Some of the geological challenges and opportunities associated with the dynamics of the Cenozoic East African Rift System

The Cenozoic East African Rift System (EARS) is the largest continental rift valley system on Earth. Extending over a total distance of approximately 4,500 km, and with an average width of about 50 km, it is home to some of East Africa's largest urban populations and some of its most important transport, energy and water supply infrastructure. Rifting commenced during the Early Miocene and crustal extension has continued to the present day, posing seismic and volcanic hazards throughout its history of human occupation. Deep-seated landslides also present significant challenges for public safety, land management and infrastructure development on the flanks of rift margins. The rift floor itself poses a range of geohazards to community livelihood and engineering infrastructure, including ground fissuring and cavity collapse, flooding and sedimentation. On the positive side, the development of the EARS has created hydrocarbon and geothermal energy resources, and geomaterials for use as aggregates and cement substitutes in road and building construction. Optimising the use of these resources requires careful planning to ensure sustainability, while land use management and infrastructure development must take full consideration of the hazards posed by the ground and the fragility and dynamism of the human and physical environment.

Tandem cavity collapse in a high-speed droplet impinging on a constrained wall

A focusing shock wave can be generated during the high-speed impact of a droplet on a $180^\circ$ constrained wall, which can be used to realise energy convergence on a small scale. In this study, to realise high energy convergence and peak pressure amplification, a configuration of droplets embedded with cavities is proposed for high-speed impingement on a $180^\circ$ constrained wall. A multicomponent two-phase compressible flow model considering the phase transition is used to simulate the high-speed droplet impingement process. The properties of the embedded cavities can influence the collapse pressure peak. The collapse of an embedded single air cavity or vapour cavity, as well as the cavities in a tandem array, is simulated in this study. The physical evolution mechanisms of the impinging droplet and the embedded cavities are investigated qualitatively and quantitatively by characterising the focusing shock wave generated inside the droplet and its interaction with different cavity configurations. The interaction dynamics between the cavities is analysed and a theoretical prediction model for the intensity of each cavity collapse in the tandem array is established. With the help of this theoretical model, the influencing factors for the collapse intensities of the tandem cavities are identified. The results reveal that the properties of the initial shock wave and the interval between the cavities are two predominant factors for the amplification of the collapse intensity. This study enhances the understanding of the physical process of shock-induced tandem-cavity collapse.

Compound jetting from bubble bursting at an air-oil-water interface

Bursting of bubbles at a liquid surface is ubiquitous in a wide range of physical, biological, and geological phenomena, as a key source of aerosol droplets for mass transport across the interface. However, how a structurally complex interface, widely present in nature, mediates the bursting process remains largely unknown. Here, we document the bubble-bursting jet dynamics at an oil-covered aqueous surface, which typifies the sea surface microlayer as well as an oil spill on the ocean. The jet tip radius and velocity are altered with even a thin oil layer, and oily aerosol droplets are produced. We provide evidence that the coupling of oil spreading and cavity collapse dynamics results in a multi-phase jet and the follow-up droplet size change. The oil spreading influences the effective viscous damping, and scaling laws are proposed to quantify the jetting dynamics. Our study not only advances the fundamental understanding of bubble bursting dynamics, but also may shed light on the airborne transmission of organic matters in nature related to aerosol production.

Jet Dynamics Associated with Drop Impact on Micropillared Substrate

The jetting phenomenon associated with droplet impact upon a hydrophilic micropillared substrate was analyzed in detail using a high-speed camera. Viscosities of the fluids were varied using differing concentrations of glycerol in deionized water. This paper aims to connect similarities between this form of capillary jetting and another well-known jetting phenomenon from the bubble bursting. Both experience a cavity collapse when opposing fluid fronts collide which causes a singularity at the liquid surface, thus leading to the occurrence of jetting. Following processes used to define scaling laws for bubble bursting, a similar approach was taken to derive scaling laws for the dimensionless jet height, jet radius, base height, and radius of the jet base with respect to dimensionless time for the jetting phenomenon associated with the droplet impact. The development of a top droplet before the breakup of the jet also allows the examination of a scaling law for the necking diameter. We find that with the proper scaling factors, the evolution of the jet profile can collapse into a master profile for different fluids and impact velocities. The time dependence of the necking diameter before the jet breakup follows the power law with an exponent of ~2/3. Contrastingly, for other jet parameters such as the radius and height, the power law relationship with time dependence was not found to have a clear pattern that emerged from these studies.

