Bearing of plate geometry and rheology on shallow-focus mega-thrust seismicity with special reference to 26 December 2004 Sumatra event     

Prosanta Kumar Khan  Partha Pratim Chakraborty 《Journal of Asian Earth Sciences》2009,34(3):480-491

We propose a model pertaining to the generation of 26th December 2004 off Sumatra mega-event in the backdrop of other similar type earthquakes along subduction zones around the world. Reconstructions of Benioff trajectories through the hypocenters of historical earthquakes including six mega-earthquakes indicate (i) confinement of hypocenters right within the descending lithosphere, and (ii) natural coincidence of foci of the mega-events around the zones of plate flexing. These observations are discussed in detail with special emphasis on the Sumatra margin considering the role of rheological anomaly across the cross-section of the descending lithosphere; yield strength envelope and residual stress accumulation through time. The intraplate origin of shallow mega-thrust earthquakes allowed us to advocate the ‘zone of flexing’ along the profiles of the subducting plates as nodal area for stress concentration. We propose here that at elevated confining pressure and temperature, loading of unidirectional cyclic stress on time-average bending stress enhanced the material yield strength (i.e., strain-hardening), and leads the semi-brittle portion of the lithosphere into near-brittle condition through rheological transformation. Under subsequent rise in neutral surface and increase in compressive stress field, non-coaxial deformation triggered shear failure on 26th December 2004 preferably at the rheological interface between strain-hardened near-brittle layer and deformed ductile layer within the sub-oceanic mantle.A two-stage fracture mechanism viz. a slow (～1.1 km/s) bilateral initiation in an essentially strain-hardened near-brittle domain and a follow-up very rapid progression (3.3 km/s) in the brittle, crustal domain was mainly involved in the generation of 2004 off Sumatra mega-event. Estimation shows an amount of 3.38 × 10²² to 4.50 × 10²² N m seismic moment (M_o) and 8.95–9.03 moment magnitude (M_w) for the southern part of the 1300 km extended rupture i.e. between the North Andaman to the north and the Sumatra at its south. The study necessitates the reassessment of other shallow-focus mega-thrust earthquakes along the subduction margins around the globe.  相似文献   

Deep learning for predicting the monsoon over the homogeneous regions of India   总被引：1，自引：0，他引：1  

Moumita Saha  Pabitra Mitra  Ravi S Nanjundiah 《Journal of Earth System Science》2017,126(4):54

Indian monsoon varies in its nature over the geographical regions. Predicting the rainfall not just at the national level, but at the regional level is an important task. In this article, we used a deep neural network, namely, the stacked autoencoder to automatically identify climatic factors that are capable of predicting the rainfall over the homogeneous regions of India. An ensemble regression tree model is used for monsoon prediction using the identified climatic predictors. The proposed model provides forecast of the monsoon at a long lead time which supports the government to implement appropriate policies for the economic growth of the country. The monsoon of the central, north-east, north-west, and south-peninsular India regions are predicted with errors of 4.1%, 5.1%, 5.5%, and 6.4%, respectively. The identified predictors show high skill in predicting the regional monsoon having high variability. The proposed model is observed to be competitive with the state-of-the-art prediction models.  相似文献   

A hypothesis on onset,advance and withdrawal of the Indian summer monsoon     

Suranjana Saha  Kshudiram Saha 《Pure and Applied Geophysics》1980,118(2):1066-1075

The paper defines the intertropical convergence zone. (ITCZ) in the Indian monsoon region during the northern summer, identifies it with the northern boundary of the advancing monsoon and suggests that its seasonal movement can serve as an indicator of onset, advance and withdrawal of the monsoon. Evidence suggesting the movement of the ITCZ which is associated with the equatorial trough of low pressure is indirectly furnished by an analysis of the isallobaric or height-tendency field which reveals a distinct gradient towards the north/south during period of advance/withdrawal of the monsoon. A comparative study of the dates of onset of monsoon during two successive years appears to suggest that some of the problems encountered in using rainfall as the sole criterion for determining the onset and advance of the monsoon may be over-come by using the ITCZ concept as proposed in the present paper. Attention is drawn to the effects of synoptic-scale disturbances on the normal dates of onset, advance and withdrawal of the monsoon.  相似文献   

