GD7.1 | Anisotropy from crust to core: Observations, models and implications
EDI
Anisotropy from crust to core: Observations, models and implications
Co-organized by EMRP1/SM5
Convener: Manuele Faccenda | Co-conveners: Tuna Eken, Judith Confal
Orals
| Tue, 25 Apr, 14:00–17:35 (CEST)
 
Room -2.47/48
Posters on site
| Attendance Tue, 25 Apr, 08:30–10:15 (CEST)
 
Hall X2
Orals |
Tue, 14:00
Tue, 08:30
Many regions of the Earth, from crust to core, exhibit anisotropic fabrics which can reveal much about geodynamic processes in the subsurface. These fabrics can exist at a variety of scales, from crystallographic orientations to regional structure alignments. In the past few decades, a tremendous body of multidisciplinary research has been dedicated to characterizing anisotropy in the solid Earth and understanding its geodynamical implications. This has included work in fields such as: (1) geophysics, to make in situ observations and construct models of anisotropic properties at a range of depths; (2) mineral physics, to explain the cause of some of these observations; and (3) numerical modelling, to relate the inferred fabrics to regional stress and flow regimes and, thus, geodynamic processes in the Earth. The study of anisotropy in the Solid Earth encompasses topics so diverse that it often appears fragmented according to regions of interest, e.g., the upper or lower crust, oceanic lithosphere, continental lithosphere, cratons, subduction zones, D'', or the inner core. The aim of this session is to bring together scientists working on different aspects of anisotropy to provide a comprehensive overview of the field. We encourage contributions from all disciplines of the earth sciences (including mineral physics, seismology, magnetotellurics, geodynamic modelling) focused on anisotropy at all scales and depths within the Earth.

Orals: Tue, 25 Apr | Room -2.47/48

Chairpersons: Manuele Faccenda, Judith Confal
14:00–14:05
14:05–14:35
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EGU23-5235
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ECS
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solicited
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On-site presentation
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Emanuel D. Kästle and the AlpArray Working Group

Making use of the dense AlpArray and SwathD networks in the eastern Alps, a large dataset of Rayleigh phase-velocity measurements is extracted. This dataset is the basis for a 3D azimuthally anisotropic shear-velocity model of the Alpine crust. A 2-step inversion approach is followed: First, phase-velocity maps are created which are inverted for the shear velocity structure at depth in a second step. In both steps, a Bayesian (rjMcMC) approach is used to find the posterior distribution of anisotropic models. The model uncertainties are propagated from the phase-velocity maps to the depth inversion to make sure that the data is not overfitted. The final model shows a 2 layer anisotropy in the Alpine crust, the upper crustal layer is mostly orogen parallel and follows the major fault structures. The lower crustal to uppermost mantle layer shows orogen-perpendicular fast axis in the Alps and an anisotropy following the curvature of the Alps in the northern foreland. The importance of microfabric such as microcracks and oriented mineral grains is difficult to estimate from the presented model on the effective regional-scale anisotropy. But the results suggest that the azimuthal anisotropy may be largely controlled by macro-scale structures. The transition from upper to lower crustal anistotropy takes place at approx. 20 km depth which is unlikely to be due to the brittle-ductile transition. But it could indicate that upper and lower crust are only weakly coupled underneath the Alps.

How to cite: Kästle, E. D. and the AlpArray Working Group: Azimuthal Anisotropy in the Eastern Alpine Crust from Ambient Noise Tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5235, https://doi.org/10.5194/egusphere-egu23-5235, 2023.

14:35–14:45
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EGU23-4746
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ECS
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On-site presentation
Junliu Suwen, Yi Lin, Li Zhao, and Qi-Fu Chen

Northeast (NE) China is located in the eastern Central Asian Orogenic Belt, and has a complex deformation history. The evolution of NE China has been controlled by the (Paleo-)Pacific Plate since the late Mesozoic and was affected by the closure of the Paleo-Asian and Mongol–Okhotsk oceans. Meanwhile, large strike-slip faults and extensive intraplate volcanisms characterize active tectonics in NE China. Different mechanisms have been proposed to interpret the origin of the intraplate volcanism, such as interactions between the lithosphere and the big mantle wedge, and the subduction-induced upwelling within the gap of the stagnant Pacific slab.

Seismic anisotropy describes the directional dependence of the seismic velocities. In NE China, seismic anisotropy not only reveals the past and present deformations in the lithosphere but also helps us clarify the possible intraplate volcanism. In this study, we apply the full-wave multi-scale anisotropy tomography method to investigate the seismic anisotropy in NE China. We measure the splitting intensities of SKS waves, which can be linearly inverted for the 3D variation of anisotropy. We employ broadband seismograms recorded at ~450 regional seismic stations (including ~250 temporary stations deployed for 2 years) of unprecedented density from teleseismic events of magnitudes greater than 5.5 occurring in 2009-2018. We obtain a total of 4249 splitting intensity measurements, and perform the multi-scale inversion using sensitivity kernels computed by normal-mode summation. The resulting 3D anisotropic model of the upper mantle in NE China shows a dominant NW-SE fast axis, which highlights a strong correlation between the intraplate volcanoes and upper-mantle seismic anisotropy, and indicates that NE China is still mainly controlled by the Pacific Plate.

How to cite: Suwen, J., Lin, Y., Zhao, L., and Chen, Q.-F.: Full-wave anisotropy tomography for the upper mantle of Northeast China using SKS splitting intensities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4746, https://doi.org/10.5194/egusphere-egu23-4746, 2023.

14:45–14:55
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EGU23-11721
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ECS
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Virtual presentation
Monumoy Ghosh, Arijit Chakraborty, Siddharth Dey, Inashua Kharjana, Shubham Sharma, Sankar N. Bhattacharya, and Supriyo Mitra

3-component regional waveforms for ~14700 raypaths sampling India, Himalaya and Tibet, have been used for multi-taper polarization analysis of surface waves between periods of 10 and 120 s. Rayleigh (LR) and Love (LQ) wave energy arriving at the station within +/- 10 degree of the theoretical back azimuth have been used to compute 1D path average fundamental mode group velocity dispersion. Theoretical dispersion of fundamental and first two higher modes have been computed using each 1D path average velocity structure constructed from CRUST1.0 over IASP91 mantle model. These are compared with the observed dispersion dataset to identify and remove those periods with higher mode overlap with the observed fundamental mode picks. The shortlisted dispersion datasets consist of ~90% of the original dataset. To ascertain the lateral variation in the group velocity, the observed dispersion has been used to compute 2D tomography maps of LR and LQ group velocities at discrete periods between 10 and 120 s. From these maps, seven 2D profiles across northern India, Himalaya and Tibet have been extracted for modeling the radially anisotropic shear-wave velocity structure of the lithosphere. Haskell-Thompson (H-T) matrix method is used to calculate synthetic LQ dispersion. For the LR dispersion, the H-T method with reduced delta matrix has been used, considering Vph≠Vpv and Eta≠1. The inversion scheme uses genetic algorithms (GA) to search the model space parameterized using Vsh, Vph, Xi and thickness for 3 crustal layers and 2 mantle layers underlain by a mantle half-space. Synthetic tests have been performed using theoretical LR and LQ dispersion curves, computed from global models with 5% and 10% anisotropy, introduced in the mantle layers. The fit to the synthetic LR and LQ dispersion data and the model recovery using GA inversion is satisfactory for such tests. This inversion scheme is being applied to the observed LR and LQ dispersion data from the seven profiles and the results will be presented.

