Thermal evolution of the early Moon

The early thermal evolution of Moon has been numerically simulated to understand the magnitude of the impact-induced heating and the initially stored thermal energy of the accreting moonlets. The main objective of the present study was to understand the nature of processes leading to core–mantle differentiation and the production and cooling of the initial convective magma ocean. The accretion of Moon was commenced over a time scale of 100 yr after the giant impact event around 30–100 million years in the early solar system. We studied the dependence of the planetary processes on the impact scenarios, the initial average temperature of the accreting moonlets, and the size of the protomoon that accreted rapidly beyond the Roche limit within the initial 1 yr after the giant impact. The simulations indicate that the accreting moonlets should have a minimum initial averaged temperature around 1600 K. The impacts would provide additional thermal energy. The initial thermal state of the moonlets depends upon the environment prevailing within the Roche limit that experienced episodes of extensive vaporization and recondensation of silicates. The initial convective magma ocean of depth more than 1000 km is produced in the majority of simulations along with the global core–mantle differentiation in case the melt percolation of the molten metal through porous flow from bulk silicates was not the major mode of core–mantle differentiation. The possibility of shallow magma oceans cannot be ruled out in the presence of the porous flow. Our simulations indicate the core–mantle differentiation within the initial 10² to 10³ yr of the Moon accretion. The majority of the convective magma ocean cooled down for crystallization within the initial 10³ to 10⁴ yr.     

Some aspects of volcanic ash layers in the Central Indian Basin

 Intercalated volcanic ash layers in two deep-sea sediment cores from the Central Indian Basin (CIB) are examined for the possibility of an in situ source of suboceanic volcanism. An in situ source has been predicated based on the bottom morphological structures and activation of seamounts during the geological past. The tentative correlation between depositional ages of the ash layer horizons and the period of global climatic cooling events reinforces the hypothesis that the suboceanic volcanic episode might have been initiated during the onset of glaciation. Received: 20 June 1995 / Revision received: 11 May 1998     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Near-field and far-field simulation of accelerograms of Sikkim earthquake of September 18, 2011 using modified semi-empirical approach

The semi-empirical approach for modeling of strong ground motion given by Midorikawa (Tectonophysics 218:287?C295, 1993) has been modified in the present paper for component wise simulation of strong ground motion. The modified approach uses seismic moment in place of attenuation relation for scaling of acceleration envelope. Various strong motion properties like directivity effect and dependence of peak ground acceleration with respect to surface projection of source model have been studied in detail in the present work. Recently, Sikkim earthquake of magnitude 6.9 (M _w ) that occurred on September 18, 2011 has been recorded at various near-field and far-field strong motion stations. The modified semi-empirical technique has been used to confirm the location and parameters of rupture responsible for this earthquake. Strong motion record obtained from the iterative modeling of the rupture plane has been compared with available strong motion records from near as well as far-field stations in terms of root mean square error between observed and simulated records. Several possibilities of nucleation point, rupture velocity, and dip of rupture plane have been considered in the present work and records have been simulated at near-field stations. Final selection of model parameters is based on root mean square error of waveform comparison. Final model confirms southward propagating rupture. Simulations at three near-field and twelve far-field stations have been made using final model. Comparison of simulated and observed record has been made in terms of peak ground acceleration and response spectra at 5?% damping. Comparison of simulated and observed record suggests that the method is capable of simulating record which bears realistic appearance in terms of shape and strong motion parameters. Present work shows that this technique gives records which matches in a wide frequency range for Sikkim earthquake and that too from simple and easily accessible parameters of the rupture plane.     

Arsenic mobility in fluvial environment of the Ganga Plain,northern India

In the northern part of the Indian sub-continent, the Gomati River (a tributary of the Ganga River) was selected to study the dynamics of Arsenic (As) mobilization in fluvial environment of the Ganga Plain. It is a 900-km-long, groundwater-fed, low-gradient, alluvial river characterized by monsoon-controlled peaked discharge. Thirty-six water samples were collected from the river and its tributaries at low discharge during winter and summer seasons and were analysed by ICP-MS. Dissolved As and Fe concentrations were found in the range of 1.29–9.62 and 47.84–431.92 μg/L, respectively. Arsenic concentration in the Gomati River water has been detected higher than in its tributaries water and characteristically increases in downstream, attributed to the downstream increasing of Fe₂O₃ content, sedimentary organic carbon and silt-clay content in the river sediments. Significant correlation of determination (r ² = 0.68) was also observed between As and Fe concentrations in the river water. Arsenic concentrations in the river water are likely to follow the seasonal temperature variation and reach the level of World Health Organization’s permissible limit (10 μg/L) for drinking water in summer season. The Gomati River longitudinally develops reducing conditions after the monsoon season that mobilize As into the river water. First, dissolved As enters into pore-water of the river bed sediments by the reductive dissolution of Fe-oxides/hydroxides due to microbial degradation of sedimentary organic matter. Thereafter, it moves upward as well as down slope into the river water column. Anthropogenically induced biogeochemical processes and tropical climatic condition have been considered the responsible factors that favour the release of As in the fluvial environment of the Ganga Plain. The present study can be considered as an environmental alarm for future as groundwater resources of the Ganga–Brahmaputra Delta are seriously affecting the human–environment relationship at present.     

Dissolved Folin Phenol Active Substances (Tannin and Lignin) in the Seawater along the West Coast of India

Information on the distribution of dissolved Folin phenol active substances (FPAS) such as tannin and lignin in the seawater along the west coast of India is provided. Notable amounts of FPAS (surface concentrations: 80 g/l to 147 g/l and bottom concentrations: 80 g/l to 116 g/l) were detected in the seawater along the coast. The distribution pattern brings about a general depth-wise decrease. A seaward decrease was observed in the southern stations whereas reverse was the case in northern stations. A significant negative correlation was observed between FPAS concentration and dissolved oxygen in sub-surface samples. The appreciable amounts of FPAS detected in the coastal waters indicate the presence of organic matter principally originating from terrestrial (upland and coastal marsh) ecosystems in the marine environment. In this context, they may be used as tracers to determine the fate of coastalborn dissolved organic matter in the ocean and to determine directly the relationship between allochthonous and autochthonous organic matter.     

Peculiar oscillations in the microwave emission of Jupiter during the impact of K fragment of comet Shoemaker-Levy 9

In this article we report the peculiar oscillations in the intensity of microwave (4.15 GHz) emission seen during the impact of K fragment of comet Shoemaker-Levy 9 on July 19, 1994. The oscillations begin at 10h 13m 25s UT suddenly with a frequency of ~0.3 Hz and gradually the frequency of these oscillations increases to ~ 1 Hz. The oscillations are not due to local atmosphere or the radio interference from signals of geostationary satellite. They are intrinsic to the microwave emission from Jupiter during the impact of K fragment. Peak-to-peak amplitude of the oscillations is about 34% of the total microwave emission from Jupiter. If we assume that only 50% of the microwave emission is non-thermal and only the non-thermal emission suffered oscillations, then the oscillations are about 68% of the non-thermal emission from Jupiter. The observations also indicate that there are three continuum enhancements during this event and periodic oscillations almost all through. The third enhancement was the largest and during this enhancement there were some additional aperiodic variations. The aperiodic variations were of the order of few minutes and were possibly generated by the gravity waves. The periodic oscillations could be synchrotron emission modulated by the plasma oscillation in the outer magnetosphere. kg]Key words