Analysis of the role of diffraction in topographic site effects using boundary element techniques

The role played by the diffraction field on the problem of seismic site effects is studied. For that purpose we solve and analyze simple scattering problems under P and SV in-plane wave assumptions, using two well known direct boundary-element-based numerical methods. After establishing the difference between scattered and diffracted motions, and introducing the concept of artificious and physically based incoming fields, we obtain the amplitude of the Fourier spectra for the diffracted part of the response: this is achieved after establishing the connection between the spatial distribution of the transfer function over the studied simple topographies and the diffracted field. From the numerical simulations it is observed that this diffracted part of the response is responsible for the amplification of the surface ground motions due to the geometric effect. Furthermore, it is also found that the diffraction field sets in a fingerprint of the topographic effect in the total ground motions. These conclusions are further supported by observations in the time-domain in terms of snapshots of the propagation patterns over the complete computational model. In this sense the geometric singularities are clearly identified as sources of diffraction and for the considered range of dimensionless frequencies it is evident that larger amplifications are obtained for the geometries containing a larger number of diffraction sources thus resulting in a stronger topographic effect. The need for closed-form solutions of canonical problems to construct a robust analysis method based on the diffraction field is identified.     

The Pterosaur Collection at the Natural History Museum,London, UK: an Overview and List of Specimens,with Description of Recent Curatorial Developments

The pterosaur collection at the Natural History Museum is of great historical importance. Most of the material is figured or cited, and several specimens are types or casts of types. Pterosaurs from UK localities are well represented, but foreign material is also present as original specimens and as high quality casts. The entire collection has been recorded in a database and curatorial improvements have been made, which has improved physical and virtual access to the collection. Although many of the specimens were collected as long ago as the early 1800s, they are still useful today. A full list of Natural History Museum pterosaur specimens is published, for the first time since 1888.     

Using InSAR to identify hydrological connectivity and barriers in a highly fragmented wetland

Hydrological connectivity is a critical determinant of wetland functions and health, especially in wetlands that have been heavily fragmented and regulated by human activities. However, investigating hydrological connectivity in these wetlands is challenging due to the costs of high-resolution and large-scale monitoring required in order to identify hydrological barriers within the wetlands. To overcome this challenge, we here propose an interferometric synthetic aperture radar (InSAR)-based methodology to map hydrologic connectivity and identify hydrological barriers in fragmented wetlands. This methodology was applied along 70 transects across the Baiyangdian, the largest freshwater wetland in northern China, using Sentinel 1A and 1B data, covering the period 2016–2019. We generated 58 interferograms providing information on relative water level changes across the transects that showed the high coherence needed for the assessment of hydrological connectivity. We mapped the permanent and conditional (temporary) barriers affecting connectivity. In total, 11% of all transects are permanently disconnected by hydrological barriers across all interferograms and 58% of the transects are conditionally disconnected. Areas covered by reed grasslands show the most undisturbed hydrological connectivity while some of these barriers are the result of ditches and channels within the wetland and low water levels during different periods of the year. This study highlights the potential of the application of Wetland InSAR to determine hydrological connectivity and location of hydrological barriers in highly fragmented wetlands, and facilitates the study of hydrological processes from large spatial scales and long-time scales using remote sensing technique.     

A Mariner/MESSENGER global catalog of mercurian craters

Using MESSENGER and Mariner 10 flyby images, we have compiled a global catalog of impact structures with diameters D > 10 km. The distribution of impact structures shows a factor of 10 range in areal crater density. Most regions of low crater density are located within large impact basins, consistent with the idea that these were low-lying areas that have been filled by subsequent volcanism over an extended period.     

Three-dimensional joint elements applied to concrete-faced dams

In the study of concrete-face dams it is essential to provide joint elements; otherwise the calculated stresses at the facing will be non-allowable. Three-dimensional joint elements must be simple so as not to increase in excess the CPU time. Seven non-incremental joint elements have been formulated or developed, and their relative advantages discussed. It is shown that quasi-linear or linear joint elements at the perimeter of the solid elements can have calculation advantages, specially for interfaces with ‘no tension’ resistance. A system of thin solid finite elements (FE) joints at the face may reproduce qualitatively the displacements and stresses, but not quantitatively. With a system of quasi-linear elements placed at the boundaries of the solid FE we have been able to predict correctly the displacements, strains and joint openings at Cethana concrete-faced rockfill dam.     

New theoretical approaches to black holes

Quite recently, some new mathematical approaches to black holes have appeared in the literature. They do not rely on the classical concept of event horizon—which is very global, but on the local concept of hypersurfaces foliated by trapped surfaces. After a brief introduction to these new horizons, we focus on a viscous fluid analogy that can be developed to describe their dynamics, in a fashion similar to the membrane paradigm introduced for event horizons in the seventies, but with a significant change of sign of the bulk viscosity.     

