0016-7622/2018-92-3-259/$ 1.00 © GEOL. SOC. INDIA | DOI: 10.1007/s12594-018-1002-y JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.92, September 2018, pp.259-264 Shear Wave Velocities in the Estimation of Earthquake Hazard Over Alluvium in Seismically Active Region Narasimman Sundararajan 1* and T. Seshunarayana 2 1 Department of Earth Sciences, Sultan Qaboos University, Post box 36, Postal Code 123, Muscat, Sultanate of Oman 2 Engineering Geophysics, National Geophysical Research Institute, Uppal road, Hyderabad – 500 007, India E-mail: sundararajan_N@yahoo.com*; tanseshu@gmail.com ABSTRACT It is well known that the potential hazard during an earth- quake is mainly in alluvium or alluvium filled basins; shear wave velocity plays a significant role in estimating the possible hazard during an earth quake in such an area. This paper presents shear wave velocity profile from Jabalpur, Central India mainly dominated by alluvial soil that was moderately affected by an earthquake of magnitude 6.5 in May, 1997. The acquired shear wave velocity by Multichannel Analysis of Surface Wave (MASW) in as many as 36 sites over alluvial soil ranges from 200 m/sec to 400 m/sec and in a few sites marginally less than 200 m/sec corresponding to a sub-surface depth of 30-35 m. Further, the computed N values vary as low as near zero to less than 25. The study is substantiated by the estimation of P-wave velocity by refraction seismic method at the same locations of MASW which ranges from 350 m/sec to 2200 m/sec. The results suggest that the damage during an earth quake appears to be highly unlikely in view of the marginally high Vs up to depth of 30 m. This study on seismic hazard is substantiated by the estimation of frequency of the ground as well as amplification which is found to be a maximum of about 2.5 in the frequency band of 2-6 Hz in west and north western portion of the study area. INTRODUCTION The natural hazards like tsunami, earthquakes are the biggest and ever formidable challenge to the mankind. It is indeed the local ground condition that is one of the major risk factors that cause the havoc and not earthquakes as such. Therefore, it is imperative to study the ground conditions which are termed as site characteristics. In this direction, geophysical methods are increasingly applied to geotechnical investigations, as they can identify material properties and natural boundaries as well as variation in space and time of relatively large area. The potential hazard is mainly in alluvium filled basins (Field, et al., 2000). Under National Earthquake Hazards Reduction Program(NEHRP), provisions of Building Seismic Safety Council (BSSC, 1998) prepared Uniform Building Code (UBC) in which sites are categorized for shaking hazard with shear wave velocity up to a depth of 30 m (V S30 ). It is an important parameter in building codes (NEHRP, 1997), and the earthquake engineering community widely uses shear wave velocity (V S ) in design applications (Kramer, 1996). Hazards mapping methodology is advancing to incorporate local V S information into hazard estimation, particularly, in urban areas (Cramer, 2003 and Cramer, et al., 2004). This trend is expected to accelerate with future expansion of these efforts (Applegate, 2004). Incorporation of earthquake scenario into future hazard characterization will also depend on reliable V S determinations in the upper several tens of meters. Therefore, it is an urgent need to rapidly and economically estimate shallow V S over very large urban sedimentary basins that will become critical to accurately represent site response in future urban hazard maps. In general, borehole sonic logging is considered to be the standard practice for obtaining (V S ) data, as drilling and logging to the required depths for earthquake ground motion investigations is very expensive, time consuming and it is becoming increasingly problematic in heavily urbanized settings. This, in part, has led to avoiding the cost of drilling a hole, but has a constraint that cone penetration depth may be restricted if gravels or cemented sands are encountered. However, recent developments in seismic techniques applicable to geotechnical investigations can be grouped into four categories (Asten and Boore, 2005) viz. 1. invasive measurements, 2. active source surface seismic measurements, 3. combined active passive seismics and 4. passive seismic methods (Park et al., 1999, Sundararajan and Seshunarayana, 2011 and Trupti et al., 2013). Shallow shear wave velocity in engineering surveys were traditionally estimated through boreholes, such as cross hole (American Society for Testing and Material-ASTM, D4428/D4428M-00, 2005), down hole, and suspension logger surveys, which are quite expensive. The Cone Penetration Test (CPT) specified by ASTM, D5778, (2005) some times correlates well with borehole tests and is used widely for soft sites (Ghose and Goudswaard, 2004). However, it should be noted that the CPT is limited to measure soil strength obtained at a single geographical point and thus is a poor predictor of the average soil strength over an area of interest. For all practical applications, shear wave suspension log is found to be the best. Traditionally, several boreholes are required to be drilled for estimating the shear wave velocity with depth at an exorbitantly high cost. Further, it is time consuming and also very difficult to generate shear waves. Processing and identification of shear waves are also cumbersome. The surface methods using specially designed shear wave sources and receivers also suffer from the above problems to a great extent. On the other hand, body wave seismic methods suffer from noise related problems, particularly in urban areas. For a study like microzonation, several spatial shear wave velocity values are required at a quicker pace. Therefore, the non- invasive active/passive methods like Spectral Analysis of Surface Waves (SASW), MASW, Refraction Mirotremor (ReMi), etc. will play a crucial role. Currently, the MASW technique is gaining great importance and it is relatively free from the drawbacks enlisted above, however ensures the shear wave velocity with added accuracy and at cheaper cost. Details of data acquisition, processing and interpretation besides other salient features of this elegant method is available in literature (Park et al., 1999; Sundararajan and Seshunarayan, 2011; Tirupti et al. 2013). The main objective of this paper is to map the shear wave velocity up to a depth of 30 m in alluvial soil near Jabalpur, Central India which was affected by a moderate earth quake of magnitude 6.5 during 1997 which comes under seismic zone III.