How to cite: Ghosh, M., Chakraborty, A., Dey, S., Kharjana, I., Sharma, S., Bhattacharya, S. N., and Mitra, S.: Radially anisotropic shear-wave velocity structure of northern India, Himalaya and Tibet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11721, https://doi.org/10.5194/egusphere-egu23-11721, 2023.

14:55–15:05
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EGU23-15777
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ECS
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On-site presentation
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Abolfazl komeazi, Ayoub Kaviani, Georg Rümpker, Christian Weidle, and Thomas Meier

The obduction of the Semail Ophiolite onto the Arabian continental margin during the convergence of the Arabian and Eurasian plates has left a significant impact on the lithospheric structure beneath the Oman Mountains. However, there remains a degree of uncertainty concerning the extent to which the inherited structures (pre-existing features of the lithosphere) contribute to the obduction of ophiolites. To gain a deeper understanding of the impact of the obduction process on the mantle structure beneath northern Oman, we analyze seismic anisotropy beneath this region using splitting analysis of teleseismic shear wave data collected from a dense network of 40 seismic stations that have been operational for approximately 3 years since 2013. 

Based on azimuthal distribution of the shear wave splitting (SWS) parameters, φ and δt, we are able to divide the study area into two subregions. The stations located to the west of the Semail gap exhibit relatively azimuthally invariant SWS parameters suggesting a single anisotropic layer. On the other hand, at most of the stations located in the central and eastern regions we observe a 90-degree periodicity versus back-azimuth, indicative of a depth-dependent anisotropic medium. 

In the western part, the fast axes are aligned with the strike of the collision between the continental and oceanic plates, where the oceanic lithosphere is believed to be obducted over the continental lithosphere. We also invert the azimuthal variation of the SWS parameters from the central and eastern stations for two layers of anisotropy. The fast axes of the upper layer exhibit a predominantly NW-SE trend, in good agreement with the anisotropy directions of the one-layer models obtained in the western region. The fast axes of the lower layer display a NE-SW trend, possibly representative of the large-scale mantle flow resulting from the present-day plate motion. 

How to cite: komeazi, A., Kaviani, A., Rümpker, G., Weidle, C., and Meier, T.: Signatures of the subduction/obduction processes in the lithosphere and asthenosphere beneath the Semail Ophiolites in Oman revealed by seismic anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15777, https://doi.org/10.5194/egusphere-egu23-15777, 2023.

15:05–15:15
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EGU23-6183
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ECS
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On-site presentation
Derya Keleş, Tuna Eken, Pan Wang, Zhouchuan Huang, and Tuncay Taymaz

Crustal scale deformation along the fault zone and dipping Moho structures can be constrained by azimuthal seismic anisotropy. Reliable knowledge of the geometry of fault and its vertical extent in the crust and uppermost mantle that often controls observed seismic anisotropy parameters is of great importance for proper seismic hazard assessments in active tectonic settings. The North Anatolian Fault Zone (NAFZ) extending from Karlıova Triple Junction in the east to the Aegean Sea in the west poses actively deforming areas. To elucidate the crustal anisotropy along the NAFZ we will apply a novel receiver function method that simultaneously measures the Moho orientation and average bulk crustal anisotropy. It employs an algorithm in which transverse polarization component minimization (TPCM) applied on the Pms (Moho converted phase) is being integrated into a joint objective method (JOF). The method is advantageous as it restrains the random and coherent noise in the data. Prior to raw data-based anisotropic parameter estimations, we performed synthetic tests mainly considering two hypothetic models devised to be analogous to the NAFZ case. Model-1 assumes S-anisotropy in the crust oriented along N45°E with 4% of strength with flat Moho. The Model 2 involves the same anisotropic properties but with 25° of dipping Moho. Our synthetic tests show that this new approach is able to exactly resolve true model parameters assumed for Model 1 resulting in a 0.987 per cent of model accuracy. The resultant 0.225 s of time delay corresponds to ~4% of anisotropic strength considering a crustal thickness with 30 km and 4.8 km/s average isotropic S-wave velocity for the medium. Our results obtained for Model-2 still tend to converge the true model parameters but show slight discrepancies, in particular, for anisotropic parameters resolved with 0.3 s of time delay and N40°E oriented fast wave azimuth. The test for Model-2 achieves 27.5° as the dipping angle of Moho which is fairly close to its true model parameter. We observe relatively low model accuracy with 0.710 per cent in the case of Model-2. At the further stage of this work, we will utilize digital waveforms of teleseismic earthquakes recorded at the KOERI and AFAD permanent seismic station networks along the NAFZ.   

How to cite: Keleş, D., Eken, T., Wang, P., Huang, Z., and Taymaz, T.: Novel Application of Receiver Function Analyses Dependent on Splitting Measurement: Crustal Anisotropy along the NAFZ, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6183, https://doi.org/10.5194/egusphere-egu23-6183, 2023.

15:15–15:25
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EGU23-16313
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ECS
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On-site presentation
Ceyhun Erman, Seda Yolsal-Çevikbilen, Tuna Eken, and Tuncay Taymaz

The eastern Mediterranean which is one of the most tectonically active collisional regions where Eurasian, African and Arabian plates converge, provides an excellent opportunity to investigate the evolution of various scales of deformation throughout the Earth. In such a region with highly complex and active tectonic structures, a detailed study of geodynamic processes and related mantle kinematics is required to better understand the development of complex structures at the surface. For example, the region of study, the Anatolian plate and surroundings host several complicated deformation regimes with two large transform faults (North and East Anatolian Faults; NAF and EAF, respectively), regions of extensional and compressional tectonics in the west and east of Anatolia. Seismic anisotropy provides a robust link between seismic observations and geodynamic processes which play a key role for controlling the past and/or present deformations in the mantle lithosphere and asthenosphere. In this study, we perform shear wave splitting analyses on teleseismic core-refracted S-waves (e.g. SKS and SKKS phases) recorded by ~600 broad-band seismic stations located in the region. We estimate seismic anisotropy parameters (e.g., fast polarization direction; FPD and delay time; DT) beneath each seismic station by employing conventional shear wave splitting (e.g., transverse energy minimization and eigenvalue) and splitting intensity approaches. Exploiting a large earthquake dataset, spanning through 2000-2022 with Mw ≥ 5.5 events, that covers a wide range of back-azimuths enables the reliable estimates of complex anisotropic models, such as two-layer and dipping anisotropy models. Our preliminary results largely indicate the NE-SW directed FPDs throughout the study area, except for SW Turkey (NW-SE) and central parts of Anatolia (E-W) that can be mainly explained by the lattice-preferred orientation (LPO) of olivine minerals in the upper mantle induced by the mantle flow related to the roll-back process of the Hellenic slab. Findings from our two-layer grid search algorithm indicated strong evidences for two-layer anisotropy models beneath the seismic stations in eastern Aegean and western Anatolia, in particular close to the western branches of NAF in the Aegean.

