A WKBJ spectral method for computation of SV synthetic seismograms in a cylindrically symmetric medium     

Chang-Eob Baag  Charles A. Langston 《Geophysical Journal International》1985,80(2):387-417

Summary. A method of synthetic seismogram computation for teleseismic SV -waves is developed in order to treat quantitatively SV -waves in problems of body wave source inversion and source—receiver structure studies. The method employs WKBJ theory for a generalized ray in a vertically inhomogeneous half-space and the propagator matrix technique for waves in near-surface homogeneous layers. Wavenumber integration is done along the real axis of the wavenumber plane and anelasticity is included by using complex velocity in all regions of the earth model. The near-surface source structure is taken into account in the computation for the case of the shallow source by allowing a point source to be located in the homogeneous layers. Source and receiver area structures are also allowed to differ. A general moment tensor point source is considered.  相似文献   

Lower-crustal rifting in the Rukwa Graben, East Africa     

Ming Zhao  Charles A. Langston  rew A. Nyblade  Thomas J. Owens 《Geophysical Journal International》1997,129(2):412-420

An M_w 5.9 earthquake occurred in the Lake Rukwa rift, Tanzania, on 1994 August 18, and was well recorded by 20 broad-band seismic stations at distances of 160 to 800 km and 21 broad-band stations at teleseismic distances. The regional and teleseismic waveforms have been used to investigate the source characteristics of the main shock, and also to locate aftershocks that occurred within three weeks of the main shock. Teleseismic body-wave modelling yields the following source parameters for the main shock: source depth of 25 ± 2 km, a normal fault orientation, with a horizontal tension axis striking NE-SW and an almost vertical pressure axis (Nodal Plane I: strike 126°–142°, dip 63°–66°, and rake 280°–290°; Nodal Plane II: strike 273°–289°, dip 28°–31°, and rake 235°–245°), a scalar moment of 4.1 times 10¹⁷ N m, and a 2 s impulsive source time function. Four of the largest aftershocks also nucleated at depths of 25 km, as deduced from regional sPmp–Pmp times. The nodal planes are broadly consistent with the orientation of both the Lupa and Ufipa faults, which bound the Rukwa rift to the northeast and southwest, respectively. The rupture radius of the main shock, assuming a circular fault, is estimated to be 4 km with a corresponding stress drop of 6.5 MPa. Published estimates of crustal thickness beneath the Rukwa rift indicate that the foci of the main shock and aftershocks lie at least 10 km above the Moho. The presence of lower-crustal seismicity beneath the Rukwa rift suggests that the pre-rift thermal structure of the rifted crust has not been strongly modified by the rifting, at least to depths of 25 km.  相似文献   

Discrete element modelling of material non‐coaxiality in simple shear flows     

Jun Ai  Paul A. Langston  Hai‐Sui Yu 《国际地质力学数值与分析法杂志》2014,38(6):615-635

We investigate the quasi‐static simple shear flow of a two‐dimensional assembly of cohesionless particles using discrete element method (DEM) simulations. We focus on the unsteady flow regime where the solid would experience significant evolution of stresses, mobilised shear strength and dilation. We construct the DEM model using a discretised‐wall confined granular cell where the apparent boundary is allowed to dilate or contract synchronously with the confined solid. A rather uniform simple shear field is achieved across the whole assembly, which benefits rheological studies in generalising constitutive laws for continuum methods. We examine two aspects of the simple shear behaviour: macroscopic stress and strain rate evolution, particularly the non‐coaxiality between the principal directions of the two; and micromechanics such as evolution of fabric. For an initially anisotropic specimen sheared under constant normal pressure, the direction of principal stress rotates towards that of the principal strain rate, gradually reducing the degree of non‐coaxiality from about 45° to fluctuating around 0°. The rate in approaching coaxiality is slower in samples with larger initial porosity, stress ratio and mean stress. Generally, a faster rate in approaching coaxiality in simple shear is observed in a more dilatant sample, which often shows a larger degree of mobilised fabric anisotropy, suggesting the possible important role of instantaneous internal friction angle. The evolution of principal fabric direction resembles that of the principal stress direction. © 2013 The Authors. International Journal for Numerical and Analytical Methods in Geomechanics published by John Wiley & Sons, Ltd.  相似文献   

