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| Author(s): |
Felipe Proença Corral
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto Astronômico e Geofísico (IAG/SBD) |
| Defense date: | 2024-04-26 |
| Examining board members: |
Carlos Alberto Moreno Chaves;
Aderson Farias do Nascimento;
Cíntia Rocha da Trindade
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| Advisor: | Carlos Alberto Moreno Chaves; George Sand Leão Araújo de França |
| Abstract | |
In this master\'s dissertation, we focus on analyzing the influence of mantle heterogeneities on the traveltime of P-to-S wave conversions at the 410 km and 660 km discontinuities, which delineate the mantle transition zone (MTZ). This region is characterized by phase changes in the mineral olivine, the main component of the mantle, and plays a crucial role in mantle convection. Understanding the structure of the MTZ can provide significant constraints on the predominant convection pattern in the Earth\'s mantle, a topic still open in geodynamics. P-waves and their conversions at the 410 km and 660 km discontinuities, P410s and P660s, are widely used to determine topography variations of these discontinuities, especially in regional studies. Here, we obtain spectral-element method waveforms using the PREM model and recent 3D global seismic tomography models of P and S waves for 12 events distributed in a spiral shape from coordinates -100° E and 40° N, which are recorded by 1848 virtual seismic stations, spaced 1° apart, located in the United States. With this experiment, we map traveltime variations between the P410s-P and P660s-P phases and analyze how theoretical simplifications affect the estimation of the topography of the MTZ discontinuities. Additionally, we verify the resolvability of lateral variations in mantle discontinuity topography using the receiver function method, by distorting the spectral element mesh of SPECFEM3D_GLOBE, allowing the analysis of how harmonic variations in topography, of different wavelengths, are recovered. Using the Common Conversion Point (CCP) stacking technique to sum the receiver function seismograms to enhance the amplitudes of the P410s and P660s phases, our results reveal that the heterogeneous mantle velocity structure can produce topography values comparable to those observed in the literature, i.e., as high as 15 km. Therefore, the moveout correction process must take into account the P, P410s, and P660s traveltimes calculations from 3D seismic tomography models. However, we observe that time corrections based on ray theory are inadequate to estimate the influence of the heterogeneous mantle velocity structure, potentially introducing short-wavelength artifacts (< 200 km) in the MTZ topography maps, with amplitudes that may exceed 10 km, which could be erroneously interpreted as real structures. During the analysis, we find that although the time correction process should be performed with seismic tomography models, this procedure is not robust, meaning that the use of different models leads to different results, thus affecting the final estimation of topographies. Finally, we verify that the receiver function method is sensitive to detecting variations in MTZ topography, but the accuracy of recovering these features in-depth significantly depends on the bin size used to select depth conversion points for stacking. For the station array and event distribution used in the experiment, we find that only lateral topographic variations with wavelengths longer than approximately 111 km and amplitudes greater than approximately 5 km can be accurately recovered. Therefore, caution is recommended in interpreting MTZ topography maps with short-wavelength features that have a station spacing larger than that used in our experiment and do not take into account traveltime corrections performed with 3D seismic tomography models. (AU) | |
| FAPESP's process: | 22/05766-0 - Analysis of the mantle heterogeneous velocity structure effect on converted teleseismic waves used to image the transition zone. |
| Grantee: | Felipe Proença Corral |
| Support Opportunities: | Scholarships in Brazil - Master |