EXTENDED FRAMEWORK FOR EARTHQUAKE AND TSUNAMI RISK ASSESSMENT: PADANG CITY A CASE STUDY

The Great Sumatra earthquake and resulting devastating tsunami in 2004 have highlighted the significance of earthquake risk assessment that can effectively include all main associated hazards, such as tsunami. An integrated quantification of earthquake and tsunami risk is challenging due to mathematical and computational issues as well as limited data, and as such it has not been done before in developing countries. In addition, the conventional Probabilistic Seismic Hazard Assessment (PSHA) generally assumes a Poissonian earthquake model with a stationary rate of hazard. However, hazard rates increase with elapsed time since last large earthquake, or when a seismic gap is present along a fault, and thus, a time dependent (non-Poissonian) PSHA model is generally more suitable. The Earthquake Risk Assessment (ERA) Framework developed at the University of Sheffield is extended in this study to account for tsunami and to include a time dependent hazard assessment. The ERA Framework is based on a stochastic approach that utilizes readily available seismological information. Hence, it is suitable for use in developing countries. The extended framework is used to carry out earthquake and tsunami hazard assessments for Sumatra. This study finds that a maximum PGA of 0.65g on bedrock is expected in the area, a value comparable with that found in other recent studies, but higher than the Indonesian seismic code SNI 03-1726-2002 (the code uses a value of 0.3g). In addition, the time dependent model of this study captured the increased rate of hazard in the middle segment of the Sumatra Subduction Zone, which is consistent with the location of the seismic gap. Currently, the seismicity of this region has increased about 2 times above that of the long term rates. An average tsunami height of 20.7 m is estimated for Padang city at 10% probability of exceedance in 50 years. This study estimates that total earthquake and tsunami risks for buildings in Padang are approximately £54.5 million and £30.8 million per annum, respectively. The annual fatality risk due to tsunamis is much higher than that due to earthquakes, which is approximately 2000 and 8 fatalities, respectively. Total earthquake premiums for the existing building stock of Padang are estimated to be 36.4‰, 16.6‰, 8.1‰ and 3.1‰ for UBM, CBM, RCI and steel structures, respectively. For tsunami hazard, the recommended premium rates are 11.7‰, 9.9‰ and 7‰ for UBM, CBM, RCI/steel structures, respectively. For seismically designed structures, the premium rates decrease about 80% and 25% for earthquake and tsunami hazards, respectively. Earthquake insurance rates applied by insurance companies in Indonesia are comparable with those estimated for seismically designed structures (1.9‰ for RC/steel buildings and 4.7‰ for other buildings). Mitigation strategies to minimize the risks are proposed including the enforcement of seismic design provisions for all buildings in Padang, nationwide obligatory seismic insurance for buildings, tsunami vertical evacuations shelters and tsunami energy dissipation efforts (e.g. offshore barriers, coastal vegetation). At least 17 points of tsunami evacuations refuges are proposed for Padang city, to increase the survival probability of the residents in the area