In the recent earthquake in Turkey you see modern buildings crumble or pancake but a 500 year old mosque seems to be survived with only minor damage.
In: 2
1. Modern buildings were built cost effective. Meaning they were built the least ammount of materials necessary.
2. Its not the first earthquake in the region. Meaning if there were older buildings weak enough to be destroyed they would be gone by now.
3. Today, we use reinforced concrete for buildings. Its strong, but also hard and rigid. In the past they used weaker and more flexible materials. They had to build thicker walls from it,but because its flexibility its less prune to damage caused by earthquakes.
Answer: earthquakes of different magnitude shake the earth at different frequencies. Buildings (and our structures) can have frequencies at which they inherently have surprising stability or massive instability. These “resonance frequencies” can be dependent on the shape, weight distribution, materials, and much more.
It’s likely that some surviving buildings in Turkey happened to have a unique stability at he frequency of the recent earthquake – or the converse – the downed buildings had a unique vulnerability to the frequency experienced. It’s quite likely that an earthquake in the same location but with different soil mechanics or magnitude could have collapsed a different set of buildings (leaving some that collapsed in the first scenario, and toppling some that stand today).
The challenge of engineers who “earthquake-proof” buildings is to have a building that has none of those highly unstable resonance frequencies, typically by putting “shock absorbers” at the base, which makes the building’s naturally unstable frequency lower than what earthquakes produce.
Older buildings tends to be overengineered. They did not have the advantage of accurate calculations and simulators when designing the buildings so they tended to build on the safe side and make the buildings stronge then they needed. It does not take a great engineer to build a bulding that can stand for a century, but it does to build a building that will bearly stand for a century. With modern calulations and simulators we can more easily make buildings that cost a fraction of the price and which will stand for a long time. But of course this means that there are less room for error. Some contractors might even have saved money during construction by not including reinforcement that is only needed to handle earthquakes. By the time the earthquake hit the warantee on the contract have expired, the company is bankrupt or the people in charge have taken their bonuses and dividends and left.
There is also a fair bit of survivorship bias. This is an earthquake prone area, the last major earthquake was in 1998 and killed 145 people. The mosques that are standing today have been thrugh several such earthquakes. However the modern houses are built on ruins of houses that did not survive these earthquakes. So for a lot of these houses this is the first earthquake they have experienced, or at least the biggest.
That 500yr old mosque has already survived previous earthquakes, so it makes sense that it survives this one too. Not all of the buildings that old survived – hence why there are so few older buildings still standing.
Modern buildings are often built cheaply. As long as it’s ‘good enough’ to survive as long as it’s likely to be needed, that’ll do.
Old buildings had fewer options and less finely calculated costs. They were built out of the best people could find, in the hopes it would last forever.
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