Hierarchical interlocked orthogonal faulting in the 2019 Ridgecrest earthquake sequence

A nearly 20-year hiatus in major seismic activity in southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (M_w) of 6.4 followed by a M_w 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults. This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm. The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards

Community Code Verification Exercise for Simulating Sequences of Earthquakes and Aseismic Slip (SEAS)

Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code‐verification exercise for SEAS simulations, supported by the Southern California Earthquake Center. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well‐defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike‐slip fault obeying rate‐and‐state friction, embedded in a 2D homogeneous, linear elastic half‐space. Sequences of quasi‐dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long‐term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics

Community Code Verification Exercise for Simulating Sequences of Earthquakes and Aseismic Slip (SEAS)

Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code‐verification exercise for SEAS simulations, supported by the Southern California Earthquake Center. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well‐defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike‐slip fault obeying rate‐and‐state friction, embedded in a 2D homogeneous, linear elastic half‐space. Sequences of quasi‐dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long‐term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics

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

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 ≳ 0.7RJup) 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.</p