Some of the most voluminous mafic potassic provinces of the world are located in SE and Central Brazil emplaced into proterozoic mobile belts and whose magmas date ~85 Ma. Their origins are controversial, some interpreted these magmatisms... more
Some of the most voluminous mafic potassic provinces of the world are located in SE and Central Brazil emplaced into proterozoic mobile belts and whose magmas date ~85 Ma. Their origins are controversial, some interpreted these magmatisms as an end-stage of Paraná volcanism. On the other hand, reconstructions of plate motion suggest that, at ~ 85 Ma ago, the location
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ABSTRACT The lithospheric structure of the mid-plate South American continent has been studied with surface and body wave tomography, as well as receiver functions, in the last few years, using several temporary seismic station... more
ABSTRACT The lithospheric structure of the mid-plate South American continent has been studied with surface and body wave tomography, as well as receiver functions, in the last few years, using several temporary seismic station experiments. Joint inversion of group velocities and waveform data are beginning to delimit the Amazon and San Francisco cratons at depth. Despite the low density of stations in northern Brazil, both cratons seem to be less thick than the North American craton. A mid-continent strong low-velocity anomaly at 100-300 km depth beneath the Pantanal basin, also seen at other global-scale tomographic studies, reveals a weak lithosphere (consistent with low Te and higher seismicity); the origin of this major anomaly is not fully understood yet. In the intracratonic Parana basin, joint inversions of receiver function and Rayleigh-wave dispersion, integrated with results from teleseismic body-wave tomography, indicate a fragmented nature of the crust beneath the basin as opposed to previous models of a single cratonic block. Future plans for more detailed lithospheric studies will be discussed in view of the installation of the Brazilian permanent seismic network, as well as the operation of the Brazilian Equipment Pool for temporary deployments.
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ABSTRACT New geological mapping in the central Taiwan mountain belt, together with a new collapsed data set of relocated seismicity, indicate that the basal detachment beneath the thin-skinned Western Foothills possibly ramps down in the... more
ABSTRACT New geological mapping in the central Taiwan mountain belt, together with a new collapsed data set of relocated seismicity, indicate that the basal detachment beneath the thin-skinned Western Foothills possibly ramps down in the crystalline basement along their eastern flank. Consequently, the basement appears to be involved in the deformation in the more internal Hsuehshan Range. Furthermore, seismic tomography data indicate a shallowing of higher Vp velocity material beneath the Hsuehshan Range than beneath the Western Foothills. Therefore, we propose that the basement is possibly being uplifted to form a basement-cored culmination beneath the Hsuehshan Range. These basement rocks appear to be located at progressively shallower structural levels than the basement rocks beneath the Western Foothills. This structural architecture is similar to that in other orogens worldwide, where the thin-skinned foreland fold-and-thrust belt ends hinterlandward with a ramp down into the middle to lower crust and, subsequently, the involvement of basement rocks in the deformation. However, we are uncertain if this basement culmination is somehow linked to the outcropping basement rocks of the Tananao Complex in the Central Range or if two different types of basement rocks are juxtaposed across the Lishan fault.
We used the regional seismic tomography to study the upper mantle beneath SE and Central Brazil. This method is based on the inversion of P- and S-wave relative travel time residuals (VanDecar, 1991) obtained from more than 80 stations in... more
We used the regional seismic tomography to study the upper mantle beneath SE and Central Brazil. This method is based on the inversion of P- and S-wave relative travel time residuals (VanDecar, 1991) obtained from more than 80 stations in an area of 20 x 20 degrees. The ~11000 P and PKP residuals and ~8000 S, ScS, SKS, and SKKS residuals have been obtained from waveform cross-correlations for up to 12 simultaneous stations. Our results show correlations of seismic anomalies with the main tectonic structures and reveal new anomalies not yet observed in previous works. High velocity anomalies in the western portion of the Sao Francisco Craton support the hypothesis that this craton was part of a major Neoproterozoic plate. Low velocity anomalies beneath the Tocantins Province (mainly fold belts between the Amazon and Sao Francisco cratons) are interpreted as due to lithospheric thinning. Assumpcao et al. (2004) showed a good correlation between intraplate seismicity and low velocity a...
