Torsional wave under initial stress     

Sukhendu Dey 《Pure and Applied Geophysics》1972,94(1):53-59

Summary In this paper, the frequency equation for phase velocity of torsional wave in a rod of circular cross section under initial tension has been obtained. The graphs are plotted to compare the velocity with that in the absence of initial tension. Some analytical informations have been obtained for instability of the body under initial compression.  相似文献   

Seismic vulnerability assessment of earthquake-prone mega-city Shillong,India using geophysical mapping and remote sensing     

Saurabh Baruah  Goutam Kashyap Boruah  Sangeeta Sharma  Washim Akram Hoque  Timangshu Chetia  Chandan Dey 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2020,14(2):112-127

ABSTRACT

The concept of seismic vulnerability is a yard-stick of damage estimation from a probable earthquake considering physical cum social dimension and enables a basis for decision-makers to develop preparedness and mitigation strategies. We aim at vulnerability assessment of the typical urban system of capital city Shillong situated on hilly terrain. High-resolution satellite imagery of Shillong facilitates analysis of building footprints, communication network, and open ground. Different building typologies are identified taking into account the building’s structural configuration assessed through a rapid visual survey of more than 15% of total residential households. Slope map demarcates the landslide-prone area through discrete elevation modelling. A methodology incorporating several parameters e.g. building typology, slope angle, shear wave velocity characteristics, geomorphology, and the number of occupants in correlation with a physical measurement of vulnerability is presented and is applied to estimate the dimension of vulnerability. Additionally, MASW survey indicates lithology up to 30?m deep along with the existence of stiff soil and rocks at different depths whereas resonant frequency is identified to be in the range of 6–8?Hz through H/V ratio. Integrating all, it is observed that more than 60% of Shillong city falls under moderate to higher vulnerability and the rest is less vulnerable.  相似文献   

Landscape disequilibrium on 1000–10,000 year scales Marsyandi River, Nepal, central Himalaya     

Beth Pratt-Sitaula  Douglas W. Burbank  Arjun Heimsath  Tank Ojha 《Geomorphology》2004,58(1-4):223-241

In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10⁵–10⁷ years. Over shorter times, <10⁵ years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 10⁴-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and ¹⁴C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic ¹⁰Be and ²⁶Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 10⁴-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.  相似文献   

Quantifying sediment transport across an undisturbed prairie landscape using cesium-137 and high resolution topography     

James M. Kaste  Arjun M. Heimsath  Matthew Hohmann 《Geomorphology》2006,76(3-4):430-440

Soil erosion is a global environmental problem, and anthropogenic fallout radionuclides offer a promising tool for describing and quantifying soil redistribution on decadal time scales. To date, applications of radioactive fallout to trace upland sediment transport have been developed primarily on lands disturbed by agriculture, grazing, and logging. Here we use ¹³⁷Cs to characterize and quantify soil erosion at the Konza Prairie Long-Term Ecological Research (LTER) site, an undisturbed grassland in northeastern Kansas. We report on the small scale (< 10 m) and landscape scale (10 to 1000 m) distribution of fallout ¹³⁷Cs, and show significant variability in the concentrations and amounts of ¹³⁷Cs in soils at our site. ¹³⁷Cs soil concentrations and amounts typically vary by 10% to 30% on small scales, which most likely represents the spatial heterogeneity of the depositional processes. Landscape scale variability of soil ¹³⁷Cs was significantly higher than small scale variability. Most notably, soils collected on convex (divergent) landforms had ¹³⁷Cs inventories of 2500 to 3000 Bq m^− 2, which is consistent with the expected atmospheric inputs to the study area during the 1950s and 1960s. Concave landforms, however, had statistically lower inventories of 1800 to 2300 Bq m^− 2. The distribution of ¹³⁷Cs on this undisturbed landscape contrasts significantly with distributions observed across disturbed sites, which generally have accumulations of radioactive fallout in valley bottoms. Because the upslope contributing area at each sampling point had a significant negative correlation with the soil inventory of ¹³⁷Cs, we suggest that overland flow in convergent areas dominates soil erosion at Konza on time scales of decades. Very few points on our landscape had ¹³⁷Cs inventories significantly above that which would be predicted from direct deposition of ¹³⁷Cs on the soil surface; we conclude therefore that there is little net sediment storage on this undisturbed landscape.  相似文献   

Reply to Comments by J. H. Chandler and Others     

Arjun M. Heimsath  Hany Farid 《Mathematical Geology》2003,35(3):351-352

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Assessment of bearing capacity of interfering strip footings located near sloping surface considering artificial neural network technique     

Rana Acharyya  Arindam Dey 《山地科学学报》2018,15(12):2766-2780

The bearing capacity of interfering footings located near the slope face suffers from reduced bearing capacity due to the formation of the curtailed passive zone. Depending upon the position of the footing, their spacing and steepness of the slope, different extents of bearing capacity reduction can be exhibited. A series of finite element investigation has been done with the aid of Plaxis 3D vAE.01 to elucidate the influence of various geotechnical and geometrical parameters on the ultimate bearing capacity of interfering surface strip footings located at the crest of the natural soil slope. Based on the large database obtained from the numerical simulation, a 6-8-1 Artificial Neural Network architecture has been considered for the assessment of the ultimate bearing capacity of interfering strip footings placed on the crest of natural soil slope. Sensitivity analyses have been conducted to establish the relative significance of the contributory parameters, which exhibited that for the stated problem, apart from shear strength parameters, the setback ratio and spacing of footing are the prime contributory parameters.  相似文献   

