A stochastic methodology for risk assessment of a large earthquake when a long time has elapsed

We propose a stochastic methodology for risk assessment of a large earthquake when a long time has elapsed from the last large seismic event. We state an approximate probability distribution for the occurrence time of the next large earthquake, by knowing that the last large seismic event occurred a long time ago. We prove that, under reasonable conditions, such a distribution is exponential with a rate depending on the asymptotic slope of the cumulative intensity function corresponding to a nonhomogeneous Poisson process. As it is not possible to obtain an empirical cumulative distribution function of the waiting time for the next large earthquake, an estimator of its cumulative distribution function based on existing data is derived. We conduct a simulation study for detecting scenario in which the proposed methodology would perform well. Finally, a real-world data analysis is carried out to illustrate its potential applications, including a homogeneity test for the times between earthquakes.     

Biocrust landscape-scale spatial distribution is strongly controlled by terrain attributes: Topographic thresholds for colonization in a semiarid badland system

Biological soil crust, or biocrust communities, are the dominating life form in many extreme habitats, such as arid and semiarid badlands, where water scarcity and highly erodible substrates limit vegetation cover. While climate, soil and biotic factors have been described as environmental filters influencing biocrust distribution in such biomes, little is known about the effect of terrain attributes on creating specific microhabitats that promote or restrict biocrust colonization. This study aimed to identify the main terrain attributes controlling biocrust distribution in the driest badland system in Europe, the Tabernas Badlands (SE Spain). To do this, we analysed the influence of different terrain attributes related to landscape stability and microclimate formation on the spatial distribution of lichen and cyanobacteria, using field measurements and topographical information from a LiDAR survey. Our results showed that the spatial distribution of cyanobacteria-dominated biocrusts, which are physiologically and morphologically adapted to extreme drought and high UVA radiation, was mostly associated with areas of high potential incoming solar radiation. The exception was bare south-aspect hillslopes with very high sediment transport potential, where bare physically crusted soils were the dominant ground cover. Lichen-dominated biocrusts, in contrast, colonized near the top of north-aspect hillslopes, characterized by low potential incoming solar radiation and potential evapotranspiration, and their cover decreased downstream, as conditions became good enough for vascular plants. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Laboratory studies on the sputtering contribution to the sodium atmospheres of Mercury and the Moon

To ascertain the importance of sputtering by solar wind ions on the formation of a sodium exosphere around Mercury and the Moon, we have irradiated with 4 keV He ions, the Na bearing tectosilicates: albite, labradorite, and anorthoclase, as well as adsorbed Na layers deposited on albite and on olivine (a neosilicate that does not contain Na). Sodium at the surface and near surface (<40 Å) was quantified with X-ray photoelectron spectroscopy before and after each irradiation to determine the depletion cross section. We measured a cross section for sputtering of Na adsorbed on mineral surfaces, σ_s ≈ 1 × 10^?15 cm² atom^?1. In addition, mass spectrometric analyses of the sputtered flux show that a large fraction of the Na is sputtered as ions rather than as neutral atoms. These results have strong implications for modeling the sodium population within the mercurian and the lunar exospheres.     

Analysis of horizontal and vertical ring structure based on analytical model and satellite data: Application to the North Brazil Current Rings

A theoretical analysis of the principal oceanic ring parameters (tangential and angular velocities, shape function, horizontal and vertical scale of the ring) is described. The theoretical model consists of a reduced gravity model of the lens-like vortices with solid body rotation. The application of this approach is tested by comparison with data from the North Brazil Current Rings Experiment and remote sensing data. Specifically, we used the data corresponding to the surfaceintensified North Brazil Current ring (R-3) surveyed in February–March 1999, using direct velocity and hydrographical measurements. The theoretical model was used for evaluating the geometrical structure of the surface-intensified rings that produce remarkable signals in satellite data. The principal ring parameters from the model were compared with those from satellite data (altimetry and drifter information), which were estimated by using the method of minimization of the multivariable objective functions. Although the proposed model is linear in its conception, a good agreement was observed between the model and the primary characteristics of the observed rings. The model, however, allows for improvement in its assumptions, since its application is rather limited to intense ocean rings.     

Ranking of Placer Gold Prospects in Chile Through Analytic Hierarchy Process

Chile has a rich, but poorly known history of placer gold mining. At present, this sector is almost nonexistent and there are some restrictions for its revival: disperse and partial information on existing resources and limited technical expertise to assess the potential of placer gold mine sites. This paper presents the background, methodology and results of the prioritization process of known prospects of this kind in Chile. This research was part of a publicly funded project aimed to incentivize the development of this industry. The ranking was carried out using the analytic hierarchy process, which allowed to include different quantitative and qualitative variables related to the economic potential, technical aspects, contextual viability and socioeconomic factors in the analysis. The results show that, despite the increasing relevance of environmental and community issues in mining development, the business potential and the economic/technical aspects are the main factors in the early selection of a site to advance in exploration and development activities. Both variables represented around 40% and 37% of weights in the final selection, respectively. In contrast, contextual viability and local socioeconomic impacts only accounted for the remaining 23%. This study also shows that the inclusion of experts with different backgrounds in the process enriches the analysis and does not significantly distort the final outcome of the prioritization. Finally, the relevance of using MCDM tools when assessing the attractiveness of mine sites for their development is highlighted, particularly when public funds for subsequent exploration activities are committed.