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Research Interests: Geology, Geochemistry, Geophysics, Seismology, Tectonophysics, and 3 moreBody Waves, Slab, and Transition Zone
ABSTRACT Surface geological data indicate that the structural architecture and kinematics of the thrust belt in central Taiwan, from the Central Range to the frontal thrust in the west, is not that of a simple west-verging imbricate... more
ABSTRACT Surface geological data indicate that the structural architecture and kinematics of the thrust belt in central Taiwan, from the Central Range to the frontal thrust in the west, is not that of a simple west-verging imbricate stack. In particular, the Hsuehshan Range appears to be an oblique thrust system that is inverting an Eocene-age extensional basin, resulting in a complex interaction between faults and folds. Field mapping indicates that there is a major structural break between the Hsuehshan Range and the Western Foothills in the west, and with the Central Range in the east. Because of the complexity caused by oblique thrusting, determining the structure at depth across the thrust belt is difficult from surface data alone. Therefore, we have incorporated seismicity data to see how deep the major faults in the thrust belt penetrate and, where possible, what their kinematics are. Moment magnitudes for each event have been recalculated to energy release, and maps and cross sections cut through the volume to determine where the major energy release has been taking place. In Central Taiwan, these data indicate that a significant portion of the energy release in the thrust belt is within or along the boundaries of the Hsuehshan Range. The Western Foothills show patchy energy release with a local concentration at between 8 and 10 km depth. Focal mechanisms here suggest that roughly west-directed thrusting is dominant. At the transition into the Hsuehshan Range, across the Shanghua and Shuili-Keng faults, there is a sudden deepening of the seismic activity, with major energy release taking place to depths of about 20 km. The majority of focal mechanisms suggest highly oblique to strike-slip faulting, in keeping with field observations. There is a marked drop in seismic activity at the eastern flank of the Hsuehshan Range, along the Lishan Fault. This integration of field geological observations with seismicity data has proven to be an important step in interpreting the crustal structure, kinematics and evolution of the Taiwan thrust belt.
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Research Interests: Geology and Cape Verde
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Research Interests: Mathematics, Computer Science, Signal Processing, Localization, Multidisciplinary, and 14 moreTime-Frequency Analysis, Wavelet Transform, Discretization, Time Frequency Analysis, Fourier transform, Harmonic wavelet transform, Side Effect, Discrete Fractional Fourier Transform, Fourier Transformation, Hartley Transform, Short Time Fourier Transform, Constant-Q Transform, local time, and Discrete Hartley Transform
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A novel technique is implemented to search for weak amplitude upper mantle phases that arrive in the P-wave coda. Cross-correlation and stacking techniques are applied in order to detect waveform similarity and eliminate the source... more
A novel technique is implemented to search for weak amplitude upper mantle phases that arrive in the P-wave coda. Cross-correlation and stacking techniques are applied in order to detect waveform similarity and eliminate the source influence from the vertical and radial component of records from single stations. A pilot wave is selected from the vertical component, this wavelet contains the P-wave and part of its coda. Phase cross-correlation (PCC) and geometrically normalized cross-correlation (CCGN) are performed between this pilot and the vertical, and the radial component of each event. It is expected that this procedure detects P to s conversions, and reflections at different mantle discontinuities (such as 410-km and 660-km depth discontinuities). Stacking is used to enhance signals which arrive consistently (near receiver conversions and reflections) and attenuate isolated depth phases and also spurious arrivals. Besides the source equalization, PCC and CCGN provide relative ...
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The S-transform is a powerful tool for non stationary signals which allows the visualisation of local spectra. In this communication, we modify the definition of an inverse of the S-transform to make it exact. Then, we study the influence... more
The S-transform is a powerful tool for non stationary signals which allows the visualisation of local spectra. In this communication, we modify the definition of an inverse of the S-transform to make it exact. Then, we study the influence of discretising the S-transform and its inverses. Finally, one of the main interests of the S-transform is the possibility to filter in frequency and time at the same time. We show the different side effects generated by the two existing inverse S-transforms when filtering.
