Nat Hazards (2014) 74:1829–1851 DOI 10.1007/s11069-014-1283-4 ORIGINAL PAPER Soft sediments and damage pattern: a few case studies from large Indian earthquakes vis-a-vis seismic risk evaluation Mithila Verma • R. J. Singh • B. K. Bansal Received: 27 November 2012 / Accepted: 2 June 2014 / Published online: 19 June 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract India is prone to earthquake hazard; almost 65 % area falls in high to very high seismic zones, as per the seismic zoning map of the country. The Himalaya and the Indo- Gangetic plains are particularly vulnerable to high seismic hazard. Any major earthquake in Himalaya can cause severe destruction and multiple fatalities in urban centers located in the vicinity. Seismically induced ground motion amplification and soil liquefaction are the two main factors responsible for severe damage to the structures, especially, built on soft sedimentary environment. These are essentially governed by the size of earthquake, epi- central distance and geology of the area. Besides, lithology of the strata, i.e., sediment type, grain size and their distribution, thickness, lateral discontinuity and ground water depth, play an important role in determining the nature and degree of destruction. There has been significant advancement in our understanding and assessment of these two phenomena. However, data from past earthquakes provide valuable information which help in better estimation of ground motion amplification and soil liquefaction for evaluation of seismic risk in future and planning the mitigation strategies. In this paper, we present the case studies of past three large Indian earthquakes, i.e., 1803 Uttaranchal earthquake (Mw 7.5); 1934 Bihar–Nepal earthquake (Mw 8.1) and 2001 Bhuj earthquake (Mw 7.7) and discuss the role of soft sediments particularly, alluvial deposits in relation to the damage pattern due to amplified ground motions and soil liquefaction induced by the events. The results presented in the paper are mainly focused around the sites located on the river banks and experienced major destruction during these events. It is observed that the soft sedimentary sites located even far from earthquake epicenter, with low water saturation, experienced high ground motion amplification; while the sites with high saturation level have under- gone soil liquefaction. We also discuss the need of intensifying studies related to ground M. Verma (&) Á B. K. Bansal Geoscience Division, Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi 110003, India e-mail: [email protected] R. J. Singh 101, Shiv Vihar, Janakipuram-I, Lucknow 226020, India 123 1830 Nat Hazards (2014) 74:1829–1851 motion amplification and soil liquefaction in India as these are the important inputs for detailed seismic hazard estimation. Keywords Earthquakes Á Soft sediments Á Ground motion amplification Á Soil liquefaction Á Seismic risk evaluation 1 Introduction Earthquakes are considered to be the worst natural calamities in comparison with other natural events such as cyclones, floods and droughts, as they strike without any notice and cause immediate loss of life and property. The areas far from human habitation rarely get affected by these events; while they cause widespread damage and multiple fatalities, when occur in the vicinity of urban centers. The damage caused during large earthquakes is governed by geology, subsurface soil and ground water conditions of the affected area; however, the degree of damage largely depends upon the state of social development of the area, such as population density, construction practices and emergency preparedness. The damage caused by the earthquakes therefore, may be categorized into two types; (1) Direct damage and (2) Indirect damage. Direct damage may result from primary effects of the earthquake, i.e., intense ground shaking and fault displacement or from secondary effects like landslides and soil liquefaction generated by primary effects. The magnitude of damage due to strong shaking depends upon intensity and frequency of motion, whereas landslides and liquefaction depends upon slope instability, bearing capacity failure, lateral spreading etc. (3) Indirect damage generally refers to the socioeconomic and environ- mental impact of the earthquake. It includes loss in terms of life, economy and environ- mental side effects. Cities which are built on soft/unconsolidated sediments are more vulnerable, as the soft sediments are susceptible to ground motion amplification and liq- uefaction. The severe damage due to ground motion amplification and soil liquefaction has also been observed at sites (comprised of soft sediments), even at larger epicentral dis- tances. For example, the Ahmadabad city (Gujarat), located about 330 km from the epi- center