ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
Filter
  • Articles  (21,112)
Collection
  • Articles  (21,112)
Publisher
Years
Journal
Topic
  • 1
    Publication Date: 2021-10-30
    Description: Summary Objective assessment of seismic hazard and understanding of the Himalayan arc's tectonics requires detailed information on the crustal structure and geometry of the underthrusting Indian Plate beneath the Himalaya. Here, we present high-resolution three-dimensional P- wave velocity (Vp) and P-to-S wave velocity ratio (Vp/Vs) images of the Kumaun-Garhwal Himalaya, a proposed potential region for the future great earthquake. We generate these images by inverting arrival times of 515 local earthquakes recorded by 41 broadband stations during November 2006-June 2008. The tomographic images show a heterogeneous structure in the upper-mid crust. These images, along with available geophysical and geological information, indicate the presence of quartz-rich felsic rocks in the uppermost crust; and the occurrence of saline-rich aqueous fluid/partial melt in the upper-mid crust. We propose that the Main Himalayan Thrust (MHT), having a flat-ramp-flat geometry, lies at the base of these fluid zones. The small and moderate-to-strong magnitude earthquakes are mainly confined to the fluid-rich zones along the MHT and quartz-rich rocks in the upper crust. Such an interpretation implies that the earthquake occurrence in the Kumaun-Garhwal Himalaya is largely controlled by the geometry of the MHT and crustal lithology.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 2
    Publication Date: 2021-10-30
    Description: Summary Constitutive theory for viscoelasticity has broad application to solid mantle or ice deformations driven by tides, surface mass variations, and post-seismic flow. Geophysical models using higher order viscoelasticity can better accommodate geodetic observations than lower-order theory, typically provided by tensor versions of Maxwell, 4-parameter Burgers or standard linear (Zener) rheology. We derive, for the first time, a mathematical description of a compressible version of the extended Burgers material (EBM) model paradigm which has a distribution function of relaxation spectra. The latter model is often used for parameterizing high temperature background transient responses in the rock physics and mechanics laboratory setting and have demonstrated application to low frequency seismic wave attenuation. A new generalization of this practical anelastic model is presented and applied to the glacial isostatic adjustment momentum equations, thus providing useful guidance for generating initial-value boundary problem-solving software for quite general coding strategies. The solutions for the vertical motion response to a suddenly imposed surface load reveal a short-term transience of substantial amplitude.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 3
    Publication Date: 2021-10-30
    Description: Summary Seismic wave scattering dispersion and attenuation can be significant in cracked reservoirs. Many scattering models have been proposed, and the fractal (self-similar) features of the medium need to be further incorporated and analyzed. We solve the P-wave scattering caused by fluid-saturated aligned cracks of finite thickness embedded in fractal media. The model is based on crack displacement discontinuities by using the Foldy approximation and representation theorem. The frequency dependence of velocity and attenuation are analyzed as a function of the incidence angle and the crack and fluid properties. The results show that the crack density, thickness and radius can have a significant influence on the wave properties, as well as the fluid bulk modulus and saturation. The model requires three parameters to describe self-similar cracked media, and can be relevant in seismology, oil exploration and non-destructive testing of materials.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 4
    Publication Date: 2021-10-29
    Description: Summary The Bayesian slip inversion offers a powerful tool for modeling the earthquake source mechanism. It can provide a fully probabilistic result and thus permits us to quantitively assess the inversion uncertainty. The Bayesian problem is usually solved with Monte Carlo methods, but they are computationally expensive and are inapplicable for high-dimensional and large-scale problems. Variational inference is an alternative solver to the Bayesian problem. It turns Bayesian inference into an optimization task and thus enjoys better computational performances. In this study, we introduce a general variational inference algorithm, automatic differentiation variational inference (ADVI), to the Bayesian slip inversion and compare it with the classic Metropolis-Hastings (MH) sampling method. The synthetic test shows that the two methods generate nearly identical mean slip distributions and standard deviation maps. In the real case study, the two methods produce highly consistent mean slip distributions, but the ADVI-derived standard deviation map differs from that produced by the MH method, possibly because of the limitation of the Gaussian approximation in the ADVI method. In both cases, ADVI can give comparable results to the MH method but with a significantly lower computational cost. Our results show that ADVI is a promising and competitive method for the Bayesian slip inversion.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 5