Discrimination of Seismic Events (2006–2020) in North Korea Using P/Lg Amplitude Ratios from Regional Stations and a Bivariate Discriminant Function

Two events of magnitude (mb) 3.6–3.8 occurred in southern North Korea (NK) on 27 June 2019 and 11 May 2020. Although these events were located ∼330–400 km from the known nuclear test site, the fact that they occurred within the territory of NK, a country with a recent history of underground nuclear tests, made them events of interest for the monitoring community. We used P/Lg ratios from regional stations to categorize seismic events that occurred in NK from 2006 to May 2020, including these two recent events, the six declared NK nuclear tests, and the cavity collapse and triggered earthquakes that followed the 3 September 2017 nuclear explosion. We were able to separate the cavity collapse from the population of nuclear explosions. However, based on P/Lg ratios, the distinction between the earthquakes and the cavity collapse is ambiguous. The performed discriminant analyses suggest that combining Pg/Lg and Pn/Lg ratios results in improved discriminant power compared with any of the ratio types alone. We used the two ratio types jointly in a quadratic discriminant function and successfully classified the six declared nuclear tests and the triggered earthquakes that followed the September 2017 explosion. Our analyses also confirm that the recent southern events of June 2019 and May 2020 are both tectonic earthquakes that occurred naturally.

Imaging the subsurface structure of pit craters

<p>Pit craters are enigmatic sub-circular depressions observed on rocky and icy planetary bodies across the Solar System. These craters do not primarily form during catastrophic impact or the forcible eruption of subsurface materials, but likely due to collapse of subsurface cavities following fluid (e.g., magma) movement and/or extensional tectonics. Pit craters thus provide important surficial records of otherwise inaccessible subsurface processes. However, unlocking these pit crater archives is difficult because we do not know how their surface expression relates to their subsurface structure or driving mechanisms. As such, there is a variety of hypotheses concerning pit crater formation, which variously relate cavity collapse to: (i) opening of dilatational jogs during faulting; (ii) tensile fracturing; (iii) karst development; (iv) permafrost melting; (v) lava tube evacuation; (vi) volatile release from dyke tip process zones; (vii) pressure waning behind a propagating dike tip; (viii) migration of magma away from a reservoir; and/or (ix) hydrothermal fluid movement inducing host rock porosity collapse. Validating whether these proposed mechanisms can drive pit crater formation and, if so, identifying how the physical characteristics of pits can be used to infer their driving mechanisms, is critical to probing subsurface processes on Earth and other planetary bodies.</p><p>Here we use seismic reflection data from the North Carnarvon Basin offshore NW Australia, which provides ultra-sound like images of Earth’s subsurface, to characterize the subsurface structure of natural pit craters. We extracted geometrical data for 61 pits, and find that they are broadly cylindrical, with some displaying an inverted conical (trumpet-like) morphology at their tops. Fifty-six pit craters, which are sub-circular and have widths of ~150–740 m, extend down ~500 m to and are aligned in chains above the upper tips of dikes; crater depths are  ~12–225 m. These dike-related pit craters occur within long, linear graben interpreted to be bound by dyke-induced normal faults. Five pit craters, which are ~140–740 m wide and ~32–107 m deep, formed independent of dykes and are associated only with tectonic normal faults. Our preliminary data reveal a moderate, positive correlation between crater width and depth but there is no distinction between the depth and width trends of pit craters associated with dikes and those with tectonic normal faults. To test whether our quantitative data can be used to inform interpretation of pit craters observed on other planetary bodies, we compare their morphology to those imaged in Noctis Labyrinthus on Mars; there are >200 pit craters here, most of which occur in chains, with widths ranging from 369–11743 m and depths from 1–1858 m.</p><p>Overall, we show reflection seismology is a powerful tool for studying the three-dimensional geometry of pit craters, with which we can test pit crater formation mechanisms. We anticipate future seismic-based studies will improve our understanding of how the surface expressions of pit craters (either in subaerial or submarine settings) can be used to reconstruct subsurface structures and processes on other planetary bodies, where such subsurface information is not currently available.</p>

Boundary Vorticity Analysis and Shedding Dynamics of Transient Cavitation Flow Around a Twisted Hydrofoil

The objective of this paper is to investigate the dynamic characteristics of transient cavitating flow over a twisted NACA0009 hydrofoil. The large eddy simulation (LES) approach is selected for the computation of fluid flow and the Zwart model is used for the mass transfer due to cavitation. Moreover, the skin-friction coefficient and boundary-vorticity flux (BVF) are used to study the flow separation. Numerical results show that the attached shear layer separates from the boundary layer and then squeezes to form the separation line under the obstruction of the re-entrant jet. The analysis based on the terms of vorticity transport equation demonstrates that vortex stretching and vortex dilation terms dominate the evolution of multi-scale vortex. Moreover, the secondary shedding induced by the side-entrant jet enhances the instability of partial cavities and the underlying mechanism is comprehensively revealed. Furthermore, the feature of the pressure fluctuation indicates that high pressure generated by the cavity collapse at the tail simultaneously propagates to the leading edge and downstream of the hydrofoil. This enhances the intensity of the re-entrant jet and side-entrant jet, promoting occurrences of flow separation near the suction surface and cavity shedding to a certain extent.