The week before last, the southeastern US was pummelled by a swarm of tornadoes that killed more than 300 people in 6 states, including Alabama (which appears to have borne the brunt of the damage) and Mississippi. Although tornadoes are hardly an uncommon occurrence in these states, if you were simply assessing the hazard based on the frequency of tornadoes, you might expect to be in more danger in states like Kansas and Oklahoma to the west. However, a recent study has shown that this is not the case: the most fatalities caused by tornadoes do not occur in the regions with the highest frequency of tornados, but in the states further to the east – like Alabama and Mississippi.
(For another way of looking at the dataset plotted above, the New York Times has a nice interactive map/timeline of deadly tornados since 1950)
There are several reasons for this mismatch, the most important of which seem to be the higher density of mobile homes, which are much more susceptible to tornado damage, in the more easterly states; and less appreciation of tornado risks because of the lack of a focussed tornado season, and the perception of safety due to ‘Tornado Alley’ being further to the west. Sadly, the reduced risk of tornadoes occurring is more than matched by the lower resilience and preparedness of communities, which magnifies tornadoes’ impact when they do occur.
I’ve been wondering if similar factors are also an issue when considering the risk from earthquakes. Not unreasonably, a lot of attention is focussed on the seismic hazard in places which are located on plate boundaries, and where damaging earthquakes strike fairly regularly: places like California, or New Zealand, or Japan. However, while most large earthquakes do occur at the edges of plates, earthquakes large enough to do significant damage can and do occur in the interior of plates, the most famous example in the mainland US being the three magnitude 7.5-7.7 earthquakes that struck the New Madrid region between December 1811 and February 1812. In general, the risks from ‘intraplate’ earthquakes within plates are much more poorly known than the risks from ‘interplate’ earthquakes between plates: they follow less regular patterns in time and space, and because strain build up is much, much slower away from plate boundaries, there are much longer periods between one big earthquake and the next in any one area. If an active fault capable of producing large earthquakes only ruptures every few centuries or millennia rather than every few decades, they may not have left their mark in our historical records, and we may be almost completely ignorant of the danger they pose.
In the absence of this knowledge, similar factors to those that increase fatalities from tornadoes in less active areas may be important. The damage and potential casualties from even a moderate intraplate earthquake will be amplified because it will be striking an area with many, many more buildings that are not designed to withstand seismic shaking. Second, away from the regular, low level seismic activity near plate boundaries, people will not only not realise that large earthquakes are a possibility, they will we far less aware of how to respond if one does strike. Events like the recent Central US Shakeout, timed to coincide with the 200-year anniversary of the New Madrid earthquakes, are attempting to at least partially address the public awareness problem. But the fact remains that for many natural hazards, the actual risk is not purely a function of frequency and magnitude: politics, regulation and psychology are also a large influence on the potential human impact.