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I'm working on a fantasy story that takes place on a moon that orbits a gas giant, but is not tidally locked. I've worked through a lot of the other considerations of this already—the wider astronomy is kinda beside the point here, other than its direct impact on the moon's tectonics—but from my understanding this is a moon that would have quite regular earthquakes due to tidal forces. In terms of the severity, I've arbitrarily decided that they'll be from a 3 to a 5 on the Richter scale, with 6 and above being a bit rarer (maybe once or twice a year.)

So the kicker of the question: in a relatively temperate, Earth-like world, how would societies with about 12th and 13th century technology design their buildings to withstand earthquakes? What materials and designs would be best to last a long time without being torn down and put back up? And, lastly, are there any real world innovations in anti-earthquake architecture that could have feasibly happened earlier in history if the entire world was facing this problem?

I appreciate any answers you guys can give!

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    $\begingroup$ You should know that Japan actually existed as a civilized country in the 12th and 13th centuries... And that a pissant earthquake of 3 to 5 magnitude on the Richter scale won't do any damage to any reasonably well-built building. (3 on Richter scale means "felt by some people, no damage", 4 means "felt by most people, some objects may rattle, some objects may fall off shelves", 5 means "felt by everyone, may cause damage to particularly poorly built structures". Richer magnitude 6 means "a moderate number of well-built structures may be damagaed".) $\endgroup$
    – AlexP
    Commented Jan 31, 2022 at 1:55
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    $\begingroup$ Look into post and lintel Japanese architecture. It used wood as the main building material and developed in conditions similar to your desired environment. $\endgroup$
    – Otkin
    Commented Jan 31, 2022 at 7:45
  • $\begingroup$ Greece is another part of the world (actually almost all the Mediterranean) with frequent earthquakes. The Parthenon in Acropolis is still standing after 2500 years (ok, the roof was destroyed but that was 700-800 years later than built and only by fire, not earthquake). $\endgroup$ Commented Jan 31, 2022 at 11:40
  • $\begingroup$ @morningknight try looking at questions and answers at: worldbuilding.stackexchange.com/search?q=earthquakes+building $\endgroup$ Commented Jan 31, 2022 at 17:31
  • $\begingroup$ Look at Roman concrete construction. They discovered "real" concrete in about the 1st century AD more or less and various Roman concrete constructions have survived ~= 2000 years. Colusseum, Trajans column, The Pantheon, aquaducts, ... . These survive BECAUSE they are NOT steel reinforced. Other reinforcing is possible but the Romans used no stranded reinforcing. The design of the Pantheon shows that they had an extremely good understanding of the material's properties and limitations. $\endgroup$ Commented Feb 1, 2022 at 10:02

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Low level expert here. I have been involved in seismic qualificaiton of nuclear reactors

There are a few basic methods of having a building live through an earthquake. The specifics will depend on the exact character of the quakes. Is the motion side-to-side, up-and-down, one big shock with a bunch of little ones, more or less steady shocks for several minutes, etc.

One method is to make the structure sufficiently rigid such that it does not get damaged by the earthquake. This is basically brute force. You determine the maximum acceleration and make sure the structure can survive that when applied in any direction. This might be the strategy for the one-big-shock earthquake. It's also the strategy of reactors, at least in my locality. We build every component and the connections between to survive the maximum credible 1000-year-return earthquake.

This method produces very chunky and very expensive buildings. Thick walls and floors, many braces and posts. And it gets rapidly harder as the building gets taller. So for a one or two story building it might be acceptable. For a 20 story building, not so much.

Another method is for the building to let the energy pass over it in some way. Usually this works for side-to-side vibration better than other possibilities. One possible method is for the building to be built to sway at a frequency that won't produce damage in the quake. If the quakes are reliably in a particular frequency, you could test this out by building a few buildings with different sway frequency, and see which ones fall down and which stand. It means you have to build in means by which the building can flex in particular directions with specific amounts of elasticity. You can tune the frequency by making the building taller or adding mass.

Another possible means of tolerating the energy is to put the building on a big slab that can slide across the ground in a quake. Careful of your toes! And the building will need to sway somewhat though less than without the slab. This lets the earth move under the building and so reduce the acceleration the building feels. It does tend to screw up the alignment of your city streets.

Another method is to tolerate flex damage. This may be the way your context chooses. A wooden building will tend to live through substantial flexing. You get some small gaps, some small cracks, some bending. The floors can become a little bit warped. Some windows may need to be repaired so the glass fits in properly.

In the city of Winnipeg, Manitoba, Canada, there used to be a store owned by the compy Eaton's. It was 8 stories tall and entirely made of wood. Thousands and thousands of wood beams. The floors were uneven and creaked when you walked on them. But when Eaton's went bankrupt and they decided to tear it down, they had a problem. They could not use dynamite. The wood absorbs the energy and stops propagation of cracks. They had to put guys to work with crowbars and power saws. It took months and months. A big earthquake might require them to do some major repair, maybe replace some beams and push things back in place. But it would require a gargantuan earthquake to cause this thing to crumble.

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Take a look at the architecture of ancient habitations at Thera for inspiration, they lived with regular earthquakes and minor eruptions and built houses and other structures that have stood the test of time and many earthquakes in the bronze age.

Interestingly enough they used some very similar techniques to our modern approach to building in places like Haiti. In the absence of sophisticated base isolation technology they rely instead on monolithic doorways and ring beams at the top of walls to keep most of the buildings from collapsing during strong earthquakes.

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With being torn down and put back up!

hut falls down

http://thissenegaleselife.blogspot.com/2012/09/falling-down-huts.html

It will be hard, in the course of your story, to convey the clever architectural tricks your people have used to earthquake proof their dwellings. It will be more interesting if they have none. Have the dwellings routinely fall apart. It is expected. The people roll with it. Buildings are only one story. The thatch roofs fall in on sleeping people, who have stout canopies over their beds to shield them. People throw water on the fire (the bucket is hanging by the fire), trundle outside and wait for the shaking to stop. Goats pick their way out of the collapsed barn.

Then walls that fall down are propped back up. Roofs that cave in are re-erected. These people are always rebuilding. It does not take long. It doesn't make sense to put a lot of work into it either.

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    $\begingroup$ I love this concept - it's like shovelling the sidewalk for Canadians. You know it's going to just snow again in a few days, but you just do it and then get on with your day. $\endgroup$
    – corsiKa
    Commented Jan 31, 2022 at 16:39
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Look at Roman concrete construction.
They discovered "real" concrete in about the 1st century AD more or less and various Roman concrete constructions have survived ~= 2000 years.
Coluseum, Trajan's column, The Pantheon, aqueducts, ... .

These all survive BECAUSE they are NOT steel reinforced. Other reinforcing is possible but the Romans used no stranded reinforcing. The design of the Pantheon shows that they had an extremely good understanding of the material's properties and limitations.

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