Numerical modeling of asteroid survivability and possible scenarios for the Morokweng crater‐forming impact

The fate of the impactor is an important aspect of the impact‐cratering process. Defining impactor material as surviving if it remains solid (i.e., does not melt or vaporize) during crater formation, previous numerical modeling and experiments have shown that survivability decreases with increasing impact velocity, impact angle (with respect to the horizontal), and target density. Here, we show that in addition to these, impactor survivability depends on the porosity and shape of the impactor. Increasing impactor porosity decreases impactor survivability, while prolate‐shaped (polar axis > equatorial axis) impactors survive impact more so than spherical and oblate‐shaped (polar axis < equatorial axis) impactors. These results are used to produce a relatively simple equation, which can be used to estimate the impactor fraction shocked to a given pressure as a function of these parameters. By applying our findings to the Morokweng crater‐forming impact, we suggest impact scenarios that explain the high meteoritic content and presence of unmolten fossil meteorites within the Morokweng crater. In addition to previous suggestions of a low‐velocity and/or high‐angled impact, this work suggests that an elongated and/or low porosity impactor may also help explain the anomalously high survivability of the Morokweng impactor.     

Ganymede crater dimensions – Implications for central peak and central pit formation and development

The morphology of impact craters on the icy Galilean satellites differs from craters on rocky bodies. The differences are thought due to the relative weakness of ice and the possible presence of sub-surface water layers. Digital elevation models constructed from Galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of Ganymede. Measurements were made from multiple profiles across each crater, so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed. The additional depth, slope and volume information reported in this work has enabled study of central peak formation and development, and allowed a quantitative assessment of the various theories for central pit formation. We note a possible difference in the size-morphology progression between small craters on icy and silicate bodies, where central peaks occur in small craters before there is any slumping of the crater rim, which is the opposite to the observed sequence on the Moon. Conversely, our crater dimension analyses suggest that the size-morphology progression of large lunar craters from central peak to peak-ring is mirrored on Ganymede, but that the peak-ring is subsequently modified to a central pit morphology. Pit formation may occur via the collapse of surface material into a void left by the gradual release of impact-induced volatiles or the drainage of impact melt into sub-crater fractures.     

Upper crustal structure of the western Corinth Gulf, greece, inferred from arrival times of the January 2010 earthquake sequence

The western part of the Corinth Gulf attracts attention due to its seismically active fault system and considerable seismic hazard. A moderate size earthquake occurred close to the town of Efpalio on January 18, 2010, followed by a sequence of smaller earthquakes. In the present paper we use this sequence to derive a local structural model for the region in the vicinity of Efpalio. The model is based on the minimization of traveltime residuals. In particular, we used arrival times from 51 selected events recorded on January 19 and 20 by at least 5 stations at epicentral distances less than about 25 km. A variant of the method of conjugate gradients has been used for this purpose. In comparison with several previous models, the new model is characterized by higher velocities to a depth of about 8 km. The velocity ratio in the model is v_P / v_S = 1.83. The hypocentres of the selected earthquakes lay at depths between about 5 and 9 km, but their distribution is rather irregular.     

Wadati method as a simple tool to study seismically active fault zones: a case study from the West-Bohemia/Vogtland region,central Europe

The ratio of P- to S-wave velocities, V_P/V_S, is an important parameter characterizing rock composition and fluid saturation. We have studied properties of the ratio in the West-Bohemian seismically active region, using data from the earthquake swarm which occurred here in 2008. The earthquake swarm was well recorded by 23 seismic stations from epicentral distances less than 25 km. We selected a subset of 158 events with local magnitudes between 1.5 and 3.8. Applying the Wadati method to the measured arrival times of P and S waves, we arrived at an average value of V_P/V_S =1.68 ± 0.01. This differs a little from the value of V_P/V_S = 1.70, which is routinely used for earthquake locations in the region at present. Moreover, it was recognized that the points in the Wadati graphs for some stations were systematically deviated from the mean straight lines. In particular, the stations with the largest positive deviations (above the mean straight lines) are situated close to the Mariánské Lázně Fault and to some intensive mofettes. Further analyses revealed reduced P- and S-wave velocities along the seismic rays toward these anomalous stations. In our opinion, the seismic waves arriving at the anomalous stations probably propagated along a fault or another zone of weakness. In this way, our results support the hypothesis that the Mariánské Lázně Fault is a deep-seated fault continuing down to the seismically active zone of local earthquakes. From a general point of view, this study demonstrates that even some narrow structural anomalies in the crust, such as fault zones, can be recognized by the simple Wadati method if data from a dense seismic network are available.     

