Investigation of seismic anisotropy in the D'' layer and at the CMB regarding intense magnetic flux regions

Fröhlich, Yvonne; Dillah, Muhammad Iman Fath; Dorn, Fiona; Ritter, Joachim R. R.

doi:10.5281/zenodo.12658821

Investigation of seismic anisotropy in the D'' layer and at the CMB regarding intense magnetic flux regions

Fröhlich, Yvonne; Dillah, Muhammad Iman Fath; Dorn, Fiona; Ritter, Joachim R. R.

Publisher: Zenodo
DOI: 10.5281/zenodo.12658821
Repository: zenodo.org
Language: en

Abstract

Abstract Within the Priority Program 2404 “Reconstructing the Deep Dynamics of Planet Earth over Geologic Time” (DeepDyn, https://www.geo.lmu.de/deepdyn/en/) we investigate possible seismic signatures at magnetic high-latitude flux lobes (HLFLs). The focus is on four target regions on the Northern Hemisphere: Siberia, Canada, North Atlantic and Indonesia. While Siberia and Canada show the HLFLs, the North Atlantic should be the location of a third postulated HLFL, but this area shows no intense-flux signal in the magnetic field. The region beneath Indonesia and the Indian Ocean is characterized by an area of intense magnetic flux that changes direction and moves westwards over time. Our aim is to understand whether mineralogy and seismic structure (i.e., thermal constraints) could be responsible for the different magnetic signatures at the core mantle boundary (CMB). This is done by combining two approaches: seismic anisotropy (KIT) and seismic reflections (University of Münster) near the CMB. To study anisotropy, we measure shear wave splitting of SKS, SKKS, and PKS (XKS) phases as well as of S and ScS phases. Thereby, we determine the splitting parameters, the fast polarization direction φ and the delay time δt as well as the splitting intensity SI. Especially, we search for phase pair discrepancies, e. g., between SKS and SKKS phases, as they are a clear indication for a lowermost mantle contribution to the splitting signal. For the target region underneath Siberia, we present first shear wave splitting measurements (SWSMs) on XKS phases recorded at the Finnish station KEF, including SKS-SKKS pairs. Based on our shear wave splitting measurements, we will derive structural and mineralogical anisotropy models using the MATLAB Seismic Anisotropy Toolbox (Walker and Wookey 2012). To test these models, we will simulate synthetic seismograms using AxiSEM3D (Leng et al. 2016, 2019). Besides comparing synthetic and observed seismograms, we plan to measure the shear wave splitting of the synthetic phases and compare splitting parameters and splitting intensity to the observed values. See also Proceedings of the18th Symposium of Study of the Earth's Deep Interior DeepDyn - DFG Priority Programm 2404 Funding Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 521545943under Germany’s Priority Programme "Reconstructing the deep dynamics of planet Earth over geologic time (DeepDyn)" SPP 2404 – 500707704.

Full text

KIT –The Research University in the Helmholtz Association Investigation of seismic anisotropy in the D’’ layer and at the CMB regarding intense magnetic flux regions Fröhlich Yvonne, Dillah Muhammad I. F., Dorn Fiona, Ritter Joachim R. R. Karlsruhe Institute of Technology (KIT), Geophysical Institute (GPI), Karlsruhe, Germany References & Acknowledgements Shear wave splitting (SWS) PKS, SKS, SKKS (XKS) phases S, ScS phases → Splitting parameters, splitting intensity Phase pair discrepancies SKS vs. SKKS → LMM mantle contribution •YF is supported by DFG grant RI1133/14-1 (SPP 2404 DeepDyn). •Shear wave splitting measurements were performed with SplitLab 1.2.1 (Porritt, 2014). •Correction of error calculation in SplitLab after Walsh et al., 2013, https://doi.org/10.1002/jgrb.50386. •Correction of temporal alignment in SplitLab after Fröhlich et al., 2022, https://doi.org/10.4401/ag-8781. •Geographic maps were generated with PyGMT v0.12.0 (Tian et al., 2024) and GMT 6.4.0 (Wessel et al., 2022). •Color-coding was accomplished using colormaps of cmocean (Thyng et al., 2016) and Scientific colour maps v8.0.1 (Crameri, 2023). Method Inclusion of Existing Models and Databases Nowacki et al., 2011; Fig. 2, modified. First Results Ray paths and travel times after iasp91 Earth model (Kennet et al., 1991). Calculation with the Java TauP Toolkit (Crotwell et al., 1999). Modeling MATLAB Seismic Anisotropy Toolbox → Walker & Wookey, 2012 Consideration of different anisotropy sources → Nowacki et al., 2011 Consideration of phase transitions and slip systems → Pisconti et al., 2021 → Set up different anisotropy / splitting and reflection scenarios AxiSEM2D / AxiSEM3D → Leng et al., 2016, 2019 Simulations of wave propagation Generation of synthetic seismograms → Measure shear wave splitting for SKS-SKKS, S-ScS phase pairs → Calculate reflection coefficients for P, S phases Library of elastic tensors for LMM anisotropy → Creasy et al., 2020 Shear wave splitting database → Wüstefeld et al., 2009 Deep mantle anisotropy database →Wolf et al., 2023 S i b e r i a A t l a n t i c Atlantic Siberia •Creasy et al., 2020, G3, https://doi.org/10.1029/2019GC008883. •Crotwell et al., 1999, SRL, https://doi.org/10.1785/gssrl.70.2.154. •Grund & Ritter, 2019, Geology, https://doi.org/10.1130/G45514.1. •Kennet et al., 1991, Terra Nova https://doi.org/10.1111/j.1365-3121.1991.tb00863.x. •Leng et al., 2016, GJI, https://doi.org/10.1093/gji/ggw363. •Leng et al., 2019, GJI, https://doi.org/10.1093/gji/ggz092. Observations at KEF Complex splitting pattern Discrepant SKS-SKKS pairs Analyse of data from further stations Search for differences between the target regions AlpArray ScanArray USArray by FD by MIFD •Nowacki et al., 2011, J. of Geodyn., https://doi.org/10.1016/j.jog.2011.04.003. •Pisconti et al., 2021, JGR Solid Earth, https://doi.org/10.1029/2018JB016993. •Walker & Wookey, 2012, Comput. Geosci., https://doi.org/10.1016/j.cageo.2012.05.031. •Wolf et al., 2023, G3, https://doi.org/10.1029/2023GC011070. •Wüstefeld et al., 2007, Comput. Geosci., https://doi.org/10.1016/j.cageo.2007.08.002. •Wüstefeld et al., 2009, Phys. Earth Planet. Int., https://doi.org/10.1016/j.pepi.2009.05.006. Shear wave splitting can be used to detect seismic anisotropy in the lowermost mantle (LMM) based on phase pair discrepancies, especially using SKS and SKKS phases. Within the priority program DeepDyn (SPP 2404), for details please refer to the poster Thiyagarajan & Thomas (2024), we study target regions related to intense magnetic flux on the northern hemisphere. Here we show first results, including shear wave splitting measurements at the recording station KEF (network HE) in Finland. Pisconti et al., 2021; Fig.5, slightly modified. minerals structures LPO SPO SKKS – split SKKS – split SKS – null SKS – null