# Seismic Resilience in Critical Infrastructures: A Power Station Preparedness Case Study
## 4 Case Study
### Development and Evaluation of Alternative Mitigation Strategies  
Three factors are considered to quantify the risk associated with seismic events: the estimated cost of damage, the probability of an earthquake occurrence, and the total probability of damage, given that an earthquake has occurred. The _risk_ is calculated using the following general equation:  
\[Risk=C\times P(EQ)\times P(D|EQ) \tag{1}\]  
where \(C\) represents the estimated cost of damage, \(P(EQ)\) represents the probability of exceedance (i.e., the probability of an earthquake occurring), and \(P(D|EQ)\) represents the probability of damage, given that an earthquake has occurred. To evaluate the estimated cost of damage, this analysis follows the structural repair cost ratio for heavy industrial facilities, using a value of 15.7% for complete damage, in accordance with the Hazus Technical Manual [47]. This percentage reflects the relative cost of repairs compared to the total replacement cost of the facility. In our case, the estimated cost of damage to the power station due to an earthquake is USD 188.4 million, based on the construction cost of USD 1.2 billion. The probability of exceedance is calculated by considering the probability of exceeding a PGA of 0.2 g, which is determined to be one in 2475 years. For critical infrastructures, a design life cycle of 120 years is typically used, and the probability of exceedance for this duration is calculated to be 4.85%. When considering a design life cycle of 50 years, the probability of exceedance is found to be 2.02%. As found in the previous chapter, the probability of damage for a given earthquake that generated a PGA of 0.2 g at the site is 0.573 (57.3%). Therefore, the risk for a 120-year life cycle is estimated to be approximately USD 5.23 million and for a lifespan of 50 years is USD 2.18 million. A possible mitigation strategy is anchoring the subcomponents of the power station. Anchoring reduces the vulnerability to seismic events, which is expressed by updated fragility parameters for the relevant subcomponents. Table 6 presents the updated probability of damage for a PGA of 0.2 g, calculated after updating the parameters and conducting an FTA. For anchored subcomponents, the average damage probability is 0.0397 and the standard deviation is 0.036. After conducting an FTA, the total probability of damage for the power station in the anchored condition at a PGA of 0.2 g is determined to be 0.2733 (or 27.33%). Based on the updated calculations, the risk assessment for the power station can be updated to reflect the reduced probability of damage when the subcomponents are anchored. The risk for the anchored condition is determined to be USD 2.53 million considering a 120-year life cycle and USD 1.05 million for a 50-year lifespan. This reduction represents a mitigation of 52% of the risk, which amounts to USD 2.7 million and USD 1.12 million for the 120-year and 50-year lifespans, respectively. In order to determine the cost-effectiveness of a mitigation strategy, it is essential to conduct a BCA. Thus, if the cost of implementation is lower than the reduction in risk expectancy, the mitigation strategy is cost-effective. In the given scenario, for a 120-year lifespan, anchoring the subcomponents reduces the risk by USD 2.7 million. Therefore, if the implementation cost of this anchoring strategy is lower than USD 2.7 million, it can be considered a cost-effective solution. Additionally, the design life cycle of the power station significantly influences the evaluation of mitigation alternatives' effectiveness. The longer the design life cycle, the more advantageous the mitigation strategy is due to the increase in cumulative risk reduction over time. Anchoring subcomponents within critical infrastructure, like power stations, is widely recognized as a strategy to enhance seismic resilience by ensuring the physical stability of crucial equipment during earthquakes.