In quantitative risk assessment (QRA), what is the key difference between fault tree analysis (FTA) and event tree analysis (ETA)?

The key difference between Fault Tree Analysis (FTA) and Event Tree Analysis (ETA) in quantitative risk assessment (QRA) lies in their direction of analysis and the questions they answer. QRA is a systematic process for evaluating the likelihood and consequences of potential hazards. Fault Tree Analysis (FTA) is a top-down, deductive approach that starts with an undesired event (the "top event") and works backward to identify the possible causes or contributing factors that could lead to that event. It uses logic gates (AND, OR, etc.) to represent the relationships between different events. FTA answers the question: "How can this undesired event occur?" An example of FTA would be starting with the top event of a pump failure and tracing back to potential causes such as motor failure, power loss, or blockage in the suction line. Event Tree Analysis (ETA), on the other hand, is a bottom-up, inductive approach that starts with an initiating event and works forward to identify the possible outcomes or consequences that could result from that event, considering the success or failure of various safety systems or mitigating measures. ETA answers the question: "What happens if this initiating event occurs?" For example, starting with an initiating event like a gas leak, ETA would trace the possible outcomes depending on whether the gas detection system functions correctly, whether the fire suppression system activates, and whether evacuation procedures are followed effectively. Therefore, FTA focuses on identifying causes, while ETA focuses on identifying consequences.