Influence of magnetic fabric anisotropy on seismic wave velocity in paramagnetic granites from NW Himalaya: Results from preliminary investigations     

Koushik Sen  Ruchika Sharma  B. R. Arora  Vikram Gupta 《Journal of the Geological Society of India》2010,76(5):322-330

Anisotropy of Magnetic Susceptibility (AMS) and seismic wave velocity studies of some paramagnetic Himalayan granitoids show good correlation between magnetic fabric anisotropy and P wave velocity (Vp). Vp shows strong positive correlation with magnetic lineation (L) and degree of magnetic anisotropy (P′) having correlation coefficient (r) values of 0.93 and 0.89 respectively. Both Vp and Vs show positive correlation with the SiO₂ content of Proterozoic and Paleozoic granitoids. Velocity of S wave (Vs) shows negative correlation with mean magnetic susceptibility (K_m) having ‘r’ value of 0.86. The correlation between Vs-K_m, V_p-P′, V_p-L also shows >95% probability in Spearman’s rank correlation. Based on the results from the present sample size it is suggested that, in paramagnetic granites, V_p is proportional to intensity of deformation and preferred orientation of minerals as well as the mineralogy. On the other hand, Vs is more dependent on the mineralogy alone.  相似文献   

Structural and magnetic fabric studies of recess structures in the western Himalaya: Implications for 1905 Kangra earthquake     

R. Jayangondaperumal  A. K. Dubey  K. Sen 《Journal of the Geological Society of India》2010,75(1):225-238

Kinematic information from deformation structures and magnetic fabrics are used to infer recent tectonics around Kangra and Dehradun recesses, western Himalaya. Three types of magnetic fabrics (Type I & II, III, and IV) are identified based on the angle between K3 axis and bedding pole. It was observed that Kangra recess shows more mature fabric type IV as compared to Dehradun recess, where orientation of K3 axes and tectonic fabric reveals dominance of superposed deformation. In the vicinity of Dehradun recess, normal faults occur in un-indurated Quaternary fan deposits confirming their recent formation. The observations are in conformity with earlier studies that during the 1905 Kangra earthquake, the Main Boundary Thrust (MBT) (or one of its subsidiary thrusts) near Kangra showed thrusting whereas the MBT near Dehradun underwent either normal faulting or post seismic adjustments characteristic of normal faulting. The thrusting could be a result of subsurface processes whereas the normal faulting was a result of prevailing surface strains.  相似文献   

Influence of magnetic fabric anisotropy on seismic wave velocity in paramagnetic granites from NW Himalaya: Results from preliminary investigations     

Koushik Sen  Ruchika Sharma  B. R. Arora  Vikram Gupta 《Journal of the Geological Society of India》2010,76(4):322-330

Anisotropy of Magnetic Susceptibility (AMS) and seismic wave velocity studies of some paramagnetic Himalayan granitoids show good correlation between magnetic fabric anisotropy and P wave velocity (Vp). Vp shows strong positive correlation with magnetic lineation (L) and degree of magnetic anisotropy (P′) having correlation coefficient (r) values of 0.93 and 0.89 respectively. Both Vp and Vs show positive correlation with the SiO₂ content of Proterozoic and Paleozoic granitoids. Velocity of S wave (Vs) shows negative correlation with mean magnetic susceptibility (K_m) having ‘r’ value of 0.86. The correlation between Vs-K_m, V_p-P′, V_p-L also shows >95% probability in Spearman’s rank correlation. Based on the results from the present sample size it is suggested that, in paramagnetic granites, V_p is proportional to intensity of deformation and preferred orientation of minerals as well as the mineralogy. On the other hand, Vs is more dependent on the mineralogy alone.  相似文献   

基于PSO-PP的围岩稳定性评价   总被引：1，自引：0，他引：1  

徐飞  徐卫亚  温森  刘造保  赵延喜 《岩土力学》2010,31(11):3651-3655

围岩的稳定性评价是一个复杂的不确定系统问题。结合投影寻踪方法、粒子群算法和逻辑斯谛曲线函数,建立了围岩稳定性评价的粒子群优化投影寻踪(projection pursuit based on particle swarm optimization,PSO-PP)模型。该模型一方面采用粒子群算法优化投影指标函数及逻辑斯谛曲线函数参数,确保了模型的准确性;另一方面利用逻辑斯谛曲线函数建立投影值与经验等级之间的非线性关系。模型的测试结果显示了良好的精度,实例分析结果与实际状态完全一致,表明该模型在围岩稳定性评价中的可行性和有效性。  相似文献   

Geochemical signatures of Mesoproterozoic siliciclastic rocks of the Kaimur Group of the Vindhyan Supergroup,Central India     

