Microtremor Testing for Ground Response Analysis in Visakhapatnam City, India Akhila Manne Research Scholar Geotechnical Engineering Laboratory Earthquake Engineering Research Centre International Institute of Information Technology Hyderabad Gachibowli, Hyderabad-32. e-mail: [email protected] Dr. D. Neelima Satyam Assistant Professor Geotechnical Engineering Laboratory Earthquake Engineering Research Centre International Institute of Information Technology Hyderabad Gachibowli, Hyderabad-32. e-mail: [email protected] ABSTRACT The city of Visakhapatnam is one of the important and largest port city of India and is also the most densely populated. Site effects due to sandy and clay sediments are characteristic of the whole city area. Microzonation studies of the city are required as per the survey by National Disaster Management Authority of India (NDMA). The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied for measuring the free-field response in the city to assess the fundamental frequency and amplitude of the sediments. To overcome the error in recordings, testing has been performed during early morning and late evenings. Nakamura (1989) method has been selected to analyse the recorded data as it simplifies the recording process and provides accurate results. Very clear HVSR peaks were obtained in the entire central parts of the city, whereas in the eastern and western parts the site response is in general lower due to lower impedance contrast of gravel with the bedrock. The iso-frequency map of sediments shows a distribution in the range of 0.44–10.1 Hz. Frequency of the HVSR peaks is considerably higher in the central part (>4Hz) than in the eastern and western parts (<4.0Hz) of the city, indicating a high impedance contrast with the bedrock. The amplitude of vibration is higher (>1) in the north eastern and south western locations. KEYWORDS: Microtremor; Nakamura Method, Peak Frequency, Amplitude INTRODUCTION Dynamic soil properties form an important basis in estimation of deformations and strains during cyclic loading due to earthquakes. Frequency, amplitude and time period are the - 3035 - Vol. 18 [2013], Bund. O - Manne.A & Satyam.D.S. 3036 parameters that define the strong ground motion. When the dynamic earth period equals that of the strong ground motion it results in resonance. Therefore, it is required to estimate the resonant frequency of the ground and amplitude of vibration. Methods to determine dynamic soil properties can be divided into invasive and non-invasive. Invasive methods (SPT, DCPT, downhole, cross hole etc) contain source located either on the surface or in a downhole. The non- invasive methods can be classified as single station and multiple station methods. Microtremor method falls under single station method whereas SASW, MASW etc (active sources); ReMi, SPAC, FK etc (passive source) are multiple source methods. Microtremor method has been gaining a lot of importance due to its feasibility to perform test in densely populated areas and the speed of processing the information. The instrument works on the principle of recording micro vibrations in the ground of specific amplitude termed as microtremors. Microtremors are low amplitude (10-4-10-2 mm) vibrations of the ground of artificial origin. Microseisms and microtremors can be distinguished from the frequencies and type of origin. Microtremors are caused due to man-made activities and have frequencies >1Hz. Microseisms are caused due to oceanic activity, local meteorology, winds etc and have frequencies <1Hz. Microtremors have gained importance in assessment of dynamic properties of ground such as frequency and amplitude. Gao et al., (2012) used microtremors to study the health conduction of concrete tunnel lining. From the analysis of records from multiple sensors of same site, they can be used to estimate the shear wave velocity and sediment thickness. Kuo et al., (2009) by comparing three different methods for evaluation of shear wave velocity, evidenced that microtremor is efficient in estimation of the values. The Microtremor peak which denote the frequency of first mode of vibration can be obtained either from Rayleigh wave ellipticity (Lachet and Bard, 1994; Kudo, 1995; Bard, 1998; Bonnefoy-Claudet, Cornou, et. al., 2006) or S-wave resonance (Nakamura, 1989, 2000) or Airy phase of love waves (Bonnefoy-Claudet et al., 2008). Rayleigh dominate the records in case of high impedance (I) contrast or frequency range (6-20Hz) and Love waves when I<3 or in the case of low frequency (3.6-6Hz) (S. Bonnefoy-Claudet et al., 2008; Chávez-García & Luzón, 2005). Basin geometry can have effects on resonance frequency from HVSR technique (Guéguen et al., 2007) particularly in narrow sediment basins. In this study, microtremor testing has been performed in Visakhapatnam city to develop iso- frequency maps for the city. Though the city has no record of significant