Current motion on faults of the San Andreas system in central California inferred from recent GPS and terrestrial survey measurements

The San Andreas fault system of California comprises dominantly right-lateral strike-slip faults and forms part of the Pacific-North American plate boundary. This fault system has been studied extensively using geological and geophysical methods since it was first brought into prominence by the 1906 M = 8(^1)(_4) San Francisco earthquake. Observations of surface deformation thought to define an earthquake deformation cycle have been inferred from terrestrial and space-based geodetic methods. The observed relative motion in these networks has also been used to constrain the distribution of motion across the plate boundary. Sites in three profiles extending across the fault system in the San Francisco bay region were measured up to 7 times between March 1990 and February 1993 using the Global Positioning System (GPS). The data were processed using the Bernese V3.2 software. The GPS data were combined with trilateration and VLBI data to create a spatially dense sample of the deformation field in the region. Approximately 35±3 mm/yr of fault-parallel (N33ºW) shear is distributed across a deforming zone that increases in width northwards from 60 to 100 km and in style from fault-concentrated deformation in the south to near-linear trends in the north. No systematic convergence upon the fault is observed. Both two- and three-dimensional models of dislocations in an elastic half-space were used to model the deformation and to investigate the effects of structural complexities such as a low-rigidity fault zone, the depth to which surface creep extends, geometrical complexities of the fault system and along-strike variations in slip rate. The models produce a remarkably close fit to the deformation despite such a rheologically simple Earth structure. Approximately half of the observed deformation is accommodated along faults to the east of the San Andreas fault. A zone of concentrated deformation across the San Andreas fault zone in the north of the region may be the result of a 1-2 km wide low-rigidity fault zone there. Surface creep rates, although highly variable, appear to increase to the south. An increase in depth of the surface creep zone to the south may also accompany this. The variations in slip rate at depth along strike are consistent with connectivity between the major faults of the system. Quasi-steady slip on discrete fault planes or shear zones may occur down to 2-3 times the seismogenic depth and deformation rates are probably almost constant throughout much of the earthquake cycle. The present earthquake potential calculated from the estimated slip rates indicate that several fault segments may have an earthquake potential equivalent in magnitude to the "characteristic" earthquake assumed for that segment. The estimates of relative motion indicate that deformation across the San Andreas fault system, plus that observed to the east of the Sierra Nevada mountains, can account for all of the Pacific-North American plate motion rate

Theory of planned behaviour variables and objective walking behaviour do not show seasonal variation in a randomised controlled trial

BACKGROUND: Longitudinal studies have shown that objectively measured walking behaviour is subject to seasonal variation, with people walking more in summer compared to winter. Seasonality therefore may have the potential to bias the results of randomised controlled trials if there are not adequate statistical or design controls. Despite this there are no studies that assess the impact of seasonality on walking behaviour in a randomised controlled trial, to quantify the extent of such bias. Further there have been no studies assessing how season impacts on the psychological predictors of walking behaviour to date. The aim of the present study was to assess seasonal differences in a) objective walking behaviour and b) Theory of Planned Behaviour (TPB) variables during a randomised controlled trial of an intervention to promote walking. METHODS: 315 patients were recruited to a two-arm cluster randomised controlled trial of an intervention to promote walking in primary care. A series of repeated measures ANCOVAs were conducted to examine the effect of season on pedometer measures of walking behaviour and TPB measures, assessed immediately post-intervention and six months later. Hierarchical regression analyses were conducted to assess whether season moderated the prediction of intention and behaviour by TPB measures. RESULTS: There were no significant differences in time spent walking in spring/summer compared to autumn/winter. There was no significant seasonal variation in most TPB variables, although the belief that there will be good weather was significantly higher in spring/summer (F = 19.46, p < .001). Season did not significantly predict intention or objective walking behaviour, or moderate the effects of TPB variables on intention or behaviour. CONCLUSION: Seasonality does not influence objectively measured walking behaviour or psychological variables during a randomised controlled trial. Consequently physical activity behaviour outcomes in trials will not be biased by the season in which they are measured. Previous studies may have overestimated the extent of seasonality effects by selecting the most extreme summer and winter months to assess PA. In addition, participants recruited to behaviour change interventions might have higher levels of motivation to change and are less affected by seasonal barriers. TRIAL REGISTRATION: Current Controlled Trials ISRCTN9593290

