Seismic source zonation for NE India on the basis of past EQs and spatial distribution of seismicity parameters

Journal of Seismology - The North East (NE) of India is an earthquake (EQ) prone zone and has experienced considerable damages in the past due to high intensity EQs. With rapidly growing...     

Mineralogical study of gabbro-anorthosite from Dumka, Chhotanagpur Gneissic Complex, Eastern Indian Shield

The Chhotanagpur Gneissic Complex (CGC), bearing imprints of widespread high grade metamorphic and magmatic history since Palaeoproterozoic, represents an integral crustal segment of Eastern Indian Shield. The gabbroanorthosite intrusives constitute a part of mafic-ultramafic magmatism in the CGC. The study area around Dumka (24°16?? to 24°20??N: 87°13?? to 87°22??E) predominantly comprises of granite gneiss and charnockitic country rocks within which gabbro-anorthosite intrusions occur as lenses. Field relations and structural studies reveal that the country rocks of Dumka have suffered three phases of deformation represented by F₁, F₂ and F₃ folds. The gabbro-anorthosite intrusives maintain a sharp contact with the host rocks, deformed and metamorphosed. Relict igneous layering or primary igneous foliation (S_ig) is recorded where metamorphic overprint is minimal. Mineral phases of gabbro-anorthosite rocks suggest that clinopyroxene compositions from gabrro correspond to diopside and clinoferrosilite, while those from anorthosite are clinoferrosilite. Amphiboles from the gabbro-anorthosite rocks are calcic, and range from ferroan pargasite in gabbro to ferroan pargasitic hornblende in anorthosite. Plagioclase from gabbro and anorthosite belong to bytownite and andesine respectively. Chemical composition of garnet in gabbro is almandine. Thermobarometric estimates for Dumka gabbroanorthosites correspond to 511°C to 915°C and 5.0?C7.5 kb pressure, comparable to that estimated for Bengal Anorthosite (593?C795°C, 4.1?C7.3 kb). Fractionation trend of plagioclase substantiates a single parental magma in the evolution of Dumka gabbro-anorthosite intrusives.     

Effect of Corrugation and Reinforcement on the Dispersion of SH-wave Propagation in Corrugated Poroelastic Layer Lying over a Fibre-reinforced Half-space

The presence of porosity and reinforcement in a medium is an important factor affecting seismic wave propagation and plays vital role in many geophysical prospects. Also, the presence of salt and ore deposits, mountains, basins, mountain roots, etc. is responsible for the existence of corrugated boundary surfaces of constituent layers. Such facts brought motivation for the present paper which deals with the propagation of SH-wave in a heterogeneous fluid-saturated poroelastic layer with corrugated boundaries lying over an initially stressed fibre-reinforced elastic halfspace. Closed form of dispersion relation has been obtained and is found in well agreement to classical Love wave equation for isotropic case. The effect of corrugation, wave number, undulation, position parameter, horizontal compressive/tensile initial stress and heterogeneity on phase velocity has been analysed through numerical computation and graphical illustration. Moreover, comparative study exploring the effect of presence and absence of reinforcement in half-space on dispersion curve is the major highlight of the current study.     

On the tidally driven circulation in the South China Sea: modeling and analysis

The South China Sea is a large marginal sea surrounded by land masses and island chains, and characterized by complex bathymetry and irregular coastlines. An unstructured-grid SUNTANS model is employed to perform depth-averaged simulations of the circulation in the South China Sea. The model is tidally forced at the open ocean boundaries using the eight main tidal constituents as derived from the OSU Tidal Prediction Software. The model simulations are performed for the year 2005 using a time step of 60 s. The model reproduces the spring-neap and diurnal and semidiurnal variability in the observed data. Skill assessment of the model is performed by comparing model-predicted surface elevations with observations. For stations located in the central region of the South China Sea, the root mean squared errors (RMSE) are less than 10 % and the Pearson’s correlation coefficient (r) is as high as 0.9. The simulations show that the quality of the model prediction is dependent on the horizontal grid resolution, coastline accuracy, and boundary locations. The maximum RMSE errors and minimum correlation coefficients occur at Kaohsiung (located in northern South China Sea off Taiwan coast) and Tioman (located in southern South China Sea off Malaysia coast). This may be explained with spectral analysis of sea level residuals and winds, which reveal dynamics at Kaohsiung and Tioman are strongly influenced by the seasonal monsoon winds. Our model demonstrates the importance of tidally driven circulation in the central region of the South China Sea.     

Nonlinear soil response to ground motions during different earthquakes in Nepal,to arrive at surface response spectra

Catastrophic damages reported during an earthquake include building damages, excessive ground shaking, uneven settlements and liquefaction. While most of the seismic hazard studies map the probable level of ground shaking at the bedrock level, their use in assessing the above damages is very limited until the response of the local soil is also taken into account. Determination of the local soil response needs regionally recorded ground motions, dynamic soil properties, in situ geotechnical details, etc., which most of the time are not readily available for the region under study. In the present work, the response of local soil for Nepal has been studied indirectly taking into account the surface level of ground shaking during various past as well as recent EQs observed at various locations. Based on the present analysis, a low value of amplification factor for high peak horizontal acceleration and vice versa is observed in central, western as well as southern parts of Nepal. These observations suggest nonlinear soil behavior and are in accordance with the available literature. Further, the ground motion records during 2015 Nepal EQ show maximum soil response at 0.3 s which is exactly matching with the site class C obtained from in situ data for the above locations. Based on the above observations, various correlations between the high peak horizontal acceleration and the surface spectral acceleration are proposed to obtained site specific surface response spectrum for Nepal.     

