What size earthquake can a typical building built to modern building codes withstand? That question is not an easy one to answer. As with most complex issues, there are many things that will factor into the answer.

Building Codes

The first part of the answer requires us to know what modern building codes require. Building codes are legal documents that specify certain minimum requirements of how buildings will be built. One way that it does that is by specifying minimum design loads for engineers to use. In some cases this is very simple. Floor loading is one example. Most building codes have decided that a typical floor live load (the load of all the things besides the floor structure itself) is 40 pounds per square foot, psf, for homes. And 100 psf for auditoriums. And 250 psf for storage areas. OK, so you need to know what kind of building you have, but it’s still simple enough.

When it comes to earthquakes, it isn’t that simple. Earthquakes aren’t a load that you place on something. Earthquakes are the ground shaking the building, making it move from side to side and up and down. If it tries to move it from side to side really slowly, it won’t have a big force, but if it tries to move it from side to side really quickly, then it’s a much bigger force.

Earthquake Size

OK, so can we just look at the Magnitude like they report on the news and convert that to a force somehow? Short answer, no. Magnitude is the total energy released by an earthquake, but it doesn’t reflect how that energy interacts with the surface of the earth. If the energy is released really deep in the ground, it has to go through a bunch of soil and bedrock, maybe water, before it hits your building. That will make the movement of your building smaller and slower than a shallow earthquake that may be closer to your building.

So, instead of looking at Magnitude, engineers look at ground accelerations. This is how quickly the ground surface is changing direction where your building is. Since force equals mass times acceleration, we can turn acceleration into a force on the building pretty easily.

So, how do we get the ground accelerations: Science and statistics. The USGS (United States Geographical Survey) has done the work to produce some maps that show the ground accelerations for earthquakes with a return period of roughly 2,475 years. There are reasons that they picked that number, but I’m not going into that now. The USGS has developed these maps based on historical earthquake data and surface maps and other geography things that I don’t understand. But I don’t need to. Because the Building Code has an easy way for engineer-me to transform this acceleration number into a force on my building. The building code even has a formula for how to apply this to my building.

OK, so great, that means that my building can survive an earthquake with a return period of 2,475 years. That sounds pretty good. Except that we’re not finished yet.

Damage Levels

What do you mean when you say your building will survive? Building Code earthquake design forces provide ‘Life Safety’ for building occupants. Well, great, Life Safety sounds like something I’d like. And, yes, Life Safety is awesome, and it’s also what many third world countries that have tens of thousands of deaths in an earthquake should definitely adopt as their minimum standards. Life Safety means that the occupants of the building will be able to safely exit the building after an earthquake and won’t be killed by something falling on them in the meantime, either. This is an awesome standard. I think we can all agree that we want to live through earthquakes.

What it does not mean is that your building is guaranteed to be habitable following an earthquake. The building may no longer be structurally sound. You may not even be able to return to the building. Or you may find that structurally the building is fine, but all the interior finishes are so damaged that they must be replaced. Or the building may look completely fine and have no discernable damage. And all of these example buildings (provided there were no casualties) performed ‘up to code.’

The variations in the performance of different buildings have to do with so many things, including the building type (some buildings inherently perform better in earthquakes than others), architectural elements and their installation (anchoring non-structural elements can prevent a lot of cosmetic damage), construction quality (the biggest issue in developing countries is building code enforcement, not the building code itself), even the dominant direction ground shaking (is shaking predominantly N-S or E-W and in which direction is your building stronger?).