Network of earthquakes and recurrences therein     

T. R. Krishna Mohan  P. G. Revathi 《Journal of Seismology》2011,15(1):71-80

We quantify the correlation between earthquakes and use the same to distinguish between relevant causally connected earthquakes. Our correlation metric is a variation on the one introduced by Baiesi and Paczuski (Phys Rev E 69:066,106, 2004). A network of earthquakes is constructed, which is time-ordered and with links between the more correlated ones. Data pertaining to the California region has been used in the study. Recurrences to earthquakes are identified employing correlation thresholds to demarcate the most meaningful ones in each cluster. The distribution of recurrence lengths and recurrence times are analyzed subsequently to extract information about the complex dynamics. We find that the unimodal feature of recurrence lengths helps to associate typical rupture lengths with different magnitude earthquakes. The out-degree of the network shows a hub structure rooted on the large magnitude earthquakes. In-degree distribution is seen to be dependent on the density of events in the neighborhood. Power laws are also obtained with recurrence time distribution agreeing with the Omori law.  相似文献   

Effects of rotation and tidal distortions on the periods of adiabatic oscillations of composite models of stars     

C. Mohan  S. R. Agarwal 《Astrophysics and Space Science》1987,129(1):73-94

Mohan and Saxena's approach of using the averaging technique of Kippenhahn and Thomas in conjunction with Kopal's method of evaluating various parameters on the Roche equipotentials has been used to compute the effects of rotation and tidal distortions on the periods of small adiabatic radial and nonradial modes of oscillations of a series of composite models of stars. In these stars the density decreases slowly in the core from the centre to the interface and then falls of rapidly in the envelope from the interface to the outer surface.  相似文献   

The occultation of MKE 31 by Neptune on September 12, 1983     

A. K. Pandey  H. S. Mahra  Vijay Mohan 《Earth, Moon, and Planets》1984,31(2):217-219

The predicted occultation of the star No. 31 (Mink et al., 1981) by Neptune on September 12, 1983 was observed photoelectrically. Four secondary occultations were recorded before the immersion event.  相似文献   

A high galactic latitude HI 21 cm-line absorption survey using the GMRT: I. Observations and spectra     

Rekhesh Mohan  K. S. Dwarakanath  G. Srinivasan 《Journal of Astrophysics and Astronomy》2004,25(3-4):143-183

We have used the Giant Meterwave Radio Telescope (GMRT) to measure the Galactic HI 21-cm line absorption towards 102 extragalactic radio continuum sources, located at high (|b| > 15°) Galactic latitudes. The Declination coverage of the present survey is δ}> - 45°. With a mean rms optical depth of ∼ 0.003, this is the most sensitive Galactic HI 21-cm line absorption survey to date. To supplement the absorption data, we have extracted the HI 21-cm line emission profiles towards these 102 lines of sight from the Leiden Dwingeloo Survey of Galactic neutral hydrogen. We have carried out a Gaussian fitting analysis to identify the discrete absorption and emission components in these profiles. In this paper, we present the spectra and the components. A subsequent paper will discuss the interpretation of these results.  相似文献   

Photon escape probabilities in expanding atmospheres     

A. Peraiah  K. E. Rangarajan  D. Mohan Rao 《Journal of Astrophysics and Astronomy》1981,2(1):81-86

A comparison of mean number of scatterings and escape probabilities has been made in isotropic scattering and dipole scattering by using the angle-averaged partial frequency redistribution functionR _I. We have solved the equations of radiative transfer and statistical equilibrium simultaneously in a spherically symmetric expanding atmosphere. Two cases of atmospheric extension (i.e.)B/A=3 and 10 (whereB andA are the outer and inner radii of the atmosphere) have been treated. We find that the partial frequency redistribution gives a larger mean number of scatterings compared to that given by complete redistribution. Velocities tend to reduce the mean number of scatterings and in crease the mean escape probabilities.  相似文献   

A high galactic latitude HI 21 cm-line absorption survey using the GMRT: II. Results and interpretation     

Rekhesh Mohan  K. S. Dwarakanath  G. Srinivasan 《Journal of Astrophysics and Astronomy》2004,25(3-4):185-201

