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Surendar Kumar ¹, A. K. Mahajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun 248001, IN

Abstract

A moderate earthquake of magnitude 5.5 on Richter scale rocked the Dharamshala area in Himachal Pradesh on 26th April 1986 at 07.35 : 17 GMT. The earthquake was felt in Kangra valley, Kullu, Mandi and Chamba areas. Dharamshala town, Macleodganj and Naddi etc., were subjected to maximum damage where the mud houses, older buildings and some of the Govt. buildings suffered maximum loss. Cracks along mortar joints, fall of plaster and relative movement of walls were observed in a number of residential buildings. On the basis of study, four isoseisrnals of VII, VI, V, IV intensities on the MMscale have been drawn. The shape of these isoseismals is elliptical and based on this, the NW/SE trending epicentral tract of this earthquake has been estimated to bel oeated just at Dharamshala Town. With the land levelHng survey and field observation of cracks and fracture, the direction of movement and direction of fracture is also shown. Local depth distribution of earthquake in the Dharamshala area suggests that fractures causing this earthquake have. direct relationship with the local thrust faults.

Keywords

Geophysics (Seismology), Earthquake, Dharamshala, Himachal Pradesh.

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A. K. Mahajan ¹, Surendar Kumar ², S. K. Chabak ²

Affiliations
1 Scientist Incharge, Kangra Seismic Array, c/o K.C. Mahajan, Shyamnagar Dharamsala, Dist. Kangra - 176215. (H.P.), IN
2 Wadia Institute of Himalayan Geology, Dehra Dun-248 00, IN

Abstract

Gravity survey in the station spacing of about 2 km was carried out in Dhararnsala-Palampur area (36 x 20 km) of the northwest Himalaya to study the seismotectonics of the region. The regional trend of Bouguer anomaly shows general decrease from -230 mGal to -250 mGal from south to north. The present gravity survey in DhararnsaIa-Palampur area has brought out some gravity highs trending transverse to the Himalayan trend. The presence of gravity high zone in the central portion of the study area near Yol may indicate either an anticlinal structure or emplacement of denser material from below, probably along the southeast dipping transverse feature noticed in the area. Another gravity high is noticed southwest of Palampur, also named as Paror anticline. The Bouguer anomaly map has helped to detect some transverse faults trending northeast-southwest and north-south i.e. oblique to the major thrust in the Dharamsala-Palampur region.

Keywords

Geophysics, Tectonics, Gravity Anomaly, Himalaya.

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A. K. Mahajan ¹

Affiliations
1 CRSSN, Wadia Institute of Himalayan Geology, Naddi, Dharamsala, IN

Abstract

On 24th March, 1995 at 17:22 hr. IST (11:52 UTC) an earthquake of magnitude 4.9 mb (USGS) struck the Chamba region in northwest Himalaya. The epicentral zone lies 8-10 km northeast of Chamba township. Based on the isoseist characteristics, the source mechanism of this earthquake has been evaluated. The longest axis of the isoseismal is aligned in NW-SE direction, which is parallel to the trend of strike of rocks in the area. The pattern of isoseismals suggest that the causative fault for the 1995 Chamba earthquake is parallel to the local Himalayan trends and the strain has been released at a very shallow depth.

Keywords

Seismology, Earthquake, Chamba, Himalaya.

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A. K. Mahajan ¹, Subash Chandra ², V. S. Sarma ², B. R. Arora ³

Affiliations
1 School of Earth and Environmental Sciences, Central University of Himachal, Dharamshala 176 207, IN
2 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN
3 Ministry of Earth Sciences, Government of India, New Delhi 110 003, IN

Abstract

Geophysical studies using multichannel analysis of surface waves (MASW) and high-resolution electrical resistivity tomography (HERT) have been jointly carried out on an experimental basis in the field. The motive is to study shallow subsurface features (i.e. faults traces, cavities and palaeo-channels) in the foothill zone of Northwest Himalaya. These techniques have shown their potentiality in successfully identifying shallow (0-24 m) fault traces and dissolution features/palaeo-channels. Depending on the sensitivity of the MASW and HERT techniques, geophysical signatures of the subsurface features were recorded and further resolved with the help of synthetic simulation. The synthetic simulation of 2D electrical response has been carried out over the initial model for subsurface fault traces as well as palaeo-channels. The initial model has been refined iteratively to bring the synthetic response close to the field response and hence the final refined model is considered to be the true representation of the subsurface.

Keywords

Fault Traces, High-Resolution Electrical Resistivity Tomography, Multichannel Analysis of Surface Waves, Synthetic Simulation, Palaeo-Seismology.

