Statements of Past and Recent Research

As a global seismologist, I use seismic waves to understand the Earth’s interior structures and seismic sources using mathematical tools, such as signal processing, numerical modeling, and geophysical inference. I have been interested in diverse research topics in seismology, including structures and processes a few kilometers beneath the surface, such as polar ice sheets and volcanic or nuclear explosions down to the Earth’s deep interior, including the cores. To date, one of my most visible contributions is to help understand better the architecture of the seismic wavefield several hours after large earthquakes and use it to decipher several long-lasting puzzles regarding the Earth’s inner core. With this work, I was awarded the prestigious Zatman lectureship by the Study of Earth’s Deep Interior (SEDI) Committee in the 2024 biannual meeting held in Great Barrington (MA). In current and near-future research, I aim to expand my seismological toolbox, particularly towards better-utilizing machine learning tools, to advance research on Earth’s deep interiors and the Antarctic ice sheets. 

New method to image shallow discontinuities: teleseismic P-wave coda autocorrelation

Using digital seismic waveforms to characterize shallow structures beneath a seismic station, approximated as stratified layers, is a classic problem in passive seismology. During my early years of Ph.D., I developed a new single-station method, namely the teleseismic P-wave coda autocorrelation (Phạm & Tkalčić 2017), that manifests the reverberation of the steep arrivals from distant earthquakes interacting with sharp interfaces beneath a seismic receiver. As highlighted by the JGR editor (link), the method is a welcome methodological contribution with clear advantages to existing methods, such as ambient noise autocorrelation and receiver function methods. The novel earthquake-based autocorrelation method has been applied to imaging the polar ice caps (Phạm & Tkalčić 2018, 2021a), sedimentary basins (Wang et al. 2020), and crustal structures (Phạm & Tkalčić 2017).

Earth’s coda coda-correlation wavefield as a new tool for studying the deep Earth

Despite the ever-expansion of the global seismograph network and modern computational capacity, studying the Earth’s deep interiors remains at the discovery stage. That is because of the inherent limitation in a volumetric sampling of the existing seismological probes. On the one hand, travel times and amplitudes of PKIKP waves have been the primary short-period tools used to obtain inferences on spatially distributed properties, such as in the 3D mantle model. However, the sampling coverage of body ray paths is inherently limited due to the confinement of large subduction-zone earthquakes in the quasi-equatorial belt and the limited seismic deployments in the oceans and remote areas. On the other hand, normal modes have limited lateral and radial resolution because of their long-period nature, and their sensitivity approaches zero in the Earth’s center.

On the way forward, my Ph.D. thesis contributed breakthroughs in theory and methodology to the rise of a new paradigm in deep Earth seismology: global coda-correlation wavefield (Kennett & Phạm 2018; Phạm et al. 2018; Tkalčić et al. 2020). Large earthquakes can illuminate the Earth’s entire volume with reverberations lasting hours after the origin time. The record of this reverberating energy, known to semiologists as earthquake coda, had not been effectively used to study the Earth’s deep interior. In the correlation wavefield, the similarity between weak seismic signals is manifested by cross-correlating the digital records from the global seismic network for new insights into the Earth’s interior. This line of work has stirred two new PhD projects and has underpinned several discoveries regarding the Earth's (see details below) and other planets’ deep structures (Wang & Tkalčić, 2022, Nat. Astro.).

Discoveries of the Earth’s inner core: J-waves and innermost inner core

The inner core (IC), which accounts for less than 1% of the Earth’s volume, is a time capsule of the Earth’s history. As the IC grows, latent heat and light elements released by the solidification process drive the convection of the liquid outer core, which, in turn, maintains the geodynamo and is currently responsible for generating the geomagnetic field. Although the geomagnetic field might have preceded the IC’s birth, detectable changes in its structures with depth could signify shifts in its operation, which could have profoundly influenced the Earth’s evolution and ecosystem. Therefore, probing the solidity hinting about the solidification process and internal boundaries within the IC hinting possible change in the solidification regime is critical to further understanding the IC’s chemical compositions and physical states as well as Earth’s evolution in the distant past.

