The unconsolidated materials of young sedimentary basins can have a profound effect on the spatial distribution of earthquake ground motion amplification, resulting in a large variation in the severity of damage to buildings, transportation corridors and other lifeline infrastructures. To be able to carry out a seismic microzonation study for such sedimentary basins a large amount of input data is required on the three dimensional structure of the basin, the stratigraphy of the unconsolidated deposits and their geotechnical and geophysical characteristics.
The city of Dehradun – located on unconsolidated sediments (Doon gravels) underlain by Siwalik sediments in the broad intermontane depression known as the Doon valley within the Siwalik foreland basin of Garhwal Himalaya (Fig 1), has been chosen for this study.
The Doon valley is a crescent shaped, longitudinal, synclinal valley bounded on the north by the Main Boundary Thrust (MBT), where pre-tertiary rocks of the Lesser Himalayan zone override the tertiary rocks from the Siwalik Group. The southern margin of the Doon valley is marked by a sudden break in topography defined by the Himalayan Frontal Thrust (HFT), locally known as the Mohand Thrust where the rocks from the Siwalik Group are overriding the recent alluvial sediments towards the south. On the seismic hazard map of India, this area is located in Zone IV, the second highest category.
The understanding of geotechnical characteristics of the near surface material is of fundamental interest in seismic microzonation. Shear wave velocity is one of the most important parameters for soil response modelling. Shear wave velocity of the near surface soil at about 50 sites in the sub Himalayan mountain exit covering Doon fan deposits, were determined using multi-channel analysis of surface waves (MASW), a seismic technique. In accordance with the provisions of the National Earthquake Hazards Reduction Programme (NEHRP) and based on the average shear wave velocity of the upper 30 m of soil column, sites located on the north western, eastern and south eastern side of the Dehradun fan deposit have shear wave velocities greater than 360 m/s, thereby classifying them as soil class ‘C’ (360-760 m/s). Similarly, the sites in the fan are predominantly classified as class ‘D’ (180-360 m/s). However, some sites towards the south eastern side of the fan deposit have average shear wave velocities less than 180 m/s and could be classified as class ‘E’ (Fig 2).
The seismic hazard assessment carried out using the probabilistic technique for the north west Himalayan region shows peak ground acceleration of the order of 0.2 g for Dehradun city. The input motion derived from the strong motion data shows 0.05 g at the bed rock for the horizontal component of the Chamoli earthquake (M 6.8) data recorded at Tehri. The spectral acceleration shows the acceleration value in the range of 0.14 to 0.36 g for single storey buildings (10 Hz natural frequency) and 0.24 to 0.74 g for two storey buildings (5 Hz natural frequency) (Fig 3).
The 1D site effects studied for the suite shows largest amplification of the ground motion near the eastern end of the city. In general, the amplification function of the ground motion decreases from north east to south west across the study area. This decreasing trend of the ground motion amplification is due to the increased depth of the bedrock and the subsequent increase in attenuation – i.e. deamplifcation.
However, the impedance contrast between the near surface material and the deeper bed rock of the north and the north western part are not generally present in the central part of the city. As a consequence, the amplification of the bedrock motion in the central part of the city is less than the amplification in the north-north western part and the south eastern part. Studies such as this will be useful in land use planning if one considers the one dimensional site effect while designing new engineering structures in the city.