Vulnerability assessment through index modeling: a case study in Muriganga–Saptamukhi estuarine interfluve,Sundarban, India

The coastal regions, deltas, and estuaries are severely affected by the sea level rise and cyclonic activities and climate changes. Sundarban delta is one of the most mysterious landscapes in the world, which has successively evolved due to sediment accumulation by the great Ganga and Brahmaputra river system. The area is characterized by low-lying islands and a flat topography coupled with macro-tidal activities, powerful surges, and seasonal cyclonic events. All these conditions put together this landscape defenseless to frequent flood and erosion. Since the last hundred years, the face of Sundarban has been changed remarkably from wildest to human-occupied territory by protecting this low-lying flat plain from tidal inundation through artificial embankment. In this background, the current study attempts to highlight the spatial extent and magnitudes of internal risk factors of the region using the composite vulnerability index. Coastal vulnerability defines a system’s openness to flood and erosion risk due to hydrogeomorphic exposures and socio-economic susceptibility in conjunction with its capacity/incapacity to be resilient and to cope, recover, or adapt to an extent. Coastal vulnerability assesses the potential risk from erosion and flooding of any low-lying coastal region due to its physiographical and hydrological exposures, socio-economic and political susceptibility, and resilience capacity. A natural system affects the socio-economic scenario of any region. Hence, multidimensional databases can be more effective to understand the extent of exposure, susceptibility, and resilience of any system. To throw some light on the situation of vulnerability of western estuarine Sundarban, between Muriganga and Saptamukhi interfluve, the composite vulnerability index has been carried out to delineate the magnitude and spatial extent of vulnerability with the help of quantitative techniques and geospatial tools. The estuarine tracts and coastal parts of the Ganga delta are two of the most densely populated areas in the world. The study highlights the critical situation of the population under different potential risk classes residing in the study area with the intention of suggesting some proper course of action of planning and management to conserve coastal communities in their original habitat.     

Application of physical scaling towards downscaling climate model precipitation data

Physical scaling (SP) method downscales climate model data to local or regional scales taking into consideration physical characteristics of the area under analysis. In this study, multiple SP method based models are tested for their effectiveness towards downscaling North American regional reanalysis (NARR) daily precipitation data. Model performance is compared with two state-of-the-art downscaling methods: statistical downscaling model (SDSM) and generalized linear modeling (GLM). The downscaled precipitation is evaluated with reference to recorded precipitation at 57 gauging stations located within the study region. The spatial and temporal robustness of the downscaling methods is evaluated using seven precipitation based indices. Results indicate that SP method-based models perform best in downscaling precipitation followed by GLM, followed by the SDSM model. Best performing models are thereafter used to downscale future precipitations made by three global circulation models (GCMs) following two emission scenarios: representative concentration pathway (RCP) 2.6 and RCP 8.5 over the twenty-first century. The downscaled future precipitation projections indicate an increase in mean and maximum precipitation intensity as well as a decrease in the total number of dry days. Further an increase in the frequency of short (1-day), moderately long (2–4 day), and long (more than 5-day) precipitation events is projected.     

Implications of REE incorporation and host sediment influence on the origin and growth processes of ferromanganese nodules from Central Indian Ocean Basin

This study presents new major, trace and REE data for thirty-five ferromanganese nodules recovered from areas representing three different sediment types (silic...     

Geochemistry of core sediments from the southeastern Bay of Bengal:Inferences on weathering and early diagenetic changes

A sediment core(ABP24/05),collected at a water depth of 3520 m from the southeastern Bay of Bengal was studied to determine the change in chemical weathering during the last glacial to deglacial periods and the factors of sedimentary environment which controlled earliest diagenetic changes in the sediment after its deposition.High ratios of K/Rb,Ti/Al and Zr/Rb during~45 to~18 cal kyr B.P.in the core sediments may be attributed to the stronger physical erosion and turbidity currents activity during this period.This might have brought a higher quantity of unaltered minerals to the study area.Low ratios of K/Rb,Zr/Rb,and Ti/Al and increase of SiO₂/TiO₂,Rb/Al and Cs/Al from~18 cal kyr B.P.to present may be indicating an increase in the rate of chemical weathering during this period.The time of increased chemical weathering in the study area is consistent with deglaciation warming in the tropical Indian Ocean and strengthening of river runoff into the Andaman Sea.Climate change during the interglacial period by increased solar insolation thereby strengthened the summer monsoon which might have led to intensified chemical weathering in the source region since~18 cal kyr B.P.The low organic carbon(OC),high Mn/Al,Fe/Al and the Mn-oxides minerals precipitation indicate prevailing of oxic conditions during~11 cal kyr B.P.in the core sediments,which is contradictory to suboxic conditions developed in the deep ocean sediments in the western Bay of Bengal and the equatorial Indian Ocean.The low terrigenous influx and export of less OC to the bottom sediments might have created a favorable condition for the formation of Mn-oxides in the study area during Holocene.     

Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

The Naga Hills Ophiolite(NHO) belt in the Indo-Myanmar range(IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene.Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO, northeastern India to address(i) the mantle processes and tectonic regimes involved in their genesis and(ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle.The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Washelo and Lacham areas.The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase.The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber.Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE.These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions.Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone(SSZ) fore-arc regime coherent with the subduction initiation process.The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process:(i) a depleted fore arc basalt(FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle;(ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration.The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.