Evaluation of seismic hazard in low to moderate seismic regions, Sri Lankaa case study

An assessment of seismic hazard based on the probabilistic approach was undertaken for a low seismic island region, Sri Lanka. Potential source zones in both local and regional contexts were identified in a comprehensive manner based on active tectonic structures and past evidence of seismic activities in the region. This source zonation, because of dormant to mild seismic nature of the region, led to characterize most of the regionally dominant seismic sources simply into large area zones. Recurrence rates of seismicity in each of the identified source zones were obtained for pre-estimated periods of completeness, unless otherwise data of a source zone were clearly incomplete due to lack of earthquake records, in which case recurrence parameters were found based on a subjective evaluation concerning the need of conservative recurrence rates for the area. Computed hazard values in terms of expected ground motions (peak ground acceleration (PGA) and spectral accelerations (SAs) at 0.1, 0.5, and 1.0s natural periods) at rock sites in Sri Lanka for 10% (475-year return period), 5% (975-year return period), 2% (2475-year return period), and 0.5% (9975-year return period) probabilities of exceedance in 50years' time were depicted in separate raster maps. Hazard values showed that the area around the capital Colombo possesses maximum expected PGA (in rock sites) which is about 0.11g for a 475-year return period. Most of the other areas indicated relatively low values.