The Acheron rock avalanche, Canterbury, New Zealand—morphology and dynamics

The 1100-year-old Acheron rock avalanche deposit lies in an active tectonic setting in Canterbury, New Zealand, and has a volume of ten million cubic metres and a runout distance 3.5 km. The deposit comprises intensely fragmented greywacke rock, and the processes of intense rock fragmentation during runout are postulated to have generated an isotropic dispersive stress. Dynamic simulation shows that the runout can be explained as a flow of dry granular material with a normal coefficient of friction, if the presence of an isotropic dispersive stress within the moving rock debris throughout the runout is assumed. The dispersive stress distribution required to model the rock avalanche runout and match velocities calculated from run-up traces is closely similar to that used to simulate the runout of the much larger Falling Mountain rock avalanche in a similar lithologic and tectonic setting. Both events thus behaved in a fundamentally similar fashion.     

Toxicity and compositional analysis of aromatic hydrocarbon fractions of two pairs of undegraded and biodegraded crude oils from the Santa Maria (California) and Vienna basins

The monoaromatic and total aromatic hydrocarbon fractions of two pairs of undegraded and moderately biodegraded crude oils from the Santa Maria basin (California) and the Vienna basin (Austria), all dominated by unresolved complex mixtures, were studied regarding their composition and toxicity towards the feeding rate of the marine mussel Mytilus edulis. Total aromatic and monoaromatic hydrocarbon fractions from sulphur-rich Monterey Formation crude oils were slightly more toxic than the fractions isolated from sulphur-lean Vienna basin oils. The ecotoxicity tests did not show any significant differences in toxicity of aromatic compounds from undegraded or in-reservoir biodegraded crude oils from the same oilfield although some differences in composition were observed. Organic sulphur compounds are suspected to cause the slightly higher toxicity of the aromatic hydrocarbon fractions from the Monterey oils.     

Analysis of radionuclide migration through a 200-m Vadose zone following a 16-year infiltration event

Atomic weapons testing at the Nevada Test Site has introduced many tracers for quantifying subsurface hydrologic transport processes in arid climates. In 1975, groundwater adjacent to the Cambric test, conducted beneath Frenchman Flat 10 years earlier, was pumped steadily for 16 years to elicit information on the migration of residual radioactivity through the saturated zone. Radionuclides in the pumping well effluent, including tritium, ¹⁴C, ³⁶Cl, and ⁸⁵Kr, were extensively monitored prior to its discharge in an unlined ditch, where approximately a third of the flow infiltrated over a distance of 1 km. Radionuclide infiltration through a 220-m thick vadose zone created a second, and rather unique long-term field experiment. Effluent data have been utilized in conjunction with geologic data, new radionuclide measurements, isotopic age-dating estimates, and vadose zone flow and transport models to better understand the movement of radionuclides between the ditch, the water table and a nearby groundwater monitoring well. Detection of tritium in the monitoring well occurred approximately 16 years after its initial discharge into the ditch. Modeling and tritium age dating have suggested 3–5 years of this 16-year transit time occurred solely in the vadose zone. They also suggest considerable recirculation of the pumping well discharge back into the original pumping well. Notably, there have been no observations of ¹⁴C or ⁸⁵Kr in the monitoring well, suggesting their preferential retention or volatilization during transit to the water table.     

iSPITM— the improved source parameter imaging method

Interpretation of an anomalous magnetic response involves determining the parameters that characterize the source of the anomaly. The depth to the top of the structure is a parameter that is commonly sought, and the Source Parameter Imaging^TM (SPI^TM) method is one way of determining this depth estimate. One advantage of the SPI method is that the depths can be displayed on an image. Typically there can be one image for an assumed contact (fault) model and another image for an assumed dipping thin sheet (dike) model. The depth estimate obtained will depend on the model assumed. An improvement to the source parameter imaging method extends the method to horizontal cylinders and at the same time allows the most appropriate model to be determined automatically. This model can be displayed on an image and the correct depth estimate for each anomaly can also be determined. The depth estimates can therefore be summarized on one map independent of an assumed model. The images generated from synthetic and field data show that the improved SPI method makes the task of interpreting magnetic data significantly easier.     

Relationship between point rainfall, average sampled rainfall and ground truth at the event scale in the Sahel

Geostatistical techniques are used to quantify the reference mean areal rainfall (ground truth) from sparse raingaugenetworks. Based on the EPSAT-Niger event cumulative rainfall, a linear relationship between the ground truth considered as the mean area rainfall estimated from the densely available raingauge network and the area rainfall estimated from sparse network are derived. Also, a linear relationship between the ground truth and point rainfall is established. As it was reported experimentally by some authors, the slope of these relationships is less than one. Based on the geostatistical framework, the slope and the ordinate at the origin can be estimated as a function of the spatial structure of the rainfall process. It is shown that the slope is smaller than one. For the special case of one gauge inside a fixed area or a Field Of View (FOV), an areal reduction factor is derived. It has a limit value which depends only on the size of the area and the spatial structure of the rainfall process. The relative variance error of estimating the FOV cumulative rainfall from point rainfall is also given.     

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (²³⁰Th/²³²Th) (≥1.06) with low (²³⁸U/²³⁰Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (²³⁰Th/²³²Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (²³⁸U/²³⁰Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion ⁴⁰Ar/³⁹Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (²³⁰Th/²³²Th)₀ ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997