The ExaVolt Antenna: A large-aperture, balloon-embedded antenna for ultra-high energy particle detection

We describe the scientific motivation, experimental basis, design methodology, and simulated performance of the ExaVolt Antenna (EVA) mission, and planned ultra-high energy (UHE) particle observatory under development for NASA’s suborbital super-pressure balloon program in Antarctica. EVA will improve over ANITA’s integrated totals – the current state-of-the-art in UHE suborbital payloads – by 1–2 orders of magnitude in a single flight. The design is based on a novel application of toroidal reflector optics which utilizes a super-pressure balloon surface, along with a feed-array mounted on an inner membrane, to create an ultra-large radio antenna system with a synoptic view of the Antarctic ice sheet below it. Radio impulses arise via the Askaryan effect when UHE neutrinos interact within the ice, or via geosynchrotron emission when UHE cosmic rays interact in the atmosphere above the continent. EVA’s instantaneous antenna aperture is estimated to be several hundred m² for detection of these events within a 150–600 MHz band. For standard cosmogenic UHE neutrino models, EVA should detect of order 30 events per flight in the EeV energy regime. For UHE cosmic rays, of order 15,000 geosynchrotron events would be detected in total, several hundred above 10 EeV, and of order 60 above the GZK cutoff energy.     

Lichenometric Dating of Rock Surfaces in the Northern Cascade Range,USA

This study presents a growth curve developed from direct and indirect growth rates of Rhizocarpon geographicum lichens from study sites on Mounts Baker, Rainier, Adams, and Hood in the northern Cascade Range of the western USA. Our observations of direct growth rates are based on 31 measurements of 11 lichens growing on different surfaces. This direct growth rate dataset is complemented by indirect growth rates based on measurements of the largest lichen observed on 20 different surfaces over 24–33‐yr periods. The direct and indirect datasets produce statistically indistinguishable mean radial growth rates of 0.48 and 0.50 mm yr^?1, respectively. Statistical analysis of zero and first order fits of our growth rate data suggests that lichen growth is best characterized by the average of our mean growth rate (zero order) models at 0.49 mm yr^?1. Our revised growth curve for the study area extends the applicable range for dating rock surface in the study area to the seventeenth century, approximately 175 years longer than previous calibrated curves.     

Memories of Donald V. Helmberger: Mentor and seismologist extraordinaire

I grew up in one of the most aseismic regions of the planet but decided as a child that I wanted to be a seismologist.Ignoring all the naysayers who claimed tha...     

Am improved source mechanism for the 1935 Timiskaming,Quebec earthquake from regional waveforms

The Timiskaming earthquake, which occurred near the Quebec-Ontario border at the northwest end of the Western Quebec seismic zone in 1935, is one of the five largest instrumentally recorded southeastern Canadian earthquakes. Previous studies of this earthquake concentrated on modeling teismograms recorded at regional distances, a better constrained focal mechanism is obtained. The waveforms indicate thrust faulting on a moderately dipping northwest striking plane at a depth of 10 km. TheM _w of 6.1 determined in this study is in good agreement with previous magnitude estimates (m _b 6.1,M _s 6.0, andm _bLg 6.2–6.3). The focal mechanism is similar to those of many recent small to moderate earthquakes in the region, and the inferred (from theP axis) acting stress of northeast compression is consistent with the overall eastern North American stress field. The Lake Timiskaming Rift Valley in which the earthquake occurred, comprises several northwest striking faults consistent with the strike of the 1935 event. Thus, the 1935 earthquake appears to be a result of faulting on the reactivated Timiskaming graben.     

Developments in a non-destructive method of determining rock strength: A reply

In a previous publication (Allison, 1989), a non-destructive method for indirectly determining rock strength by measuring Dynamic Young's Modulus was described. Data were presented to assess the Grindosonic apparatus in relation to standard laboratory techniques. A further Short Communication (Allison, 1990) evaluated the non-destructive test as a field technique, in part achieved by comparing the Grindosonic results with data collected using the Schmidt hammer. The Schmidt hammer is a widely used field technique in geomorphology for determining rock strength (see for example Day and Goudie, 1977; Day, 1981). Allison (1989, 1990) also suggested that the elastic properties of materials are becoming increasingly important in geomorphological studies. The opportunity to provide additional information and comments here is appreciated.     

The East Antarctic sea ice zone: Ice characteristics and drift

Results from studies of the surface energy balance and the ocean structure in the presence of fast ice near Mawson on the Antarctic coast are used to illustrate the important ways in which sea ice interacts with the ocean and atmosphere. Away from the coast, ship and drifting buoy observations are used to characterize the E Antarctic sea ice zone in a study area between 60° E and 120° E and S of 61° S. Divergent drift over most of the region plays a dominant role in expanding the ice extent in autumn and in determining the characteristics of the pack. Much of the sea ice in the region is young thin ice which forms in leads and polynyas, and in late spring in the study area, the ice thickness averaged over the total ocean surface within the ice edge less than 0.4 m. Even in winter the majority of ice floes off E Antarctica are probably less than 1 m thick.     

