Periodic Reporting for period 4 - ATUNE (Attenuation Tomography Using Novel observations of Earth's free oscillations)
Okres sprawozdawczy: 2020-12-01 do 2022-05-31
Tectonic processes at Earth's surface, like volcanic eruptions and earthquakes,are driven by convection deep in Earth's mantle. Seismic tomography using earthquakes is the main tool to directly image the Earth's mantle by making pictures which are very similar to an MRI scan of our brain. Such images show regions with slow and fast seismic velocity anomalies, including two large continental size regions just above the core mantle boundary, one located under the Pacifc and the other one under Africa (the so called 'LLSVPs' or Large Low Shear Velocity Provinces). These two regions have low shear wave velocity, but their role in mantle convection as either a thermal plume or a stable compositional pile had been heavily debated.The problem was that most models only studied seismic velocity, which is insufficient to obtain robust estimates of temperature and composition, and make direct links with mantle convection.
The main innovation of ATUNE was to add seismic attenuation models to the velocity models as extra information. Seismic attenuation, or loss of energy, is key to mapping partial melt, water and temperature variations, and interpret the images in terms of mantle convection. Unfortunately, attenuation has only been imaged using short- and intermediate-period seismic data, showing little similarity even for the upper mantle and no reliable lower mantle models exist. In ATUNE, we developed novel full-spectrum techniques and applied them to Earth’s long period free oscillations to observe global-scale regional variations in seismic attenuation from the lithosphere to the core mantle boundary. Whole Earth oscillations occur after large earthquakes and make the Earth `ring like a bell'. Studying Earth's free oscillations is similar to listening to the tones of a musical instrument. By investigating how much the tones of the Earth are `out of tune', we were able to determine the 3D variations in velocity, density and attenuation in our planet. ATUNE's overall delivery is the first ever full-waveform global tomographic model of 3D attenuation variations in the lower mantle, providing essential constraints temperature and grain size and helping us in understanding the complex dynamics of our planet. We found that the LLSVPs are weakly attenuating and that the surrounding slab graveyards are strongly attenuating and attribute this to the LLSVPs having a much larger grain size which makes them stable. This observation is also in agreement with the base of the LLSVPs being dense, which also makes them more stable.
The main innovation of ATUNE was to add seismic attenuation models to the velocity models as extra information. Seismic attenuation, or loss of energy, is key to mapping partial melt, water and temperature variations, and interpret the images in terms of mantle convection. Unfortunately, attenuation has only been imaged using short- and intermediate-period seismic data, showing little similarity even for the upper mantle and no reliable lower mantle models exist. In ATUNE, we developed novel full-spectrum techniques and applied them to Earth’s long period free oscillations to observe global-scale regional variations in seismic attenuation from the lithosphere to the core mantle boundary. Whole Earth oscillations occur after large earthquakes and make the Earth `ring like a bell'. Studying Earth's free oscillations is similar to listening to the tones of a musical instrument. By investigating how much the tones of the Earth are `out of tune', we were able to determine the 3D variations in velocity, density and attenuation in our planet. ATUNE's overall delivery is the first ever full-waveform global tomographic model of 3D attenuation variations in the lower mantle, providing essential constraints temperature and grain size and helping us in understanding the complex dynamics of our planet. We found that the LLSVPs are weakly attenuating and that the surrounding slab graveyards are strongly attenuating and attribute this to the LLSVPs having a much larger grain size which makes them stable. This observation is also in agreement with the base of the LLSVPs being dense, which also makes them more stable.
Objective 1: Method development
We first developed new methods to measure attenuation and anisotropy using whole Earth oscillations. We studied how much cross-coupling would be needed (Akbarashrafi et al, 2018) and developed a direct spectrum one-step inversion method (Jagt & Deuss, 2021). We also developed new software to collect the data that we needed for our measurements and modeling (Schneider et al, 2022) and new Monte Carlo techniques to use a trans dimensional approach for inner core structure (Brett et al, 2022) and Hamiltonian inversion for mantle density structure (van Tent et al, in prep, 2022).
Objective 2: Data measurement and model development
We then proceeded to make the actual measurements of attenuation (Talavera-Soza & Deuss, in review Geophysical Journal International, 2022) and make a model of 3D velocity and density Koelemeijer et al, Nature Communications, 2017, van Tent et al, in prep, 2022) and 3D attenuation variations in the earth's mantle (Talavera-Soza & Deuss, in review Nature, 2022). We also found that radial modes provide us with new constraints in 1D attenuation but also, quite unexpectedly, on the radial anisotropy structure of the inner core (Talavera-Soza & Deuss, 2020, 2021). The resonance between spheroidal and toroidal modes shows evidence for the existence of strong azimuthal anisotropy in the upper mantle, which will help us in the future to constrain mantle flow (Schneider & Deuss, 2021). because the research on the mantle was going so well, we also extended ATUNE to study outer core structure and its stratification (van Tent et al, 2020) and made new observations of ultra polar body wave paths traveling the Earth's inner core (Brett & Deuss, 2020) and used these two make a new 3D topographic model of the inner core (Brett et al, 2022). We also collaborated in an international research effort to make database of long period multimode surface wave observations and contributed our new data measurements (Moulik et al. 2021).
