A multi-tiered earthquake hazard model for Australia

Earthquakes result from tectonic processes, and their distribution is strongly influenced by large-scale geology and the tectonic stress field. However, earthquake hazard estimates, particularly ground motion recurrence, have traditionally been computed using source models based primarily on instrumental and historical seismicity. In areas of low to moderate seismicity such as Australia, large earthquakes commonly occur in areas which have experienced little or no recent activity, making it difficult to develop source models based solely on seismicity.

The seismotectonic model developed for Australia that is presented here (AUS5) is based on geology, geophysics, tectonics and seismicity. The model was developed using a number of tiers of information, so that new information can easily be incorporated. The information used includes, but is not limited to, tectonic provinces, basins and ranges, gravity, magnetic, topography, and seismicity, all on a regional scale. On a local scale, for a site-specific earthquake hazard study, active faulting can be incorporated to provide fault source zones.

An earthquake hazard map showing peak ground acceleration with a 10% chance of exceedance in 50 years for southeastern Australia using the geologically defined seismotectonic model AUS5 is presented as an indication of how the model performs.     

Depth variation of seismic source scaling relations: implications for earthquake hazard in southeastern Australia

Advances in earthquake data acquisition and processing techniques have allowed for improved quantification of source parameters for local Australian earthquakes. Until recently, only hypocentral locations and local magnitudes (M_L) had been determined routinely, with little attention given to the inversion of additional source parameters. The present study uses these new source data (e.g. seismic moment, stress drop, source dimensions) to further extend our understanding of seismicity and the continental stress regime of the Australian landmass and its peripheral regions.

Earthquake activity within Australia is typically low, and the proportion of small to large events (i.e. the b value) is also low. It is observed that average stress drops for southeastern Australian earthquakes appear to increase with seismic moment to relatively high levels, up to approximately 10 MPa for M_L 5.0 earthquakes. This is thought to be indicative of high compressive crustal stress, coupled with strong rocks and fault asperities. Furthermore, the data indicates that shallow focus earthquakes (shallower than 6 km) appear to produce lower than average stress drops than deeper earthquakes (between 6 and 20 km) with similar moment.

Recurrence estimates were obtained for a discrete seismogenic zone in southeastern Australia. Decreasing b values with increasing focal depth for this zone indicate that larger earthquakes (with high stress drops) tend to occur deeper in the crust. This may offer an explanation for the apparent increase of stress drop with hypocentral depth. Consequently, earthquake hazard estimates that assume a uniform Gutenburg–Richter distribution with depth (i.e. constant b value) may be too conservative and therefore slightly overestimate seismic hazard for surface sites in southeastern Australia.     

Pool-fills: a window to palaeoflood history and response in bedrock-confined rivers

Channel‐scale sedimentary units associated with bedrock‐controlled riffle‐pool morphology are examined in detail along Sandy Creek gorge, an ephemeral stream in arid south‐eastern central Australia. Pool‐fills comprise cut‐and‐fill assemblages of poorly sorted sediments ranging in texture from muds to boulders. Five unit types are defined based on particle size, sedimentary structures, geometry and bounding surface character: (1) coarse‐grained bar platform; (2) fine‐grained bar supraplatform; (3) fine‐grained pool‐fill; (4) fine‐grained bench; and (5) modern pool‐fill. The last coarse‐grained unit currently lining the pools suggests an altered sedimentation style over the post‐settlement period (post‐ad 1860s). Situated at bedrock valley constrictions, pool‐fills are compared with other sedimentary units associated with recirculating currents: eddy bars and slackwater deposits. But only the fine‐grained bench units reflect eddy recirculation; the pool‐fills are principally forced‐bars associated with bedrock‐controlled or ‘forced’ riffle‐pool morphology. A late Holocene palaeoflood history is proposed based on radiocarbon ages from the pool‐fills: multiple phases of cut‐and‐fill activity were preceded by a superflood 3400–1900 years ago that eroded the pool‐fills to bedrock. The resilience of the pool‐fills was illustrated by the passage of a 1‐in‐100‐year flood in 1992, which caused only minor erosion. The presence of pool‐fills may provide a window to past phases of river activity that cannot be extracted from either historical records/observations or palaeoflood slackwater sediment analyses. The formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences, but the occurrence of high‐magnitude floods exerts the dominant control.     