Contrasting definitions for the term ‘karst aquifer’

It is generally considered that karst aquifers have distinctly different properties from other bedrock aquifers. A search of the literature found five definitions that have been proposed to differentiate karst aquifers from non-karstic aquifers. The five definitions are based upon the presence of solution channel networks, hydraulic conductivities >10^?6 m/s, karst landscapes, channels with turbulent flow, and caves. The percentage of unconfined carbonate aquifers that would classify as ‘karst’ ranges from <1 to >50%.     

Anomalous propagation of refracted waves beneath the Orlík water reservoir, Czech Republic

The Orlík water reservoir was built on the Vltava River in the places where the river crosses the geological unit called the Central Bohemian Pluton. Previous geological studies have disclosed no significant lateral inhomogeneity or fault zone in the close vicinity of the reservoir. However, repeated levellings, performed there in the 1950s and 1960s, revealed a systematic subsidence of the Vltava River valley. Using these measurements, the idea of the tectonic origin of the valley was even formulated. Recent seismic observations have also attracted attention to this region. Namely, a macroseismically felt earthquake occurred there on January 13, 2007, and many weaker earthquakes have been recorded by local seismic networks since 1992. To increase the accuracy of locating these seismic events, recently we have carried out refraction measurements along a short profile across the Orlík reservoir. These authors smoothed the observed travel times and derived a 1-D velocity model of the shallow crustal structure for the Orlík region. In the present paper, using the same refraction measurements, we study some anomalies in wave propagation beneath the Orlík reservoir. In particular, it is shown that the passage of seismic waves beneath the reservoir leads to their time delays and spectral changes. Moreover, we have also recognised similar time delays in earlier data from the nearby international profile CEL09 in the places where the profile crosses the Orlík reservoir.     

Reconciled bedload sediment transport rates in ephemeral and perennial rivers

It has been thought for some time that bedload sediment transport rates may differ markedly in ephemeral and perennial rivers and, supporting this thought, there has been observation of very high rates of bedload transport by flash floods in the ephemeral river Nahal Yatir. However, until now, there has been no quantitative model resolving the observation, nor a theory capable of explaining why bedload transport rates by unsteady flash floods can be reasonably well described by bedload transport capacity formulae initially derived for steady flows. Here a time scale analysis of bedload transport is presented as pertaining to Nahal Yatir, which demonstrates that bedload transport can adapt sufficiently rapidly to capacity determined exclusively by local flow regime, and accordingly the transport capacity formulations developed for steady flows can be applied even under unsteady flows such as flash floods. Complementing the time scale analysis, a series of computational exercises using a coupled shallow water hydrodynamic model are shown to adequately resolve the observation of the very high rates of bedload transport by flash floods in Nahal Yatir. While bedload transport rates in ephemeral and perennial rivers differ remarkably when evaluated against a pure flow parameter such as specific stream power, they are essentially reconciled if assessed with a physically sensible parameter incorporating not only the flow regime but also the sediment particle size. The present finding underpins the practice of fluvial geomorphologists relating measured bedload transport to local flow and sediment characteristics only, irrespective of whether the flow is unsteady or steady. Copyright © 2010 John Wiley & Sons, Ltd.     

A numerical assessment of simple airblast models of impact airbursts

Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy‐based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving‐source airburst model where the meteoroid's energy is partitioned as two‐thirds internal energy and one‐third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving‐source model predicts overpressures two times greater than the static‐source model, whereas the cylindrical line‐source model based on the pancake model predicts overpressures two times lower than the static‐source model. Given other uncertainties associated with airblast damage predictions, the static‐source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.     

Element mapping the Merensky Reef of the Bushveld Complex

The Merensky Reef hosts one of the largest PGE resources globally.It has been exploited for nearly 100 years, yet its origin remains unresolved.In the present study, we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy.Our results indicate that the Merensky Reef formed through a range of diverse processes.Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth.The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow.Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation.The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts.Olivine-hosted melt inclusions containing Cl-rich apatite, sodic plagioclase, and phlogopite suggest the presence of highly evolved, volatile-rich melts.Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase, possibly triggered by flux of heat, acidic fluids, or hydrous melt.Together, these data suggest that the reef formed through a combination of magmatic, hydrodynamic and hydromagmatic processes.