How to cite: Erman, C., Yolsal-Çevikbilen, S., Eken, T., and Taymaz, T.: Seismic Anisotropy in the Upper Mantle Beneath the Anatolian Plate and Surroundings Inferred from Shear wave Splitting Analyses, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16313, https://doi.org/10.5194/egusphere-egu23-16313, 2023.

15:25–15:35
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EGU23-12807
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ECS
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Virtual presentation
Laura Petrescu, Andrei Mihai, and Felix Borleanu

We investigate the complex flow field around the Vrancea slab in Romania, a steeply sinking seismogenic lithospheric block that experienced lateral tear-off and possible rotation. The Vrancea slab is located beneath the South-East Carpathians and generates frequent seismicity despite its remote location from active collisional boundaries. We analyse core-refracted shear wave (SKS) splitting recorded by permanent broadband seismic stations from the Romanian Seismic Network for periods up to 10 years, and compare our results with seismic tomography of the upper mantle. We identify several stations with large backazimuthal variations of SKS fast axis polarization and delay times both in the slab hinge zone, the back-arc and the circumslab region, indicating complex mantle deformation patterns. To investigate the effect of a two-level rotated slab we invert SKS waveforms using cross-convolutional misfit combined with a neighbourhood search algorithm to model two layers of anisotropy. In the shallow mantle, anisotropy aligns with the upper slab strike and reorients along the strike of the lower slab at depths below the hinge zone. In the backarc trench-perpendicular anisotropy switches to trench-parallel, due to the recent retreat and roll-back of the slab. Our results have important implications for understanding SKS interference from subducted slab fragments and provide evidence of the recent retreat, break-off and rotation of two Vrancea slab levels sinking into the upper mantle.

How to cite: Petrescu, L., Mihai, A., and Borleanu, F.: Slab tear and rotation imaged with core-refracted shear wave anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12807, https://doi.org/10.5194/egusphere-egu23-12807, 2023.

Coffee break
Chairpersons: Tuna Eken, Judith Confal
16:15–16:45
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EGU23-15771
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solicited
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Highlight
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On-site presentation
Arwen Deuss, Hen Brett, and Jeroen Tromp

The Earth's inner core is one of the most strongly anisotropic regions of our planet. On average, the anisotropy appears to be aligned with the Earth's rotation axis with a larger wave velocity in the polar (North-South) direction than in the equatorial (East-West) direction. Over de last few decades, seismic studies of inner core anisotropy have revealed regional variations with ever increasing detail, suggesting that the top 60-80 km of the inner core is isotropic, the western hemisphere is more strongly anisotropic than the eastern hemisphere and that the anisotropy in the innermost inner core has an anomalous slow direction. Most previous studies assumed that the symmetry axis of the anisotropy is aligned with the rotation axis axis and then attributed regional variations to variations in the magnitude of the anisotropy. 

Here, we make a tomographic model of inner core anisotropy using seismic body waves observations using a different approach. We assume that the inner core is made of cylindrically symmetric anisotropy crystals that all have the same magnitude of anisotropy, and instead we allow the symmetry axis to vary. We find that our model fits the body wave data equally well as models in which the magnitude varies, with the advantage that our model requires fewer parameters. In our model, the anisotropy in the central part of the inner core is still mainly aligned with the rotation axis. In the upper part of the inner core we find two caps around South-East Asia and Central America with anisotropy aligned parallel to the inner core boundary.

Inner core anisotropy is most likely due to alignment of hcp iron crystal  formed either (i) during solidification at the inner core boundary or (ii) afterwards by deformation deeper in the inner core. Thus, our new model may be related to flow in the inner core or solidification processes at the inner core boundary and constrain geodynamic processes in the inner core. 

How to cite: Deuss, A., Brett, H., and Tromp, J.: Anistropy in the Earth's inner core, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15771, https://doi.org/10.5194/egusphere-egu23-15771, 2023.

16:45–16:55
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EGU23-3947
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ECS
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On-site presentation
Jonathan Wolf and Maureen D. Long

It has been suggested that the present-day locations of ultra-low velocity zones (ULVZs), which are thin features just above the core-mantle boundary (CMB), are influenced by mantle convection; however, apart from their preferential locations, there is little direct evidence for this connection. Observations of deep mantle anisotropy can be used to infer mantle dynamics but are not usually jointly analyzed with ULVZ structure. We newly detect and characterize a ULVZ beneath the Himalaya, located approximately at the edge between an (almost) isotropic and a large anisotropic region in the lowermost mantle. Using global wavefield simulations to model realistic mineral physics scenarios, we show that the seismic anisotropy is indicative of northeast-southwest flow directions. The southwestwards flow is likely induced by slab remnants at the CMB, and the ULVZ is located at the southwestern edge of the anisotropic province, which is indicative of slab-induced ULVZ displacement. 

How to cite: Wolf, J. and Long, M. D.: Slab-driven transport of ultra-low velocity material in the deep mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3947, https://doi.org/10.5194/egusphere-egu23-3947, 2023.

16:55–17:05
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EGU23-10460
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On-site presentation
Jeffrey Park, Xiaoran Chen, and Vadim Levin

Many researchers have used S-to-P (Sp) converted waves to detect the Moho discontinuity, the lithosphere-asthenosphere boundary (LAB) and mid-lithospheric discontinuities (MLD). The anisotropy of Earth’s lithosphere is typically constrained with shear-wave birefringence.  Both theory and reflectivity computations, however, argue for a substantial influence of anisotropy on the initial amplitude of the Sp converted wave.  The effects of compressional anisotropy on initial Sp amplitudes are stronger than the effects of shear anisotropy for anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations.  This Sp behavior is not typically studied, but it has the potential to test the hypothesis that the seismic lithosphere-asthenosphere boundary (LAB) is caused by a transition in anisotropic layering at the base of Earth’s tectonic plates.

We develop and apply multiple-taper correlation estimates for Sp receiver functions, applicable to either SV or SH incoming polarization, or for a linear combination of SV and SH. In the context of incoming SV-polarized body waves, e.g., SKS phases, algorithms from multiple-taper Ps RFs can be borrowed to apply moveout corrections before the Fourier transform to target a particular interface depth in the crust or mantle.  With synthetic seismograms, we find that SH “receiver functions” can be computed from incoming SV waves, promising a diagnostic detecting SKS birefringence and to estimate an average splitting signal from a station. The SV and SH waveforms can be “unsplit” in the frequency domain by the estimated average birefringence to reconstruct the S waves that impinge the lithosphere from the deep mantle.  We will report analyses with data from permanent stations of the Global Seismographic Network and the USGS ANSS.

How to cite: Park, J., Chen, X., and Levin, V.: Detecting Anisotropy from Back-Azimuth Amplitude Dependence of Sp Converted Waves, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10460, https://doi.org/10.5194/egusphere-egu23-10460, 2023.

