Seismic Hazard Maps and Design Response Spectra for Sri Lanka - A Generic Approach

Seismic hazard maps and design acceleration response spectra for rock and soil sites in Sri Lanka are presented in this chapter. These were derived from a combination of the standard Probabilistic Seismic Hazard Approach (PSHA), the deterministic approach and site response analyses. Six seismic zones were identified in and around Sri Lanka (with Colombo considered as a separate zone). The de-clustered earthquake data showed Gutenberg-Richter b values in the range 0.3 to 0.9. About 180 records of broadband ground motion data obtained from three recording stations in Sri Lanka were analysed to derive seismological parameters such as the wave transmission quality factor (Q0), upper crustal amplification factor ( ) and stress drop ( ). Using these parameters, synthetic accelerograms [GENQKE (Lam, 1999)] were developed and compared with recorded ground motions. Based on the satisfactory comparisons, ground motion prediction equations (GMPE) were developed for the region. All these parametric combinations were utilized to produce seismic hazard maps for return periods of 500, 1000, 2500 and 10,000 year return periods. Furthermore, target ground motions of Peak Ground Accelerations (PGA) of 0.08 g and 0.05 g were generated for site response studies. Site amplification factors were derived using five borehole records that were consistent with the definitions of notional site classes C and D according to standard building codes. Results obtained from the above approaches were compared and benchmarked with international codes of practice. Based on these observations, design response spectra are presented in accordance with the Eurocode format for the benefit of practicing engineers. Simple manual calculations for site specific spectra are also presented. Our study shows that Sri Lanka should be considered as a low to moderate seismic region, similar to regions such as Australia.