Meenal Mishra  Shinjana Sen 《中国地球化学学报》2010,29(1):21-32

The Upper Kaimur Group of the Vindhyan Supergroup in Central India, primarily consists of three rock types-DhandraulSandstone, Scarp Sandstone and Bijaigarh Shale. Mineralogically and geochemically, they are quartz arenite, sublitharenite to litharenite and litharenite to shale in composition, respectively. The A-CN-K ternary plot and CIA and ICV values suggest that the similar source rocks suffered severe chemical weathering, under a hot-humid climate in an acidic environment with higher P _{CO 2}, which facilitated high sediment influx in the absence of land plants. Various geochemical discriminants, elemental ratios like K₂O/Na₂O, Al₂O₃/TiO₂, SiO₂/MgO, La/Sc, Th/Sc, Th/Cr, Gd_N/Yb_N and pronounced negative Eu anomalies indicate the rocks to be of post-Archean Proterozoic granitic source, with a minor contribution of granodioritic input, in a passive margin setting. The sediments of the Upper Kaimur Group were probably deposited in the interglacial period in between the Paleoproterozoic and Neoproterozoic glacial epochs.  相似文献   

Decadal changes in the relationship between the Indian and Australian summer monsoons     

Karumuri Ashok  C. Nagaraju  A. Sen Gupta  D. S. Pai 《Climate Dynamics》2014,42(3-4):1043-1052

In this study, we investigate a long-term modulation in the relationship between Indian summer monsoon rainfall with the subsequent Australian summer monsoon rainfall. The two monsoon rainfall time series are significantly correlated at 0.3 at the 99 % confidence level. However, the relationship weakens during the 1932–1966 period, with the inter-monsoon correlation for the period falling below statistical significance. We find that this modulation is consistent with a breakdown of the typical El Niño-Southern Oscillation (ENSO) influence on sea surface temperature in the northern region of Australia, during this period. In addition, a change in the relative influences of ENSO and Indian Ocean Basin-wide Warming sea surface temperature anomalies on the Australian summer monsoon rainfall is also apparent across different time periods.  相似文献   

Diurnal variation in the initiation of rainfall over the Indian subcontinent during two different monsoon seasons of 2008 and 2009   总被引：1，自引：1，他引：0  

Shouraseni Sen Roy  Soma Sen Roy 《Theoretical and Applied Climatology》2014,117(1-2):277-291

In the present study, the diurnal variations in the time of initiation of rainfall, during two contrasting monsoon seasons of 2008 (below normal) and 2009 (normal) over the Indian subcontinent and surrounding oceanic areas has been analyzed. Harmonic analysis was used to detect the spatial variation of the diurnal cycle of the time of initiation of rainfall, as obtained at half-hourly intervals from the Kalpana 1 satellite. In general, the diurnal cycle in the time of initiation is strongest in regions where convective clouds are predominant, while it is weaker in regions where the clouds are predominantly stratiform with long-lived medium to high cloud cover. In the interior of the subcontinent, the time of maximum mainly occurred in the afternoon to evening hours, with a distinct southeast to northwest gradation. Substantial spatial variations were detected in the diurnal patterns between a normal and below normal monsoon years. Spatially, rainfall is initiated later in 2009 compared to 2008 over most of the interior of the Indian subcontinent. The most distinct difference was observed over the core monsoon region in central India, where the diurnal patterns were stronger in 2009 compared to 2008. On the other hand, over the oceans surrounding the Indian subcontinent, the initiation times are generally earlier in 2009.  相似文献   

Optimization of Nowcast Software WDSS-II for operational application over the Indian region     

Soma Sen Roy  Subhendu Brata Saha  S. K. Roy Bhowmik  P. K. Kundu 《Meteorology and Atmospheric Physics》2014,124(3-4):143-166

