Seismic Hazard and Risk Assessment in Central Indo-Gangetic Plains, India

M C, Raghucharan (2020) Seismic Hazard and Risk Assessment in Central Indo-Gangetic Plains, India. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

The exact location and time of an earthquake cannot be predicted with the present scientific knowledge, which has diverted the scientific community to move towards disaster mitigation to overcome its after-effects. Effective disaster mitigation involves predicting the seismic hazard/risk for a most probable earthquake that can occur at a given location so that the infrastructure can be built/repaired to sustain the earthquakes with bare minimal damage. Seismic hazard at a location can be computed from the recorded ground motions. However, ground motion records in India are scarce, and not available for every region, and certainly not for all magnitude and distance ranges. Employing ground motions from other regions may lead to inaccurate prediction. Hence, the viable alternative is to simulate ground motions in the broadband frequency range from the validated seismological model. Seismic hazard computed in previous studies has the following limitations. IS 1893:2016 reports seismic hazard values for only four zones covering the entire India. The National Disaster Management Authority (NDMA) developed a probabilistic seismic hazard contour map for India from the Ground Motion Prediction Equation (GMPE) using synthetic ground motions only. Similar small scale studies that were taken up in the present study region either computed hazard in terms of PGA or at city levels. Also, there is a possibility of potential earthquake of magnitude more than 8.0 in the seismic gap between the rupture zones of 1905 Kangra and 2015 Gorkha earthquakes. Hence, in this study, the seismic hazard for the entire Central IndoGangetic Plains (CIGP) is computed at district level, employing recorded and synthetic ground motions validated from the 2015 Nepal earthquake records. Furthermore, two new GMPEs were derived from the recorded and combined dataset (recorded as well as synthetics) to predict PGA and PSA at 25 periods between 0.01 and 4 s for the Himalayas and the IGP regions, employing Artificial Neural Network (ANN) methodology. Also, seismic risk in terms of probability of damage to buildings, economic losses, and casualties is not available for CIGP so far. Hence, vii utilizing the derived GMPEs and seismic hazard, the seismic risk is computed for the first time at 54 districts of Uttar Pradesh state covering the central part of IndoGangetic plains (CIGP). The response spectrum prediction of the validated seismological model is close to that of the recording, when compared to widely used existing GMPEs of IGP in literature. Hence, the model is used to generate the synthetic ground motion at data gap regions of CIGP. The two GMPEs derived in this study have a better fit with recorded data at eight stations and reported the least standard deviation of the error, V(H), than the existing GMPEs in Himalayas and IGP. Further, with the GMPEs derived in this study, the seismic hazard in terms of 10% and 2% probability of exceedance in 50 years, is computed at 54 districts of the study region. These hazard results obtained are comparable to the results of previous works available in the literature. Seismic risk assessment in CIGP acknowledges several key findings. Allahabad district, even though demarcated as Zone II in IS 1893: 2016, has expected economic losses around 16 billion dollars and the highest number of homeless and uninhabitable dwellings. Model Building Classes (MBCs) MMB (Mud Mortar Bricks with temporary roof) and BSR (Bricks with Stone Roof), comprising of 16.5 and 9.5% of total buildings, have collapse probability of 0.6 and 0.45 respectively These building types need immediate retrofitting or reconstruction for effective disaster mitigation. Also, 36% and 11% of buildings in CIGP might collapse for MCE and DBE earthquakes, respectively. Further, for a scenario magnitude range of Mw 7.5 to 8.5, the expected economic losses vary from 60 to 150 billion dollars, and the human casualties vary between 0.8 and 2.8 lakhs, respectively. The poor quality and low seismic resilience of buildings in CIGP region is the principal reason for the substantial economic losses and casualties. Finally, it was found that the most influential parameter is the magnitude, followed by source location and GMPE.

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