     

Bioerosion and encrustation rates in recent mollusk death assemblages on a supratidal siliciclastic setting,Playa Norte,Veracruz State,Mexico

This study presents both qualitative and quantitative data regarding marine mollusk (gastropods and bivalves) shell bioerosion and encrustation based on death assemblages obtained from a recent supratidal environment in Playa Norte, Veracruz State. The objectives of this study were to assess the nature of bioerosion and encrustation processes and to investigate the role of these taphonomic features contributing to the deterioration of natural shell accumulations within a tropical siliciclastic tidal environment. The assemblage comprises 31 species: 13 gastropods and 18 bivalves. The bioerosion and encrustation degrees were low to moderate for both types. The most abundant traces were predatory gastropod structures (Oichnus paraboloides and O. simplex), whereas sponge borings (Entobia isp.), polychaete dwellings (Caulostrepsis taeniola), and echinoid raspings (Gnatichnus isp.) were less frequent. The encrusting organisms include polychaete serpulids, bryozoans, and rare foraminifers (Homotrema rubrum). Because of the low bioerosion and encrustation degrees occurring in this area, accumulation is expected to predominate over biotic destruction. As deposition conditions (richly fossiliferous carbonate sandstone beds) were similar to those prevailing in the Tuxpan Formation during the Miocene (Langhian), it is suggested that this study provides an equivalent reference to interpret mollusk fossil assemblages located in this site.     

Pockmarks on either side of the Strait of Gibraltar: formation from overpressured shallow contourite gas reservoirs and internal wave action during the last glacial sea-level lowstand?

Integrating novel and published swath bathymetry (3,980 km²), as well as chirp and high-resolution 2D seismic reflection profiles (2,190 km), this study presents the mapping of 436 pockmarks at water depths varying widely between 370 and 1,020 m on either side of the Strait of Gibraltar. On the Atlantic side in the south-eastern Gulf of Cádiz near the Camarinal Sill, 198 newly discovered pockmarks occur in three well localized and separated fields: on the upper slope (n=14), in the main channel of the Mediterranean outflow water (MOW, n=160), and on the huge contourite levee of the MOW main channel (n=24) near the well-known TASYO field. These pockmarks vary in diameter from 60 to 919 m, and are sub-circular to irregularly elongated or lobate in shape. Their slope angles on average range from 3° to 25°. On the Mediterranean side of the strait on the Ceuta Drift of the western Alborán Basin, where pockmarks were already known to occur, 238 pockmarks were identified and grouped into three interconnected fields, i.e. a northern (n=34), a central (n=61) and a southern field (n=143). In the latter two fields the pockmarks are mainly sub-circular, ranging from 130 to 400 m in diameter with slope angles averaging 1.5° to 15°. In the northern sector, by contrast, they are elongated up to 1,430 m, probably reflecting MOW activity. Based on seismo-stratigraphic interpretation, it is inferred that most pockmarks formed during and shortly after the last glacial sea-level lowstand, as they are related to the final erosional discontinuity sealed by Holocene transgressive deposits. Combining these findings with other existing knowledge, it is proposed that pockmark formation on either side of the Strait of Gibraltar resulted from gas and/or sediment pore-water venting from overpressured shallow gas reservoirs entrapped in coarse-grained contourites of levee deposits and Pleistocene palaeochannel infillings. Venting was either triggered or promoted by hydraulic pumping associated with topographically forced internal waves. This mechanism is analogous to the long-known effect of tidal pumping on the dynamics of unit pockmarks observed along the Norwegian continental margin.     

Dynamic faulting under rate-dependent friction

We discuss the effects of rate-dependent friction on the propagation of seismic rupture on active faults. Several physicists using Burridge and Knopoff's box and spring model of faulting have proposed that fault complexity may arise from the spontaneous development of a self-similar stress distribution on the fault plane. If this model proves to be correct, it has important consequences for the origin of the complexity of seismic sources. In order to test these ideas on a more realistic earthquake model, we developed a new boundary integral equation method for studying rupture propagation along an antiplane fault in the presence of nonlinear rate-dependent friction. We study rupture dynamics of models with single and twin asperities. In our models, asperities are places on the fault with a higher value of prestress. Othewise all fault parameters are homogeneous. We show that for models with such asperities, a slip velocity weakening friction leads to the propagation of supersonic healing phases and to the spontaneous arrest of fracture if the prestress outside the asperities is low enough. For models with asperities, we can also observe narrow slip velocity pulses, qualitatively similar to the so-called Heaton pulses observed in some earthquake accelerograms. We also observe a complex distribution of stress after the rupture that depends on details of the initial distribution of asperities and on the details of the friction law.