Seismic hazard assessment for Colombo city with local site effects

Abstract

floods, droughts, landslides etc. Due to the experiencing of ground shaking in last few decades and the disaster on 26th December, 2004 due to tsunami, Sri Lanka cannot further treated as an earthquake damage free country. Few research has been carried out to investigate seismic hazards at Colombo city area and the response spectrum at rock level for Colombo city area has been proposed with the PGA (Peak Ground Acceleration) of 0.1g. However, no studies have been carried out to develop earthquake response spectrum for Colombo city with local soil variations. This study discusses generalized soil profiles for Colombo city and earthquake response spectrum with local site effects for Colombo city area. 77 borehole logs done at the study area are collected and locations were plotted on a map of study area. Eight vertical sections were obtained through the study area and using them eight soil profiles which have horizontal soil layers were developed. All eight profiles highlight that the average soil cover in Colombo city area is about 20m. Just above the basement rock dense to very dense silty sand/ sand layer is present. Topmost layer also having loose to dense sandy soil. In between both of sand layers, very loose to loose clay/ silt or organic material layer is encountered in all boreholes. The developed soil profiles were used to analyze with earthquake motions using the computer software called EduShake. The six earthquake motions are applied at rock level as input motions. The response spectra at rock level and the response spectra with the local soil effects were obtained as the output file. The program was run for the eight soil profiles and obtains the average value of both output files as final result. Finally, earthquake response spectrum has been proposed for Colombo city area and it was compared with existing response spectrum at rock level for Colombo city. According to this study, the PGA with local site effects is 0.13g for Colombo city area.