A 10-Year Comparison of Water Levels Measured with a Geodetic GPS Receiver versus a Conventional Tide Gauge

A standard geodetic GPS receiver and a conventional Aquatrak tide gauge, collocated at Friday Harbor, Washington, are used to assess the quality of 10 years of water levels estimated from GPS sea surface reflections. The GPS results are improved by accounting for (tidal) motion of the reflecting sea surface and for signal propagation delay by the troposphere. The RMS error of individual GPS water level estimates is about 12 cm. Lower water levels are measured slightly more accurately than higher water levels. Forming daily mean sea levels reduces the RMS difference with the tide gauge data to approximately 2 cm. For monthly means, the RMS difference is 1.3 cm. The GPS elevations, of course, can be automatically placed into a well-defined terrestrial reference frame. Ocean tide coefficients, determined from both the GPS and tide gauge data, are in good agreement, with absolute differences below 1 cm for all constituents save K1 and S1. The latter constituent is especially anomalous, probably owing to daily temperature-induced errors in the Aquatrak tide gauge

Scattering by wave-bearing surfaces under fluid loading

Wave-bearing surfaces and compressible fluids are often adjacent, the subsequent interactions are of substantial interest in structural acoustics, acoustic microscopy, seismology and many other fields. Here we take a broad view and discuss a variety of problems, both time harmonic and transient, which are amenable to exact solution. These in turn highlight physical effects and can additionally form the basis of asymptotic solutions. In structural acoustics the interaction of plate waves with defects is Cl major source of underwater noise. A model problem of two semi-infinite elastic plates (made of different material) joined in a variety of ways is considered for obliquely incident flexural plate waves. Asymptotic results for 'light' and 'heavy' fluid loading are extracted. In addition reciprocity and power flow relations, besides being of independent interest, provide a useful check on the results. There are many closely related problems involving a fluid loaded elastic solid. The situation here is somewhat similar, but often more complicated, due to the number of waves that an elastic solid supports, mode conversion at interfaces, and interfacial waves. We first address the scattering effects of low frequency waves by very small interfacial defects, that is, small relative to a typical wavelength. In this limit, and in related water wave or acoustic work, matched asymptotic expansions are used. An important aspect, that has not been noticed before, is the natural separation that occurs in the inner problem into fluid and solid pieces. A matching argument may now be used to give a useful physical interpretation of these defects and far field directivity patterns show the distinctive beaming that occurs along the Rayleigh angles in the light fluid loading limit. In many areas of interest embedded defects are imaged by pulses and we therefore require a transient analysis. In this case our problem involves a combination of compressional and shear source loadings beneath a fluid-solid interface. The exact solution is found and a full asymptotic analysis of this solution is performed with an emphasis upon wavefront expansions and leaky waves, and in particular, for 'light' and 'moderate' fluid loading. In some situations, when the sources are near the interface, a pseudo-compressional wavefront is generated and the limit as the loading approaches the interface is investigated. These non-geometric wave arrivals may be important in seismology and elastic wave studies related to the non-destructive evaluation of structures. This study is generalised to investigate the dynamic stress loading of subsurface cracks in either homogeneous or non-homogeneous media. An iterative method of solution based on physical considerations is developed and quantities of interest such as the scattered displacement fields and the stress intensity factors are determined. The problems considered here are ideally suited to analysis by transform methods and the Wiener-Hopf and Cagniard-de Hoop techniques

Fell bundles and imprimitivity theorems

Our goal in this paper and two sequels is to apply the Yamagami–Muhly–Williams equivalence theorem for Fell bundles over groupoids to recover and extend all known imprimitivity theorems involving groups. Here we extend Raeburn’s symmetric imprimitivity theorem, and also, in an appendix, we develop a number of tools for the theory of Fell bundles that have not previously appeared in the literature