17:05–17:15
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EGU23-13670
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On-site presentation
William Sturgeon, Ana M.G. Ferreira, and Matthew Fox

The seismic imaging of radial anisotropy can be used as a proxy for the direction of mantle flow. Previous studies have imaged radial anisotropy throughout the mantle on a global scale and are starting to show some consistent features. However, the interpretation of existing models is hindered by the lack of uncertainties provided from the employed inversion method. To address this, we build a new global radially anisotropic model of the Earth’s upper mantle which consists of two main stages. Firstly, we build global Rayleigh and Love wave phase and group velocity maps using ~47 million measurements, including fundamental mode and up to 5th overtone measurements, and compute their associated uncertainties. Weights according to similar paths and data uncertainties are employed in the inversions. We construct a total of 310 2D maps, at periods between T16-375 s, expanded in spherical harmonics up to degree lmax=60 and observe many relevant small-scale structures, such as e.g. the curvature of the Tibetan plateau at T~40s (fundamental mode). As expected, uncertainties are higher in regions of poor data coverage (e.g., southern hemisphere and oceans). Then, we invert for 1D profiles of radial anisotropy using two Monte Carlo based inversion methods: the Neighbourhood Algorithm (NA) and the Reversible-Jump Markov Chain Monte Carlo algorithm (RJMCMC). The NA has been widely used for seismic inversion, as it efficiently explores the parameter space. However, the advantage of the RJMCMC is that in addition to constraining e.g. radial anisotropy, it can also constrain e.g. layer thickness. We compare the 1D profiles from both methods, and their associated uncertainties, which will lead to a new global 3D model of radial anisotropy.

How to cite: Sturgeon, W., Ferreira, A. M. G., and Fox, M.: Towards constraining mantle flow through imaging of radial anisotropy, with full uncertainty quantification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13670, https://doi.org/10.5194/egusphere-egu23-13670, 2023.

17:15–17:25
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EGU23-9927
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ECS
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On-site presentation
Gianmarco Del Piccolo, Brandon VanderBeek, Manuele Faccenda, Andrea Morelli, and Joseph Byrnes

The implementation of stochastic methods in seismic tomography arises as a response to the limitations introduced by traditional non-linear optimization solvers. Since tomographic problems are generally ill-conditioned, additional constraints on the model are set in the misfit function, and the weight given to each minimization term has a level of arbitrariness; different solutions are obtained with different choices for the damping/smoothing factors. Non-linear optimization solvers are based on a perturbative approach which linearizes the forward modelling locally around a reference model, updated at each iteration until convergence. These methods need the evaluation of the derivatives of the predictions with respect to the parameters of the model, which is not always an easy task, and they generally do not provide the uncertainties associated with the solution model.
The Reversible-Jump Markov-Chain Monte Carlo is a stochastic method which performs a random walk in the model space sampling the posterior probability distribution associated with the model in the light of the observations. This method is a trans-dimensional Metropolis-Hastings where the number of parameters used to represent the continuous fields (as interpolation nodes) is treated as a parameter itself of the inversion, as the positions of the nodes. Using statistical estimators on the ensemble of models produced by the algorithm it is possible to extract a reference model, typically as an average of the ensemble. With this method no regularization is needed, and uncertainty can be estimated using the ensemble of models sampled. The limitations of non-linear optimization solvers are overcome at the cost of an increase in the computational time required.

The presented applications of this method involve seismic tomography in subduction zones, where the anisotropic component of the seismic velocity field is relevant, and the inversion of seismological data could provide an interesting insight into the dynamics of these regions. 

How to cite: Del Piccolo, G., VanderBeek, B., Faccenda, M., Morelli, A., and Byrnes, J.: Reversible-Jump, Markov-Chain Monte Carlo seismic tomographic inversion for anisotropic structure in subduction zones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9927, https://doi.org/10.5194/egusphere-egu23-9927, 2023.

17:25–17:35
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EGU23-2613
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On-site presentation
Alexey Stovas

The singularity points are very important for elastic waves propagation in low-symmetry anisotropic media (Stovas et al., 2021a). Being converted into the group velocity domain, they result in internal refraction cone with anomalous amplitudes and very complicated polarization fields. In elastic orthorhombic (ORT) media, there is always one singularity point in the essential symmetry plane, (0,1,2) singularity points in remaining non-essential symmetry planes and (0,1) points in-between the symmetry planes (Stovas et al., 2021b).

I analyze the conditions for existence of a singularity point in-between the symmetry planes.

In order to do that I fix the diagonal elements of the stiffness coefficient matrix, cjj , j=1,6, and introduce new variables d12 = c12 +c66, d13 = c13 +c55 and d23 = c23 +c44. I also assume that the symmetry plane 2-3 is the essential one by introducing the inequality c55 < c44 < c66 . If c66 < c44 < c55, the 2-3 plane is still essential one but the properties of non-essential planes will interchange. In case of other inequalities for “S wave” stiffness coefficients, the corresponding properties of singularity points can be obtained by a cyclic rotation of stiffness coefficients and symmetry planes (Stovas et al., 2023).

For selected essential symmetry plane (2-3), I propose to fix two variables d12 and d13, and set the variable d23 as a free variable. By changing d23 only, I can define the trajectory of a singularity point in-between the symmetry planes. This trajectory is given by a continuous line connecting the symmetry planes. Then I define the traces of this trajectory on symmetry planes (maximum two points for each plane) by a specific value of the variable d23. These 6 values can be used for intervals of d23 where the singularity point in-between the symmetry planes exists. Analysis shows that there are 7 zones in (d12 , d13) plane with different intervals of d23, which guarantee the existence of singularity point in-between the symmetry planes. There are 3 intervals of d23 in one zone, two intervals in two zones and one interval in three zones. There is no singularity point in-between the symmetry planes for any d23 in remaining zone. These zones are separated by three straight lines that defined by d12 = d12(critical) , d13 = d13(critical)  and d13 = α d12, where α guarantees that trajectory of singularity point meets the essential symmetry plane.

 References

Stovas, A., Roganov, Yu., and V. Roganov, 2021a, Geometrical characteristics of P and S wave phase and group velocity surfaces in anisotropic media, Geophysical Prospecting, 68(1), 53-69.

Stovas, A., Roganov, Yu., and V. Roganov, 2021b, Wave characteristics in elliptical orthorhombic medium, Geophysics, 86(3), C89-C99.

Stovas, A., Roganov, Yu., and V. Roganov, 2023, On singularity points in elastic orthorhombic media, Geophysics, 88(1), C11-C32.

How to cite: Stovas, A.: On singularity point in-between the symmetry planes in elastic orthorhombic media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2613, https://doi.org/10.5194/egusphere-egu23-2613, 2023.

Posters on site: Tue, 25 Apr, 08:30–10:15 | Hall X2

Chairpersons: Judith Confal, Tuna Eken, Manuele Faccenda
X2.210
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EGU23-2522
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ECS
Shijie Hao, Ping Tong, and Jing Chen

Surface waves contain critical information on seismic azimuthal anisotropy, which is directly related to underground geological dynamics. However, despite seismic azimuthal anisotropy, the topographic variation may also contribute to the azimuthal dependence of surface wave speed. To our knowledge, most surface wave traveltime tomography methods ignore the topographic variation in forward modeling. Furthermore, the theoretical propagation path is also calculated in isotropic media with flat surface. Undoubtably, inaccurate forward simulations could introduce artefacts to imaging results 

To address these problems, we develop a novel surface wave tomography method which tracks the surface wave propagation path in anisotropic media and incorporates the topographic variation. An elliptically anisotropic eikonal equation is used to describe the traveltime field of surface wave propagation, and sensitivity kernels with respect to shear wave velocity and azimuthal anisotropy are derived using the adjoint-state method. This new tomography method is tested and verified in Po Basin and adjacent regions, including the central Alps and northern Apennines.  