Nowcasting in the India Meteorological Department (IMD) is being provided for T + 0 to T + 2 h, using the Warning Decision Support System (WDSS-II) software. Prior to operational nowcasting over the Indian region, the parameters of the nowcast algorithm tool of the software were optimized, and accuracy was evaluated for various weather systems over Delhi. This optimization is demonstrated in this study with reference to three weather systems over Delhi, with each case representing one of three typical types of cloud systems over the region. These are—(a) convective lines associated with winter and early pre-monsoon weather systems, (b) deep convective cells that form in the pre-monsoon (April–June) and post-monsoon season (October–November) and (c) wide convective echoes that form during the monsoon season. The efficacy of the algorithm was assessed on a frame-by-frame basis as well as holistically for entire convective episodes. The important findings of the frame-by-frame study are (1) the inability of the inbuilt growth-decay algorithm to capture the evolution of storm cells, (2) setting of the threshold of detection of storms and tracking storms and (3) number of scales through which storms should be tracked. The holistic capabilities of the nowcast algorithm were tested for entire convective episodes using Model Evaluation Tools software. The results indicate that the advection algorithm tends to move the convective areas faster than observed at all time scales. Hence the multi-scale segmentation approach (over the two-scale approach) increases the smoothening of the output, at the cost of decreased nowcast skill. The inter-event comparison indicates that the low-intensity convective line zones, which are characteristic of winter and early pre-monsoon weather systems, have the most rapid temporal change in the overall area under convection. This leads to larger area errors during nowcasting of these systems. On the other hand, pre-monsoon systems comprised mostly isolated cells that reach great heights and move very fast, but do not have much horizontal area growth. The error in the nowcasting of these systems is mostly in respect of location error, as well as error in forecast of the intensity of the cells. The overall error in nowcasting is least for the monsoon systems over the Delhi region.  相似文献   

Simulation of the annual and diurnal cycles of rainfall over South Africa by a regional climate model     

Benjamin Pohl  Mathieu Rouault  Shouraseni Sen Roy 《Climate Dynamics》2014,43(7-8):2207-2226

The capability of a current state-of-the-art regional climate model for simulating the diurnal and annual cycles of rainfall over a complex subtropical region is documented here. Hourly rainfall is simulated over Southern Africa for 1998–2006 by the non-hydrostatic model weather research and forecasting (WRF), and compared to a network of 103 stations covering South Africa. We used five simulations, four of which consist of different parameterizations for atmospheric convection at a 0.5 × 0.5° resolution, performed to test the physic-dependency of the results. The fifth experiment uses explicit convection over tropical South Africa at a 1/30° resolution. WRF simulates realistic mean rainfall fields, albeit wet biases over tropical Africa. The model mean biases are strongly modulated by the convective scheme used for the simulations. The annual cycle of rainfall is well simulated over South Africa, mostly influenced by tropical summer rainfall except in the Western Cape region experiencing winter rainfall. The diurnal cycle shows a timing bias, with atmospheric convection occurring too early in the afternoon, and causing too abundant rainfall. This result, particularly true in summer over the northeastern part of the country, is weakly physic-dependent. Cloud-resolving simulations do not clearly reduce the diurnal cycle biases. In the end, the rainfall overestimations appear to be mostly imputable to the afternoon hours of the austral summer rainy season, i.e., the periods during which convective activity is intense over the region.  相似文献   

Lithospheric structure below transantarctic mountain using receiver function analysis of TAMSEIS data   总被引：1，自引：0，他引：1  

P. Kumar  Karabi Talukdar  Mrinal K. Sen 《Journal of the Geological Society of India》2014,83(5):483-492

The lithospheric structure of Antarctica has been investigated from P- (PRF) and S- receiver functions (SRF) using the seismological data from Trans-Antarctic Mountain Seismic Experiment (TAMSEIS). For the stations deployed on the thick ice sheet, estimation of crustal parameters from PRF may be erroneous as the Moho conversions may interfere with the reverberations within the thick ice sheet. However, the free surface multiples are well observed in PRF. On the other hand, in SRFs, the primary conversions of interest and multiples are separated by the mother S-phase. Therefore, it is advantageous to interpret PRF and SRF jointly for the regions where we have thick low velocity layer at the top such as ice or sediments. The crustal structure and corresponding parameters have already been estimated by various workers, but here we interpret the PRF and SRF jointly to minimize the ambiguity and map the lithospheric architecture below TAM. Our analysis reveals that the average crustal thickness beneath the east Antarctica craton is ～44 km with Vp/Vs ranging between ～1.7 and 1.9. Below Trans-Antarctic Mountain (TAM), the average crustal thickness is ～36 km with higher Vp/Vs of ～1.8–2.0. The rift and the volcanic affected coastal region show erratic depths and Vp/Vs, primarily due to the absence of either primary conversion or multiples in the receiver functions. A small number of stations far from the volcano show that the crust is thinnest (～26 to 34 km thick) in the coastal part. The contribution of this study is the mapping of the lithospheric configuration, not done so far using SRF. The SRF section along a profile spanning E-, W- Antarctica and TAM reveals that the lithospheric thickness in the coast is ～80 km and below TAM it is ～120 km. In the central thick ice cover region, the lithosphere thickens upto ～150 km towards Vostok highlands. The most intriguing feature in our SRF section is that the crust and lithosphere are shallow below TAM compared to the E- Antarctica. Further, we observe a mid-lithospheric low velocity layer confined mostly below TAM, suggesting that the thermal buoyancy could be the prime cause for the upliftment of TAM.  相似文献