Unsteady boundary layer flows induced by a wavy plate     

L. M. Saha  L. Debnath 《Pure and Applied Geophysics》1973,105(1):802-809

Summary Unsteady boundary layer flows generated in an incompressible, homogeneous, nonrotating viscous fluid bounded by a rigid wavy plate are studied theoretically. The Laplace transform method is employed to obtain exact solutions of the unsteady boundary layer equations in a wavy plate configuration. The structures of the unsteady velocity distribution and the associated boundary layers are determined explicitly and several particular solutions are recovered as special cases of this analysis. The physical interpretation of the mathematical results are examined.  相似文献   

DETERMINATION OF PHOSPHOROUS IN FIFTY-NINE GEOCHEMICAL REFERENCE SAMPLES BY EXTRACTIVE SPECTROPHOTOMETRY     

Partha CHATTOPADHYAY 《Geostandards and Geoanalytical Research》1993,17(2):219-222

A digestion procedure with sequential use of HF-HCl and HNO₃-HClO₄ involving yellow vanadium molybdophosphate complex and its extraction into isobutylmethylketone (IBMK) has been employed for estimation of phosphorus in 59 geochemical reference samples. The agreement between the results obtained from IBMK-extraction and the recommended values has been found to be better than those available from the usual procedures (i.e., estimation in the aqueous phase).  相似文献   

Kinetics of cadmium adsorption on aluminum precipitation products formed under the influence of tannate     

G. Yu  U.K. Saha  P.M. Huang 《Geochimica et cosmochimica acta》2006,70(20):5134-5145

The impeding effects of organic substances on the crystallization of Al precipitation products have been studied for more than three decades. However, the impacts of organics-induced structural perturbation and the resultant surface alteration of Al transformation products on their kinetics and mechanisms of the adsorption of trace metals still remain to be uncovered. This paper describes the kinetics of Cd adsorption on the short-range-ordered (SRO) Al precipitation products formed under the influence of tannate. The kinetics of Cd adsorption on the SRO Al precipitation products formed in the presence of tannate at initial tannate/Al molar ratios (MRs) of 0, 0.001, 0.01 and 0.1 was studied at an initial Cd concentration of 0.89 μM, pH 5.5, background electrolyte of 10⁻² M NaNO₃, and solid phase concentration of 0.08 g L⁻¹ at 278, 288, 298 and 313 K using the conventional batch method. The results show that, among the six empirical kinetic models tested, the second-order rate equation best described the kinetic data. Cadmium adsorption on SRO Al precipitation products was a multi-step process involving an initial fast reaction (0.083-0.75 h) followed by a slow reaction (0.75-4 h). The structural perturbation of Al precipitation products by tannate and the resultant development of their microporosity and alteration of surface and charge properties substantially enhanced the rate constants of both the fast and slow reaction processes of Cd adsorption. The heat of activation values for the fast reaction ranged from 27 to 41 kJ mol⁻¹, indicating that diffusion is the rate-limiting step in Cd adsorption. Except for the Al precipitation products formed at a tannate/Al MR of 0 and 0.001, the heat of activation was >47 kJ mol⁻¹ for the slow reaction, indicating that the rate-limiting step is a chemical process, which apparently involves bond breaking and formation on the surfaces of the SRO Al precipitation products. The pre-exponential factor, an index of the frequency of Cd collision with the surface of Al precipitation products, significantly increased with the increase of the initial tannate/Al MR, which is attributed to the development of microporosity and the increase in the specific surface area of the Al precipitation products formed under the influence of tannate ligands. The findings merit attention to the role of natural organics, which vary in the structure and functionality, in developing the microporous structure of Al precipitation products and the impact on the dynamics and mechanisms of Cd transformation and transport in natural environments.  相似文献   

Theoretical development of the differential scattering decomposition for the 3D resistivity experiment     

Greg A. Oldenborger  Partha S. Routh 《Geophysical Prospecting》2006,54(4):463-473

In any numerical solution of the DC resistivity experiment, care must be taken to deal with strong heterogeneity of electrical conductivity. In order to examine the importance of conductivity contrasts, we develop a scattering decomposition of the DC resistivity equation in the sparse differential domain as opposed to the traditional dense integral formulation of scattering‐type equations. We remove the singularity in the differential scattered series via separation of primary and secondary conductivity, thereby avoiding the need to address the singularity in a Green's function. The differential scattering series is observed to diverge for large conductivity contrasts and to converge for small contrasts. We derive a convergence criterion, in terms of matrix norms for the weak‐form finite‐volume equations, that accounts for both the magnitude and distribution of heterogeneity of electrical conductivity. We demonstrate the relationship between the differential scattering series and the Fréchet derivative of the electrical potential with respect to electrical conductivity, and we show how the development may be applied to the inverse problem. For linearization associated with the Fréchet derivative to be valid, the perturbation in electrical conductivity must be small as defined by the convergence of the scattered series. The differential scattering formulation also provides an efficient tool for gaining insight into charge accumulation across contrasts in electrical conductivity, and we present a derivation that equates accumulated surface charge density to the source of scattered potential.  相似文献   

