A qualitative appraisal of geomagnetic manifestations of the geological structures in the Indian equatorial region

From the data of the geomagnetic survey of the Indian equatorial region conducted during 1971, it has been possible to deduce the subsurface magnetic anomalies at each survey station. The vertical magnetic anomaly profiles seem to reflect some of the known geological structures in the region. The association between magnetic anomalies and the deep-seated structures in this area emphasises the importance of future intensive studies.     

Velocity Structure of the West-Bengal Sedimentary Basin, India along the Palashi-Kandi Profile Using a Travel-time Inversion of Wide-angle Seismic data and Gravity Modeling – An Update

The 2-D shallow velocity structure along the north-south Palashi-Kandi profile in the West Bengal sedimentary basin has been updated by travel-time inversion of seismic refraction, wide-angle reflection and gravity data. A six-layer shallow model up to a depth of about 7 km has been derived. The first layer, which has an average velocity of 2.0 kms^?1, represents the alluvium deposit, which rests over the shale formation with average velocity of 3.0 kms^?1. The thin (200 m) Sylhet limestone, observed at a nearby Palashi well, remains hidden in the present data set. Hence a 200-m thin layer with a velocity of 3.7 kms^?1, corresponding to the Sylhet limestone, has been assumed to be present throughout the profile. The fourth layer with a velocity of 4.5–4.7 kms^?1 at a depth of 1.7–2.4 km represents the Rajmahal traps. The ‘skip’ phenomenon and rapid amplitude decay of first arrivals indicate a low-velocity zone (LVZ) in the study area. Using the ‘skip’ phenomena and wide-angle reflection data, identified on seismograms, the LVZ with a velocity of 4.0 kms^?1, indicating the Gondwana sediments, has been delineated below the Rajmahal traps. The next layer with a velocity 5.4–5.6 kms^?1 overlying the crystalline basement (5.8–6.25 kms^?1) may be associated with the Singhbhum group of meta volcanic rock that has been exposed in the western part of the basin. The basement lies at a variable depth of 4.9 to 6.8 km. The overall uncertainties of various velocity and boundary nodes are ± 0.15 kms^?1 and ± 0.5 km, respectively. The elevated basement feature in the north might have acted as a structural barrier for the deposition of Sylhet limestone during the Eocene epoch. The seismically derived shallow structure correctly explains the observed Bouguer gravity anomaly along the profile. The addition of reflections in the present analysis provides a stronger control on the depths and velocities of basement and overlying sedimentary formations, compared to the earlier model derived mainly by the first arrival seismic data.     

The Lohawat howardite: Mineralogy,chemistry and cosmogenic effects

Abstract— The Lohawat meteorite is a texturally heterogeneous breccia having a variety of mineral and lithic fragments. Among mineral fragments, pyroxenes show a wide range of composition (Wo_0.011–0.17En_0.37–0.78Fs_0.21–0.60) whereas plagioclase is anorthitic (An_0.92Ab_0.07Or_0.007). Abundant rounded “chondrule‐like” objects ranging in size up to ～7 mm, some with concentric layering, have been observed. Petrographic features, trace element composition and rare earth element patterns show the presence of eucritic and diogenitic components confirming that it is a typical howardite. Cosmogenic tracks, rare gases (He, Ne, and Ar) and radionuclides (²²Na and ²⁶Al) were measured. Track density in olivine and plagioclase varies between 0.7 to 6 times 10⁶/cm². ³⁸Ar exposure age is estimated to be ～110 Ma, being the highest among howardites. The track production rates correspond to ablation of 9 to 15 cm, implying a radius for its preatmospheric size of ～27 cm. ²²Na/²⁶Al ～ 1, as expected from the production models and solar modulation of galactic cosmic‐ray fluxes before its fall, suggesting that the meteoroid did not undergo any fragmentation during the past ～2 Ma in interplanetary space. The radiogenic age based on K‐Ar method is 4.3 Ga while the U‐Th‐⁴He age is 3.3 Ga indicating partial loss of He.     

Subcrustal Low Velocity Layers in Central India and their Implications

A 2-D subcrustal velocity model for the central Indian continental lithosphere has been derived by travel time and relative amplitude modeling of a digitally normalized analog seismic record section of the Hirapur-Mandla DSS profile, using a ray-tracing technique. Some prominent wave groups with apparent velocities slightly higher than the Moho reflection phase (P_MP) are identified on the normalized record sections assembled with a reduction velocity of 6 km s⁻¹. We interpret these phases as the wide-angle reflections from subcrustal lithospheric boundaries. Comparison of synthetic seismograms with the observed record section shows that the observed phases cannot be explained either by multiples or by the P-to-S converted phase (P_MS) from the Moho. Subcrustal velocity models either with a velocity increase or with a single low velocity layer (LVL) also do not provide a satisfactory fit. We infer that a subcrustal velocity model with two alternate LVLs (velocity 7.2 km s⁻¹), separated by a 6-km thick high velocity layer (velocity 8.1 km s⁻¹), can satisfy both the observed travel times and amplitudes. The prominent reflection phases are modeled at depths of 49, 51, 57 and 60 km. It is inferred that the subcrustal lithosphere in the central Indian region has a lamellar structure with varying structural and mechanical properties. The alternating LVLs, occurring at relatively shallow depths below Moho, may be associated with the zones of weakness and lower viscosity suggesting continued mobility, with a possible thermal source in the upper mantle. This explains the source of observed high heat flow values in the central Indian region.     

