Earthquakes and the New Paradigm of Diluted Cores in Gas Giant Planets

Citation

Idini Zabala, Benjamín Rodo (2022) Earthquakes and the New Paradigm of Diluted Cores in Gas Giant Planets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hqtw-ka38. https://resolver.caltech.edu/CaltechTHESIS:05252022-055331911

Abstract

In this thesis, I present results on two distinct topics within geophysics: earthquake mechanics and the core of gas giant planets. A common element connecting this work is the similar research approach that I use to address each topic. Each chapter in this thesis attempts to provide a simple physical understanding on the fundamental aspects relevant to the system in question. Further, I use numerical models to expand my arguments in some cases, while in others I build up my case with mathematical modeling only.

Chapters II-IV focus on the gravitational field of Jupiter and connect radio science observations from NASA's Juno mission to the structure of Jupiter's dilute core. In Chapter II, I use dynamical tides to interpret a nonhydrostatic component in Jupiter's degree-2 tidal response -- represented by the Love number k₂ -- observed by Juno at the mid-mission perijove (PJ) 17. The results presented here show how the Coriolis acceleration contributes with a dynamical effect to Jupiter's tidal response, providing a satisfactory fit to Juno's observed k₂. From these results, I conclude that Juno obtained the first unambiguous detection of the gravitational effect of dynamical tides in a gas giant planet.

In Chapter III, I build a perturbation theory to show that the high-degree tidal gravitational field of Jupiter is dominated by spherical harmonic coupling promoted by Jupiter's oblate figure as forced by the centrifugal effect. Based on this novel understanding of Jupiter's high-degree tidal gravitational field, I establish that Juno observed a 7σ nonhydrostatic component in k₄₂ at mid-mission.

In Chapter IV, I invoke a core-orbital resonance between internal gravity waves trapped in Jupiter's dilute core and the orbital motion of Io to explain the 7σ nonhydrostatic component in the high-degree tidal response of Jupiter as observed by Juno at mid-mission -- namely the Love number k₄₂. These results suggest that an extended dilute core in Jupiter (r ≳ 0.7R_Jup) reconciles the k₄₂ nonhydrostatic component. This explanation of Juno's observation requires two ingredients: a dilute core in Jupiter that becomes smoother or shrinks over geological time, alongside with a high amount of dissipation provided by resonantly excited internal gravity waves.

In Chapter V, I connect observations of earthquake modes of propagation to the damaged rock often found around tectonic fault zones. Previous work showed that pulse-like rupture -- a propagation mode where slip propagates as a narrow pulse -- can be induced by the dynamic effect of seismic waves reflected at the boundary of a cavity formed by the damaged material in fault zones. My main result shows that pulses are easier to produce than previously thought; pulses can appear in a highly damaged fault zone even in the absence of reflected seismic waves. In addition, these results provide a new explanation for back-propagating rupture fronts recently observed during large earthquakes and the rapid-tremor-reversal slip patterns observed in Cascadia and Japan.

In summary, the results contained in these four chapters advance our knowledge in fundamental problems related to geophysics. In relation to gas giant planets, my results include the development of a novel technique to reveal the structure of Jupiter's core using spacecraft observations of the tidal gravitational field. In relation to earthquakes, my results connect earthquake ruptures to observable fault zone properties.

Thesis Files