Littérature scientifique sur le sujet « Global tomography,surface waves,wavelets »

Abstract Ambient seismic noise tomography is one of the most widely used methods in seismological studies today, especially after a comprehensive Earth noise model was published and noise analysis was performed on the IRIS Global Seismographic Network. Furthermore, the Power Spectral Density technique was introduced to identify background seismic noise in the United States. Many studies have been carried out using the ambient seismic noise tomography method which can be broadly grouped into several groups based on the objectives and research targets, such as to determine the structure of the earth’s crust and the upper mantle, to know the thickness of the sedimentary basins, to know the tectonic settings and geological structures, to know volcanic systems and geothermal systems, knowing near-surface geological features and as a monitoring effort the Ambient Noise Tomography method carried out by repeated measurements or time lapse. In this study, we investigate the characteristics of the ambient noise seismic tomography method, both its advantages and limitations of the method by utilizing synthetic data modeling using a simple geological model. Synthetic data is generated based on 1D dispersion curve forward modelling and the forward modeling of surface waves travel time for each period, which is then convoluted with the wavelets of each periods, then doing reverse correlation using a reference signal to produce synthetic recording data. We found that the estimate target depth and vertical resolution depend on the recorded data periods and the synthetic data modeling can be used as a basis in determining the acquisition design.

Full-waveform seismic inversions based on minimizing the distance between observed and predicted seismograms are, in principle, able to yield better-resolved earth models than those minimizing misfits derived from traveltimes alone. Adjoint-based methods provide an efficient way of calculating the gradient of the misfit function via a sequence of forward-modeling steps, which, using spectral-element codes, can be carried out in realistically complex media. Convergence and stability of full-waveform-difference adjoint schemes are greatly improved when data and synthetics are progressively presented to the algorithms in a constructive multiscale approximation using a (bi)orthogonal wavelet transform. Wavelets provide the nonredundant spectral decomposition that paves the way for the inversion to proceed successively from long-wavelength fitting to detailed exploration of the phases in the seismogram. The choice of wavelet class and type, the initial depth of the multiscale decomposition, and the minimization algorithms used at every level continue to play crucial roles in our procedure, but adequate choices can be made that test successfully on 2C elastic seismograms generated in toy models, as well as in the industry-standard 2D Marmousi model. Although for simplicity our inversion ignored surface waves by prior tapering and filtered removal, those also appeared to be very well matched in the final model.

Abstract The knowledge of lateral heterogeneities is crucial for path corrections in moment tensor inversions using surface waves. After some attempts to use regionalized Earth models for very long-period surface-wave moment-tensor inversions, recent tomographic Earth models offer the possibility to make short-period path corrections and therefore retrieve more reliable moment tensors for teleseismic earthquakes. First we try to evaluate the precision required for path corrections in comparison with source effects. Some selected Earth models are tested to evaluate how their results compare to those using multiple-frequency filtering techniques. Some real cases illustrate the sensitivity of moment-tensor solutions to the different path corrections, and it appears clearly that regionalized Earth models and tomographic models deduced from long-period data alone (greater than 150 sec) cannot lead to trustworthy broadband moment-tensor inversions. Recent tomographic models using phase velocities at much shorter periods (40 to 200 sec) offer a precision comparable to that of the multiple-frequency filtering technique. Both methods lead to acceptable source mechanisms, using a small number of stations, in more than two cases out of three. The use of recent global tomographic models based upon shorter-period surface waves might thus be a useful alternative to heavy multiple-frequency filtering techniques to automate source studies, especially for rapid determinations using a small number of stations.

The interaction of ocean surface waves produces pressure fluctuations at the seafloor capable of generating seismic waves in the solid Earth. The accepted mechanism satisfactorily explains secondary microseisms of the Rayleigh type, but it does not justify the presence of transversely polarized Love waves, nevertheless widely observed. An explanation for two-thirds of the worldwide ambient wave field has been wanting for over a century. Using numerical simulations of global-scale seismic wave propagation at unprecedented high frequency, here we explain the origin of secondary microseism Love waves. A small fraction of those is generated by boundary force-splitting at bathymetric inclines, but the majority is generated by the interaction of the seismic wave field with three-dimensional heterogeneity within the Earth. We present evidence for an ergodic model that explains observed seismic wave partitioning, a requirement for full-wave field ambient-noise tomography to account for realistic source distributions.

Abstract. The relative contributions of scattering and viscoelastic attenuation to the apparent attenuation of seismic body waves are estimated from synthetic and observed S waves multiply reflected from Earth's surface and the core–mantle boundary. The synthetic seismograms include the effects of viscoelasticity and scattering from small-scale heterogeneity predicted from both global tomography and from thermodynamic models of mantle heterogeneity that have been verified from amplitude coherence measurements of body waves observed at dense arrays. Assuming thermodynamic models provide an estimate of the maximum plausible power of heterogeneity measured by elastic velocity and density fluctuations, we predict a maximum scattering contribution of 43 % to the total measured attenuation of mantle S waves having a dominant frequency of 0.05 Hz. The contributions of scattering in the upper and lower mantle to the total apparent attenuation are estimated to be roughly equal. The relative strength of the coda surrounding observed ScSn waves from deep focus earthquakes is not consistent with a mantle having zero intrinsic attenuation.