With the development of the coda-correlation wavefield, I contributed to several discoveries regarding the Earth’s inner core. In a Science article (Tkalčić & Phạm 2018), we reported robust detection of shear waves (i.e., J-waves) transversing the Earth’s IC, regarded as the Holy Grail in global seismology. There has been elusive evidence for the IC’s solidity since it was discovered 80 years ago (Tkalčić et al. 2022), as it is so feeble and often lies under the observational threshold in raw seismic records. With recent advances in understanding the correlation wavefield, the estimate of shear wave speeds has recently been refined in a Nature Communications publication (Costa de Lima et al. 2023) with a new, crystal-clear observation, which is exclusively sensitive to the shear wave speeds in the Earth’s inner core.

In an independent attempt, we reported unprecedented observations of compressional waves ricocheting along the Earth’s diameters multiple times in a Nature Communications article (Phạm & Tkalčić 2023). Due to their particular sensitivity to the Earth’s very centers, they provide novel evidence to strengthen the existence of the Earth’s innermost inner core (Tkalčić et al. 2023), a hypothesis proposed more than 20 years ago. The article was a welcome promotion for deep Earth seismology, as it was featured by an array of international media outlets, including the front page of the New York Times. It gained an Altmetric score of 1884 and was included in Nature Communications’ Top 25 Earth, Environmental, and Planetary Sciences Articles of 2023.

Forefront source inversion research: Embracing Earth’s model uncertainty

Understanding the force representation of a seismic source is critical to deciphering its nature, which can be of tectonic or volcanic origins or artificial explosions. The main practical challenge is the incomplete knowledge of Earth’s structures to explain seismological records adequately. I contributed new methods to incorporate the Earth’s model uncertainty in the mathematical solution of source mechanics (Phạm & Tkalčić 2021b; Phạm 2024). The technique has been deployed to study various seismic sources near the earth’s surface, such as volcanic earthquakes in Long Valley Caldera (Phạm & Tkalčić 2021b), nuclear tests in the Democratic People of Republic Korea (Hu et al. 2023a), and the 2022 climatic eruption at the Hunga volcano (Hu et al. 2023b; Henley et al. 2024).

Deep learning for deep Earth seismology: Exploring under-utilized seismic archives

Machine learning has been a ubiquitous tool for boosting productivity in various aspects of modern life, and it has made its way into geoscientific research with profound impacts. Despite its exploding use in characterizing micro seismicity at shallow depths, using such tools in seismology to study the Earth’s interior on a global scale has been limited, probably due to the relatively small community size. We are leading the effort to develop an automatic phase onset picker dedicated to PKIKP waves (Zhou et al. 2024), compressional waves transversing the Earth’s entire diameter. It could unleash the massive potential of existing seismic archives to shed light on unanswered questions about the Earth’s deep interior, including its solid inner core.

Bibliography

Costa de Lima, T., Phạm, T.-S., Ma, X. & Tkalčić, H., 2023. An estimate of absolute shear-wave speed in the Earth’s inner core. Nat. Commun., 14, 1–10, Nature Publishing Group. doi:10.1038/s41467-023-40307-9

Henley, R.W., Ronde, C.E.J. de, Arculus, R.J., Hughes, G., Pham, T.-S., Casas, A.S., Titov, V., et al., 2024. The 15 January 2022 Hunga (Tonga) eruption: A gas-driven climactic explosion. J. Volcanol. Geotherm. Res., 451, 108077. doi:10.1016/j.jvolgeores.2024.108077

Hu, J., Phạm, T.-S. & Tkalčić, H., 2023. Seismic moment tensor inversion with theory errors from 2-D Earth structure: implications for the 2009–2017 DPRK nuclear blasts. Geophys. J. Int., 235, 2035–2054. doi:10.1093/gji/ggad348

Hu, J., Phạm, T.-S. & Tkalčić, H., 2024. A Composite Seismic Source Model for the First Major Event During the 2022 Hunga (Tonga) Volcanic Eruption. Geophys. Res. Lett., 51, e2024GL109442. doi:10.1029/2024GL109442

