The failure to date of all attempts to identify a reliable quake precursor does not ipso facto imply that such precursors do not exist. Each of these four outcomes will also have a consequence, characterised by a pure number L quantifying the loss resulting from a specific combination of event and decision. Before we can calculate its numerical value, however, we must tackle the question of setting the various quantities it contains. For example, one might piously argue that the loss of one single life is worth any number of false alarms, making K huge. It would be difficult to argue that any civic authority would feel justified in calling for the far more disruptive action of total evacuation of a city if posterior odds on a quake striking were lower than this figure.
Evacuation is, however, the most drastic action than can be based on a quake prediction.
Many other ameliorative measures, such as isolation of gas supplies and preparation of blood banks, might be deemed appropriate at a much lower risk level of concern. I now consider the two types of precursor-based decision in turn, and examine their implications for the minimum level of accuracy demanded of the precursor. As such, it seems highly unlikely that there exist quake precursors capable of giving predictions as accurate as those currently achieved by meteorologists, let alone 200 times more so. Alerts are considerably less demanding, and the ability to make them would be motivation for continuing with the search for precursors. To see what accuracy is required, we again require an appropriate timescale over which an alert could be sustained, and for which the precursor is deemed to be accurate.