Objective 3: Model interpretation
Finally, we collaborated with mineral physicists to interpret our 3D mantle attenuation model (Talavera-Soza et al, Nature, in review, 2022) and also collaborated with an expert in paleomagnetism to interpret our 3D inner core anisotropy model and link the different layers to changes in magnetic field reversal rates in the past (de Jong et al, in prep, 2022). We also collaborated with mantle convection modelers in an attempt to constrain the presence of post-perovskite in the lowermost mantle using our 3D mantle models (Koelemeijer et al, 2018).
All our results discussed above have been presented at international conferences, including:
American Geophysical Union Fall meeting in December 2016, 2017, 2018, 2019, 2020, 2021
European Geosciences Union meeting in Vienna in 2016, 2019, 2021, 2022
Studies of Earth's deep Interior meeting (SEDI) in Edmonton, Canada in 2018
Studies of Earth's deep Interior meeting (SEDI) in Zurich, Switzerland in 2022
Gordon Research Conference 'Interior of the Earth', Mount Holyoke, June 2017
The PI has also given a number of invited talk about ATUNE and its results:
CIDER Community workshop, Point Reyes, US, 2016
4D Earth European Space Agency meetings, Noordwijk, The Netherlands, 2017
Inaugural Lecture, Utrecht University, The Netherlands, 2019
ESA 4D deep Earth meeting, online, 2021
American Chemical Society, online, 2022
We first developed new methods to measure attenuation and anisotropy using whole Earth oscillations. We studied how much cross-coupling would be needed (Akbarashrafi et al, 2018) and developed a direct spectrum one-step inversion method (Jagt & Deuss, 2021). We also developed new software to collect the data that we needed for our measurements and modeling (Schneider et al, 2022) and new Monte Carlo techniques to use a trans dimensional approach for inner core structure (Brett et al, 2022) and Hamiltonian inversion for mantle density structure (van Tent et al, in prep, 2022).
Objective 2: Data measurement and model development
We then proceeded to make the actual measurements of attenuation (Talavera-Soza & Deuss, in review Geophysical Journal International, 2022) and make a model of 3D velocity and density Koelemeijer et al, Nature Communications, 2017, van Tent et al, in prep, 2022) and 3D attenuation variations in the earth's mantle (Talavera-Soza & Deuss, in review Nature, 2022). We also found that radial modes provide us with new constraints in 1D attenuation but also, quite unexpectedly, on the radial anisotropy structure of the inner core (Talavera-Soza & Deuss, 2020, 2021). The resonance between spheroidal and toroidal modes shows evidence for the existence of strong azimuthal anisotropy in the upper mantle, which will help us in the future to constrain mantle flow (Schneider & Deuss, 2021). because the research on the mantle was going so well, we also extended ATUNE to study outer core structure and its stratification (van Tent et al, 2020) and made new observations of ultra polar body wave paths traveling the Earth's inner core (Brett & Deuss, 2020) and used these two make a new 3D topographic model of the inner core (Brett et al, 2022). We also collaborated in an international research effort to make database of long period multimode surface wave observations and contributed our new data measurements (Moulik et al. 2021).
Objective 3: Model interpretation
Finally, we collaborated with mineral physicists to interpret our 3D mantle attenuation model (Talavera-Soza et al, Nature, in review, 2022) and also collaborated with an expert in paleomagnetism to interpret our 3D inner core anisotropy model and link the different layers to changes in magnetic field reversal rates in the past (de Jong et al, in prep, 2022). We also collaborated with mantle convection modelers in an attempt to constrain the presence of post-perovskite in the lowermost mantle using our 3D mantle models (Koelemeijer et al, 2018).
All our results discussed above have been presented at international conferences, including:
American Geophysical Union Fall meeting in December 2016, 2017, 2018, 2019, 2020, 2021
European Geosciences Union meeting in Vienna in 2016, 2019, 2021, 2022
Studies of Earth's deep Interior meeting (SEDI) in Edmonton, Canada in 2018
Studies of Earth's deep Interior meeting (SEDI) in Zurich, Switzerland in 2022
Gordon Research Conference 'Interior of the Earth', Mount Holyoke, June 2017
The PI has also given a number of invited talk about ATUNE and its results:
CIDER Community workshop, Point Reyes, US, 2016
4D Earth European Space Agency meetings, Noordwijk, The Netherlands, 2017
Inaugural Lecture, Utrecht University, The Netherlands, 2019
ESA 4D deep Earth meeting, online, 2021
American Chemical Society, online, 2022
We made normal mode measurements that have not been made before of inelastic splitting functions for the Earth's mantle and used these to make the first global lower mantle model of 3D variations in attenuation using whole Earth oscillations.