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A. K. Mundepi ¹, A. K. Mahajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehradun, IN

Abstract

The technique of ground ambient noise (micro tremor) measurement and analysis has been successful for site characterization in many places around the world. This technique has the advantage of being a fast and easy way to estimate the effect of ground motion characteristics due to an earthquake. Single station ground ambient noise (micro tremor) measurements were carried out at 136 sites in the municipal limit of Jammu city, NW Himalaya. This extensive survey allows the estimation of fundamental resonance frequencies (0.432 to 7 Hz) of the region and identifies the areas prone to site amplification. The thickness of the soft sediments has been derived using empirical relationship that comes out to be 14 to 295 mts above the bedrock. The results are in good agreement with the 1-D profile derived using MASW measurements from representative sites. The resonance frequency and sediment thickness is in good agreement with the geological distribution of sedimentary units, indicating a progressive decrease of the fundamental resonance frequencies from the northeastern part (where the bedrock outcrops is exposed) to the southwestern and southern side where a thick sedimentary cover is estimated.

Keywords

Himalaya, Resonance Frequency, HVSR, Soil Thickness, Seismic Microzonation, Jammu City.

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G. K. Ghosh ¹, A. K. Mahajan ²

Affiliations
1 Oil India Limited, Duliajan, Assam - 786 602, IN
2 Wadia Institute of Himalayan Geology, 33, GMS Road, Dehradun - 248 001, IN

Abstract

Seismic intensity information of historical earthquake of NW Himalayas has been utilized for evaluating the attenuation of the intensity with the epicentral distance. Purposefully, the observed intensity of big historical earthquake may implicitly incorporate some site effects arising due to the near surface geology or deposited soil. The isoseismal map of the 1905 Kangra earthquake yields an area of perceptibility defined by the intensity VII (RF Scale), which is mainly elongated in the NNW-SSE direction. However, the higher isoseismals of intensity VIII and IX are more symmetrical and elongated in the SE direction. These isoseismals are more compressed in the northeast and northwest direction indicating fast attenuation characteristics, which may be either due to the presence of any transverse geotectonic feature or change in lithology. In the present study the regression relation has been derived to incorporate the attenuation of intensity with variable magnitudes at different epicentral distances. The derived attenuation relation is useful for assessing damage of a potential future earthquake (earthquake scenario-based planning purposes) for the Kangra region, Northwest Himalaya. The derived relation is mentioned below:

               I(R, Ms) = 2.856 + 1.31^* Ms - 0.0017 R^* - 0.9598 ^*ln(R)

The decrease in intensity with distance has been used to calculate the PGA using the relation of Atkinson and Sonley (2000). The calculated values are quite in agreement with the values obtained by others around its epicentral zone. Another derived relation is used to calculate the size of the Kangra earthquake (Mw = 8.4) using intensity attenuation characteristic.

Keywords

Intensity, Attenuation, Seismic Hazard, Isoseismal, Meizoseismal, Magnitude and Epicenter.

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A. K. Mahajan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33, GMS Road, Dehradun - 248001, IN

Abstract

The presence of natural voids, cavities and palaeochannels under the surface hinders the extension of urbanization particularly in the new upcoming cities in the frontal part of the Himalaya. The Doon valley is characterized by such features. In this context, multichannel analysis of surface waves technique is used as a cost-effective solutions for investigating subsurface cavities, voids, shallow weathered zones and dissolution features in the Doon valley, NW Himalaya. Given the sensitivity of the deduced shear wave velocity to lithology, clast composition, degree of saturation, velocity distribution to a target depth of 40–50 m is a useful guide to demarcate different erosional/ dissolution features, cavities and palaeo-channels. 2D shear wave velocity data sets have been acquired along several profiles in the upcoming region of the Doon valley. Data processing has been carried out at 50 sites and shear wave velocity and profiles were validated taking into account field observations and bore hole data. The subsurface karst features or palaeo-channels are always a matter of concern to engineers before any development starts, because these features could cause subsidence in the region. Anomalies, that include caverns, dissolution features, and erosional channels etched into the Dehradun Fan sediments at shallow depth, have effectively been identified in the 2D shear wave velocity (Vs) field from >50 sites. The subsurface architecture further suggests variation in tectono-climatic condition and shifting of provenance during the fan building processes.

Keywords

Caverns and Palaeo-Channels, Dehradun Fan, Dissolution Features, Multichannel Analysis of Surface Waves, Sub-Surface Investigations.

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