    Publication Date: 2021-10-29
    Description: Summary The numerical simulation of seismic wave propagation in realistic heterogeneous media, as sedimentary basins, is a key element of seismic hazard estimation. Many numerical methods in two dimensions are based on unstructured triangular meshes and explicit time schemes. However, the presence of thin layers and tangential stratigraphic contacts in sedimentary basins entails poorly shaped mesh elements: some triangle heights are extremely small compared to the edge lengths, which requires small time steps in the simulations and thus leads to prohibitive computation times. We compare manual and automatic geological model simplification techniques to modify problematic areas of the domain, so as to improve the quality of the triangulated mesh. We modify the shape and the connectivity between rock units in the basin, with the objective to reduce the computation time without significantly changing the physical response of the geological medium. These simplification techniques are applied in an investigation of site effects in the lower Var valley, a densely urbanized area located near the city of Nice (South-East of France). Numerical simulations of plane wave propagation in a heterogeneous 2D profile are carried out with a discontinuous Galerkin finite element method. Five simplified meshes are generated and the impacts of the simplifications are analyzed in comparison to the reference model. We compare the time solutions and the transfer functions obtained on the surface of the basin. The results show that the simplification procedures, in particular automatic modifications of the model, yield a significant performance gain, with a ratio higher than 55, while having a negligible impact on the ground motion response.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 6
    Publication Date: 2021-10-29
    Description: Summary Stoneley modes are a special subset of normal modes whose energy is confined along the core-mantle boundary. As such, they offer a unique glimpse into Earth structure at the base of the mantle. They are often observed through coupling with mantle modes due to rotation, ellipticity and lateral heterogeneity, though they can be detected without such coupling. In this study, we explore the relative sensitivities of seismic spectra of two low-frequency Stoneley modes to several factors, taking as reference the fully coupled computation up to 3 mHz in model S20RTS. The factors considered are (i) theoretical, by exploring the extent to which various coupling approximations can accurately reproduce reference spectra; and (ii) model-based, by exploring how various Earth parameters such as core-mantle boundary topography, attenuation, and S-wave and P-wave structures, and the seismic source solution may influence the spectra. We find that mode-pair coupling is insufficiently accurate, but coupling modes within a range of ±0.1 mHz produces acceptable spectra, compared to full coupling. This has important implications for splitting function measurements, which are computed under the assumption of isolated modes or at best, mode-pair or group coupling. We find that uncertainties in the P-wave velocity mantle model dominate compared to other model parameters. In addition, we also test several hypothetical models of mantle density structure against real data. These tests indicate that, with the low-frequency Stoneley mode spectral data considered here, it is difficult to make any firm statement on whether the large-low-shear-velocity-provinces are denser or lighter than their surroundings. We conclude that better constraints on long wavelength elastic mantle structure, particularly P-wave velocity, need to be obtained, before making further statements on deep mantle density heterogeneity. In particular, a dense anomaly confined to a thin layer at the base of the mantle (less than ∼100-200 km) may not be resolvable using the two Stoneley modes tested here, while the ability of higher frequency Stoneley modes to resolve it requires further investigations.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 7
    Publication Date: 2021-10-29
    Description: Summary We deployed a seismic network near the source region of the 2017 Mw 6.5 Jiuzhaigou earthquake to monitor aftershock activity and to investigate the local fault structure. An aftershock deployment of Array of small Arrays (AsA) and a Geometric Mean Envelop (GME) algorithm are adopted to enhance detection performance. We also adopt a set of association, relocation, and matched-filter techniques to obtain a detailed regional catalog. 16,742 events are detected and relocated, including 1,279 aftershocks following the Mw 4.8 aftershock. We develop a joint inversion algorithm utilizing locations of event clusters and focal mechanisms to determine the geometry of planar faults. Six segments were finally determined, in which three segments are related to the Huya fault reflecting a change in fault dip direction near the mainshock hypocenter, while the other segments reflect branches showing orthogonal and conjugate geometries with the Huya fault. Aftershocks were active on branching faults between the Huya and Minjiang faults indicating that the mainshock may have ruptured both major faults. We also resolve a fault portion with ‘weak strength’ near the mainshock hypocenter, which is characterized by limited co-seismic slips, concentrated afterslip, low aftershock activities, high b-value, and high sensitivity to stress changes. These phenomena can be explained by fault frictional properties at conditional stable sliding status, which may be related to the localized high pore-fluid pressure produced by the fluid intrusion.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 8