Shear-coupled PL     

Chang-Eob Baag  Charles A. Langston 《Geophysical Journal International》1985,80(2):363-385

Summary. Observed teleseismic shear-coupled PL -waves ( SPL ) display a variety of waveforms depending on factors such as source depth, source type and velocity structure. Using a WKBJ spectral method for SV -wave propagation, synthetic seismograms of SPL are produced to examine the factors important in SV and SPL excitation. Results show that SPL is preferentially excited by shallow sources compared to deep sources. This is due to large source area reverberations which consequently leak as SV -waves into the mantle. Interaction at the receiver area then sets up the classic prograde elliptical motion by which SPL can be identified. This is in accordance with the teleseismic observations and indicates that most previous models for the propagation of SPL were not appropriate for shallow source since emphasis was placed on wave interactions occurring only near the receiver.  相似文献   

Spatial distribution and structure of benthic polychaete communities of Essaouira intertidal rocky shores (Atlantic coast of Morocco)     

Chouikh  Nor-eddine  Gillet  Patrick  Langston  W. J.  Cheggour  Mohamed  Maarouf  Abdelmalek  Mouabad  Abdelfattah 《中国海洋湖沼学报》2020,38(1):143-155

Polychaetes are one of the most important groups of macrobenthic organisms in coastal and marine environments, in terms of diversity and abundance, and play an important role in the functioning of ecosystem. This study aims to describe the structure and spatial distribution of polychaete communities along Essaouira's rocky shoreline(Atlantic coast of Morocco) in relation to the major environmental variables such as granulometry, salinity, temperature, pH, and intertidal range. Twelve sites from these intertidal rocky shores were sampled using a quadrat method during the summer 2016. A total of 4 435 individuals belonging to 34 taxa were recorded. Among the families, Sabellaridae(24%) and Nereididae(23%) were ranked ?rst in terms of abundance of individuals. Sabellaria alveolata(24%) and Perinereis cultrifera(13%) were the dominant species. Diversity( H′) values varied from 2.28 to 3.95. Pielou's index( J′) varied between 0.62 and 0.89. This was essentially due to the low dominance of few species. Cluster analysis was used to characterise Essaouira's rocky shores on the basis of benthic polychaete communities. SIMPER analysis con?rmed the presence of three distinct communities. According to canonical correspondence analysis, the structure and distribution of faunal assemblages of benthic polychaetes was mainly related to topographical complexity, water temperature, hydrodynamic conditions and upwelling. We conclude that these rocky shores show typically high benthic polychaete diversity compared to the taxonomic list of other rocky coasts. In addition, these results constitute a baseline data for the development of a sustainable network for long-term monitoring of benthic polychaete community changes due to ecological factor in?uences and anthropogenic activity impacts.  相似文献   

A numerical investigation of scattering effects for teleseismic plane wave propagation in a heterogeneous layer over a homogeneous half-space     

Gregory S. Wagner  Charles A. Langston 《Geophysical Journal International》1992,110(3):486-500

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Bioavailability and effects of sediment-bound TBT in deposit-feeding clams, Scrobicularia plana     