The outstanding geosites in Satun UNESCO Global Geopark, Thailand are mainly karst topography. Sinkhole which is originated from the dissolution of karst rocks by groundwater or acidic rainwater is one of the potential natural disasters in these geosites. To gain the confident among geotourism, detecting karst features, cavities and surficial dissolution is crucial in risk assessment and sustainable geopark management. As a part of geohazard assessment, non-invasive geophysical methods were applied for detecting near-surface defects and karst features. In this study, electrical resistivity tomography (ERT), seismic tomography and multichannel analysis of surface waves (MASW) have been integrated to understand the mechanism of an existing sinkhole formation in Satun Geopark region. ERT appeared to be an effective approach to investigate the cavity development at shallow subsurface. MASW and seismic tomography were combined to help constrain the interpretation of lithology and karst features in vicinity of the sinkhole. The results indicated that the sinkhole occurrence in this area was probably developed by forming of cavity due to an increased dissolution of the fractured limestone bedrock. This carbonate layer is in contact with the overlying groundwater and weathering shale or cohesive soil layer. The changing of water table and infiltration of surface water by heavy rainfall allowed for a sudden vertical downward of overlying sediments into the empty voids, leading to the sinkhole hazard.

In order to analyze the dynamic processes of the Earth interior and the effect of the propagation of the seismic waves to the surface, a comprehensive study of the Earth crust kinematics is necessary. Although the Global Positing System (GPS) is a powerful method to measure ground displacements and velocities both horizontally and vertically as well as to infer the tectonic stress regime generated by the subsurface processes (from local fault systems to huge tectonic plate movements and active volcanoes), the complexity of the deformation pattern generated during such movements is not always easy to be interpreted. Therefore, it is necessary to work on new methodologies and modifying the previous approaches in order to improve the current methods and better understand the crustal movements. In this paper, we focus on western Alaska area, where many complex faults and active volcanoes exist. In particular, we analyze the data acquired each 30 seconds by three GPS stations located in western Alaska (AC31, AB09 and AB11) from January 1, 2012 to December 31, 2012 in order to compute their displacements in horizontal and vertical components by vectorial summation of the average daily and annual velocities components. Furthermore, we design non-parametric DMeyer and Haar wavelets for horizontal and vertical velocities directions in order to identify significant and homogenous displacements during the year 2012. Finally, the non-parametric decomposition of total horizontal and vertical normalized velocities based on level 1 and level 2 coefficients have been applied to compute normal and cumulative probability histograms related to the accuracy and statistical evolution of each applied wavelet. The results present a very good agreement between the designed non-parametric wavelets and their decomposition functions for each of the three above mentioned GPS stations displacements and velocities during the year 2012.

SUMMARY SPiRaL is a joint global-scale model of wave speeds (P and S) and anisotropy (vertical transverse isotropy, VTI) variations in the crust and mantle. The model is comprised of >2.1 million nodes with five parameters at each node that capture velocity variations for P- and S-waves travelling at arbitrary directions in transversely isotropic media with a vertical symmetry axis (VTI). The crust (including ice, water, sediments and crystalline layers) is directly incorporated into the model. The default node spacing is approximately 2° in the lower mantle and 1° in the crust and upper mantle. The grid is refined with ∼0.25° minimum node spacing in highly sampled regions of the crust and upper mantle throughout North America and Eurasia. The data considered in the construction of SPiRaL includes millions of body wave traveltimes (crustal, regional and teleseismic phases with multiples) and surface wave (Rayleigh and Love) dispersion. A multiresolution inversion approach is employed to capture long-wavelength heterogeneities commonly depicted in global-scale tomography images as well as more localized details that are typically resolved in more focused regional-scale studies. Our previous work has demonstrated that such global-scale models with regional-scale detail can accurately predict both teleseismic and regional body wave traveltimes, which is necessary for more accurate location of small seismic events that may have limited signal at teleseismic distances. SPiRaL was constructed to predict traveltimes for event location and long-period waveform dispersion for seismic source inversion applications in regions without sufficiently tuned models. SPiRaL may also serve as a starting model for full-waveform inversion (FWI) with the goal of fitting waves with periods 10–50 s over multiple broad regions (thousands of kilometres) and potentially the globe. To gain insight to this possibility, we simulated waveforms for a small set of events using SPiRaL and independent waveform-based models for comparison. For the events tested, the performance of the traveltime-based SPiRaL model is shown to be generally on par with regional 3-D waveform-based models in three regions (western United States, Middle East, Korean Peninsula) suggesting SPiRaL may serve as a starting model for FWI over broad regions.