Relative Sea Level Changes in Maldives and Vulnerability of Land Due to Abnormal Coastal Inundation

Oceanic Islands in the Pacific and Indian Oceans have extremely small land areas, usually less than 500 km2, with maximum height about 4 m above sea level. The Republic of Maldives is an independent island nation in the Indian Ocean south of Sri Lanka which stretches vertically in the Indian Ocean from 07° 06'N - 0° 42'S. The land area of this island country is about 300 km2, and none of Maldives' 1190 islands has an elevation more than 3 m above sea level. In fact the Maldives has the distinction of being the flattest country on earth, making it extremely vulnerable to the effects of global warming. Of the south Asian countries, the Maldives is the most vulnerable nation, facing severe consequences as a result of global warming and sea level rise (SLR). Because of their obvious vulnerability to SLR, the Government of Maldives is very much concerned about climate change. As global warming and the related SLR is an important integrated environmental issue, the need of the hour is to monitor and assess these changes. The present article deals mainly with the analysis of the tidal and Sea Surface Temperature (SST) data observed at Male and Gan stations along the Maldives coast in the northern and southern hemispheres, respectively. The objective of the analysis is to study the trends of these parameters. Trend analysis is also performed on the corresponding air temperature data of both stations. The results show that Maldives coastal sea level is rising in the same way (rising trend) as the global sea level. The mean tidal level at Male has shown an increasing trend of about 4.1 mm/year.Similarly at Gan, near the equator,it has registered a positive trend of about 3.9 mm/year.Sea level variations are the manifestations of various changes that are taking place in the Ocean-Atmosphere system. Therefore, the variations in SST and air temperature are intimately linked to sea level rise. It is found that SST and air temperature have also registered an increasing trend at both stations. The evidence of rising trends suggest that careful future monitoring of these parameters is very much required. Tropical cyclones normally do not affect the Maldives coast. However, due to its isolated location, the long fetches in association with swells generated by storms, that originated in the far south have resulted in flooding. Thus the rising rate of sea level with high waves and flat topography have increased the risk of flooding and increased the rate of erosion and alteration of beaches.     

Extreme Sea Levels Associated With Severe Tropical Cyclones Hitting Orissa Coast of India

Extreme sea levels associated with severe cyclonic storms are common occurrences along the east coast of India. The coastal districts of Orissa have experienced major surges in the past. The recent Paradip super cyclone is one of the most severe cyclones, causing extensive damage to property and loss of lives. Extreme sea levels are major causes for coastal flooding in this region. Damages can be minimized if the extreme sea levels are forecast well in advance. In the present study, we develop a location specific, fine resolution model for the Orissa coast on the lines similar to that of IIT-D storm surge model (Dube et al. 1994). The model runs on a personal computer. The bathymetry for the model is extracted from very fine resolution naval hydrographic charts for the region extending from the south of Orissa to south of West Bengal. A simple drying scheme has also been included in the model in order to avoid the exposure of land near the coast due to strong negative sea surface elevations. An attempt was made in this study to simulate extreme sea levels along the Orissa coast using the data of past severe cyclones. The model results reported in the present study are in good agreement with available observations or estimates.     

Stochastic Modeling of Short Term Variations of Sea Level in Eastern Canada

Hourly sea level records from three stations in eastern Canada (Québec-Lauzon, Harrington-Harbour and Halifax) are analyzed both in frequency domain from 1970 to 1979 and in time domain during 1973. At the three stations, the deterministic model explains 90 to 96% of the total variability of sea level. The semidiurnal and diurnal tides contribute largely to its variations. The residual series, less than 10% of the initial variations of sea level, contain irregular values including extreme values of seiches and storm surges. Such random variations are analyzed and modeled following the method described by Box and Jenkins (1976). The long period variations (2 to 30 days) can be attributed to meteorological forcing (atmospheric pressure and winds). The short period variations (some hours to one day) can be attributed to longitudinal seiches, semidiurnal and diurnal atmospheric tides, and inertial oscillations. The water discharge from the St. Lawrence River contributes 29% of the monthly residual sea level at Qué