IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 05 Issue: 04 | Apr-2016, Available @ http://www.ijret.org 77 SITE RESPONSE IN GUWAHATI REGION USING STANDARD SPECTRAL RATIO Sasanka Borah 1 , Diganta Goswami 2 , Jayanta Pathak 3 1 Research Scholar, Department of Civil Engineering, Assam Engineering College, Assam, India 2 Associate Professor, Department of Civil Engineering, Assam Engineering College, Assam, India 3 Professor, Department of Civil Engineering, Assam Engineering College, Assam, India Abstract This study attempts, through a very limited number of data to explain the requirement of a detailed seismic analysis of the western part of the Guwahati metropolitan region of Assam, India, which falls under seismic zone-V(IS:1893-2002) and consists of mainly alluvial deposits. This study carries out a Site Response study of Western part of Guwahati by employing Standard Spectral Ratio (SSR) method, which is a reference site method, where the ratio of Fourier spectrum of ground motions recorded at a soil site to that of ground motions recorded at a nearby reference site (rock site) is obtained to define the site response. Significant site response in terms of site amplification is observed in the frequency range of 0.1 to 1.00 Hz, which concretes the necessity of a detailed analysis program for the region. Keywords: Site Response, Standard Spectral Ratio (SSR) ---------------------------------------------------------------------***--------------------------------------------------------------------- 1. INTRODUCTION One of the most easy, popular, widely accepted and used reference site method to evaluate site response is the Standard Spectral Ratio (SSR) method (Mittal et. al., 2013). It has been used by many authors to determine the site response (Field, E. and Jacob, K.H., 1995, Reipl, et. al., 1988, Lermo & Chavez Garcia, 1994a, Chavez-Garcia & Cuenca, 1998, Raptakis et. al., 1998a). The SSR is a technique where the site response is defined as the ratio of the Fourier amplitude spectrum of ground motions recorded at a soil-site to that of ground motions recorded at a rock- site record located nearby, from the same earthquake and component of motion (Borcherdt, 1970, Pitilakis, 2012). 1.1 BASIC ASSUMPTIONS OF SSR In this method it is assumed that earthquake records obtained from the reference site (an earthquake recording station located on a hard rock outcrop) are free from site effects and contains the same source properties and when the two sites are closely located, the path or propagation effects are also same for the pair of records. Hence, the ratio of the Fourier amplitude spectra of sedimentary site to reference site expresses the local site effects or in other terms amplification at the sedimentary site. Alternatively, it can be said that the technique depends on two basic assumptions (Lang, 2004): 1. The source and paths effects at both sites are similar, 2. The reference site has a negligible site response, that is, its spectrum is flat. The first assumption requires that the distance between both sites is limited in order to have a similar wave field (with similar incidence angles and azimuths) arriving at the two sites. This effect can also be reached with increasing source- receiver distance compared to the distance between the two recording sites (Field et. al., 1992). The second assumption implies that the earthquake record on the rock site, which usually is a free field station on the ground surface, is almost equivalent to the input motion at some depth, not taking into account the free-surface effects (Lermo and Chavez-Garcia, 1994). 1.2 THE SSR METHOD The Standard Spectral Ratio (SSR) method is defined as the ratio of ground motions records of that of a soil-site to that of a rock site, where the rock site is used as a reference site (Figure 1). It is assumed that earthquakes recorded on the reference site (i.e. recorded on a station placed on outcropping hard bedrock) contains the same source and propagation effects as that of the records from the other sites that is:  ݌ ݋  =   (ݔ, ݕ)   (ݔ, ݕ) =   ݔ(ݔ)    (ݔ, ݕ)   ݔ(ݔ)    (ݔ, ݕ) Where, U ij (x,y) = Fourier Amplitude of the ground motion observed at a soil site j for an event i, S i (x) = source function. Z i (x) = response of the site. A ij (x,y) = Attenuations function. x = the frequency, y = distance from epicenter and k = reference station.