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Research Interests: Mathematics, Signal Processing, Algorithm, Multidisciplinary, Wavelet Transform, and 9 moreAdaptive Filter, Time Frequency Analysis, discrete Yang-Fourier transforms, Discrete wavelet transform, Harmonic wavelet transform, Point of View, Wavelet Packet Decomposition, Continuous wavelet transform, and Constant-Q Transform
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In order to detect possible signal redundancies in the ambient seismic wavefield, we develop a new method based on pairwise comparisons among a set of synchronous time-series. This approach is based on instantaneous phase coherence... more
In order to detect possible signal redundancies in the ambient seismic wavefield, we develop a new method based on pairwise comparisons among a set of synchronous time-series. This approach is based on instantaneous phase coherence statistics. The first and second moments of the pairwise phase coherence distribution are used to characterize the phase randomness. For perfect phase randomness, the theoretical values of the mean and variance are equal to 0 and $\sqrt{1-2/\pi }$, respectively. As a consequence, any deviation from these values indicates the presence of a redundant phase in the raw continuous signal. A previously detected microseismic source in the Gulf of Guinea is used to illustrate one of the possible ways of handling phase coherence statistics. The proposed approach allows us to properly localize this persistent source, and to quantify its contribution to the overall seismic ambient wavefield. The strength of the phase coherence statistics relies in its ability to qua...
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ABSTRACT Seismic noise recorded in the frequency band 0.1 and 0.33 Hz is called secondary microseisms. They are dominantly Rayleigh waves which are generated by the interaction of ocean gravity waves. A statistical analysis of the noise... more
ABSTRACT Seismic noise recorded in the frequency band 0.1 and 0.33 Hz is called secondary microseisms. They are dominantly Rayleigh waves which are generated by the interaction of ocean gravity waves. A statistical analysis of the noise polarization at broadband stations in Greenland (GLISN network), Canada and Europe shows that the detected noise sources are frequency dependent. Stations in Western Canada record low frequency noise generated in the North Atlantic and Pacific oceans and higher frequency noise only from the North Atlantic. Greenland stations do not detect the Pacific sources. Sea ice in the Labrador Sea in winter is well correlated with the decrease of high frequency seismic noise and with the change of the source azimuths. Indeed, in winter the sea ice prevents the generation of noise sources in that area. We model seismic noise using an oceanographic model that takes into account coastal reflection and show that we are able to accurately model the noise spectra temporal variations and frequency content. The strongest sources are generated in deep ocean close to the mid ocean ridge axis. Sources generated by coastal reflection are negligible along the Western Canada coast and more important along Greenland and European coasts. We further show that the location of the strongest noise sources are consistent with the back azimuths derived for the polarization analysis and that they depend on both, frequency and bathymetry.
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Research Interests: Geology, Computer Science, Multidisciplinary, Time series analysis, Noise, and 15 moreFrequency, Coherence, Adaptive Filter, Analytic Signal, Adaptive Filters, Frequency Dependence, Geophysical, Signal Detection, Body Waves, Seismic Noise, Attenuation, Time Frequency Representation, Amplitude, seismic array, and Discontinuities
In geophysics, the frequency-wave number (fk) filter is a common tool which allows to distinguish between seismic phases through their apparent velocity. However, this filter is based on a two dimensional Fourier transform, so it is not... more
In geophysics, the frequency-wave number (fk) filter is a common tool which allows to distinguish between seismic phases through their apparent velocity. However, this filter is based on a two dimensional Fourier transform, so it is not well adapted to the non-...
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Ocean gravity waves travelling in opposite directions create second order pressure fluctuations which are the sources of secondary microseisms. These microseisms are mainly of Rayleigh waves and they have the double of the frequency of... more
Ocean gravity waves travelling in opposite directions create second order pressure fluctuations which are the sources of secondary microseisms. These microseisms are mainly of Rayleigh waves and they have the double of the frequency of the incident ocean waves. The ocean regions where these microseisms are generated are not well known. We consider both amplitude spectra and polarization spectra of seismic data to identify the strongest sources and we combine back azimuths recorded by stations in and around a given ocean to determine likely source positions. Ocean wave directional and frequency spectra are then used to determine the wave-wave interactions as a function of frequency and the corresponding second order pressure fluctuations at the ocean surface. We observe a good agreement between the seismic source likelihood maps and the source locations computed with the ocean wave model. The location of the most abundant sources is changing over the year. Following Longuet-Higgins (1950) and Kedar et al. (2007) we further compute the synthetic seismic spectra. The IOWAGA ocean wave model enables to take into account coastal reflection and we compare data with synthetic spectra considering different coastal reflection coefficients. In many cases, we observe less than 20% difference between synthetic seismic amplitude spectra with and without coastal reflection and an excellent correlation with the real spectra.