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231.
Nicholas W. Hayman Lucie Ducloué Kate L. Foco Karen E. Daniels 《Pure and Applied Geophysics》2011,168(12):2239-2257
It is a long-standing question whether granular fault material such as gouge plays a major role in controlling fault dynamics
such as seismicity and slip-periodicity. In both natural and experimental faults, granular materials resist shear and accommodate
strain via interparticle friction, fracture toughness, fluid pressure, dilation, and interparticle rearrangements. Here, we
isolate the effects of particle rearrangements on granular deformation through laboratory experiments. Within a sheared photoelastic
granular aggregate at constant volume, we simultaneously visualize both particle-scale kinematics and interparticle forces,
the latter taking the form of force-chains. We observe stick-slip deformation and associated force drops during an overall
strengthening of the shear zone. This strengthening regime provides insight into granular rheology and conditions of stick-slip
periodicity, and may be qualitatively analogous to slip that accompanies longer term interseismic strengthening of natural
faults. Of particular note is the observation that increasing the packing density increases the stiffness of the granular
aggregate and decreases the damping (increases time-scales) during slip events. At relatively loose packing density, the slip
displacements during the events follow an approximately power-law distribution, as opposed to an exponential distribution
at higher packing density. The system exhibits switching between quasi-periodic and aperiodic slip behavior at all packing
densities. Higher packing densities favor quasi-periodic behavior, with a longer time interval between aperiodic events than
between quasi-periodic events. This difference in the time-scale of aperiodic stick-slip deformation is reflected in both
the kinematics of interparticle slip and the force-chain dynamics: all major force-chain reorganizations are associated with
aperiodic events. Our experiments conceptually link observations of natural fault dynamics with current models for granular
stick-slip dynamics. We find that the stick-slip dynamics are consistent with a driven harmonic oscillator model with damping
provided by an effective viscosity, and that shear-transformation-zone, jamming, and crackling noise theories provide insight
into the effective stiffness and patterns of shear localization during deformation. 相似文献
232.
Peter Schürch Alexander L. Densmore Nicholas J. Rosser Michael Lim Brian W. McArdell 《地球表面变化过程与地形》2011,36(14):1847-1859
Detection of surface change is a fundamental task in geomorphology. Terrestrial laser scanners are increasingly used for monitoring surface change resulting from a variety of geomorphic processes, as they allow the rapid generation of high‐resolution digital elevation models. Irrespective of instrument specifics, survey design or data processing, such data are subject to a finite level of ambiguity in position measurement, a consideration of which must be taken into account when deriving change. The propagation of errors is crucial in change detection because even very small uncertainties in elevation can produce large uncertainties in volume when extrapolated over an area of interest. In this study we propose a methodology to detect surface change and to quantify the resultant volumetric errors in areas of complex topography such as channels, where data from multiple scan stations must be combined. We find that a commonly proposed source of error – laser point elongation at low incidence angles – has a negligible effect on the quality of the final registered point cloud. Instead, ambiguities in elevation inherent to registered datasets have a strong effect on our ability to detect and measure surface change. Similarly, we find that changes in surface roughness between surveys also reduce our ability to detect change. Explicit consideration of these ambiguities, when propagated through to volume calculations, allows us to detect volume change of 87 ± 5 m3, over an area of ~ ?4900 m2, due to passage of a debris flow down a 300 m reach of the Illgraben channel in Switzerland. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
233.
Alison R. Pawley Nicholas J. Chinnery Simon M. Clark Michael J. Walter 《Contributions to Mineralogy and Petrology》2011,162(6):1279-1289
The 10-Å phase (TAP) is a hydrous magnesium silicate that forms from the reaction of talc with H2O at high pressures. Its high-pressure, low-temperature stability means that it could be a storage site for H2O in subduction zones. We have determined the position of the TAP dehydration reaction, TAP = enstatite + coesite + H2O, in phase-equilibrium experiments from 5.0 to 7.1 GPa. Because previous studies had suggested that the composition of TAP is a function of synthesis duration, we used a TAP sample that was synthesised for 392 h. Over the pressure interval of our experiments, the dehydration reaction is isothermal, occurring at a temperature of ~690°C. It is coincident, within experimental uncertainty, with the position of the dehydration reaction of TAP synthesised in short experiments (up to 46 h). Above 7.5 GPa, TAP breaks down to enstatite + stishovite + H2O. This reaction has a negative dP/dT and terminates at an invariant point involving the 3.65-Å phase at ~9.5 GPa, 500°C. The zero volume change implied by the isothermal reaction TAP = enstatite + coesite + H2O was used to calculate the interlayer H2O content of TAP along the reaction. A best-fit H2O content of 1 H2O pfu was obtained. This H2O content is independent of TAP synthesis conditions, suggesting that variations in previously measured H2O contents of TAP occur during quenching and decompression of the samples. The stability of TAP in the Earth is probably limited to cold subduction zones, but in these, it could persist to 300 km depth. 相似文献
234.
