Evaluation of Site Effects Using HVSR Microtremor Measurements in Vishakhapatnam (India)

Dynamic soil properties form an important base for estimation of strains and deformations due to dynamic loading. Frequency, amplitude and time period are the prominent parameters that define a strong ground motion. When the frequency of the soil and strong ground motion reaches to same value during a seismic event, it results in resonance effect leading to huge damage of the structures. Therefore, it is required to estimate the predominant frequency of the soil and amplitude of vibration. Invasive and non-invasive techniques can be employed to determine the dynamic soil properties. Invasive methods, including 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 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. Therefore, evaluation of site effects has been attempted using microtremor testing in Vishakhapatnam (India). The city of Visakhapatnam is one among the prominent and largest functional port cities of India. Site effects due to sand and clay soil layers are characteristic of the whole city area. Seismic hazard studies of the study area are required as per the survey by Disaster Management Authority of India (NDMA 2012). The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied for measuring the free-field response in the city to assess the peak frequency and amplitude of the soil sediments. Nakamura (A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Railway Technical Research Institute/Tetsudo Gijutsu Kenkyujo, Tokyo, 1989) method has been used to analyses the recorded data as it simplifies the recording process and provides accurate results. It has been evident from the results that clear H/V spectral ratio peaks were observed in the entire central parts of the city, whereas in the eastern and western parts lower site response has been observed due to low impedance contrast of gravel with the rock. The predominant frequency hazard map shows frequency distribution of 0.43–10 Hz throughout the city. Frequency is considerably higher in the central part (> 4 Hz) than in the eastern and western parts (> 1 Hz) in the north eastern and south western locations.