Monday, 12 January 2015

Medice, cura te ipsum

On 23rd November 1980 a magnitude 6.8 earthquake occurred in southern Italy. It caused 3,006 deaths and 8,841 injuries and damaged 400,000 houses in 630 municipalities. I was one of the survivors left homeless by this event. Almost exactly 34 years later, on 22rd November 2014, I was in Japan when a magnitude 6.8 earthquake occurred near where I was staying. It destroyed 37 houses and injured 57 people. Some 22 people were trapped under the rubble of collapsed buildings, but all of them were rescued alive. The physical parameters of the two events were remarkably similar, but the states of vulnerability were very different.

I spent December 2014 in central Italy. In mid-month, an earthquake swarm began. At 10:36 a.m. one day, my house rocked and swayed in a rippling motion that was the most precise earth tremor I have ever felt. Well it might have been: the epicentre was almost exactly 10 km away, on a bend in the road that connects two villages, both of which are in the same municipality as my house. This magnitude 4.1 event briefly cut electricity supplies, and it certainly caused a great deal of fear and alarm locally. Schools and offices were immediately closed and many people took to the streets.

This may well have been the first occasion in a millennium in which the area of Tuscany that contains my home was affected by a locally generated earthquake swarm, albeit not one of very high power. We experienced up to 100 earthquakes a day. Although low power events are not in themselves particularly dangerous, they remind one that a swarm could contain at least on event of significantly higher magnitude, as has been the case in many earthquake sequences in peninsular Italy; for example, at L'Aquila in 1703 and 2009, and Ferrara in 1570-4. This causes a very uncomfortable feeling of being exposed to danger. After 34 years the old, familiar sensation of acute sensitivity to vibrations suddenly returned.

My first act was to consult the catalogue of past earthquakes. The map of seismicity is based on the record of previous events, which is, of course, more and more vague the further one goes back in time. On expert suggested to me that to construct a realistic picture of the magnitudes and frequencies one might need a catalogue of 50,000 past events, and this assumes that there is no significant temporal trend in the data. Hence, the record is incomplete and the map is a hypothesis. It suggests that I live in a broad area of medium seismicity, with a local "trough" of lower hazard in the immediate area of the town in which I live.

One item of concern is that the tower of a local church, built in AD 880, leans fairly precipitously (over an occupied house) as a result of an earthquake in 1895. The epicentre for this was 25 km away. As areas of much higher seismicity exist in the Mugello, Garfagnana and Casentino, parts of the Apennine mountains, this begs the question as to what effect seismicity of distant origin would have on local structures such as my house. My home, incidentally, was built in 1909 in unreinforced masonry. It has been strengthened (at my expense), but to make it fully anti-seismic would cost something like 30 per cent of the value of the whole building, a fairly enormous sum.

The catalogue includes a magnitude 5.6 event with epicentre so close that I can walk out of my front door, cross the adjacent park, and look down on it in the valley nearby. This may be the millennial event in the area and details of its effects are sketchy, as it occurred in 1812, before the advent of systematic records of earthquakes. From what is known I deduce, as a rule of thumb, that the risk of significant earthquake damage to my house is about 0.4 per cent per lifetime. The risk of catastrophic damage is too low to be estimated.

I have a family disaster plan, and the local civil protection service has a plan that designates muster areas and prescribes organised assistance in the event of a serious earthquake. In December 2014 information leaflets were widely distributed. Whether this is sufficient is difficult to assess. As a scholar of disasters, I am aware of the need to set a good example, and disseminate information that is comprehensible and valid to those local people who ask me about the situation. However, there are few very useful guidelines about how to prepare for earthquakes in areas of relatively low risk, in which the benefits of expensive or restrictive measures are highly debatable. One can, at least, monitor the progress of seismic events and try to determine whether there is anything in the pattern that would stimulate one to be more active in preparedness. As I write, the swarm continues intermittently with events whose magnitude is lower than 2.5, most of which are imperceptible. May this be the worst we have to deal with!