previous sampling algorithms.
Abstract: S U M M A R Y In geophysical inversion, inferences of Earth’s properties from sparse data involve a trade-off between model complexity and the spatial resolving power. A recent Markov chain Monte Carlo (McMC) technique formalized by Green, the so-called trans-dimensional samplers, allows us to sample between these trade-offs and to parsimoniously arbitrate between the varying complexity of candidate models. Here we present a novel framework using transdimensional sampling over tree structures. This new class of McMC sampler can be applied to 1-D, 2-D and 3-D Cartesian and spherical geometries. In addition, the basis functions used by the algorithm are flexible and can include more advanced parametrizations such as wavelets, both in Cartesian and Spherical geometries, to permit Bayesian multiscale analysis. This new framework offers greater flexibility, performance and efficiency for geophysical imaging problems than previous sampling algorithms. Thereby increasing the range of applications and in particular allowing extension to trans-dimensional imaging in 3-D. Examples are presented of its application to 2-D seismic and 3-D teleseismic tomography including estimation of uncertainty.

Journal ArticleDOI
TL;DR: In this article, the authors combine high-resolution reconstructions of Eurasia-North America and Nubia-Eurasia plate motions to determine rotations that describe the past 20 Myr.
Abstract: SUMMARY Reconstructions of the history of convergence between the Nubia and Eurasia plates constitute an important part of a broader framework for understanding deformation in the Mediterranean region and the closing of the Mediterranean Basin. Herein, we combine high-resolution reconstructions of Eurasia-North America and Nubia-North America Plate motions to determine rotations that describe Nubia-Eurasia Plate motion at ∼1 Myr intervals for the past 20 Myr. We apply trans-dimensional hierarchical Bayesian inference to the Eurasia-North America and Nubia-North America rotation sequences in order to reduce noise in the newly estimated Nubia-Eurasia rotations. The noise-reduced rotation sequences for the Eurasia-North America and Nubia-North America Plate pairs describe remarkably similar kinematic histories since 20Ma, consisting of relatively steady seafloor spreading from 20 to 8Ma, ∼20 percent opening-rate slowdowns at 8–6.5Ma, and steady plate motion from ∼7Ma to the present. Our newly estimated Nubia-Eurasia rotations predict that convergence across the central Mediterranean Sea slowed by ∼50 percent and rotated anticlockwise after ∼25Ma until 13Ma. Motion since 13Ma has remained relatively steady. An absence of evidence for a significant change in motion immediately before or during the Messinian Salinity Crisis at 6.3–5.6Ma argues against a change in plate motion as its causative factor. The detachment of the Arabian Peninsula from Africa at 30–24Ma may have triggered the convergence rate slowdown before 13Ma; however, published reconstructions of Nubia-Eurasia motion for times before 20Ma are too widely spaced to determine with confidence whether the two are correlated. A significant discrepancy between our new estimates of Nubia-Eurasia motion during the past few Myr and geodetic estimates calls for further investigation.

Journal ArticleDOI
TL;DR: In this paper, the authors compared simulations by the Fourier pseudo-spectral method (FPSM), the Legendre spectral element method (SEM), and two formulations of the finite-difference method up to 4Hz.
Abstract: SUMMARY Differences between 3-D numerical predictions of earthquake ground motion in the Mygdonian basin near Thessaloniki, Greece, led us to define four canonical stringent models derived from the complex realistic 3-D model of the Mygdonian basin. Sediments atop an elastic bedrock are modelled in the 1D-sharp and 1D-smooth models using three homogeneous layers and smooth velocity distribution, respectively. The 2D-sharp and 2D-smooth models are extensions of the 1-D models to an asymmetric sedimentary valley. In all cases, 3-D wavefields include strongly dispersive surface waves in the sediments. We compared simulations by the Fourier pseudo-spectral method (FPSM), the Legendre spectral-element method (SEM) and two formulations of the finite-difference method (FDM-S and FDM-C) up to 4Hz. The accuracy of individual solutions and level of agreement between solutions vary with type of seismic waves and depend on the smoothness of the velocity model. The level of accuracy is high for the body waves in all solutions. However, it strongly depends on the discrete representation of the material interfaces (at which material parameters change discontinuously) for the surface waves in the sharp models. An improper discrete representation of the interfaces can cause inaccurate numerical modelling of surface waves. For all the numerical methods considered , except SEM with mesh of elements following the interfaces, a proper implementation of interfaces requires definition of an effective medium consistent with the interface boundary conditions. An orthorhombic effective medium is shown to significantly improve accuracy and preserve the computational efficiency of modelling. The conclusions drawn from the analysis of the results of the canonical cases greatly help to explain differences between numerical predictions of ground motion in realistic models of the Mygdonian basin. We recommend that any numerical method and code that is intended for numerical prediction of earthquake ground motion should be verified through stringent models that would make it possible to test the most important aspects of accuracy.