of 2001 Bhuj earthquake (Mw 7.7) got severely affected by amplified ground motions due to presence of thick soft sedimentary cover. Similarly, Mexico city located about 300 km from the epicenter of 1985 Mexico earthquake (Mw 8.3) suffered great devastation by ground motion amplification. On the other hand, during the Niigata and Alaska earthquakes of 1964 (Mw 7.6 and Mw 9.2) and Loma Prieta earthquake of 1989 (Mw 6.9), soil liquefaction was the major factor to cause maximum destruction. The damage due to ground motion amplification and liquefaction may, however, vary from region to region depending upon the geological and lithological conditions of the area. Most of the highly populated and urbanized centers are located either on the river banks or near sea shores (comprised of soft sediments) all over the world. But, in earthquake prone areas, these centers have gained importance in terms of urban safety due to rapid urbanization and population growth. In India, the Himalaya and the Indo-Gangetic plains (lying south of Himalaya), where more than 40 % of the Indian population resides are particularly vulnerable to high seismic hazard. As per the seismic zoning map of the country, prepared by Bureau of Indian Standards (IS 2002), about 65 % area of Indian landmass is under threat of moderate to severe seismic hazard, i.e., prone to shaking of MSK Intensity VII and above. Several important cities are lying in seismic zone III, IV and 123 Nat Hazards (2014) 74:1829–1851 1831 V, particularly, in Himalayas and Indo-Gangetic plains (Fig. 1). Moreover, in recent years, cities in Indian alluvial plains have seen rapidly increasing number of high-rise buildings and massive civil engineering facilities. Occurrence of large earthquake in the vicinity may cause extensive damage to the built environment, either due to amplified ground motions or liquefaction of soft sediments. Therefore, understanding the relation between soft sediments and damage pattern due to seismically induced ground motion amplification and soil liquefaction is a basic step toward seismic hazard assessment. Various studies have been carried out toward understanding the influence of soft sediments on the damage pattern during earthquakes. The influence of local geology on the amplitude and duration of ground motions was first observed and reported by Wood (1908) after the California earthquake 1906 (Mw 7.8). Since then, there has been significant development in this area. It is worth mentioning a few important studies here. For example; Borcherdt (1970) observed that the horizontal ground velocities generally, increased with the thickness of younger sediments and were as much as 10 times greater than those recorded at nearby bedrock in the San Francisco Bay area. Seed and Idriss (1982) estimated the magnitude of amplification for different sedi- ments as a function of peak ground acceleration. Singh et al. (1988) studied the ground motion amplification in and around Mexico city using strong motion data of the 1985 Mexico earthquake (Mw 8.3). The effect of topography on amplitude and frequency content of ground motions was studied by Geli et al. (1988) and Faccioli (1991). They reported that the hills produce scattering, focusing, or defocusing of incident energy (topographic effect) and thick alluvium-filled terrain causes reverberations due to trapped energy (basin effect). These effects were further quantified by Pedersen et al. (1994). Ground motion amplification was computed during the 1989 Loma Prieta earthquake (Boatwright et al. 1991; Chin and Aki 1991) and from the aftershock sequence (Field et al. 1992). Various techniques like spectral ratio, ambient noise survey or Nakamura ratio and even the nonlinear elastic behavior have been used in investigating the local site effects and dynamic soil behavior in different geographic and geologic conditions (Field and Jacob 1993; Aki 1993; Beresnev et al. 1995; Atakan and Havskov 1996; Teves-Costa et al. 1996; Atakan et al. 1997; Field et al. 1997; Safak 2001; Albarello 2001; Parolai et al. 2002). In contrast, a very few studies related to ground motion amplification have been taken up in India as a part of seismic microzonation exercise of different urban centers, viz, Guwahati (DST report 2007), Sikkim (Nath et al. 2000), Garhwal Himalaya (Nath et al. 2002), and Delhi region (Nath et al. 2003), Ahmedabad (Govindaraju et al. 2004) and Gandhinagar, Gujarat (Sairam et al. 2011). Raghukanth and Lyenger (2007) estimated the seismic spectral acceleration on various geological conditions in peninsular India based on standard spectral ratio (SSR) technique. Chopra and Choudhury (2011) reported that the response spectra are influenced by the local geological and lithological conditions despite having the same regional