Microstructural and chemical transformations accompanying deformation of granite in a shear zone at Miéville,Switzerland; with implications for stress corrosion cracking and superplastic flow

At Miéville, in the Aiguilles-Rouges Massif, granitic rocks of the basement are deformed into mylonites within a major subvertical shear zone. The ambient temperature during translation is estimated at 250° C±30° C from fluid inclusion filling temperatures in syntectonic microveins, from ¹⁸O quartzilmenite of+15%, and from mineralogical criteria. Porphyroclasts of both oligoclase and orthoclase feldspar decrease from initial diameters of 20 mm and assume elliptical shapes during progressive deformation, due to recrystallisation of the margins to ultra-fine polygonal grains which extend out from the porphyroclasts in thin trails: the final stable grain size is <5 . The recrystallised feldspar has a composition of the parent porphyroclast,+albite, requiring relative gains of Na and losses of K+Ca compared to the precursor, and implying short range redistribution of the components during deformation. Decrease of free energy associated with the deformation catalysed change in feldspar composition, coupled with stored strain energy in the porphyroclasts may account for recrystallisation to a stable aggregate of ultrafine grain size. The long trails imply exceptionally high ductility, which, coupled with microstructural criteria, and admixture of quartz from neighbouring pure quartz aggregates by grain boundary sliding, is interpreted in terms of superplastic flow. Estimated temperatures of T/T _m0.2 for the inferred superplastic deformation is lower by a factor of 2 than previously recorded for this flow michanism in silicates. The feldspar and quartz probably accomodated grain boundary sliding by intercrystalline diffusion.Biotite responds to deformation by bendgliding, kinking, and recrystallisation in mantles. The reaction of high-Ti parent grains to low-Ti biotite+Fe-muscovite+ ilmenite+chlorite is catalysed at all of these microstructural sites. Progressive deformation of the fine-grained products in the mantles is coupled with steady reaction to low-Fe muscovite+epidote+ sphene+rutile resulting in exceptionally ductile trails, as for the case of feldspar.Biotite grains have pervasive networks of nondisplacive intragranular fractures. At the fracture tips increase of the stress intensity has catalysed the reaction of high-Ti parent grains to low-Ti biotite+muscovite+ ilmenite which occupy the fractures. The fractures propagate and coalesce resulting in mechanical breakdown of the parent grains: these microstructures are believed to be examples of natural stress corrosion cracking. These features are also abundant in feldspar porphyroclasts where at fracture tips orthoclasesecondary orthoclase+albite, and oligoclasesecondary oligoclase+albite. Stress corrosion cracking may be significant in the steady state deformation of crustal rocks at low temperatures when intracrystalline plasticity is not generally dominant.Two way mass balance calculations utilising major and selected trace element data, reveal that deformation of the granite was essentially isochemical, involving average additions of <1 % H₂O+CO₂, at approximately constant specific gravity. The parameters Fe²⁺/Fe and ¹⁸O_{whole rock} maintain relatively constant values across the shear zone, and this also implies limited participation of fluids in the deformation. Alkali elements and titanium display the largest percentage variation during progressive deformation, whereas SiO₂, Al₂O₃, and P₂O₅, together with V, Ni, Cr,Y,Zr, and Nb remain relatively constant. All variations decrease at increasing states of deformation and this is interpreted in terms of mechanical mixing of chemical inhomogeneities of the granite precursor within the shear zone. Constraints imposed by variations in abundance of the relatively immobile elements imply that volume changes accompanying deformation in the shear zone were less than ±10%.     

Sources of organic matter in surface sediments of the Louisiana Continental margin: Effects of major depositional/transport pathways and Hurricane Ivan

Lignin and pigment biomarkers were analyzed in surface sediments of the Louisiana Continental margin (LCM) to distinguish differences in the degradative state of sedimentary organic matter along and between two major depositional pathways (along shore and offshore to the Mississippi Canyon) from Southwest (SW) Pass in July 2003. Barataria Bay, an inter-distributary estuary, was also assessed as a potential source of terrestrial organic matter to the LCM. Sediment signatures taken along the same pathways after Hurricane Ivan (October 2004) were compared with the pre-Ivan signature to elucidate carbon dynamics after major hurricane events. Density fractions were investigated at key stages across the LCM. Mississippi Canyon sediments are a depocenter for labile and refractory organic matter derived from river and previously deposited shelf sediments. Barataria Bay material may be a contributing source of sedimentary organic matter in shallow shelf areas bordering the bay and is thus potentially important in carbon cycling in sediments of these shallow areas; however, our results show that organic matter inputs from the bay were likely rapidly decomposed and/or diluted. Hurricane Ivan mobilized sedimentary organic carbon (SOC) offshore and homogenized terrestrial sediment parameters and gradients. As observed through pigment concentrations sediments tended to equilibrate to a more steady-state condition within months of the disturbance. Insights from density fractions show that selective degradation and aggregation/flocculation processes were also very important processes during cross-shelf transport. Zooplankton grazing, largely on diatoms and other algae, was a shelf wide phenomenon, however, grazing products dominated the marine-derived SOC in margin sediments west of the birdsfoot delta indicated by the abundance of steryl chlorin esters (SCEs).