    Publication Date: 2021-10-28
    Description: Summary We have studied the active and recent tectonics of New Guinea, using earthquake source modelling, analysis of gravity anomalies, seismic reflection profiles, and thermal and mechanical models. Our aim is to investigate the behaviour and evolution of a young continental deformation belt, and to explore the effects of lateral variations in foreland rheology on the deformation. We find that along-strike gradients in the lithosphere thickness of the southern foreland have resulted in correlated changes in seismogenic thickness, likely due to the effects on the temperature structure of the crust. The resulting variation in the strength of the foreland means that in the east, the foreland is broken through on thrust faults, whereas in the west it is relatively intact. The lack of correlation between the elevation of the mountain belt and the seismogenic thickness of the foreland is likely to be due to the time taken to thicken the crust in the mountains following changes in the rheology of the underthrusting foreland, as the thinned passive margin of northern Australia is consumed. The along-strike variation in whether the force exerted between the mountains and the lowlands is able to break the foreland crust enables us to estimate the effective coefficient of friction on foreland faults to be in the range of 0.01-0.28. We use force-balance calculations to show that the recent tectonic re-organisation in western New Guinea is likely to be due to the development of increasing curvature in the Banda Arc, and that the impingement of continental material on the subduction zone may explain the unusually low force it exerts on western New Guinea.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 9
    Publication Date: 2021-10-28
    Description: Summary Distributed Acoustic Sensing (DAS) networks promise to revolutionize observational seismology by providing cost-effective, highly dense spatial sampling of the seismic wavefield, especially by utilizing pre-deployed telecomm fiber in urban settings for which dense seismic network deployments are difficult to construct. However, each DAS channel is sensitive only to one projection of the horizontal strain tensor and therefore gives an incomplete picture of the horizontal seismic wavefield, limiting our ability to make a holistic analysis of instrument response. This analysis has therefore been largely restricted to pointwise comparisons where a fortuitious coincidence of reference three-component seismometers and co-located DAS cable allows. We evaluate DAS instrument response by comparing DAS measurements from the PoroTomo experiment with strain-rate wavefield reconstructed from the nodal seismic array deployed in the same experiment, allowing us to treat the entire DAS array in a systematic fashion irrespective of cable geometry relative to the location of nodes. We found that, while the phase differences are in general small, the amplitude differences between predicted and observed DAS strain-rates average a factor of 2 across the array and correlate with near-surface geology, suggesting that careful assessment of DAS deployments is essential for applications that require reliable assessments of amplitude. We further discuss strategies for empirical gain corrections and optimal placement of point sensor deployments to generate the best combined sensitivity with an already deployed DAS cable, from a wavefield reconstruction perspective.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 10
    Publication Date: 2021-10-28
    Description: Summary On 2020 December 29, the Mw 6.4 Petrinja earthquake hit the Kupa Valley region and set a record for the largest earthquake in northwestern (NW) Croatia. The coseismic surface displacements are well obtained on three pairs of interferometric synthetic aperture radar (InSAR) images from Sentinel-1 satellites. The interferograms exhibit coseismic ground deformation with a maximum line-of-sight (LOS) displacement of 0.4 m. Based on the coseismic deformation field, we investigate both the fault geometry and the coseismic slip distribution. The results show a dextral event with a peak slip of 3.50 m at a depth of 3.47 km. The shallow depth and unusually large coseismic slip correspond to obvious ground deformation and serious damage in the epicentral zone. The 2020 earthquake highlights an unmapped, steeply dipping strike-slip fault, which possibly enabled a potential ‘curve cut-off’ process on the bending segment of the Pokupsko fault in the context of ∼N-S compression in NW Croatia. The large coseismic slip and high stress drop associated with the Mw 6.4 Petrinja earthquake are likely products of the geometrically complex fault zones and immature seismotectonic environment in NW Croatia.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...