A fantastic guy at our church, Jan Wright, wrote the following to me as I prepared for last night’s Heart for Haiti event at the Apollo…
1. Earthquakes can be natural (caused by movement of the earth’s tectonic plates in fault regions) or manmade (e.g. due to mining / drilling / constructing dams etc. – and one does wonder about the short and long term effect of underground nuclear explosions!) – the vast majority of earthquakes are natural.
2. Earthquakes involve shaking, and sometimes rupture, of the earth surface – the accelerations of the ground at the foundation of buildings leads to vibration, potential structural damage and consequent loss of life.
3. The size of an earthquake is measured by the ‘moment magnitude scale’ which is a measure of the energy released at the earthquake source (replacing the now redundant Richter scale) – it is a logarithmic scale (help I hear you say!!) which means that small increases in scale make big differences in energy released and therefore potential damage e.g. going from 6 to 7 means a 31.6 fold energy increase (or 10 fold shaking amplitude increase) and from 6 to 8 a 1000 fold energy increase. It is not a direct measure of the earthquake intensity as experienced at any location.
4. The Haiti earthquake was of magnitude 7.0 – in the 21st century there have already been over 60 earthquakes reported that have been greater in magnitude but only the Tsunami in 2004 (229000) was near to the estimated Haiti loss of life – Kashmir in 2005 (75000) and China in 2008 (69000 – thought to be manmade) came next. The damage and loss of life experienced depend not only on the scale but also upon how near the epicentre (centre of earthquake at the surface) is to major areas of population and how far the hypocenter (actual centre of the earthquake) is below the earth’s surface – fortunately most earthquakes are centred some good distance from large population areas and so have zero or only small loss of life but in Haiti’s case the epicenter was only 10 miles away and the hypocenter about 9 miles below the surface – this was one reason why the impact was so great.
5. The damage and loss of life also depend upon the quality of the building construction, the infrastructure (e.g. medical / fire services), the efficiency of the government, poverty etc. – these areas where Haiti scored badly I suspect and aggravated the impact of the earthquake. The efficiency of the outside world’s response also has an impact on the loss of life and there is much room for improvement here! 6. In the developed world, considerable effort is made by engineers to predict the likely structural response to the level of earthquakes expected of proposed new buildings in earthquake prone areas– and the design of many buildings (e.g. in USA and Far East) is now such that they can withstand many earthquakes because of their quality and the adoption of a range of vibration reduction strategies. It is also critical to use the right materials and construction methods. However, it is not possible to predict future earthquakes so the design process aims that a building can withstand the most severe earthquake anticipated (on a statistical basis i.e. estimate) with only partial destruction.
7. Haiti needs a rebuild program that erects buildings of reasonable quality design, materials and construction (no short cuts and substitution of inferior materials). It is unlikely that the situation will ever be reached where another earthquake like that in Haiti will cause no damage / loss of life but it could be a massive amount better – the same goes for wind loading in hurricanes. We should pray not only for the current rescue / aid effort but for the sustained long term investment in the country – with adequate oversight of funds provided aiming to minimise corruption. Do bear in mind that I am basically an aerospace engineer who is an expert on vibration but not a structural engineer!
My involvement in structural engineering has been primarily on research into crowd induced vibration – how to assess what happens when people jumping up and down in stadia at pop concerts!! Cheers Jan