PROBABILISTIC SEISMIC HAZARD ASSESSMENT OF CENTRAL HIMALAYA

Abstract

In the present study Probabilistic Seismic Hazard Analysis (PSHA) has been carried out for the region of Central Himalaya which forms a part of the most severe seismic zone. The study area covers latitude 26°-31° N and longitude 82°-88° E. The region has experienced devastating earthquakes in 1833, Bihar-Kathmandu and 1934 in Nepal with magnitude greater than 8.0. Because of the increasing population and rapid growth of unplanned urban infrastructures that inhabits this region, it becomes prudent to carry out the seismic hazard analysis using the latest data and methodology available, which will assist the planners and the governments to mitigate the hazard. To carry out seismic analysis, the study area has been divided into seven seismogenic source zones based on geology, tectonics and seismicity in and around the Study area. And by using Gutenberg-Richter recurrence relationship, the seismic hazard parameters such as a, b and Magnitude of completeness, Mc have been computed for each sources zone. The study area consisted of 1223 events of which 28.12% of the events were excluded following declustering, for completeness analysis, Visual Cumulative Method proposed by Mulargia and Tinti (1985) has been incorporated. To obtain PGA values attenuation models developed by Abrahamson, Silva and Kamai (2014), Boore et al. (2014), Campbell and Bozorgnia(2014), Idriss (2008) have been used for a return period of 2475 years. Abrahamson, Silva and Kamai (2014) have shown the maximum pga values for zone 1, 4 and 6. Boore et al. (2014) have shown the maximum pga values for zone 2, 3, 5, 7. Campbell and Bozorgnia(2014) have shown the maximum pga values for zone 5. The maximum pga for zones including Vs 1200 is 0.854, for Zone 2 and 3 and minimum pga of 0.015 for zone 1 and 3. The maximum pga for zones including Vs 250 is 0.808, for Zone 7 and minimum pga of 0.0425 for zone 6. The highest PGA for Vs 1200 by Abrahamson, Silva and Kamai (2014) model was observed in Zones 1, 2,4. By Boore et al. (2014) maximum PGA was observed in Zones 2 and 3. And by Campbell and Bozorgnia(2014), have shown maximum PGA in Zones 2 and 3. Similarly Idriss model has given maximum PGA values for Zone 2 and 3. The highest PGA for Vs 250 by Abrahamson, Silva and Kamai (2014) model was observed in Zone 6. By Boore et al. (2014) maximum PGA was observed in Zone 7. And by Campbell and Bozorgnia(2014), have shown maximum PGA in Zones 7. Similarly, Idriss (2008) model has given maximum PGA values for Zone 7. Boore et al. (2014) model tends to overestimate the pga values when compared to other models. The max PGA obtained for zone 7 is relatively very low in Abrahamson model in comparision with other models. The estimated Pga values obtained by Idriss (2008) model was lesser than other models and tend to be saturated around 0.43 for Vs1200.