Approaches to surface complexation modeling of Uranium(VI) adsorption on aquifer sediments

Uranium(VI) adsorption onto aquifer sediments was studied in batch experiments as a function of pH and U(VI) and dissolved carbonate concentrations in artificial groundwater solutions. The sediments were collected from an alluvial aquifer at a location upgradient of contamination from a former uranium mill operation at Naturita, Colorado (USA). The ranges of aqueous chemical conditions used in the U(VI) adsorption experiments (pH 6.9 to 7.9; U(VI) concentration 2.5 · 10⁻⁸ to 1 · 10⁻⁵ M; partial pressure of carbon dioxide gas 0.05 to 6.8%) were based on the spatial variation in chemical conditions observed in 1999-2000 in the Naturita alluvial aquifer. The major minerals in the sediments were quartz, feldspars, and calcite, with minor amounts of magnetite and clay minerals. Quartz grains commonly exhibited coatings that were greater than 10 nm in thickness and composed of an illite-smectite clay with occluded ferrihydrite and goethite nanoparticles. Chemical extractions of quartz grains removed from the sediments were used to estimate the masses of iron and aluminum present in the coatings. Various surface complexation modeling approaches were compared in terms of the ability to describe the U(VI) experimental data and the data requirements for model application to the sediments. Published models for U(VI) adsorption on reference minerals were applied to predict U(VI) adsorption based on assumptions about the sediment surface composition and physical properties (e.g., surface area and electrical double layer). Predictions from these models were highly variable, with results overpredicting or underpredicting the experimental data, depending on the assumptions used to apply the model. Although the models for reference minerals are supported by detailed experimental studies (and in ideal cases, surface spectroscopy), the results suggest that errors are caused in applying the models directly to the sediments by uncertain knowledge of: 1) the proportion and types of surface functional groups available for adsorption in the surface coatings; 2) the electric field at the mineral-water interface; and 3) surface reactions of major ions in the aqueous phase, such as Ca²⁺, Mg²⁺, HCO₃⁻, SO₄²⁻, H₄SiO₄, and organic acids. In contrast, a semi-empirical surface complexation modeling approach can be used to describe the U(VI) experimental data more precisely as a function of aqueous chemical conditions. This approach is useful as a tool to describe the variation in U(VI) retardation as a function of chemical conditions in field-scale reactive transport simulations, and the approach can be used at other field sites. However, the semi-empirical approach is limited by the site-specific nature of the model parameters.     

Stress reorientation during folding

Intermediate principal stress, ₂, is, for mechanical reasons, taken to be parallel to the statistical direction of fold axes and traces of thrust faults during evolution of fold and thrust belts. Regionally, maximum principal stress, ₁, and least, ₃, may be taken to be the directions of maximum shortening and maximum thickening of the section, respectively. Where crystalline basement is not involved in the deformation, maximum shortening is manifestly parallel to the top of the basement, or subhorizontal, and ₃ is, therefore, subvertical. While this stress system is grossly adequate on the scale of the fold and thrust belt, it fails locally, particularly in late stages of deformation. Sinuosity develops on all scales within the belt as deformation progresses. Individual fold axes tend to be straight lines in incipient stages of folding, as shown by unrolling folds, but commonly develop with increasing curvature during deformation. The curvature resulting during deformation is a measure of extension parallel to the axial direction, if the ends of the fold are fixed points. Thus, ₂ progressively decreases. With marked sinuosity, stress parellel to the axial direction can be reduced at a given depth below the magnitude of the weight of the overburden, originally ₃. Intermediate and least principal stresses switch position, and strike-slip faulting is favored where the deformational response is failure by shear fracture. The percent axial extension can be expressed in an equation that compares the final arcuate length with the original length. With a knowledge of the physical properties of the rock, the time in the evolution of the structure at which he stresses switch can be predicted, as well as the subsequent structural response.

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Zusammenfassung Die Richtung des intermediären Hauptstresses, ₂, wird aus mechanischen Gründen als parallel zu der statistischen Richtung der Faltenachsen und der Spuren der Überschiebungsflächen angenommen während der Entwicklung von Faltungs- und Überschiebungszonen. Regional können die Richtungen des maximalen Haupstresses, ₁, und des minimalen Hauptstresses, ₃, als Richtungen der größten Verkürzung, respektive der größten Verdickung des Querschnittes betrachtet werden. Wo der kristalline Untergrund nicht in den Deformationsvorgang einbezogen wird, ist die Richtung der maximalen Verkürzung offenbar parallel zur Kristallinoberfläche oder subhorizontal und ₃ somit subvertikal orientiert. Währenddessen im großen Maßstab diese Zuordnung der Hauptstreßrichtungen zu einer ganzen Faltungs- und Überschiebungszone vorgenommen werden kann, versagt sie im lokalen Bereich, vor allem in späten Phasen der Deformation. Bei fortschreitender Deformation entwickeln sich im Deformationsgürtel Bogenformen in verschiedenem Maßstab. Individuelle Faltenachsen neigen dazu, sich an geraden Linien auszubilden in frühen Stadien der Faltung, wie dies die Abwicklung von Falten zeigt. Sie entwickeln sich aber im allgemeinen während der weiteren Deformation mit zunehmend gebogener Achsenrichtung. Die resultierende Kurvatur ist ein Maß der Dehnung parallel zur Achsenrichtung, wenn die seitlichen Endpunkte der Falte Fixpunkte darstellen. In dieser Weise nimmt der Betrag von ₂ fortschreitend ab. Bei ausgeprägter Bogenform kann der Streß parallel zur Richtung der Faltenachse in einer bestimmten Tiefe reduziert werden bis zu einem Betrag, der unterhalb des Ausmaßes der Überlast, also ursprünglich ₃, liegt. Die Richtungen des intermediären und des kleinsten Hauptstresses wechseln ihre Positionen, und Blattverschiebungen werden begünstigt, wo die Deformation Scherbrüche erzeugt. Das Ausmaß der axialen Dehnung kann durch eine Gleichung ausgedrückt werden, welche die Länge des endgültigen Faltenbogens mit der ursprünglichen Länge der Falte verknüpft. Mit der Kenntnis der physikalischen Eigenschaften des Gesteins können sowohl der Zeitpunkt in der Entwicklung der Faltenstruktur, zu welchem die Hauptstreßrichtungen ihre Positionen wechseln, als auch die nachfolgende strukturelle Entwicklung bestimmt werden.

