Research Article Mapping Sediment Thickness in Shillong City of Northeast India through Empirical Relationship Rajib Biswas, 1 Saurabh Baruah, 2 and Dipak K. Bora 3 1 Department of Physics, Tezpur University, Tezpur, Assam 784028, India 2 Geoscience Division, CSIR-North East Institute of Science and Technology, Jorhat, Assam 785006, India 3 Diphu Government College, Karbi Anglong, Assam 782462, India Correspondence should be addressed to Rajib Biswas; rajivb27@gmail.com Received 2 July 2014; Revised 11 December 2014; Accepted 21 December 2014 Academic Editor: Jose Restrepo Copyright © 2015 Rajib Biswas et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Modifed form of Nakamura method, / ratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast India. With available borehole information, an attempt is made to develop an empirical relationship between sediment thickness and resonant frequency estimated from / ratio technique. Simultaneously, shear wave velocities are computed entailing resonant frequency and sediment thickness for these boreholes. We also endeavored building another empirical relation between sediment thickness and   . With the help of this, the probable   values for other sites were also evaluated. It is observed that shear wave velocities range from 200 to 550 m/s while sediment thickness ranges from 10 to 80 m, implicating the heterogeneity prevailing in the soil layers of the Shillong city. 1. Introduction It is widely accepted amongst earthquake engineering com- munity that local geology has a dominant role in seismic motion. Sof soils, one of the constituent elements of local geology, are considered as one of the root causes of numerous geotechnical problems because of their low density, high compressibility, and low strength. As a result it becomes potentially collapsible, causing huge settlements of founda- tions even on low-magnitude loads. Tis leads to violent shaking resulting in severe damage to buildings sited over them. Moreover, damage caused by occurrence of earthquake depends not only on its magnitude and epicentral distance, but also on local site efects which are essentially frequency dependent. Tey are caused by topography, sediment thick- ness, soil conditions, and geology of the area. For seismic hazard assessment, the site efect is typically represented by resonance frequency and the associated ground motion amplifcation. Several methods exist, such as array data analysis, Nakamura method of horizontal to vertical / spectral ratio of ambient noise, site to reference spectral ratio, and receiver function type analysis. Out of these, Nakamura method, that is, use of ambient noise records for determination of fundamental resonant frequency, has recently gained worldwide acceptance because of quick data acquisition. Te amount of amplifcation depends on several factors including layer thickness, degree of compaction, and age [1]. One of the many reasons for choosing ambient noise by several authors is that it allows the quick and reliable estimate of site characteristics of any type of an area. Tere are many instances of successful utilization of the / spectral ratio estimate towards studying fundamental fre- quency from ambient microtremors in urban environments ([2–8] and many others). Te proximity of fundamental fre- quency of a site to the existing man-made structures causes damage of the later owing to resonance efects. Terefore, investigation of each site condition is an important step towards earthquake hazard mitigation. Drilling boreholes allows investigators to obtain detailed information, but they are treated as time consuming as well as very expensive process. It is however afrmed that horizontal to vertical ratio in the case of large impedance contrast Hindawi Publishing Corporation Journal of Earthquakes Volume 2015, Article ID 572619, 8 pages http://dx.doi.org/10.1155/2015/572619