235.
Nicholas R. de Sieyes Kevan M. Yamahara Adina Paytan Alexandria B. Boehm 《Estuaries and Coasts》2011,34(2):256-268
Two 14-day experiments conducted in the dry summer (July 2006) and wet winter (March 2007) seasons, respectively, examined
tidal, wave-driven, and seasonal variability of submarine groundwater discharge (SGD) at Stinson Beach, CA, using natural
radium tracers. Tide stage, tide range, breaker height, and season each explained a significant degree of radium variability
in the surf zone. A mass balance of excess radium in the surf zone was used to estimate SGD for each season, confirming larger
discharge rates during the wet season. Our results indicate median groundwater discharge rates of 6 to 8 L min−1 m−1 in July 2006 and 38 to 43 L min−1 m−1 in March 2007. SGD from 200 m of Stinson Beach in March 2007 contributed a flux of phosphate and dissolved inorganic nitrogen
approximately equal to that associated with all local creeks and streams within 6 km of the study site at that time. 相似文献
236.
Yuepeng PAN Mengna GU Yuexin HE Dianming WU Chunyan LIU Linlin SONG Shili TIAN Xuemei Lü Yang SUN Tao SONG Wendell W. WALTERS Xuejun LIU Nicholas A. MARTIN Qianqian ZHANG Yunting FANG Valerio FERRACCI Yuesi WANG 《大气科学进展》2020,37(9):933-938
正While China’s Air Pollution Prevention and Control Action Plan on particulate matter since 2013 has reduced sulfate significantly, aerosol ammonium nitrate remains high in East China. As the high nitrate abundances are strongly linked with ammonia, reducing ammonia emissions is becoming increasingly important to improve the air quality of China. Although satellite data provide evidence of substantial increases in atmospheric ammonia concentrations over major agricultural regions, long-term surface observation of ammonia concentrations are sparse. In addition, there is still no consensus on 相似文献
237.
Nicholas E. Graham Daniel R. Cayan Peter D. Bromirski Reinhard E. Flick 《Climate Dynamics》2013,40(5-6):1335-1360
A dynamical wave model implemented over the North Pacific Ocean was forced with winds from three coupled global climate models (CGCMs) run under a medium-to-high scenario for greenhouse gas emissions through the twenty-first century. The results are analyzed with respect to changes in upper quantiles of significant wave height (90th and 99th percentile HS) during boreal winter. The three CGCMs produce surprisingly similar patterns of change in winter wave climate during the century, with waves becoming 10–15 % smaller over the lower mid-latitudes of the North Pacific, particularly in the central and western ocean. These decreases are closely associated with decreasing windspeeds along the southern flank of the main core of the westerlies. At higher latitudes, 99th percentile wave heights generally increase, though the patterns of change are less uniform than at lower latitudes. The increased wave heights at high latitudes appear to be due a variety of wind-related factors including both increased windspeeds and changes in the structure of the wind field, these varying from model to model. For one of the CGCMs, a commonly used statistical approach for estimating seasonal quantiles of HS on the basis of seasonal mean sea level pressure (SLP) is used to develop a regression model from 60 years of twentieth century data as a training set, and then applied using twenty-first century SLP data. The statistical model reproduces the general pattern of decreasing twenty-first century wave heights south of ~40 N, but underestimates the magnitude of the changes by ~50–70 %, reflecting relatively weak coupling between sea level pressure and wave heights in the CGCM data and loss of variability in the statistically projected wave heights. 相似文献
238.
239.
240.
The downstream distribution of stream power is derived and analysed for 11 different streams in the upper Hunter River catchment, Australia. Stream long profiles were produced in a GIS environment using DEM data and catchment area–discharge analysis. These profiles were analysed using three approaches, namely long profile smoothing, curve fitting and a theoretical model. The methodology for deriving stream power profiles using these three approaches is discussed. The long profile smoothing method provides a good approximation of the subcatchment variability in stream power trends. The curve fitting method shows that higher-order exponential curves provide a better fit for long profile data. For the streams of the upper Hunter River catchment, second-order exponential curves fit well with significantly less error. The curve fitting method predicts a bimodal (upstream and midstream) distribution of stream power, which is a deviation from our earlier understanding of a single midstream peak. The theoretical approach provides a mathematical expression of the observed bimodal stream power distribution. The bimodal distribution emphasises the erosion potential of headwater reaches. The resultant stream power distribution provides a catchment-scale characterisation of the distribution of available energy in any given system. Using these approaches, the variability of stream power in headwater reaches is explained by discharge variability, while variability in midstream and downstream reaches is related to high variability in channel gradient. 相似文献