Age estimates of coastal terraces in the Andaman and Nicobar Islands and their tectonic implications

The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr^− 1 in the Little Andaman to 2.80 mm yr^− 1 in South Andaman and 2.45 mm yr^− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.     

On the propagation of Rayleigh waves in a transversely isotropic medium resting on a rigid or yielding base

Summary The possibility of propagation of Rayleigh waves in a transversely isotropic medium resting on (i) a rigid base and (ii) a yielding base has been studied in this paper. In the former case the relation between the wave velocity and ratio of wave length to thickness has been found and in the latter case the frequency equation and its possible solutions are obtained.     

Propagation of surface waves in heterogeneous media containing a low velocity channel

Summary Propagation of surface wave in heterogeneous media with various laws of variation of elastic properties have been discussed previously by several authors. There the laws of variations were taken on the considerations that the solutions might be had in terms of known functions. Here the approximate solutions for polynomial law of variations of elastic properties have been obtained.     

Native aluminium (spherules and particles) in the Central Indian Basin sediments: Implications on the occurrence of hydrothermal events

We have sampled particles of native aluminium (Al°) in two sediment cores from the Central Indian Basin (CIB). The cores are geographically separated but are located at the base of two seamounts. The native Al° particles occurring as grains and spherules, have an average Al content of  95% and are associated with volcanogenic–hydrothermal material. Morphologically and compositionally, the specimens are similar to those reported from the East Pacific Rise. After ruling out several processes for the presence of the native Al°, we hypothesize that during progressive melting of magma, a basaltic magma is produced which has high contents of reductants such as methane and hydrogen, and a low oxygen fugacity. During the upward migration of such magma, reduction to metallic aluminium and the formation of native Al° particles takes place.     

Collision frequency and transport properties of electrons in the ionosphere

A unified ionospheric electron collision frequency model profile 〈ν_g〉 has been calculated in the height range 50–500 km. The computed profile accounts for the electron collisions with the neutral particles as well as the ions. Experimental values of momentum transfer cross-sections have been used for electron collisions with N₂, O₂ and Ar and theoretical values for N, O, He and H. It is observed that the electron-ion collisions 〈ν_ei〉 dominate over the electron-neutral collisions 〈ν_en〉 above 170 km. However, 〈ν_e?N〉 is of the same order of magnitude as 〈ν_e?O〉 in the height range 170–210 km. Above 360 km 〈ν_e?O〉 becomes more important among the neutrals. The temporal, seasonal and solar epoch variations of 〈ν_ei〉 are also shown. A typical electron collision frequency profile 〈ν_g〉 corresponding to the exospheric temperature of 1100 K has been compared with the available experimental results for D, E and F-regions obtained from different experimental techniques. This profile has been used to determine the electrical, thermal, heat flow and current flow conductivities, the mobility and the diffusivity of electrons. The results so obtained are found to be in good agreement with the earlier results.     

Stratigraphy, sedimentology and bulk organic geochemistry of black shales from the Proterozoic Vindhyan Supergroup (central India)

Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential. The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales. These shales are thus different from Phanerozoic black shales which typically exhibit planar laminae. The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis. Total organic carbon content of many of the shales exceeds 1%. The meanT _max for the black shales translate to a vitrinite reflectance range of 2.05-2.40% Rm based on standard conversion techniques. These shales have reached the catagenetic stage near the beginning of anthracite formation.     

Determination of S-wave Site Response in Anchorage, Alaska in the 1–9 Hz Frequency Band

—?Site response was estimated at 19 sites in the Anchorage basin in south-central Alaska, using 15 local earthquakes recorded with good signal-to-noise ratio by a temporary weak motion network. The receiver-function-type horizontal-to-vertical spectral ratios (HVSR) were computed at 1–9 Hz frequency band and the resulting HVSR contour maps at 1, 5 and 9 Hz are presented here. The spatial site response distribution shows considerable variation from the foothills of the Chugach Mountains in the east to the western part of Anchorage. The site response increases by a factor of 3 and 3.5 at 1 and 5 Hz, respectively, from the area of older glacial deposits in the eastern part of the city to the area occupied by the Bootlegger Cove formation, particularly in the section adjoining Knik Arm. At 9 Hz, the variation of HVSR from the east to the west is smaller, approximately by a factor of 2. Moreover, the trend of the HVSR variation at 1 and 5 Hz shows good correlation with that of the soil class obtained from surface measurements of S-wave velocity in the 0–30?m depth range and available results on ground failure susceptibility of Anchorage.