We have carried out a sensitive high-latitude (|b| > 15°) HI 21 cm-line absorption survey towards 102 sources using the GMRT. With a 3σ detection limit in optical depth of ∼ 0.01, this is the most sensitive HI absorption survey. We detected 126 absorption features most of which also have corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen. The histogram of random velocities of the absorption features is well-fit by two Gaussians centered at V_1sr ∼ 0 km s⁻¹ with velocity dispersions of 7.6 ± 0.3 km s⁻¹ and 21 ± 4 km s⁻¹ respectively. About 20% of the HI absorption features form the larger velocity dispersion component. The HI absorption features forming the narrow Gaussian have a mean optical depth of 0.20 ± 0.19, a mean HI column density of (1.46 ± 1.03) × 10²⁰ cm⁻², and a mean spin temperature of 121 ± 69 K. These HI concentrations can be identified with the standard HI clouds in the cold neutral medium of the Galaxy. The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04 ± 0.02, a mean HI column density of (4.3 ± 3.4) × 10¹⁹ cm⁻², and a mean spin temperature of 125 ± 82 K. The HI column densities of these fast clouds decrease with their increasing random velocities. These fast clouds can be identified with a population of clouds detected so far only in optical absorption and in HI emission lines with a similar velocity dispersion. This population of fast clouds is likely to be in the lower Galactic Halo.  相似文献   

Multistage melt impregnation in Tethyan oceanic mantle: Petrochemical constraints from channelized melt flow in the Naga Hills Ophiolite     

A. Verencar  A. Saha  S. Ganguly  M. Satyanarayanan  B. Doley  M. Ram Mohan 《Chemie der Erde / Geochemistry》2022,82(1):125821

Ophiolitic sequences obducted onto continental margins allow field based observations coupled with petrochemical interrogations of upper mantle lithologies thereby aiding evaluation of compositional heterogeneity of oceanic mantle, depletion-enrichment events and geodynamic conditions governing oceanic lithosphere formation. The Naga Hills Ophiolite (NHO) suite preserves a segment of the Neotethyan oceanic lithosphere encompassing a package of mantle and crustal lithologies. This paper for the first time reports the occurrence of melt flow channels traversing the mantle section near Molen of the NHO and presents a comprehensive study involving chromite-spinel chemistry, bulk rock major, trace and PGE geochemistry to understand the petrogenesis and evolution in a geodynamic transition from mid oceanic ridge (MOR) to suprasubduction zone (SSZ). The spinel chemistry of peridotitic melt channels depicts both MOR-type and SSZ signatures underlining a transitional tectonic frame. Chromite chemistry and high Al₂O₃/TiO₂ ranging from 15.98–35.70 in concurrence with low CaO/Al₂O₃ ranging from 0.03–0.53; and chondrite normalised LREE > MREE < HREE patterns confirm the influx of boninitic melts into the refractory mantle. The boninitic signature shared by melt channels and host rock invokes a geochemical and geodynamic transition from anhydrous melting of depleted mantle to hydrated fluid flux melting resulting in boninitic melts, that subsequently impregnate and refertilise the fore arc mantle wedge in a SSZ regime at the nascent stage of subduction. The high Ba/Nb, Ba/Th, and Ba/La for the studied peridotites highlight the influx of subduction derived fluids in the supra subduction mantle. Further higher Zr/Hf and Nd/Hf with respect to primitive mantle values in concurrence with lower Nb/Ta suggest progressive refertilisation due to fluid- and melt-driven metasomatism of the refractory fore arc mantle wedge. The chondrite normalised PGE patterns suggest positive Ir and Ru anomalies stipulating the source to be refractory while enriched Pt and Pd underpins the mobilisation of these elements by subduction derived fluids and melts. The elevated abundances of PPGEs than IPGEs as cited by PPGE/IPGE > 1; and Pd/Pt avg. 0.85 for melt channels and 0.84 for host peridotites indicate fluid-fluxed metasomatism of fore arc mantle wedge with a S-undersaturated trend coupled with boninitic affinity. The mineral, trace, REE and PGE chemistry collectively emphasizes that the mantle peridotites of the NHO formed in a transitional geodynamic tectonic setting caused by fore arc extension during subduction initiation followed by rejuvenation by subduction derived fluids and boninitic melts, which typically are of the SSZ tectonic regime. The harzburgitic melt channels and host rock are refractory in nature, reflecting multiple episodes of melt extraction of about 5–15% and ~10–20% respectively from a spinel peridotite mantle source. The occurrences of these melt channels indicate segregation and percolation of melt through porous and channelized network in upper mantle peridotites.  相似文献   