How to cite: Hao, S., Tong, P., and Chen, J.: Adjoint-State Surface Wave Tomography for Azimuthally Anisotropic Media: Eikonal Equation-Based Methods and Incorporation of Surface Topography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2522, https://doi.org/10.5194/egusphere-egu23-2522, 2023.

X2.211
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EGU23-3136
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ECS
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Jungjin Lee and Haemyeong Jung

Talc and chloritoid are common metamorphic minerals observed in mafic-ultramafic rocks and/or high-pressure metapelitic rocks comprising subducting slabs. Crystallographic preferred orientations (CPOs) of elastically anisotropic minerals have been known to be important for interpreting seismic anisotropy observed in subduction zones. However, studies on the CPOs of talc and chloritoid have been very limited. In this study, CPOs of talc and chloritoid in garnet-chloritoid-talc schist samples from ultrahigh-pressure Makbal Complex (Tianshan, Kazakhstan-Kyrgyzstan) which has been regarded as a part of subducting slab were measured using SEM/EBSD technique. CPO-induced seismic properties of both talc and chloritoid were analyzed and compared. The results showed that both talc and chloritoid displayed strong CPOs characterized by the [001] axes aligned subnormal to the foliation (see also Lee et al., 2021). CPO-induced seismic properties of polycrystalline talc and chloritoid were calculated and they showed that both P-wave anisotropy (AVp = 5 – 72 %) and high S-wave anisotropy (AVs = 10 – 24 %) of talc and chloritoid were much higher than those of garnet (AVp = 0.4 %, AVs = 0.9 – 1.0 %). In addition, the AVp of polycrystalline talc was much higher than that of polycrystalline chloritoid. Analysis of S-wave delay time and fast-polarization direction based on the modelling study of subduction zone geometry showed that the CPOs of talc and chloritoid induced a long delay time of 0.3 – 0.5 s and trench-parallel polarization direction for high dip-angle subduction, which is consistent with the observation of strong trench-parallel seismic anisotropy in subduction zones. Our results suggest that the strong CPOs of talc and chloritoid would influence trench-parallel seismic anisotropy induced by subducting slab in subduction zones. Lee et al., 2021, Seismic anisotropy in subduction zones: evaluating the role of chloritoid, Frontiers in Earth Science, 9, 1-16.

How to cite: Lee, J. and Jung, H.: Crystallographic preferred orientations of talc and chloritoid and implications for seismic anisotropy in subduction zones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3136, https://doi.org/10.5194/egusphere-egu23-3136, 2023.

X2.212
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EGU23-7321
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ECS
Judith M. Confal, Paola Baccheschi, Silvia Pondrelli, Brandon P. VanderBeek, Foivos Karakostas, and Manuele Faccenda

Seismic anisotropy measurements provide a lot of information on the deformation and structure of the Earth’s interior, in particular of the upper mantle. Conventional methods of measurement of anisotropy have their limitations, especially regarding depth resolution. Splitting intensity (SI) is a seismic observable, related to the amount of energy on the transverse component waveform and, to a first order, it is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, that can be therefore inverted, allowing a high-resolution image of the upper-mantle anisotropy. Starting from synthetic SKS waveforms, we first derived high-quality SKS splitting intensity measurements; then we used the splitting intensity data as input into tomographic inversion. This approach enables high‐resolution tomographic images of horizontal upper‐mantle anisotropy through recovering vertical and lateral changes in anisotropy and represents a propaedeutic step to the real cases of subduction settings. Additionally, this approach was able to detect regions of strong dipping anisotropy by allowing a 360° periodic dependence of the splitting vector. Single and thick layers of dipping angles between 30 and 60° are clearly represented with a high dt2 value, while double layers or nearly vertical dips are more difficult to identify.

How to cite: Confal, J. M., Baccheschi, P., Pondrelli, S., VanderBeek, B. P., Karakostas, F., and Faccenda, M.: Testing the splitting intensity methodology to retrieve average, dipping, and depth dependent anisotropy from a complex subduction model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7321, https://doi.org/10.5194/egusphere-egu23-7321, 2023.

X2.213
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EGU23-7527
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ECS
James Ward, Andrew Walker, Andy Nowacki, James Panton, and Huw Davies

Seismic anisotropy in the lowermost mantle is thought to be caused by the non-random alignment of anisotropic crystals from texturing from the mantle flowfield. Therefore, seismic anisotropy observations are commonly interpreted in the context of mantle flow. It is unclear, however, how much of an influence the history of mantle convection has on lowermost mantle seismic anisotropy and whether the present-day flowfield is sufficient for interpretation.  

We investigate this by comparing the predicted anisotropy from an Earth-like mantle convection model, which includes plate motion histories from 600 Ma and a Rayleigh number of approximately 108. Therefore, these models should contain structures on similar length scales and in similar locations to the Earth. We create maps of anisotropy 50 km above the CMB using the present-day flowfield in one case and allowing the flowfield to change with time in another. For each point, we model the texture development of 500 post-perovskite crystals on their journey through the mantle to the location of interest. We then use single-crystal elastic constants to compute the full elastic tensor from the texture. To investigate what influences material properties have on the memory of mantle texture, we use three different deformation systems where we vary how easily texture can develop. 

We compare the two maps by taking the difference between radial anisotropy parameters ξ = VSH2/VSV2 and φ = VPV/VPH as this is what is often analysed from seismic tomography. We also present the difference in the final elastic tensors at each location because observations such as from shear wave splitting will be sensitive to more of the full elastic tensor. We find that no matter the deformation model, some regions show very different radial anisotropy strength (>10 % difference). Outside of these regions, there is little effect of a time-varying flowfield (<1 % difference) when assuming post-perovskite is easy to texture. If post-perovskite is hard to texture, the influence of mantle flowfield history has a greater effect on the final texture and therefore the anisotropy (>1 % difference). We find a similar pattern when comparing the full elastic tensors, though most regions do show some small differences. Comparing the most complex paths and quantifying the memory of the mantle shows varying results depending on the deformation models of post-perovskite and the flowfield sampled. Assuming an easy-to-deform material, the memory of the mantle was approximately 10 Ma along some paths. However, along other paths, the final texture is sensitive to flow it sampled at 125 Ma. These results show that, while a time-varying flowfield makes a significant difference along complex paths with difficult-to-texture minerals, a time-varying flowfield produces similar results to those when assuming the present-day flowfield. This work represents progress toward an understanding of the relationship between lower mantle seismic anisotropy and mantle convection.  

How to cite: Ward, J., Walker, A., Nowacki, A., Panton, J., and Davies, H.: The Memory of the Mantle: The Influence of a Time Varying Flow Field on Present Day Observations of Seismic Anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7527, https://doi.org/10.5194/egusphere-egu23-7527, 2023.