Predation of the beetle Rhantus sikkimensis (Coleoptera: Dytiscidae) on the larvae of Chironomus Meigen (Diptera: Chironomidae) of the Darjeeling Himalayas of India     

Gautam Aditya  Goutam Kumar Saha   《Limnologica》2006,36(4):251-257

The dytiscid beetle Rhantus sikkimensis, Regimbart, 1899 (Coleoptera: Dytiscidae), a member of the freshwater insect communities of the Darjeeling Himalayas, were noted to predate on the coexisting larvae of Chironomus sp. Meigen. Evaluation of predation by R. sikkimensis on Chironomus sp. larvae, in the laboratory, revealed that a single adult morph of R. sikkimensis could kill and consume on an average 10–90 and 10–78 numbers of small and large Chironomus sp. larvae, respectively, per day, depending on the prey density. The attack rate ranged between 520 and 537, and the handling time ranged between 4.3 and 8.6 depending on the size of the preys. The predation varied with respect to predator density also, with a maximum of 151 larvae killed by three predators per day. Two indices of predation, ingestion rate (IR) ranging between 13.33 and 74.15 larvae/day/predator and clearance rate (CR) ranging between 19.67 and 39.99 L prey/day/predator, varied with the prey size and predator density, significantly, when the predation was observed for 9 consecutive days, at two predator densities. It was also noted that R. sikkimensis predated on an average 9.8 larvae of Chironomus sp. and 1 larva of Culex sp., when the larvae of both the species are present together as preys, showing a preference for the Chironomus sp. larvae.  相似文献   

Application of remote sensing to identify coalfires in the Raniganj Coalbelt,India     

Prasun K Gangopadhyay  Kuntala Lahiri-Dutt  Kanika Saha 《International Journal of Applied Earth Observation and Geoinformation》2006

Raniganj and Jharia regions together have been for long the single largest coal supplier in India, now contributing about a quarter of the total output in the country. Numerous reasons such as improper mining techniques and policy, as well as unauthorized mining caused surface and subsurface coalfires in these areas. These coalfires burn millions of tonnes of valuable coal resources, creating severe environmental problems and posing enormous operational difficulties of mining. After first use of remote sensing as a tool to identify coalfires in 1960s, with the time, the efficiency of remote sensing to identify and monitoring coalfires has been well established by several researchers. With the knowledge of local geological setting and density sliced surface temperature image the spatial distribution of coalfires can be revealed. The present paper makes an attempt to identify temperature anomalies of the Raniganj coalbelt to locate the spatial distribution of coalfires. Landsat Thematic Mapper (TM) thermal band data was used to calculate surface temperature along with NDVI (normalized vegetation index) derived emissivity.  相似文献   

Resolution analysis of geophysical images: Comparison between point spread function and region of data influence measures   总被引：1，自引：0，他引：1  

Carlyle R. Miller  Partha S. Routh 《Geophysical Prospecting》2007,55(6):835-852

Practical decisions are often made based on the subsurface images obtained by inverting geophysical data. Therefore it is important to understand the resolution of the image, which is a function of several factors, including the underlying geophysical experiment, noise in the data, prior information and the ability to model the physics appropriately. An important step towards interpreting the image is to quantify how much of the solution is required to satisfy the data observations and how much exists solely due to the prior information used to stabilize the solution. A procedure to identify the regions that are not constrained by the data would help when interpreting the image. For linear inverse problems this procedure is well established, but for non‐linear problems the procedure is more complicated. In this paper we compare two different approaches to resolution analysis of geophysical images: the region of data influence index and a resolution spread computed using point spread functions. The region of data influence method is a fully non‐linear approach, while the point spread function analysis is a linearized approach. An approximate relationship between the region of data influence and the resolution matrix is derived, which suggests that the region of data influence is connected with the rows of the resolution matrix. The point‐spread‐function spread measure is connected with the columns of the resolution matrix, and therefore the point‐spread‐function spread and the region of data influence are fundamentally different resolution measures. From a practical point of view, if two different approaches indicate similar interpretations on post‐inversion images, the confidence in the interpretation is enhanced. We demonstrate the use of the two approaches on a linear synthetic example and a non‐linear synthetic example, and apply them to a non‐linear electromagnetic field data example.  相似文献