W. J. Langston  G. R. Burt 《Marine environmental research》1991,32(1-4)

Concentrations of tributyltin (TBT), dibutyltin (DBT), inorganic and total tin have been measured in water, sediments and deposit feeding clams, Scrobicularia plana, from 25 estuarine locations in England and Wales, in order to compare the bioavailability of different forms of the element. Abnormally high tin values in Scrobicularia from harbours and areas of high boating activity testify to the high bioaccumulation potential of organotins, notably TBT derived from anti-fouling paints, relative to inorganic tin.Infaunal deposit-feeding bivalves consistently contain higher concentrations of TBT than other benthic organisms indicating that sediments may be an important route for uptake. Significant correlations between sediment TBT values and residues in clams such as S. plana (r = 0·81, p < 0·001) support this hypothesis. Direct evidence of the bioaccumulation of sediment-bound TBT is provided from laboratory experiments in which Scrobicularia were exposed to TBT in water and in sediments, either separately or in combination. The results of these experiments clearly demonstrate a predominantly particulate component for TBT uptake in clams.The kinetics of TBT accumulation and elimination were studied in Scrobicularia exposed to sediments containing 0·3 and 1·0 μg/g TBT (as tin). Equilibrium concentrations in tissue are approached after 40 days.Sediments containing 10 μg/g TBT are acutely toxic to S. plana, although such concentrations are only likely to occur close to dockyards and large marinas. Preliminary laboratory and field observations suggest however, that clam populations could be affected at TBT concentrations in sediments of 0·3 μg/g and possibly lower.  相似文献   

Source parameters of some large earthquakes in Northern Aegean determined by body waveform inversion   总被引：1，自引：0，他引：1  

Anastasia A. Kiratzi  Gregory S. Wagner  Charles A. Langston 《Pure and Applied Geophysics》1991,135(4):515-527

Average source parameters for three large North Aegean events are obtained from body wave inversion for the moment tensor. The parameters for the events are as follows: The events exhibit dextral strike-slip faulting with theT axis striking NS and nearly horizontal, implying extension in this direction. The focal mechanisms obtained are in agreement with the seismotectonic regime of the North Aegean. It is known that the region is tectonically controlled by the existence of the strike-slip Anatolian fault and its westward continuation in the Aegean, as well as the NS extension the whole Aegean area undergoes.The components of the moment tensor show that the region is dominated by compression in the EW direction which is encompassed by extension in the NS direction. All the events were found to be shallow (10 km) with a source time function of approximately 8 s duration and small stress drop values.The teleseismic long period verticalP-waves exhibited distortions, that could be attributed to lateral inhomogeneities in the source structure or more probably to a nonflat water-crust interface.  相似文献   

基于IBOB的射电望远镜宽带数字频谱仪系统的设计与测试     

刘东亮  John Ford  Glen Langston  南仁东 《天文研究与技术》2010,7(1)

简述了基于IBOB和Xilinx System Generator平台的射电望远镜宽带数字频谱仪系统的设计与应用;给出了双路400MHz、2048通道、16IP输出数字频谱仪系统的具体设计方案,并对该数字频谱仪器的性能指标进行了分析和测试,验证了方案的可行性;最后初步讨论了滤波器设备对后端系统的影响。  相似文献   

Measurement of water content as a control of particle entrainment by wind     

Cheryl McKenna Neuman  Greg Langston 《地球表面变化过程与地形》2006,31(3):303-317

Of all controls on particle transport by wind, which include texture, crusting, vegetation cover and roughness, the role of water content is one of the most difficult to parameterize because of its high degree of spatial and temporal variability and its operation at a particle‐scale level directly at the surface. This study demonstrates that measurement of the distribution of brightness for all pixels in an image, now routinely employed in digital photography, is strongly correlated with gravimetric water content. Wind tunnel experiments further suggest that measurement of the distribution of β, as normalized against the brightness of the dry sand surface, is very useful in determining the order of magnitude of the mass transport rate (q). Finer resolution will likely never be achieved because of the heterogeneity of the particle transport phenomenon. Analysis of the variability in surface brightness does suggest that q is governed by the partitioning of momentum to particle motion that terminates in adhesion to surrounding areas of the surface that remain relatively wet. The proportion of surface particles that becomes dry appears to be of less importance. Preliminary work suggests that field application of digital photography in tracking spatial and temporal changes in the water content of beach deposits looks promising. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献