## Earthquakes and their Recurrance times(by sarah)

Last Monday I had a lecture on Fualting and Earthquake mechanics with Gerald Roberts - I'm not going to say I enjoyed this lecture becuase to be honest I found it mostly depressing.

I also both agreed and disagreed with the Gerald. Basically I will not be convinced that the apparent data skew as you back through Italy is a real data skew and not just people not recording or being in the right place to record the earthquakes - until I have seen a distrabution map of the monastrys and seen some sort of formulisation of how complete each monestries records is. Then I'd want to know what rock the monstries where standing on and therefore what sort of ground motion they would be expecting for each magnitude which I feel could dangerously biased the data - ie if they are on more ridged rock and modern towns aren't (I dont know if this is the case this is a 'suppose') then they may not have recorded the right sort of level of damage and there would be an underestermate for devastating earthquakes.

Having said all of this I do actually think he is right in the main points that he is putting across and as for those bits I doubt - I want to see the evidence and not just take his word for it - the same goes for his earthquake intensity diagram I think I understood how it was made but I don't think it was explain in a way that is going to get through to the poeple who need to listern to him.

Now the bit that I found very depressing is that earthquakes are assumed in all predictive models to have 475 yr reccurance - earthquakes can be produced by movement on a fualt now the strain builds up and up until the fualt shifts. But they are assuming that this happens at a steady rate but if you just think about snapping sticks in half this doesn't seem logical. They will break with a sort of concentrated judder. If this is happening with earthquakes then it means that you would expect a cluster or group of earthquakes to happen along a fualt one after another and then have a period of no activity whilst the strain builds up again.

Now Gerald has been measured slip rate which is I would of thought one of the first things they would have looked at with earthquake prediction - I remember doing the maths behind working the slip rates during my undergraduate so why has it been ignored? Plastic deformation ie mountian building, folding etc... is generally a precursor to the breaking of the rocks that is the fualting and thus the earthquake inducement and you measure this in strain rates which shows that for the middle of the 'crumple' zone or mountian ridge like the apanines in Italy the rate of deformation (how fast its folding up) is faster than that at the edges. So surely this means that the middle will have more earthquakes, larger and closer together.

But this is not mirrored exactly in the earthquake hazard maps produced currently as they assume this 475 yr reoccurance. This is for everywhere on the planet but they will be subject to different forces - why on earth did they get this sort of age from anyway?

Now his work shows that some fualts have longer re-occurence ages than the writen records we have - now Italy had the Romans so they have 3000yrs of records and they will still miss things - Gerald pointed this out himself. If you preject what he is saying to his risk maps then what you have is away of telling if a fault has shifted all its going to for this lot of earthquakes - this is where this become counter intuitive to poeple who haven't been looking at it in detial - areas that haven't had earthquakes for about 400 yrs are safe for thousands or at least hundreds of years rather than being about to blow (which the used maps suggest) and faults that shifted within the last century is going to go again anytime soon.

Now he showed us fualts that are behind where they should be - great you can warn those areas but then showed on the same map that there are fualts that are ahead for there earthquake quota so should be fine for ages.

This last point concerned me - if this is to be a useful tool it needs not to have faults that are ahead of themselves in the number of earthquakes they have had - the number you;ve predicted they should have had in that period of time - though over all the earthquake is expected as while the zone is active there will be strain and then slip but if there where unexpected earthquakes there then there will be when you actually project the technique into the future.

So this means you can say this area is going to have an earthquake soon but no where can actually be ruled out - now you could argue that they live in a tectonically active country they need to expect earthquakes and that is great but where should they prioritize their money for building reinforncement and the such like. Gerald thinks they should do all of it all at once but from a government perspective I doubt this will look feasible and would infact look unreasonable and there is no point in ramming facts about affects on economy of such big disastors as they are going to be getting that sort of statement from everyone about everything.

Yes I feel that as many of the buildings should be a safe as possible starting with the obvious infastructure buildings - anything new should be built properlly to begin with. Again this is heavily affected by the ground the buildings are on.

This was the bit that always makes me so depressed - he said that in Italy about 3 yrs ago there was a small mag 5 earthquake that hit a town and the only building that collapsed was the school, during the day killing about 50 kids - this sort of thing hurts me in physical scense. I don't know if it is becuase I was brought up with the nightmare shadow of the result of Aberfan where a slag heap berried a school in Wales - this happened to be where half my family where from (they were from Merthyr Tydfil to be precise though my nan was from somewhere even smaller). It was talked about with heart broken whispers when I was a child. My second cousin was involved with the rescue attempt.

I always get the guilty feeling that all this disastor management stuff or climate change stuff should be what I use my geology for but I would not be good at it - I would be doing it out of obligation and not passion. I feel I would be better off and more lickely to find something that will actually help people by researching something I am interested in and just talking to other geologist and scientists about their fields. The earthquake stuff is still depressing me though.