Kennett, B.L.N. & Phạm, T.-S., 2018. The nature of Earth’s correlation wavefield: late coda of large earthquakes. Proc R Soc A, 474, 20180082. doi:10.1098/rspa.2018.0082

Phạm, T.-S., 2024. Gradient-based joint inversion of point-source moment-tensor and station-specific time shifts. Geophys. J. Int., accepted for publication. doi:10.22541/essoar.169230267.77488140/v1

Phạm, T.-S. & Tkalčić, H., 2017. On the feasibility and use of teleseismic P wave coda autocorrelation for mapping shallow seismic discontinuities. J. Geophys. Res. Solid Earth, 122, 3776–3791. doi:10.1002/2017JB013975

Phạm, T.-S. & Tkalčić, H., 2018. Antarctic Ice Properties Revealed From Teleseismic P Wave Coda Autocorrelation. J. Geophys. Res. Solid Earth, 123, 7896–7912. doi:10.1029/2018JB016115

Phạm, T.-S. & Tkalčić, H., 2021. Constraining Floating Ice Shelf Structures by Spectral Response of Teleseismic P-Wave Coda: Ross Ice Shelf, Antarctica. J. Geophys. Res. Solid Earth, 126, 2020JB021082. doi:https://doi.org/10.1029/2020JB021082

Phạm, T.-S. & Tkalčić, H., 2021. Toward Improving Point-Source Moment-Tensor Inference by Incorporating 1D Earth Model’s Uncertainty: Implications for the Long Valley Caldera Earthquakes. J. Geophys. Res. Solid Earth, 126, 2021JB022477. doi:10.1029/2021JB022477

Phạm, T.-S. & Tkalčić, H., 2023. Up-to-fivefold reverberating waves through the Earth’s center and distinctly anisotropic innermost inner core. Nat. Commun., 14, 754, Nature Publishing Group. doi:10.1038/s41467-023-36074-2

Phạm, T.-S., Tkalčić, H., Sambridge, M. & Kennett, B.L.N., 2018. Earth’s Correlation Wavefield: Late Coda Correlation. Geophys. Res. Lett., 45, 3035–3042. doi:10.1002/2018GL077244

Tkalčić, H., Costa De Lima, T.P., Phạm, T. & Tanaka, S., 2023. Inner Core Anisotropy from Antipodal PKIKP Traveltimes. in Geophysical Monograph Series, 1st ed., pp. 165–189, eds. Nakagawa, T., Tsuchiya, T., Satish‐Kumar, M. & Helffrich, G., Wiley. doi:10.1002/9781119526919.ch10

Tkalčić, H. & Phạm, T.-S., 2018. Shear properties of Earth’s inner core constrained by a detection of J waves in global correlation wavefield. Science, 362, 329–332. doi:10.1126/science.aau7649

Tkalčić, H., Phạm, T.-S. & Wang, S., 2020. The Earth’s coda correlation wavefield: Rise of the new paradigm and recent advances. Earth-Sci. Rev., 208, 103285. doi:10.1016/j.earscirev.2020.103285

Tkalčić, H., Wang, S. & Phạm, T.-S., 2022. Shear Properties of Earth’s Inner Core. Annu. Rev. Earth Planet. Sci., 50, 153–181. doi:10.1146/annurev-earth-071521-063942

Wang, C., Tauzin, B., Pham, T.-S. & Tkalčić, H., 2020. On The Efficiency of P-Wave Coda Autocorrelation in Recovering Crustal Structure: Examples From Dense Arrays in the Eastern United States. J. Geophys. Res. Solid Earth, 125, 2020JB020270. doi:10.1029/2020JB020270

Zhou, J., Phạm, T.-S. & Tkalčić, H., 2024. Deep-Learning Phase-Onset Picker for Deep Earth Seismology: PKIKP Waves. J. Geophys. Res. Solid Earth, 129, e2024JB029360. doi:10.1029/2024JB029360