X2.214
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EGU23-7790
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ECS
Brandon VanderBeek, Rosalia Lo Bue, and Manuele Faccenda

Despite the well known anisotropic structure of Earth’s upper mantle, the effect of seismic anisotropy on the construction of body wave shear velocity models remains largely ignored. Ignoring anisotropic heterogeneity can introduce significant model artefacts that may be misinterpreted as compositional and thermal heterogeneities. While effective anisotropic imaging strategies that improve model reconstruction have been developed for P-wave delay times, no such general framework exists for S-waves partly because, unlike P-waves, there is not a simple ray-based methodology for predicting S-wave travel-times through anisotropic media. Here, we apply a new methodology for the inversion of relative shear wave delay times and splitting intensity measurements for arbitrarily oriented hexagonally anisotropic model parameters using data collected across the western United States and Cascadia subduction system. We detail the data analysis procedure required for making measurements of shear wave observables suitable for anisotropic inversions (e.g. determination of incoming polarisation directions). We then present a preliminary anisotropic shear wave velocity model for Cascadia and compare the results to purely isotropic images. The imaged anisotropic heterogeneity is compared to the well-established patterns in shear wave splitting parameters observed in the study area.

How to cite: VanderBeek, B., Lo Bue, R., and Faccenda, M.: Imaging Upper Mantle Anisotropic Structure Using Teleseismic Shear Wave Delays and Splitting Intensity: Application to the Cascadia Subduction Zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7790, https://doi.org/10.5194/egusphere-egu23-7790, 2023.

X2.215
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EGU23-8129
Paola Baccheschi, Judith M. Confal, Silvia Pondrelli, Manuele Faccenda, Brandon P. VanderBeek, and Zhouchuan Huang

Seismic anisotropy is a fundamental key to gain knowledge of the mantle dynamics and structure. The image of seismic anisotropy over the upper mantle can be obtained with several methods, including surface waves and SKS splitting measurements. Taken together, these anisotropic measurements contribute to extensively catch anisotropy at different depths, yielding insights into the structure and dynamics of the crust and upper mantle. Nevertheless, mantle images resulting from surface waves result in poor lateral resolution, while the nearly vertically propagating SKS waves, when interpreted in a ray-based framework, results in little or no depth resolution, not allowing to easily image the distribution of the anisotropy through depth. Though the anisotropic seismic nature of the upper mantle is well established by a wealth of observational research, most of common teleseismic body-wave tomography studies neglect P- and S-wave anisotropy, thus producing artefacts in tomographic models in terms of amplitude and localization of heterogeneities. To overcome this problem different tomographic methods have been implemented to invert SKS splitting observations for anisotropic structures, most of which based on finite-frequency sensitivity kernels that relate elastic model perturbations to splitting observations. In this study we adopted the tomographic method relying on the inversion of the splitting intensity, a measure of the amount of energy on the transverse component of the waveform. Since is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, SI can therefore be easily inverted, providing the basis for a better interpretation of shear wave splitting measurements. In this study, we first compute the splitting intensity (SI) and splitting parameters using teleseismic shear-wave recorded at 824 available permanent and temporary stations in Italy and surrounding regions. Then, the dataset of SI has been used as an input for the tomographic inversion. The results obtained show changes of the anisotropic properties with depth, especially for the strength of anisotropy. A progressive depth-increase in anisotropy intensity has been recovered over Italy, affecting the bulge of the Alps and Apennines chain and the southern Tyrrhenian subduction system. On the contrary, weaker anisotropy characterizes the transition zone from the Apenninic to Alps domain beneath the Po plain and the Adriatic domain. The anisotropic tomography models obtained in this study allowed us to recover for the first time a new 3D-imaging of seismic anisotropy of Italy down to the deeper layers, allowing to better understand the dynamic of asthenospheric mantle flow and its relation with subducting plate, as well as the rheology of the continental lithosphere.

How to cite: Baccheschi, P., Confal, J. M., Pondrelli, S., Faccenda, M., VanderBeek, B. P., and Huang, Z.: High-resolution imaging of the deep structure of Italy through SKS anisotropy tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8129, https://doi.org/10.5194/egusphere-egu23-8129, 2023.

X2.216
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EGU23-8301
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ECS
Francesco Rappisi, Brandon Paul VanderBeek, and Manuele Faccenda

The Mediterranean region is an active plate margin characterized by the presence of both oceanic and continental lithosphere. Its tectonic history is marked by intense seismic and volcanic activity triggered by episodes of continental collision and slab rollback leading to the formation of mountain ranges and extensional basins. Our understanding of the structural heterogeneity and tectonic complexity of this region requires accurate imaging of the subsurface. Seismic anisotropy is a key parameter commonly used to study flow in the mantle and its relations with plate motions. In this study we present a three-dimensional anisotropic seismic tomography of the entire Mediterranean area performed using travel time from the new “Global Catalog of Calibrated Earthquake Locations” by Bergman et al. (2023). We present purely isotropic and anisotropic solutions. Compared to isotropic tomography, it is found that including the magnitude, azimuth, and, importantly, dip of seismic anisotropy in the inversions simplifies isotropic heterogeneity by reducing the magnitude of slow anomalies while yielding anisotropy patterns that are consistent with regional tectonics. The isotropic component of our preferred tomography model is dominated by numerous fast anomalies associated with retreating, stagnant, and detached slab segments. In contrast, relatively slower mantle structure is related to slab windows and the opening of back-arc basins. The anisotropic patterns reveal the deformation history of the area which has been characterized by intermittent phases of collision and tectonic relaxation. A diversity of dip angles is observed with near-horizontal and more steeply dipping fabrics found in different areas of the Entire Mediterranean, probably reflecting the entrainment effect of horizontal or vertical asthenospheric flows, respectively. We interpreted the high velocity zones of our best solution as subducting lithosphere and starting from this interpretation we built a 3D reconstruction of the main slabs found in the study region. To perform the tomography, we used the method proposed by Vanderbeek and Faccenda (2021) and already used by Rappisi et al. (2022) in a similar study on the Central Mediterranean area. This work returns the first anisotropic tomography of the entire Mediterranean and demonstrates the importance of seismic anisotropy to better constrain the upper mantle.

Bergman, E. A., Benz, H. M., Yeck, W. L., Karasözen, E., Engdahl, E. R., Ghods, A., ... & Earle, P. S. (2023). A Global Catalog of Calibrated Earthquake Locations. Seismological Society of America, 94(1), 485-495.

Rappisi, F., VanderBeek, B. P., Faccenda, M., Morelli, A., & Molinari, I. (2022). Slab Geometry and Upper Mantle Flow Patterns in the Central Mediterranean From 3D Anisotropic P‐Wave Tomography. Journal of Geophysical Research: Solid Earth, 127(5), e2021JB023488.

VanderBeek, B. P., & Faccenda, M. (2021). Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 225(3), 2097-2119.

How to cite: Rappisi, F., VanderBeek, B. P., and Faccenda, M.: Seismic anisotropy tomography: new insight into upper mantle structure and dynamics beneath the Mediterranean region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8301, https://doi.org/10.5194/egusphere-egu23-8301, 2023.

