The Big Chime

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Mountains, volcanoes, deep-sea trenches: many structures on the Earth's surface are formed by processes in its interior.

Hot interior: remnants from the time of the Earth's formation

The early Earth was initially a ball of hot matter. Even though the surface of the Earth has cooled down to the point where it is solid, there is still a lot of heat left over from the time of its formation: the temperature at the center of the Earth is more than 6000 degrees Celsius - which is about as hot as the surface of our sun.

This heat drives convection currents in the Earth's interior and is dissipated into space at its surface. In the liquid iron of the Earth's core, these convection currents create a magnetic field, the so-called geodynamo. In Earth's mantle, huge rolls of mantle rock move unimaginably slowly: the material takes around a hundred million years to travel from the core-mantle boundary at a depth of around 3,000 km to the Earth's surface.

Lower mantle - direct influence on the geodynamo?

We know from geophysical measurements that there are very hot, but also colder, regions in the Earth's lower mantle. These different temperatures dictate where more and where less heat can be dissipated from the Earth's outer core. As a result, the lower mantle influences the convection in the outer core and thus our geodynamo: Depending on the position of the colder zones, the lower mantle can have both a stabilising and a destabilising effect on Earth's magnetic field. In fact, the periods of mantle convection and of the strong changes in the frequency of Earth's magnetic field reversal are similar. Can we prove this connection? Can we verify whether the temperature distribution immediately above the core-mantle boundary leads to profound changes in the properties of the geodynamo in the core?

Seismological measurements give us a picture of Earth's mantle today. But just as a meteorologist cannot draw conclusions about the climate of several decades from today's weather, the current state of the Earth's mantle does not allow us to draw conclusions about the convection of the last million years: we simply cannot observe the movements in the Earth's mantle directly, as they occur far too slowly.

Making the invisible visible

However, with the help of computer simulations, we are able to recreate the temporal evolution of the deep mantle. The starting point for these geodynamic models is a lot of data, both about the composition of the deep earth and about the plate motions on Earth's surface over the last few hundred million years. With the help of high-performance computers, models of mantle convection are calculated, which are then checked against independent seismological and geological data to see whether they are plausible.

Normal modes: Strong earthquakes are like chimes

For her doctoral thesis, Anna is testing mantle convection models using seismic data. She uses recordings of particularly strong earthquakes, such as the Tōhoku earthquake, which led to the tsunami near Fukushima in 2011. The 1994 Bolivian earthquake, the second-strongest earthquake ever recorded at great depths, and other very large earthquakes over the past 30 years are also included in her analysis. After such strong earthquakes, the earth continues to vibrate for days - like a bell that can be heard long after it has been rung. Just like the bell, the physical properties of the Earth determine its sound.

For Anna, concentrating on these so-called free oscillations of the Earth has a decisive advantage: normal modes are standing waves that span the entire body of the Earth. They therefore contain information about the entire planet, so that after an earthquake, statements can be made about the entire planet with just one recording. This allows Anna to avoid one disadvantage of classical seismology: having blind spots, as roughly two thirds of the Earth is covered by water and difficult to reach for setting up seismographs. In addition, the Earth most frequently quakes around the Pacific, while other places are almost never the source of an earthquake.

Natural oscillations have very long wavelengths: for the simplest oscillations, about half an hour passes from one full deflection to the next. As a result, the spatial resolution when evaluating free oscillations is not very high. However, this does not hinder Anna's work, as she is only interested in large-scale mantle structures.

Presentation of the first results at EGU 2024 in Vienna

At the EGU conference in Vienna, Anna showed in her presentation that the Earth's free oscillations after strong earthquakes can indeed be used to test mantle convection models. By making targeted geometric changes to a model, such as rotating the structures at depth, and then calculating synthetic normal mode data, she was able to show that the selected seismic data are suitable for evaluating the Earth-like nature of mantle convection models. However, since the data from different earthquakes did not yet allow for conclusively favouring a certain model, she is now expanding her evaluations from six to 70 strong earthquakes in order to be able to make generally valid statements.

Abstract within in the proceedings

Exploring the potential of normal mode seismology for the assessment of geodynamic hypotheses
Anna Schneider¹, Bernhard Schuberth¹, Paula Koelemeijer², Grace Shephard³, and David Al-Attar⁴

• 1: Ludwig-Maximilian University of Munich, Munich, Germany
• 2: University of Oxford, Oxford, United Kingdom
• 3: University of Oslo, Oslo, Norway
• 4: University of Cambridge, Cambridge, United Kingdom

Fluid dynamics simulations are a powerful tool for understanding processes in the Earth's deep interior. Mantle circulation models (MCMs), for example, provide important insight into the present-day structure of the mantle and its thermodynamic state when coupled with mineralogical models, which is essential information for other fields in the geosciences. The evolution of the heat flux through the core-mantle boundary, for instance, is a prerequisite for geodynamo simulations that aim to model the reversal frequency pattern of the Earth's magnetic field on geologic time scales. However, geodynamical modelling requires extensive knowledge of deep Earth properties and plate motions over time. Uncertainties in these model inputs propagate into the MCMs, which subsequently have to be evaluated with independent data, such as the seismological or geological record. Although state-of-the-art MCMs typically explain statistical properties of seismological data, they do not consistently reproduce the location of features in the mantle.

In this contribution, we explore the effect of varying the absolute position of mantle structure on seismic data by applying first-order modifications to an initial MCM. Normal mode data are particularly well suited for assessing the resulting changes in the location of mantle structure, as they capture its long-wavelength component throughout the entire mantle. In addition, the global sensitivity of normal modes reduces the drawbacks of uneven data coverage. Specifically, we use two different seismic forward modelling approaches, an iterative direct solution method for computing full-coupling spectra and a splitting function calculation that is based on the self-coupling approximation. Our goal is to quantify the effects of a limited number of large-magnitude earthquakes, the adequacy of the self-coupling approximation, and the resolvability of relevant model differences through a comprehensive data analysis. Our synthetic forward modelling framework is moreover well suited for testing the depth sensitivity associated with specific frequency intervals in the spectrum that generally is inferred from seismic 1-D profiles within the splitting function approximation.

How to cite: Schneider, A., Schuberth, B., Koelemeijer, P., Shephard, G., and Al-Attar, D.: Exploring the potential of normal mode seismology for the assessment of geodynamic hypotheses, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10128, https://doi.org/10.5194/egusphere-egu24-10128, 2024.

https://meetingorganizer.copernicus.org/EGU24/EGU24-10128.html

Further information

Here you can find more information about the project Anna is working in.