Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

J. D. Das ¹

Affiliations
1 Earthquake Engineering Department, University of Roorkee, Roorkee, IN

Abstract

The Assam basin is surrounded by hills and plateau. Structurally the basin is mainly bounded by thrust belts in the north, northeast and southeast. Bulk of the Precambrian landmass of Shillong plateau lies to the south of the river Brahmaputra between longitude 90° and 94°E and controls the whole lower part of the river course. Compressional tectonism in northeast India is responsible for the present configuration of the Assam basin which compressed gradually. Structures developed along the foothills of the Himalaya and Naga hills show migration of rock masses towards the basin. Besides, recent earthquakes have changed the basin morphology as well as river courses. It seems the basin is gradually becoming narrower and shallower under the influence of the present tectonism.

Keywords

Tectonism, Geomorphology, Assam Basin, Earthquakes, Shillong Plateau.

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J. D. Das ¹, A. R. Chandrasekaran ²

Affiliations
1 Department of Earthquake Engineering, University of Roorkee, Roorkee 247667, IN
2 Department of Earthquake Engineerjog, University of Roorkee, Roorkee 247667, IN

Abstract

Two earthquakes of April 26 and September 10 were recorded by strong motion array's in India during the year 1986 by Kangra and ShilIoog array respectively. Mean peak horizontal accelerations are in the range of 157 to 2217 mm/sec in case of the April event, whereas it is 163 to 1234 rnmlsec2 for the September one.

Epicentral information (DEQ) has been derived using P and S wave arrival times of strong motion accelerograph stations and compared with that reported by IMD and USGS. Epicentral distance and focal depth are better indicated by the strong motion array data. Focal parameter obtained for the earthquakes differ from that reported by USGS. For the April earthquake, USGS epicenter (E2) lies around 10 km southeast of DEQ epicenter (E3). Focal depth obtained in this study is only around 7 km against 33 km given by USGS. But in the case of the September event epicenter location by DEQ (E3) and USGS (E2) lie close to each other but focal depths were different around 28 km (DEQ) against 43 km (USGS). Error parameters, like RMS, ERH, ERZ are very low in both the cases.

McGuire's and other empirical formulae were used to estimate acceleration at a particuIar site. For the April event McGuire's estimate is very close to tbe observed acceleration when DEQ hypocenter was taken into consideration. However, accelerations estimated for the September event do not conform to the observed ones, eventhough some values are quite cIose.

Keywords

Earthquake, Accelograph, Dharmsala, Shillong, AcceIeration.

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N. Harijan ¹, A. K. Sen ¹, S. Sarkar ², J. D. Das ³, D. P. Kanungo ²

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Central Building Research Institute, Roorkee - 247 667, IN
3 Earthquake Engg., IIT Roorkee, Roorkee - 247 667, IN

Abstract

Sung Valley Carbonatite Complex (SVCC) of the Shillong Plateau is one of the important known carbonatite complexes in India. This complex has intruded the sub-vertical beds of Shillong Group rocks and occur along the major N-S trending pre-existing Um Ngot fault. Topographically, the carbonatite intrusion is marked by a prominent semi-circular depression formed due to weathering of soft nature of the rocks present in the complex. This characteristic is unique in relation to carbonatite and related rocks with the surrounding relatively hard lithological formations. This < feature is prominently displayed on satellite image. The drainage network of the study area is found to be strongly controlled by structure and lithology. The present study is an attempt towards morphotectonic interpretation of the SVCC and adjoining region using satellite imagery and spatial data integration in GIS environment.

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