Comparative Analysis of Observed and Estimated PGA for Himalayan Earthquakes

Seismic safety of dams constructed in seismically active regions worldwide is of prime importance for an engineer. NHPC as one of the leading hydropower developers of India is committed towards seismic safety of its projects operating throughout the Himalayan stretch spanning a length of more than 2400km. In this connection NHPC has developed a Real Time Seismic Data Center for centralized online monitoring of accelerographs installed at all its power stations. This data center has to its credit more than 320 valuable acceleration records of Himalayan earthquakes. In general, Abrahamson Liteheiser 89, Abrahamson Silva 97, NGA relation Boore Atkinson 08 for crustal events and Boore Atkinson 97 and Youngs et.al 03 for subduction zone attenuation relationships are in use for site specific seismic design parameter studies of hydro projects located in Himalayas. In this study, PGA values of events recorded by NHPC network have been compared with estimated values using these relationships. It is observed that estimated values are much higher than the observed values. This is due to the fact that all the relationships developed and employed for seismic design studies of projects in Himalayas have no representation of any Himalayan earthquake data. It is shown from analysis that percentage overestimation has progressively reduced with refinement of these relationships, however development of Himalayan specific attenuation relationships utilizing acceleration data from Himalayan events will bridge this gap further and help in accurate estimation of seismic design parameters for optimization of the design of structures.