Spectral Overlap Correction Using Weighted Deconvolution for Improved Dark Object Technique for Correction of EO1-Hyperion Data     

Shailesh S. Deshpande  Arun B. Inamdar  Krishna Mohan Buddhiraju 《Journal of the Indian Society of Remote Sensing》2018,46(3):337-344

We present a critical modification to improved dark object technique for correcting hyperspectral data (EO1-Hyperion). The modification is required in improved dark object technique as the original method does not take into account overlap of spectral response functions of two adjacent bands of hyperspectral sensor. We used weighted deconvolution for correcting the original overlap affected path radiance correction propagation factors. Further, we compared the reduction in correction factors—in different conditions—because of the overlap. We calculated the path radiance for April 22 Hyperion image and compared it with other methods such as 6SV. We found noticeable difference in corrected and uncorrected path radiance propagation factors with “clear” to “very clear” atmospheric models. For the other models (“moderate”, “hazy”, “very hazy”), the difference is negligible and can be ignored and improved dark object technique can be applied without any overlap correction.  相似文献   

Signatures of Holocene Hydrological Processes from Sedimentary Archives in Southwestern India: Case Studies from Wetlands of Kerala Coast     

K.P.N. Kumaran  D. Padmalal  Ruta B. Limaye  S. Vishnu Mohan 《Journal of the Geological Society of India》2018,91(5):596-600

This study investigates the abundance, modes of occurrence and origin of major and trace elements in the Jurassic Tazareh coal in the northern Iran. A total of 27 samples (coal, coaly shale and siltstones) were collected and analyzed for trace elements by neutron activation analysis (INAA) and for major elements by X-ray fluorescence. The abundance of these elements is discussed in relation to local geological conditions. The correlation between the various elements is demonstrated through scatter diagrams and the possible genetic associations are discussed. Most elements (Al, Si, Na, K, Fe, Mg, Ti, Ba, Cr and REE) are positively correlated with ash yields that are inorganically associated. Bromine displays statistically signiûcant negative correlations with ash yield. The Ca, Zn, Se, As and Co present mixed modes of occurrence in the coal and probably associate with organic fraction.  相似文献   

Integrating Multiple Natural Tags to Link Migration Patterns and Resource Partitioning Across a Subtropical Estuarine Gradient     

John A. Mohan  Benjamin D. Walther 《Estuaries and Coasts》2018,41(6):1806-1820

Establishing links between migration patterns and trophic dynamics is paramount to ecological studies investigating the functional role habitats provide to resident and transient species. Natural tags in fishes, such as otolith chemistry and tissue stable isotopes, can help reconstruct previous environmental and dietary histories, although these approaches are rarely combined. A novel multiproxy natural tag approach was developed to estimate immigration patterns of juvenile Atlantic croaker Micropogonias undulatus, across contrasting salinity gradients in three subtropical estuaries of the western Gulf of Mexico. Juvenile young-of-year Atlantic croaker were collected along a latitudinal gradient that included positive, neutral, and negative estuaries, based on physicochemical (temperature, salinity, dissolved element) and isotopic (δ¹⁵N and δ¹³C) parameters. Otolith elemental chronologies of Sr/Ca and Ba/Ca were used to classify migratory types within each estuary, while tissue-specific isotope ratios revealed time since recent (liver~weeks) and longer term (muscle~months) diet shifts. Nitrogen isotopes in both liver and muscle tissues were highly correlated, suggesting tissue equilibrium and estuarine residence of at least 3 months, with geographic δ¹⁵N gradients reflecting the magnitude of anthropogenic nutrient enrichment within each estuary. Differences in isotopic equilibrium of muscle-liver δ¹³C values and variation in marginal edge otolith Sr/Ca and Ba/Ca suggested recent shifts in carbon source and habitat utilization, reflecting individualized movement across seascapes and connectivity of habitat mosaics. The multiproxy approach presented here identified diverse migration patterns and linked feeding and movement on regional (inter-estuary), local (intra-estuary), and individual scales to improve our understanding of habitat function across estuarine gradients.  相似文献