X2.217
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EGU23-8453
Zheng Luo, Othmar Müntener, György Hetényi, and Klaus Holliger

Most information on the P-wave seismic velocity of the lower continental is based on controlled-source wide-angle seismic experiments, for which the direction of wave propagation in the target region is largely horizontal. Following the common practice of interpreting such data in an isotropic framework, too high P-wave velocities would therefore be inferred in an anisotropic lower continental crust, for which the long axis of the anisotropy ellipsoid is roughly aligned with the horizontal direction. This, in turn, would result in a bias of the interpretation of the lower crustal bulk composition towards the mafic side and potentially also lead to incorrect estimations of crustal thickness. Anisotropy of this type can arise from the small-scale sub-horizontal alignment of anisotropic minerals and/or from large-scale sub-horizontal layering. To assess the likely importance of lower crustal anisotropy in general and the respective contributions of mineral fabric and layering in particular in Ivrea-type lower continental crust, we analyze a range of layered canonical models. The individual layers are parameterized based on published laboratory measurements of seismic velocities from pertinent rock samples. Our preliminary results indicate that anisotropy related to the mineralogical composition and fabric prevails over the corresponding effects of layering. For the considered canonical models, these effects are particularly prominent in the presence of metapelitic rocks, where a petrological interpretation of the inferred average horizontal P-wave velocity could indeed lead to a notable overestimation of the mafic component. Conversely, our initial results also indicate that in the absence of metapelitic rocks, the anisotropy-induced velocity bias may be sufficiently benign to allow for a reasonably reliable interpretation of the bulk composition of the lower continental crust based on the P-wave velocity inferred from wide-angle seismic data.

How to cite: Luo, Z., Müntener, O., Hetényi, G., and Holliger, K.: Effects of large-scale layering and small-scale mineral fabric on the seismic anisotropy of Ivrea-type lower continental crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8453, https://doi.org/10.5194/egusphere-egu23-8453, 2023.

X2.218
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EGU23-8563
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ECS
Rosalia Lo Bue, Manuele Faccenda, Ornella Cocina, Francesco Rappisi, and Brandon Paul Vanderbeek

 Characterized by persistent eruptive activity associated with a complex interaction between magma in its plumbing system and an articulated tectonic and geodynamic context, Mt. Etna (Sicily, Italy) is one of the most hazardous and monitored volcanoes in the world. Since the late 1990s, several seismic and tomographic experiments have been performed to obtain accurate images of the shallow-intermediate P-wave velocity structures of the volcano. Unfortunately, seismic tomography models, in particular those derived from body waves, typically relies on the approximation of seismic isotropy. This is a poor assumption considering that P-waves exhibit strong sensitivity to anisotropic fabrics and neglecting anisotropic heterogeneity can introduce significant velocity artefacts that may be misinterpreted as compositional and thermal heterogeneities (VanderBeek & Faccenda,2021; Lo Bue et al, 2022). Here, we discard the isotropic approximation and invert for P-wave isotropic (mean velocity) and anisotropic (magnitude of hexagonal anisotropy, azimuth and dip of the symmetry axis) parameters using the methodology proposed by VanderBeek & Faccenda (2021). We use active and passive seismic data collected by the TOMO-ETNA experiment (Ibanez et al. 2016a, b; Coltelli et al. 2016) between June and November 2014. We present 3D anisotropic P-wave tomography models of Etna volcano and compare them with purely isotropic images. Discriminating the anisotropic structures from the velocity artifacts allows to better recover the isotropic and anisotropic crustal structures and to improve our understanding on the major regional fault systems and on the processes that control magma and fluids ascent beneath the volcanic edifice.

 

Coltelli, M., Cavallaro, D., Firetto Carlino, M., Cocchi, L., Muccini, F., D'Aessandro, A., ... & Rapisarda, S. (2016). The marine activities performed within the TOMO-ETNA experiment. Annals of Geophysics.

Ibáñez, J. M., Prudencio, J., Díaz-Moreno, A., Patanè, D., Puglisi, G., Lühr, B. G., ... & Mazauric, V. (2016a). The TOMO-ETNA experiment: an imaging active campaign at Mt. Etna volcano. Context, main objectives, working-plans and involved research projects. Annals of Geophysics, 59(4), S0426-S0426.

Ibáñez, J. M., Díaz-Moreno, A., Prudencio, J., Patené, D., Zuccarello, L., Cocina, O., ... & Abramenkov, S. (2016b). TOMO-ETNA experiment at Etna volcano: activities on land. Annals of Geophysics, 59(4).

Lo Bue, R., Rappisi, F., Vanderbeek, B. P., & Faccenda, M. (2022). Tomographic Image Interpretation and Central-Western Mediterranean-Like Upper Mantle Dynamics From Coupled Seismological and Geodynamic Modeling Approach. Frontiers in Earth Science, 10, 884100.

VanderBeek, B. P., & Faccenda, M. (2021). Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 225(3), 2097-2119.

 

How to cite: Lo Bue, R., Faccenda, M., Cocina, O., Rappisi, F., and Vanderbeek, B. P.: Joint Active and Passive P-wave Tomography reveals Mt. Etna's Seismic Anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8563, https://doi.org/10.5194/egusphere-egu23-8563, 2023.

X2.219
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EGU23-9453
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ECS
Srivatsan Vedavyas, Menno Fraters, Magali Billen, and Yuval Boneh

Olivine is a major phase in the upper mantle and its crystallographic preferred orientation (CPO) carries strong implications for the interpretation of seismic anisotropy and geodynamic models of the upper mantle. The computational model D-Rex (Kaminski et al., 2004) is often used to depict the evolution of olivine CPO under various flow patterns. In its tracing of the crystallographic orientation of olivine and orthopyroxene aggregate D-Rex includes the process of dynamic recrystallization, a fundamental process associated with deformation under dislocation creep. Dynamic recrystallization and deformation mechanism are incorporated in D-Rex via different parameters - the efficiency of nucleation of new grains, grain boundary mobility, and the threshold value below which the grains deform by grain boundary sliding (GBS) (i.e., deformation does not result in rotation or recrystallization). These parameters were benchmarked with experiments to fit the overall CPO evolution. While D-Rex is set to predict the CPO, an appraisal of other pivotal microstructural properties like grain size, dislocation density, and recrystallization fraction has been neglected. Here, we use the implementation of D-Rex within ASPECT to model the shearing of grain to first trace the microstructural properties and further test how they are affected by the dynamic recrystallization parameters. We synthesize the results of tens of runs under a range of parameter space and strains. We observe that for grain size distribution, as the nucleation and threshold value for GBS increase the spread of grain sizes is relatively low while increasing mobility causes a large spread of grain size associated with a small group of grains that dominate the overall CPO. At low strains, the intermediate-sized grains are the major contributor to the CPO while with increasing strain, a smaller fraction of large grains dominate the CPO. Further, we find that the biggest grains keep growing bigger, while the smallest grains oscillate around the GBS threshold. Our analysis highlights the gap between the natural evolution of olivine microstructure and the microstructural properties evolution in D-Rex. The use and implications of different suggested parameters will be discussed. 

  Kaminski, É., Ribe, N. M., & Browaeys, J. T. (2004). D-Rex, a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle. Geophysical Journal International, 158(2), 744–752. https://doi.org/10.1111/j.1365-246X.2004.02308.x

How to cite: Vedavyas, S., Fraters, M., Billen, M., and Boneh, Y.: Appraisal of D-Rex parameterization in simulating olivine crystallographic preferred orientation (CPO) evolution using microstructural properties, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9453, https://doi.org/10.5194/egusphere-egu23-9453, 2023.