earthquakes it has been considered for detailed seismic evaluation by the NDMA. In case of a linear to low range earthquakes, microtremor along with non-invasive active source technique best demonstrates ground response (Hardesty et al., 2010; Molnar et al., 2007). Hence, this method can be applied to locations such as Visakhapatnam with no record of significant earthquakes. ABOUT THE STUDY AREA Visakhapatnam city is located in the state of Andhra Pradesh, India and covers an area of 160sq.km (Fig.1). The city extends between 17° 40′30″–17°45′N and longitudes 83°10′–82°21′ E. The population of the city is about 17 million (2011 Census). Of the total occupied area, the major land is of urban usage of which 66% is used for residential and commercial purpose, 20% by industries and rest by wetlands. The eastern part of the city is densely populated and consists of residential areas. Characterized by Eastern Ghat Mobile Belt (EGMB), the topography of Visakhapatnam is undulating with hill ranges (320–504 m above MSL) on northern, southern and western sides, and Vol. 18 [2013], Bund. O - Manne.A & Satyam.D.S. 3037 the Bay of Bengal on eastern side, sloping towards centrally located salt marshland (4 m above MSL) from all the sides (Fig. 1). The hills consist of thick soil cover and deciduous forests. The eastern side constitutes upland with a height of 65–75 above MSL and the slope is towards Bay of Bengal. Kailasa range and Yarada range are two important hill ranges present. Kailasa hill range limits the Visakhapatnam city in the northern boundary whereas the Yarada range is located in the southern side (Figs. 1). These two ranges are being separated from each other by a vast tidal basin, a few scattered hillocks and portions of low land. The Dolphin’s nose is formed by Yarada range. The largest stream passing over the city is Naravagedda which runs from northwest towards salt marshland. The city consists of major minerals such as Khondalites, charnockites, quartzites and pegmatities (Fig.2). Visakhapatnam contains meta sediments, igneous and intrusive igneous rocks which are characteristic of Precambrian age. Younger soils such as red soils are also present upto a depth of 9m. Laterites occur as capping to the Khondalites and dune sands with black sand concentrations along the beach. The major soil types in Visakhapatnam area are gravel clay, sandy soil, clay and gravel loams. Red clays are predominant and gravel clay occupy second place. Red coastal clayey soils consist of low percentage of sand particles (<30%) and mainly fine grained sizes of silt and clay particles (<0.075 mm). These soils have coefficient of permeability less than 10-7 cm/sec. Groundwater table in the city ranges from 3m to 14m (Fig.3). The water levels in borewell are upto a maximum of 35m. The northern and eastern part of city consists of deeper water levels (12-6m) below ground level whereas western and southern locations consist of water levels above 6m i.e, upto 3m. The lower water table levels in the western locations can be attributed to the flow of Naravagedda stream. 60% of Indian landmass is considered to be prone to earthquakes. The area of study lies in the Stable Central Region (SCR) of Peninsular India. From previous earthquake case histories (1997 Jabalpur and 1993 Latur) no region of India is considered to be safe against earthquakes. Though Visakhapatnam does not consist of any major faults, abundant number of lineaments covers the area. The city falls under seismic zone II (IS 1893:2002) susceptible to a PGA of 0.1g. About 48 seismic events (1967-2013) have been identified in about 300Km radius of the city ranging from a magnitude of 2.1 to 5.2. Conversely, the regions which lack previous seismic history are assumed to be prone to excess seismic risk. Therefore, the seismic hazard of the city is proposed to be estimated in this paper. The parameters that quantify seismic hazard such as frequency and amplitude are proposed to be estimated using microtremor testing and the analysis is done using Nakamura (1989) technique. Vol. 18 [2013], Bund. O - Manne.A & Satyam.D.S. 3038 Figure 1: Area of study Figure 2: Geology Map of Visakhapatnam (Modified from Subba Rao, 2008) Vol. 18 [2013], Bund. O - Manne.A & Satyam.D.S. 3039 MICROTREMOR METHOD The method can be best described as an analog of electrically passive magneto-telluric exploration (Asten, 2004). Microtremor methods can be array based (ReMi) or single station. Array based methods require more than one sensor and can be used to correlate shear wave velocity using SPatial Auto Correlation (SPAC) or Frequency Wave number (FK) methods resulting in identification of sediment thickness. Single station method is much simple and can be used to estimate dominant earth period, amplitude and vulnerability of an area. Estimation of dynamic characteristics of soil using microtremor has been very common in the 1980s.