The manslaughter case against Italian scientists for inadequately warning local residents before the April 2009 earthquake in L'Aquila, Italy (Nature 477, 264–269; 2011), is not justified.
At the heart of this case lies one fact — the danger to L'Aquila residents at the time of the earthquake. More precisely, what was the chance that a particular individual would be killed in the subsequent 24 hours, given the frequency of low-magnitude tremors around that time and the best available science? That probability, even in L'Aquila's weakest class of building, was estimated at less than 1 in 100,000 on the night of the earthquake (T. van Stiphout et al. Geophys. Res. Lett. 37, L06306; 2010). The occurrence of many little earthquakes does not make the chance of a big earthquake very high.
Conventional wisdom suggests that roughly US$1 million needs to be spent on public-safety measures for each life that would be saved. It might have been wise to improve L'Aquila's at-risk buildings, particularly as it has been known for decades that many of them are too weak to withstand earthquakes.
But the hazard level in L'Aquila in the days before the earthquake was insufficient, by two to three orders of magnitude, to justify evacuation of even the weakest buildings. The scientists were right: sitting tight was a good recommendation and, in view of the low risk, all the published quotes from the seismologists were accurate.
The most troubling aspect is the complete absence of a quantitative assessment of risk among the people seeking to condemn the scientists. To critics, it makes no difference whether the likelihood of a magnitude-6.3 earthquake was tiny.
The critics' argument that a sterner warning should have been broadcast is based only on the fact that the event occurred, and so should have been foreseen. This logic seems to lie midway between Monty Python and Franz Kafka, and is terrifying to me as the seismologist responsible for monitoring the US Pacific Northwest, an area inhabitated by 10 million people.
See also: Italian quake: science rides politics