X2.220
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EGU23-10061
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ECS
|
Poulami Roy and Bernhard Steinberger

Seismic anisotropy is an observation that is believed to yield information on the flow pattern in the mantle. There are many studies of anisotropy in the upper mantle; however, the lower mantle is still underexplored, due to problems in seismic imaging and complexities of modelling of flow laws of
different minerals. In this study, we modelled the radially anisotropic behavior of two different geodynamic setups, one is the rising of a mantle plume from the core-mantle boundary to the surface, and another is subduction of a slab reaching the lowermost mantle. We use ASPECT for modelling large scale mantle flow and ECOMAN to simulate the development of lattice preferred orientation of mantle fabric. We use the slip system of Bridgmanite following the previous experimental study by Mainprice et al. (2008). We then couple the results from ASPECT to ECOMAN for modelling the radial anisotropy and maximum shear wave splitting direction. We show that in the part of the lowermost mantle surrounding the plume horizontally polarized shear waves (Vsh ) are faster than the vertically polarized ones (Vsv ) while the inside of the plume tail shows opposite signature. However, Vsh becomes greater than Vsv when the plume flattens out at the surface. We also find that the maximum splitting direction is horizontal outside the base of the plume and it becomes vertical inside the plume tail and again becomes horizontal at the surface. This result corroborates previous seismic observations (Wolf et al., 2019) of the Iceland plume at the core-mantle boundary. Moreover, our result for the slab setup reveals that as the slab reaches the lowermost mantle, Vsh becomes higher than Vsv and maximum splitting is horizontal at the base of the slab.

 

References
Wolf, J., Creasy, N., Pisconti, A., Long, M.D., Thomas, C., 2019. An investigation of seismic anisotropy in the lowermost mantle beneath Iceland. Geophys. J. Int. 219, S152–S166.


Mainprice, D., Tommasi, A., Ferré, D., Carrez, P., Cordier, P., 2008. Predicted glide system and crystal preferred orientations of polycrystalline silicate Mg-
perovskite at high-pressure: implicaitons for the seismic anisotropy in the lower mantle. Earth Planet. Sci. Lett. 271, 135–144.

How to cite: Roy, P. and Steinberger, B.: Modelling of seismic anisotropy in the lowermost mantle with rheologically constrained geodynamic setup, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10061, https://doi.org/10.5194/egusphere-egu23-10061, 2023.

X2.221
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EGU23-10928
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ECS
June Baek, Tae-Seob Kang, Dabeen Heo, Jin-Han Ree, Kwang-Hee Kim, Junkee Rhie, and YoungHee Kim

Shear wave splitting (SWS) is a widely used technique to study the anisotropic properties of the Earth’s interior. The geological structure of the southeastern Korean Peninsula is represented as the Yangsan fault and Ulsan fault, which is controlled by the present-day compressional stress regime in the ENE-WSW direction. We analyzed shear wave splitting to understand the anisotropic features of the upper crust above the hypocentral depth in the southeastern Korean Peninsula using the local earthquake data from the Gyeongju Hi-density Broadband Seismic Network (GHBSN). The GHBSN is a dense array composed of 200 broadband stations, which covers an area of about 60×60 km2 in the southeastern Korean Peninsula. We used the MFAST program (Savage et al., 2010) to measure the SWS parameters of fast polarization and delay time from shear waves of local earthquakes from January 2019 to December 2020. In addition, the TESSA program (Johnson et al., 2011) was employed to inspect the spatial variation in the anisotropy of the study region. To obtain reliable measurements of SWS parameters, rigorous constraints including quality control of the original waveforms were applied, and then, cycle-skipped measurements were manually removed. In final, we obtained the SWS measurements of 4260 records. Because the seismicity in the region is concentrated at the epicentral region of the 2016 Gyeongju earthquake sequence and the hanging wall of the Ulsan fault, raypaths are limited to a narrow azimuthal range. Both the raw and spatially averaged fast-polarization directions are dominant to be parallel either to major faults (structural anisotropy) or to the ENE-WSW (stress-induced anisotropy). Also, some stations and regions show bi- or multi-modal rose diagram of the SWS, representing that there is more than one factor of anisotropy to induce the SWS. The delay time of the SWS showed the right-skewed distribution. Tomographic result of the SWS delay time shows that, relatively high anisotropy is observed at the epicentral region of the 2016 Gyeongju earthquake sequence and the hanging wall of the Ulsan fault. It implies that microcracks at these regions are better developed compared to the remaining regions.

How to cite: Baek, J., Kang, T.-S., Heo, D., Ree, J.-H., Kim, K.-H., Rhie, J., and Kim, Y.: Upper crustal anisotropy in the southeastern Korean Peninsula from shear wave splitting of local earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10928, https://doi.org/10.5194/egusphere-egu23-10928, 2023.

X2.222
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EGU23-15722
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ECS
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Yijun Wang, Agnes Kiraly, Clinton Conrad, Lars Hansen, and Menno Fraters

Lattice preferred orientation (LPO) of olivine crystals occurs due to deformation in the mantle. Different parts of the upper mantle can undergo a large variety of deformation paths. During simple processes, such as simple shearing below oceans due to the movement of tectonic plates, the LPO will reflect the direction of the movement of tectonic plates. On the other hand, in areas, such as around subduction zones, the mantle undergoes more complex deformation paths, resulting in a less easily predictable LPO. Seismic anisotropy has been used as a proxy for mantle flows and the LPO formed in the mantle. To interpret the seismic anisotropy observations more accurately, we need to understand how LPO forms in different regions of subduction.

LPO has been implemented in many numerical modelling tools to predict seismic anisotropy, which places constraints on mantle dynamics. However, a few recent studies have linked olivine texture development to viscous anisotropy, resulted from the summed effect of individual crystals that are deforming anisotropically. Anisotropic viscosity can affect deformation and in turn the resulting LPO. To study the effect of anisotropic viscosity (AV) and LPO evolution in geodynamics processes, it is important to know the role of AV on LPO and the differences between the numerical methods that calculate them.

We choose three methods of olivine texture development to examine in this study. D-Rex is a polycrystal LPO model that is relatively balanced in computational efficiency and accuracy. From previous studies, D-Rex has been shown to produce faster texture development and stronger texture compared to other methods, including our second choice, the modified director method (MDM). The MDM parameterizes the olivine LPO formation as relative rotation rates along the slip systems that participate in the rotation of olivine grains due to finite deformation. We also couple the MDM with a micromechanical model for olivine AV (which makes our third choice MDM+AV), to allow the anisotropic texture to modify the viscosity and in turn affect the formation of LPO.

Here we compare the LPO evolution under subduction settings with a slowly advancing trench and a retreating trench, with and without the effect of AV. Since the mantle flow pattern in subduction zones is not homogeneous, different particles experience a variety of deformation paths. We place 60 particles in each subduction model around the slab to track the deformation and resulting olivine texture. We compute olivine texture using the above-mentioned three different methods (D-Rex, MDM, MDM+AV). With the particles, we can identify characteristic textures developed in key regions such as the mantle wedge, sub-slab area, and lateral slab edge. We then run a statistical analysis on the texture parameter and anisotropic properties of the particles from both retreating and advancing subduction models, to study where anisotropic viscosity has the largest effect on the mantle flow. We expect AV to have a larger effect in a retreating slab setting since the mantle flows feeding material to the sub-slab region is more intensive.

How to cite: Wang, Y., Kiraly, A., Conrad, C., Hansen, L., and Fraters, M.: Effect of olivine anisotropic viscosity in advancing and retreating subduction settings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15722, https://doi.org/10.5194/egusphere-egu23-15722, 2023.