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Delineation of major subsurface structural features and source depth locations using 3-D Euler deconvolution of gravity data at north-eastern part of India

Ghosh Gopal K.

Acta Geophysica

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2022

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Vol. 70, no 5

2033--2044

EN

The convergence of different major tectonic plates, namely the Eurasian, Indian and Sunda plates, may be the cause of the Assam Syntaxis due to the tectonic interaction between the Himalayan and the Indo-Burman ranges. The study covers the area between latitude 23°–28° N and longitude 88°–96° E and has experienced various types of earthquakes in the recent past. Seismic activity occurs in and around the Shillong Plateau, Mikir Hills, Arakan-Yoma Fold Belt, Naga Hills, parts of the Bengal Basin, lower and upper Brahmaputra valley, and the Mishmi Hills of the Himalayan foothills. Still, part of the study area is extremely unreachable and a limited number of thrust-faults have been identified from field geological studies and GIS maps received from the various sources. In this paper, an attempt has been made to study the delineation of thrust-fault locations using available ground gravity data of northeastern India with the help of a three-dimensional Euler deconvolution technique using the least squares method. Gravity data suggest an undulating nature throughout the area; however, high gravity values are observed at the Bengal Basin and Shillong Plateau, whereas lower gravity values are observed at Brahmaputra and Assam valley, Indo-Burman Range and Molasse Basin. The Shillong Plateau has high gravity with high elevation, whereas the Bengal Basin has high gravity with low elevation. This means that certain tectonic resettlement takes place in the Shillong Plateau, which causes the higher gravity anomaly. The use of Euler deconvolution with the help of a structural index plays a major role in gaining a better understanding of thrust-fault delineation and provides a mappable solution in this area. In this study, source depth estimation using 3- dimensional Euler deconvolution has been carried out by applying a range of structural index and window sizes. The different combinations of structural index and window size during the Euler deconvolution process generates several solutions including some unwanted spurious noise. To remove this noise, unrealistic solutions are discarded by applying filtering criteria to obtain the desired acceptable depth. The results derived using 3-dimensional Euler deconvolution correlate well with the previous finding of thrust-fault delineation. The present study validates the thrust-fault boundaries as well as providing additional thrust-fault settings in the complex tectonic area. The gravity data interpretation appears to offer a reasonable approach for source depth estimation and structural boundary identification.

2

Seismotectonic zones demarcation in the Shillong Plateau using the microearthquakes and radon emanation rate

Walia D., Lyngdoh A.C., Saxena A.

Acta Geophysica

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2010

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Vol. 58, no. 5

893-907

EN

The Shillong Plateau signifies the intense tectonic processes that the region has experienced during the Tertiary Indo-Tibetan and Indo- Burman collisions. An attempt has been made to study the microearthquake and radon emanation rate to understand and identify the seismotectonic zones. The microearthquake data was recorded along a network of seven temporary seismic stations. The epicentral map prepared using the microearthquake data indicates sparse seismic activity over the Shillong Plateau. The alignment of earthquake epicenters does indicate definite pattern of the activity disposition and hence the active fault zones. The calculated b-value over the Shillong Plateau is low which indicates asperity and the fact that the 'stress' is being built-up. Accordingly, along the few identified active crustal structures, time integrated and continuous radon monitoring was made using the LR 115 and Barasol detectors. The sites with anomalous radon concentration are demarcated as active fault zones or sensitive seismotectonic zones and are being monitored continuously using the Barasol detectors. The anomalous radon concentration may not indicate the magnitude of impending earthquake but it certainly can be used to spatially locate the earthquake preparation zones.

3

Hydrological behavior of Umshing River, East Khasi Hills, Meghalaya

Walia D., Mipun B.S.

Acta Geophysica

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2010

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Vol. 58, no. 5

908-921

EN

The morphometric and drainage basin analysis of the Umshing River is carried out quantitatively using remote sensing and GIS techniques. The results are presented concerning the hydrological behavior of Umshing river in order to define multi-scale geomorphometric landform types. The Umshing basin shows a sub-trellis drainage pattern indicating the litho-structural control on the drainage. Lithological, structural and geomorphological features control the directions of flow of the tributaries. It is observed and inferred that the Umshing river catchment is under the stage of creep or tilting and hence is vulnerable to geohazard.