Geotechnical Studies for Seismic Microzonation of Delhi

Seismic microzonation studies in Delhi have been carried out solely on the strength of the geotechnical data available with the Geological Survey of India. The main derivative products of the exercise include the shear wave velocity and predominant frequency maps on 1,50,000 scale. These maps are based on the N value data of subsurface soil columns, mostly down to a depth of 30 m, as observed at 121 Bore-Hole sites, in addition to the depth to bedrock, and other geological and geomorphological information.

The different ground response parameters, e g shear wave velocity and predominant frequency have been estimated by using empirical relationships developed elsewhere, especially m California, USA Although the tenacity of the qualitative/ semi quantitative results is yet to be affirmed, the maps, by and large conform to the effects of the past near and far events, such as the 1905 Kangra, 1960 Delhi and 1999 Chamoli earthquakes.

The amplification of the ground motion being the inverse function of impedance contrast between the underlying hard rock and the overlying soft sediments in Delhi, where the underlying rock is represented by quartzite, practically, the relative values of shear wave velocity of the soft sediments indicate the likely amplification - the low velocity promoting higher amplification A maximum ground motion amplification of 12 5 times and a minimum of 5 times, when assuming the damping as zero, has been estimated However, the actual amplifications are bound to be less because the soil does dampen the ground motions The predominant frequency map prepared on the assumption of simplified subsurface configuration, provides a generalized picture of the range of frequencies expected to get amplified -the amplification governed by the shear wave velocity of the soil cover there.