Transverse canyon incision and sedimentary basin excavation driven by drainage integration,Aravaipa Creek,AZ, USA     

Matthew C. Jungers  Arjun M. Heimsath 《地球表面变化过程与地形》2019,44(5):1077-1090

Drainage reorganization events have the potential to drive incision and erosion at high rates normally attributed to tectonic or climatic forcing. It can be difficult, however, to isolate the signal of transient events driven by drainage integration from longer term tectonic or climatic forcing. We exploit an ideal field setting in Aravaipa Creek Basin of southeastern Arizona, USA, to isolate just such a signal. Aravaipa Creek Basin underwent a period of transient incision that formed Aravaipa Canyon, evacuating a significant volume of sedimentary basin fill and Tertiary bedrock from the previously internally drained basin. We use digital terrain analyses to reconstruct the pre-incision landscapes of both Aravaipa Creek Basin and the adjacent Lower San Pedro Basin, which we use to quantify the magnitude of incision and erosion since the drainage basins integrated. Terrestrial cosmogenic nuclide burial dates from ¹⁰Be and ²⁶Al concentrations in latest stage basin fill in Aravaipa Creek enable us to calculate long-term incision and erosion rates from 3 Myr to the present. A ¹⁰Be concentration–depth profile from the Lower San Pedro Basin confirms that the San Pedro River incised into its high stand deposits prior to 350 000–400 000 years ago. Combining our landscape reconstructions with these age constraints, we determine that the transient rates of incision that created Aravaipa Canyon were 150 m/Myr or more, but that the background rate of erosion since integration is an order of magnitude lower, between 10 and 20 m/Myr. These results support our growing understanding that tectonic and climatic forcings need not apply for all episodes of rapid, transient incision and erosion during landscape evolution. © 2018 John Wiley & Sons, Ltd.  相似文献   

Resistant rock layers amplify cosmogenically-determined erosion rates     

Andrew Darling  Kelin Whipple  Paul Bierman  Brian Clarke  Arjun Heimsath 《地球表面变化过程与地形》2020,45(2):312-330

Prior numerical modeling work has suggested that incision into sub-horizontal layered stratigraphy with variable erodibility induces non-uniform erosion rates even if base-level fall is steady and sustained. Erosion rates of cliff bands formed in the stronger rocks in a stratigraphic sequence can greatly exceed the rate of base-level fall. Where quartz in downstream sediment is sourced primarily from the stronger, cliff-forming units, erosion rates estimated from concentrations of cosmogenic beryllium-10 (¹⁰Be) in detrital sediment will reflect the locally high erosion rates in retreating cliff bands. We derive theoretical relationships for threshold hillslopes and channels described by the stream-power incision model as a quantitative guide to the potential magnitude of this amplification of ¹⁰Be-derived erosion rates above the rate of base-level fall. Our analyses predict that the degree of erosion rate amplification is a function of bedding dip and either the ratio of rock erodibility in alternating strong and weak layers in the channel network, or the ratio of cliff to intervening-slope gradient on threshold hillslopes. We test our predictions in the cliff-and-bench landscape of the Grand Staircase in southern Utah, USA. We show that detrital cosmogenic erosion rates in this landscape are significantly higher (median 300 m/Ma) than the base-level fall rate (~75 m/Ma) determined from the incision rate of a trunk stream into a ~0.6 Ma basalt flow emplaced along a 16 km reach of the channel. We infer a 3–6-fold range in rock strength from near-surface P-wave velocity measurements. The approximately four-fold difference between the median ¹⁰Be-derived erosion rate and the long-term rate of base-level fall is consistent with our model and the observation that the stronger, cliff-forming lithologies in this landscape are the primary source of quartz in detrital sediments. © 2020 John Wiley & Sons, Ltd.  相似文献   

Quantifying erosional equilibrium across a slowly eroding,soil mantled landscape     

Arjun M. Heimsath  Oliver A. Chadwick  Josh J. Roering  Shaun R. Levick 《地球表面变化过程与地形》2020,45(3):499-510

The textbook concept of an equilibrium landscape, which posits that soil production and erosion are balanced and equal channel incision, is rarely quantified for natural systems. In contrast to mountainous, rapidly eroding terrain, low relief and slow-eroding landscapes are poorly studied despite being widespread and densely inhabited. We use three field sites along a climosequence in South Africa to quantify very slow (2-5 m/My) soil production rates that do not vary across hillslopes or with climate. We show these rates to be indistinguishable from spatially invariant catchment-average erosion rates while soil depth and chemical weathering increase strongly with rainfall across our sites. Our analyses imply landscape-scale equilibrium although the dominant means of denudation varies from physical weathering in dry climates to chemical weathering in wet climates. In the two wetter sites, chemical weathering is so significant that clay translocates both vertically in soil columns and horizontally down hillslope catenas, resulting in particle size variation and the accumulation of buried stone lines at the clay-rich depth. We infer hundred-thousand-year residence times of these stone lines and suggest that bioturbation by termites plays a key role in exhuming sediment into the mobile soil layer from significant depths below the clay layer. Our results suggest how tradeoffs in physical and chemical weathering, potentially modulated by biological processes, shape slowly eroding, equilibrium landscapes. © 2019 John Wiley & Sons, Ltd.  相似文献   

Spatio-temporal variations in the estimation of PM10 from MODIS-derived aerosol optical depth for the urban areas in the Central Indo-Gangetic Plain     

Shikha Chitranshi  Satya Prakash Sharma  Sagnik Dey 《Meteorology and Atmospheric Physics》2015,127(1):107-121

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