SUMMARY EarthScope's USArray seismic component provided unprecedented coverage of the contiguous United States and has therefore spurred significant advances in tomographic imaging and geodynamic modelling. Here, we present a new global, radially anisotropic shear wave velocity tomography model to investigate upper mantle structure and North American Plate dynamics, with a focus on the contiguous United States. The model uses a data-adaptive mesh and traveltimes of both surface waves and body waves to constrain structure in the crust and mantle in order to arrive at a more consistent representation of the subsurface compared to what is provided by existing models. The resulting model is broadly consistent with previous global models at the largest scales, but there are substantial differences under the contiguous United States where we can achieve higher resolution. On these regional scales, the new model contains short wavelength anomalies consistent with regional models derived from USArray data alone. We use the model to explore the geometry of the subducting Farallon Slab, the presence of upper mantle high velocity anomalies, low velocity zones in the central and eastern United States and evaluate models of dynamic topography in the Cordillera. Our models indicate a single, shallowly dipping, discontinuous slab associated with the Farallon Plate, but there are remaining imaging challenges. Inferring dynamic topography from the new model captures both the long-wavelength anomalies common in global models and the short-wavelength anomalies apparent in regional models. Our model thus bridges the gap between high-resolution regional models within the proper uppermost mantle context provided by global models, which is crucial for understanding many of the fundamental questions in continental dynamics.

In this thesis, a finite-frequency theory is developed to calculate Born sensitivity kernels for Rayleigh-wave phase and amplitude measurements that are valid in regions near seismic stations. Calculations of sensitivity kernels for inter-station measurements show that exact travelling-wave representation of Green tensor is necessary when station spacing is close to or smaller than the seismic wavelength. This finite-frequency theory will allow us to take advantage of dense seismic arrays to obtain high-resolution surface-wave tomography using inter-station measurements. The non-linear dependence of surface wave phase upon large perturbations in crustal thickness as well as finite-frequency effects in global surface-wave tomography are investigated using wave propagation simulations. Calculations show that non-linearity as well as finite-frequency effects can be accounted for by using 2D phase-velocity kernels for boundary perturbations. A 3D-reference tomographic approach is developed for iterative inversions of global crustal structure where Frechet kernels are calculated in 3D reference models. A global dataset of minor-arc and major-arc Rayleigh wave dispersion measurements at periods between 25 seconds and 100 seconds are built and global phase velocity maps based on the dataset are obtained using diffractional tomography. The phase velocity model confirms many general features associated with surface tectonics including the ocean-continent dichotomy and the signature of lithospheric cooling in oceanic plates. There are significant differences between the phase velocity model and calculations based on a current global model CRUST2.0+S20RTS in oceanic regions, Archean and Proterozoic cratons as well as orogenic belts. In addition, the high resolution phase velocity maps reveal a major change in the distribution of small scale anomalies in the Pacific at different wave periods.
Ph. D.

Every seismic event produces seismic waves which travel throughout the Earth. Seismology is the science of interpreting measurements to derive information about the structure of the Earth. Seismic tomography is the most powerful tool for determination of 3D structure of deep Earth's interiors. Tomographic models obtained at the global and regional scales are an underlying tool for determination of geodynamical state of the Earth, showing evident correlation with other geophysical and geological characteristics. The global tomographic images of the Earth can be written as a linear combinations of basis functions from a specifically chosen set, defining the model parameterization. A number of different parameterizations are commonly seen in literature: seismic velocities in the Earth have been expressed, for example, as combinations of spherical harmonics or by means of the simpler characteristic functions of discrete cells. With this work we are interested to focus our attention on this aspect, evaluating a new type of parameterization, performed by means of wavelet functions. It is known from the classical Fourier theory that a signal can be expressed as the sum of a, possibly infinite, series of sines and cosines. This sum is often referred as a Fourier expansion. The big disadvantage of a Fourier expansion is that it has only frequency resolution and no time resolution. The Wavelet Analysis (or Wavelet Transform) is probably the most recent solution to overcome the shortcomings of Fourier analysis. The fundamental idea behind this innovative analysis is to study signal according to scale. Wavelets, in fact, are mathematical functions that cut up data into different frequency components, and then study each component with resolution matched to its scale, so they are especially useful in the analysis of non stationary process that contains multi-scale features, discontinuities and sharp strike. Wavelets are essentially used in two ways when they are applied in geophysical process or signals studies: 1) as a basis for representation or characterization of process; 2) as an integration kernel for analysis to extract information about the process. These two types of applications of wavelets in geophysical field, are object of study of this work. At the beginning we use the wavelets as basis to represent and resolve the Tomographic Inverse Problem. After a briefly introduction to seismic tomography theory, we assess the power of wavelet analysis in the representation of two different type of synthetic models; then we apply it to real data, obtaining surface wave phase velocity maps and evaluating its abilities by means of comparison with an other type of parametrization (i.e., block parametrization). For the second type of wavelet application we analyze the ability of Continuous Wavelet Transform in the spectral analysis, starting again with some synthetic tests to evaluate its sensibility and capability and then apply the same analysis to real data to obtain Local Correlation Maps between different model at same depth or between different profiles of the same model.
Università di Bologna, Alma Mater Studiorum-presso INGV-Bologna.
Published
1.1. TTC - Monitoraggio sismico del territorio nazionale
open