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Résumé La contrainte principale intermédiaire, ₂, est, pour des raisons mécaniques, considérée comme étant parallèle à la direction statistique des axes du pli et des traces de chevauchement durant l'évolution du pli et des zones de chevauchement. Régionalement, on peut supposer que les contraintes principales maximum, ₁, et minimum, ₃, suivent respectivement les directions du raccourcissement maximum et de l'épaississement maximum du profil. Là, où le soubassement cristallin n'est pas entrainé dans la déformation, le raccourcissement maximum est manifestement parallèle à la surface du soubassement, ou subhorizontal, et ₃ est, par conséquent, subverticale. Tant que l'échelle de ce système de contraintes correspond à peu près à celle du pli et de la zone de chevauchement, il change de direction localement, spécialement dans les derniers stades de la déformation. Une sinuosité se développe à toutes les échelles à l'intérieur de la zone, tandis que la déformation progresse. Les axes individuels du pli ont tendance à devenir des lignes droites dans les stades embryonnaires de la déformation, comme le montre le déroulement des plis, mais ordinairement ils se développent avec une courbure croissante pendant la déformation. La courbure qui en résulte durant la déformation est une mesure de l'extension parallèle à la direction axiale. Ainsi ₂ décroit progressivement. Avec une sinuosité prononcée, la contrainte parallèle à la direction axiale peut se réduire, à une profordeur donné au dessous du poids de la surcharge, originellement ₃. Les contraintes principales maximum et minimum échangent leur position, et la composante horizontale du rejet de la faille est alors favorisée là où la réaction à la déformation devient négligeable par suite de fracture de cisaillement. Le pourcentage de l'extension axiale peut s'exprimer par une équation qui compare la longueur de la courbe finale à la longueur originelle. Avec la connaissance des propriétés physiques des roches, on peut prévoir, le moment où les contraintes s'échangent durant l'évolution de la structure, ainsi que la réaction structurale qui en résulte.

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Simulation of artificial earthquakes

A procedure is developed for the simulation of artificial earthquake accelerograms, The time variation of amplitude and frequency content is preserved in the simulation procedure. Sixteen artificial earthquake accelerograms are simulated and compared with a target accelerogram. The time variation of amplitude and frequency content for 26 historical earthquake accelerograms is characterized.     

The distribution of total nitrogen in iron meteorites

Total nitrogen abundances in 123 iron meteorites have been determined by inert carrier-gas fusion extraction-gas chromatography. The median value for the iron meteorites was found to be 18 ppm N. The N contents of Sulfide inclusions are greater, in nine cases out of ten, than the corresponding metallic phase. The N content of the iron meteorites is positively correlated with germanium content. The effects of terrestrial weathering and heat treatment by man are discussed in relation to the N contents measured for certain specimens. A correlation between N and cooling rates was found, with lower cooling rates associated with greater N abundances.     

A benthic index of environmental condition of Gulf of Mexico estuaries

An index was developed for estuarine macrobenthos in the Gulf of Mexico that discriminated between areas with degraded environmental conditions and areas with undegraded or reference conditions. Test sites were identified as degraded or reference based on criteria for dissolved oxygen levels, sediment toxicity tests, and sediment contamination. Discriminant analysis was used to identify a suite of measures of benthic community composition and diversity that would most successfully distinguish degraded from undegraded sites. The resultant benthic index was composed of a linear combination of three factors: the Shannon-Wiener diversity index, the proportion of total benthic abundance as tubificid oligochaetes, and the proportion of total benthic abundance as bivalve molluscs. This index was used to evaluate the spatial patterns of degraded benthic resources in the Gulf of Mexico.