Seismic Early Warning Systems: Procedure for Automated Decision Making

Earthquake Early Warning Systems (EWS) potentially allow mitigation measures to be carried out from the moment in which a seismic event is detected, depending on the amount of warning time available. Examples of such measures are evacuation of buildings, shut-down of critical systems (nuclear reactors, industrial chemical processes, etc.) and stopping of high-speed trains. In areas close to faults, where seismic EWS are especially relevant, only tens of seconds of warning are available. Such short warning times mean that to be effective a seismic EWS must depend on automated procedures, including those for decision making about whether to activate mitigation measures; the time is too short to require human intervention when the event is first detected. As a result of the automation, careful attention must be paid to the design of the local seismic EWS for each critical facility; in particular, a means of controlling the trade-off between false alarms and missed alarms is desirable. To investigate this trade-off, the consequences of the two alternatives of taking mitigation actions or not acting must be analyzed, accounting for significant uncertainty in the predictions. A feasibility assessment of any proposed regional EWS is of critical importance, and it should involve an examination of whether the requirements, in terms of warning time available and the probability of making wrong decisions, are met. A methodology is presented for estimating the probabilities of making wrong decisions that can be incorporated in a feasibility assessment of proposed EWS. To illustrate the methodology we consider the case of feasibility assessment of a seismic EWS for Southern California. We, also, present a decision-making procedure based on the real-time evaluation of the consequences of taking no action and of activating mitigation measures which is based on the probabilities of false and missed alerts. The threshold at which mitigating actions should be taken is quantified based on a cost-benefit analysis. The method is applied to the M 4.75 Yorba Linda seismic event occurred on 3 September 2002, in Orange County, California