I am fascinated, with designing and using innovative "inverted telescopes" to explore Earth and planetary interiors...
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Exploring Planetary Interiors on a global scale using only a single or limited instruments
How can a single instrument be sufficient for constraining planetary interiors on a global scale? This problem is critical as instrument deployments on planetary surfaces are restricted by the very high cost and challenge, and it is unlikely this will change in this century, while our generations cannot wait for planetary exploration!
We find out that a single seismograph and global-scale waveform cross-correlations between seismic events (inter-source correlations) can be used to globally constrain planetary interiors.
As shown in the right diagram, with a single planetary station (red cone) and two seismic sources (blue and white balls), the seismic waves (represented by coloured seismic rays) reverberted from the core-mantle-boundary and the free surface can be cross correlated to form a prominent feature (inter-source correlation feature). The emergence, timing, and time-distance dependency, etc., of the features are apparently sensitive to planetary internal structures. Even with a ray path simplification, a sensitivity kernel can be constructed to relate inter-source correlation observations to the structures.
Applying this principle, we showcase constraining the existence and size of the cores of Earth and Mars using a single seismograph, and we confirm a large Martian core. Please note, such detection are on a global scale, other than for a local patch of core-mantle-boundary.
This provides an effective way to investigate the structure of planetary interiors with currently realizable resources, and we eagerly anticipate harnessing its great potential in studying lunar interior during upcoming missions to the Moon.
Related papers: Wang, S., & Tkalčić, H. (2022). Scanning for planetary cores with single-receiver intersource correlations. Nature Astronomy, 6(11), 1272-1279. https://doi.org/10.1038/s41550-022-01796-8 pdf
Cross Correlation Interferometry Between Seismic Sources
Why do the observed global inter-source cross-correlograms contradict theoretical expectations, specifically, by not exhibiting any theoretically expected features, as shown in panel (a)? We have successfully resolved this problem and, furthermore, have developed an efficient and robust routine to construct global inter-source correlogram via the reciprocity principle and a rigorous selection of sources (panel b).
Significantly, our study revealed the significance of the source term (e.g., source mechanisms, source locations, etc), which was largely ignored in existing cross correlation interferometry studies. This also explains the failure, the contradiction between observations and theoretical expectations, in previously existing attempts.
Built-upon this, the inter-source correlations may potentially initiate a new “wave” in interferometry and correlation seismology, similar to what happened with the inter-receiver correlations about two decades ago.
Related publication: Wang, S., & Tkalčić, H. (2023). On the Formation of Global Inter‐Source Correlations and Applications to Constrain the Interiors of the Earth and Other Terrestrial Planets. Journal of Geophysical Research: Solid Earth, 128(8), e2023JB027236. Open Access
Global Coda Correlation Tomography on Planetary and Earth Interiors
How to use a kind of noise and seemingly chaotic waves in seismic tomography: waves that reverberated multiple times after being excited by an earthquake, also known as late coda? To address this problem, we choose late coda correlation, as an effective tool, to tomographically constrain the Earth's interior. We first identified that a correlation feature can be formed by a cross-term between two deeply reverberated body waves, and stacking multiple body-wave cross-terms can enhance the signal-to-noise ratio. Then, based on the formation mechanism, we derived the tomographic relationship between a correlation feature and the Earth's internal structure, which also depends on source terms.
Furthermore, quantitative analyses of coda-correlation observations based on the principle pinpointed a direction-dependent inner-core shear-wave speed, ~5 s faster in directions oblique to the Earth’s rotation axis than directions parallel to the equatorial plane. Our analyses demonstrat that the simplest explanation is the shear-wave anisotropy in the Earth's inner core (IC) with a minimum strength of ∼0.8%, formed through the lattice-preferred-orientation mechanism of iron. The new observations rule out one of the body-centered-cubic iron models in the IC, while we cannot uniquely determine its stable iron phase,
Related publication:
Wang, S., & Tkalčić, H. (2021). Shear‐Wave Anisotropy in the Earth's Inner Core. Geophysical Research Letters, 48(19), e2021GL094784. Open Access
Tkalčić, H., Phạm, T. S., & Wang, S. (2020). The Earth's coda correlation wavefield: Rise of the new paradigm and recent advances. Earth-Science Reviews, 208, 103285. https://doi.org/10.1016/j.earscirev.2020.103285
Wang, S., & Tkalčić, H. (2020). Seismic event coda-correlation's formation: implications for global seismology. Geophysical Journal International, 222(2), 1283-1294. Open Access
Wang, S.., & Tkalčić, H. (2020). Seismic event coda‐correlation: Toward global coda‐correlation tomography. Journal of Geophysical Research: Solid Earth, 125(4), e2019JB018848. Open Access
Exploring Secrets Beneath the Macquarie Ridge Complex with a Submarine Seismic Observatory
The Australian Macquarie archipelago surmounts the Macquarie Ridge Complex (MRC) that constitutes the boundary between Indo-Australian and Pacific plates in the southwest Pacific Ocean. The Macquarie Island features its unique geological characteristics: globally it is the only ‘normal’ oceanic crust above sea level in the ocean basin in which it formed.
Yet beneath the MRC lies a ‘factory’ for the world’s most powerful submarine earthquakes not associated with ongoing subduction (e.g., May 23, 1989, Mw=8.2). Why do they occur, and what is the physical condition facilitating the earthquakes? What is the significance of proximal intraplate earthquakes and is subduction initiating in this region? How do the dynamically changing plate boundary boundaries evolve?
To answer these questions, we designed a passive seismic experiment in the MRC. We deployed a network of ocean bottom seismometers (OBSs) in 2020-2021 (15 successfully recovered). We aim to bring together a comprehensive arsenal of seismic imaging and source detection/location techniques to understand the nature of the central MRC and its associated seismicity.
Related publication: coming soon...