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TL;DR: In this article, the authors used a single-epoch ensemble Kalman filter to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities.
Abstract: There exists a fundamental as well as practical interest in being able to accurately forecast the future evolution of Earth's magnetic field at decadal to secular ranges. This work enables such forecasts by combining geomagnetic data with an Earth-like numerical model of a convection-driven fluid dynamo. The underlying data assimilation framework builds on recent progress in inverse geodynamo modelling, a method which estimates an internal dynamic structure for Earth's core from a snapshot of the magnetic field and its instantaneous rate of change at the surface, and takes advantage of linear relationships and long-range correlations between observed and hidden state variables. Here the method is further evolved into a single-epoch ensemble Kalman filter, in order to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities. The ensemble dynamics, obtained by subsequent numerical integration of the prognostic model equations, are found to be governed by a thermal wind balance or equilibrium between buoyancy forces, the Coriolis force and the pressure gradient. The resulting core fluid flow pattern is a quasi-steady eccentric gyre organised in a column parallel to Earth's rotation axis, in equilibrium with a longitudinal hemispheric convective density anomaly pattern. The flow provides induction for the magnetic field, which also undergoes a realistic amount of diffusion. Predictions of the present magnetic field from data taken within the past century show that the ensemble has an average retaining good consistency with the true geomagnetic evolution and an acceptable spread well representative of prediction errors, up to at least a secular range. The predictability of the geodynamo thus appears to significantly exceed previous theoretical expectations based on the chaotic divergence of ensemble members. The assimilation generally outperforms the linear mathematical extrapolations from a 30-yr prediction range onwards, with a 40 per cent improvement in Earth-surface error at a secular range. The geomagnetic axial dipole decay observed over the past two centuries is predicted to continue at a similar pace in the next century, with a further loss of 1.1 ± 0.3 µT by year 2115. The focal (or minimum intensity) point of the South Atlantic geomagnetic anomaly is predicted to enter the South Pacific region in the next century, with the anomaly itself further deepening and widening. By year 2065, the minimum intensity is predicted to decrease by 1.46 ± 0.4 µT at the Earth surface and the focal point to move 12.8 ± 1.4 deg westwards with a slight northward component. This corresponds to a drift rate of 0.26 deg yr −1 , similar to the typical geomagnetic westward drift observed over the past four centuries. The same drift rate is also predicted until 2115 with a further (but more uncertain) intensity decrease

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TL;DR: In this article, an iterative quasi-Newton method based on the L-BFGS algorithm was proposed to simulate the propagation of short-period teleseismic waves through a regional 3D model.
Abstract: We present a method for high-resolution imaging of lithospheric structures based on full waveform inversion of teleseismic waveforms. We model the propagation of seismic waves using our recently developed direct solution method/spectral-element method hybrid technique, which allows us to simulate the propagation of short-period teleseismic waves through a regional 3-D model. We implement an iterative quasi-Newton method based upon the L-BFGS algorithm, where the gradient of the misfit function is computed using the adjoint-state method. Compared to gradient or conjugate-gradient methods, the L-BFGS algorithm has a much faster convergence rate. We illustrate the potential of this method on a synthetic test case that consists of a crustal model with a crustal discontinuity at 25 km depth and a sharp Moho jump. This model contains short- and long-wavelength heterogeneities along the lateral and vertical directions. The iterative inversion starts from a smooth 1-D model derived from the IASP91 reference Earth model. We invert both radial and vertical component waveforms, starting from long-period signals filtered at 10 s and gradually decreasing the cut-off period down to 1.25 s. This multiscale algorithm quickly converges towards a model that is very close to the true model, in contrast to inversions involving short-period waveforms only, which always get trapped into a local minimum of the cost function.


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TL;DR: In this paper, the authors used two decades of surface loading deformation predictions due to variations of atmospheric, oceanic and continental water mass to assess the effect on secular velocities estimated from short time-series.
Abstract: Geodetic vertical velocities derived from data as short as 3 yr are often assumed to be representative of linear deformation over past decades to millennia. We use two decades of surface loading deformation predictions due to variations of atmospheric, oceanic and continental water mass to assess the effect on secular velocities estimated from short time-series. The interannual deformation is time-correlated at most locations over the globe, with the level of correlation depending mostly on the chosen continental water model. Using the most conservative loading model and 5-yr-long time-series, we found median vertical velocity errors of 0.5 mm yr−1 over the continents (0.3 mm yr−1 globally), exceeding 1 mm yr−1 in regions around the southern Tropic. Horizontal velocity errors were seven times smaller. Unless an accurate loading model is available, a decade of continuous data is required in these regions to mitigate the impact of the interannual loading deformation on secular velocities.