Relationships between bioturbation by Tylos spinulosus (Crustacea, Isopoda) and its distribution on sandy beaches of north-central Chile

The semi-terrestrial isopod, Tylos spinulosus Dana, is a common inhabitant of the upper shore levels of sandy beaches of north-central Chile (ca. 26–30°S). During daylight hours, this isopod remains buried in the sand, while during the night emerges for feeding on stranded organic detritus, leaving exit holes on the beach surface. After feeding, isopods return to dig in their burrowing zones leaving surface irregularities such as cone-shaped mounds of sand. The burrowing preference of T. spinulosus was studied in the field, by: (i) releasing 30 isopods on artificially prepared sand circles (2 m diameter) having exit holes and mounds similar to those left by the isopods and on circles without holes and mounds, and (ii) counting active and buried isopods 15 min after their release in the experimental arenas. The circles had two densities of holes and mounds: treatments 1 and 2 had 100 and 200 holes, respectively, while treatments 3 and 4 had 100 and 200 mounds, respectively. Other 30 isopods were released on sand circles without these holes and mounds (treatment 5). A significantly higher number of isopods buried in circles with holes and mounds (either inside or outside them), compared with experimental arenas without such structures. These results show that the beach surface heterogeneity resulting from holes and mounds would be one of the processes explaining the patchiness of T. spinulosus and thus, its zonation on the intertidal zones of sandy beaches of north-central Chile.     

Evaluation of the seismic risk of the unreinforced masonry building stock in Antioquia,Colombia

This work focuses on the exploitation of very high-resolution (VHR) satellite imagery coupled with multi-criteria analysis (MCA) to produce flood hazard maps. The methodology was tested over a portion of the Yialias river watershed basin (Nicosia, Cyprus). The MCA methodology was performed selecting five flood-conditioning factors: slope, distance to channels, drainage texture, geology and land cover. Among MCA methods, the analytic hierarchy process technique was chosen to derive the weight of each criterion in the computation of the flood hazard index (FHI). The required information layers were obtained by processing a VHR GeoEye-1 image and a digital elevation model. The satellite image was classified using an object-based technique to extract land use/cover data, while GIS geoprocessing of the DEM provided slope, stream network and drainage texture data. Using the FHI, the study area was finally classified into seven hazard categories ranging from very low to very high in order to generate an easily readable map. The hazard seems to be severe, in particular, in some urban areas, where extensive anthropogenic interventions can be observed. This work confirms the benefits of using remote sensing data coupled with MCA approach to provide fast and cost-effective information concerning the hazard assessment, especially when reliable data are not available.     

Metripol birefringence imaging of unconsolidated glaciotectonized and ice keel scoured sediments: identification of unistrial plasmic fabric

In unconsolidated sediments subject to strain, clays and silts are realigned into particular optical birefringent arrangements (plasmic fabrics), which provide information on the style and intensity of sediment deformation. A relatively new, non‐destructive, optical microscopy technique for automatically recording and quantifying birefringence (previously commercialized under the name ‘Metripol’) is pioneered in this study as a valuable and innovative micromorphological tool with which to examine deformation in unconsolidated sediments. Metripol is applied to unistrial plasmic fabric in glaciotectonized and ice keel scoured sediment from the Netherlands and former Glacial Lake Agassiz (Manitoba, Canada) respectively. Colour‐coded images are produced in which colour represents relative optical retardation and thus optical anisotropy through the quantity |sinδ| and optical orientation of anisotropy through the angle Ø (also indicated by linear azimuths). In this study Metripol typically demonstrates that the better developed the unistrial plasmic fabric is, the higher the |sinδ| values, the larger the areas of high |sinδ| values, and the longer and more densely populated the azimuths. In addition, some unistrial plasmic fabrics under Metripol demonstrate lower |sinδ| than previous examples and the surrounding sediment, despite being ‘perceived’ as demonstrating higher birefringence under a standard petrographic microscope. This is particularly true in clay‐rich sediments and has implications for the way we currently describe and interpret unistrial plasmic fabrics in unconsolidated sediment. Finally, the identification and quantification of additional structures that would otherwise have gone undetected using a standard petrographic microscope (e.g. linear and circular structures that are likely to represent discrete shears and skelsepic plasmic fabric, respectively) highlight the potential for Metripol to gather information on the deformation history of unconsolidated sediments that is unavailable to standard techniques.     

Using stable isotopes to estimate travel times in a data‐sparse Arctic catchment: Challenges and possible solutions

Use of isotopes to quantify the temporal dynamics of the transformation of precipitation into run‐off has revealed fundamental new insights into catchment flow paths and mixing processes that influence biogeochemical transport. However, catchments underlain by permafrost have received little attention in isotope‐based studies, despite their global importance in terms of rapid environmental change. These high‐latitude regions offer limited access for data collection during critical periods (e.g., early phases of snowmelt). Additionally, spatio‐temporal variable freeze–thaw cycles, together with the development of an active layer, have a time variant influence on catchment hydrology. All of these characteristics make the application of traditional transit time estimation approaches challenging. We describe an isotope‐based study undertaken to provide a preliminary assessment of travel times at Siksik Creek in the western Canadian Arctic. We adopted a model–data fusion approach to estimate the volumes and isotopic characteristics of snowpack and meltwater. Using samples collected in the spring/summer, we characterize the isotopic composition of summer rainfall, melt from snow, soil water, and stream water. In addition, soil moisture dynamics and the temporal evolution of the active layer profile were monitored. First approximations of transit times were estimated for soil and streamwater compositions using lumped convolution integral models and temporally variable inputs including snowmelt, ice thaw, and summer rainfall. Comparing transit time estimates using a variety of inputs revealed that transit time was best estimated using all available inflows (i.e., snowmelt, soil ice thaw, and rainfall). Early spring transit times were short, dominated by snowmelt and soil ice thaw and limited catchment storage when soils are predominantly frozen. However, significant and increasing mixing with water in the active layer during the summer resulted in more damped steam water variation and longer mean travel times (~1.5 years). The study has also highlighted key data needs to better constrain travel time estimates in permafrost catchments.