An optical fiber Bragg grating tactile sensor

Tactile sensors are needed for many emerging robotic and telepresence applications such as keyhole surgery and robot operation in unstructured environments. We have proposed and demonstrated a tactile sensor consisting of a fibre Bragg grating embedded in a polymer "finger". When the sensor is placed in contact with a surface and translated tangentially across it measurements on the changes in the reflectivity spectrum of the grating provide a measurement of the spatial distribution of forces perpendicular to the surface and thus, through the elasticity of the polymer material, to the surface roughness. Using a sensor fabricated from a Poly Siloxane polymer (Methyl Vinyl Silicone rubber) spherical cap 50 mm in diameter, 6 mm deep with an embedded 10 mm long Bragg grating we have characterised the first and second moment of the grating spectral response when scanned across triangular and semicircular periodic structures both with a modulation depth of 1 mm and a period of 2 mm. The results clearly distinguish the periodicity of the surface structure and the differences between the two different surface profiles. For the triangular structure a central wavelength modulation of 4 pm is observed and includes a fourth harmonic component, the spectral width is modulated by 25 pm. Although crude in comparison to human senses these results clearly shown the potential of such a sensor for tactile imaging and we expect that with further development in optimising both the grating and polymer "finger" properties a much increased sensitivity and spatial resolution is achievable

Five-year impact of repeated praziquantel treatment on subclinical morbidity due to Schistosoma japonicum in China

We report the 5-year impact (1996-2001) of repeated praziquantel chemotherapy on subclinical morbidity related to Schistosoma japonicum infection. We repeated stool examinations and hepatosplenic ultrasonography in a cohort of 120 individuals living on an island with endemic infection in Dongting Lake, China. Prevalence of schistosome infection fell by 43% and intensity (geometric mean eggs per gram) declined by 80% over the 5 years. However, transmission persisted at a dangerously high rate of 13% per year for re-infection or new infection in the cohort. The prevalence of left-lobe enlargement and dilated portal vein fell significantly (P 0.05). However, endpoint infection was even more strongly associated with left-lobe enlargement (57% versus 15%, P < 0.01). The proportions of subjects with improved parenchymal and periportal fibrosis were much higher than the proportions of subjects that progressed (P < 0.05). Reduction of prevalence and intensity of infection, and improvement of subclinical morbidity, were benefits of repeated treatments. Further research is needed to understand why some patients developed fibrosis despite substantial reductions in egg counts and to evaluate the functional importance of residual subclinical morbidity after chemotherapy-based control in the lake and marshland area of Chin

Two-year impact of praziquantel treatment for Schistosoma japonicum infection in China: re-infection, subclinical disease and fibrosis marker measurements

We studied a community cohort of 193 individuals exposed to endemic Schistosoma japonicum infection in the Dongting Lake region of China to assess subclinical morbidity and the 2-year benefit of curative therapy (praziquantel) administered in 1996. Prevalence and intensity of S. japonicum infection before treatment were 28% and 192 eggs per gram faeces (epg), respectively. Two years after cure, 22% of the cohort were reinfected, but with a lighter intensity (67 epg). Sixty-four subjects (37%) showed significant improvement in ultrasound parenchyma images after treatment and 51 subjects (54%) showed significant improvement of periportal fibrosis. Left-lobe enlargement also reversed (P 0·05). The serum levels of laminin and collagen IV associated with reinfection and intensity and hyaluronic acid levels correlated with ultrasound findings (P < 0·01). Overall, treatment induced a marked decrease in subclinical hepatosplenic morbidity attributable to S. japonicum although low-intensity re-infection after treatment remained relatively frequent. Stratified analysis and logistic models evaluated potential confounding factors for assessment of treatment effects on hepatic fibrosis. S. japonicum infection and moderate-heavy alcohol intake interacted: improvement in parenchymal morbidity was impeded among drinkers (P < 0·05). Chemotherapy focused on at-risk residents controls prevalent subclinical hepatic fibrosis but re-infection indicates the need for complementary control strategie