# If an earthquake can destroy buildings why it cant kill us according to physics?

Most earthquakes with magnitude 5.5 and higher can damage or destroy buildings. However, according to my knowledge and experience, I have never seen someone dying from an earthquake itself. Rather, they die from an associated tsunami, damaged buildings, etc.

This seems counter-intuitive, since you need much more force to destroy a building or damage it than to break the human femur or cause similar damage to other species.

• Earthquakes can make both humans and buildings fall over. The humans can usually get up again by themselves. – Ilmari Karonen Aug 25 at 20:23
• bamboo houses/cities in asian countries are not affected by earthquakes. Because they can flex – RozzA Aug 25 at 21:39
• @IlmariKaronen I think the OP's point is not so much about the falling over, but rather that one crumbles during the falling over and the other doesn't. – JBentley Aug 25 at 22:39

There are several differences between humans and buildings:

1. Anchoring Suppose I were to push you backward. Just on reflex, one of your feet will move backward to catch yourself. A building, on the other hand, is anchored in place. When the ground moves, the bottom of the building has to move with it. But a human body doesn't have to move with the ground. There's only so much force that a shaking ground can apply to the human body.

2. Flexibility: The human body is designed to move (anthropomorphizing evolution a bit here). Other than bones, our organs are flexible, and the bones are connected by multiple joints and that can bend without harm to the body (that what a joint is: a part of the body that's supposed to move). A building doesn't have that level of flexibility. Nowadays buildings, especially ones in earthquake-prone regions, are often designed with some flexibility, but that can't match that of a human body.

3. Scaling: This is probably the largest factor. Things work differently at different scales. If you double every dimension, you multiply the cross-section by four, but you multiply the volume by eight. If a building is a hundred times as large as a human in each direction, then it's going to have a hundred times the volume per unit of the cross-section. And torque is proportional to both mass and distance, so you can get ten thousand times as much torque per area.

• Thank you, so if we lock our knees during earthquakes, it can break our knee bone , because our knees can bend the wrong way right? – Zheer Aug 24 at 20:37
• @Zheer You still have the first point, of anchoring. Simply locking your knees would probably result in falling over. If you were to lock you knees, and anchoring yourself to the ground in such a way that you can't bend, then broken bones would be much more likely. – Acccumulation Aug 24 at 20:46
• No because then you're just likely to fall over instead as you're still not attached to the ground – Triatticus Aug 24 at 20:46
• No, leaning against a wall doesn't really do anything, unless the wall collapses and falls on you. – infinitezero Aug 25 at 12:11
• @Zheer, the key point is number 3. Buildings are much larger than people, and that scale makes all the difference. In a sense, large things are weaker, not stronger. You can drop a small toy car from waist height and it might suffer no damage at all. Drop a real car from 20 times its length and it will be smashed. This example isn't perfect, but it illustrates the importance of scale. – Mark Foskey Aug 25 at 19:01

The maximum recorded earthquake peak ground acceleration is less than 5g, which is typically survivable, but buildings typically are not designed for such (not very short-term) acceleration.

• More precisely: x-axis is facing forward, y-axis is side to side, z-axis is down. You can survive this along the x axis. 5g is maximum survivable along the y axis. You can survive this in the z axis if you're being pushed down, but if you're being pushed up you're going to have trouble. – Aza Aug 25 at 8:27

You are right, that you need more force to break a building than to break a human body.

But an earthquake acts by acceleration (not by force). It suddenly accelerates a large part of the ground (and hence all buildings and humans there) by the same value.

The human body is quite soft and flexible, and can therefore withstand accelerations of $$10\ g$$ for a few seconds (see G-force - Human tolerance).
Also quoted from there:

The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may briefly impose hundreds of $$g$$ locally but not produce any real damage;

But hard buildings can only withstand much smaller accelerations. And they are even more sensitive for horizontal accelerations than for vertical accelerations.
According to Earthquake effects on buildings (chapter 4):

Poorly constructed buildings begin to suffer damage at about 10 percent $$g$$ (or $$0.1\ g$$).

• Thank you, according to newton's second law motion (F=MA) acceleration depends inversely upon the objects mass so if you have a building that has a big mass it will have less acceleration than a human that has less mass but high acceleration right? – Zheer Aug 24 at 20:54
• @ZHeer No. The earthquake puts the same acceleration to everything. Therefore according to Newton (F=ma) the human feels a small force, and the building feels a big force. – Thomas Fritsch Aug 24 at 21:01

Force is the rate of change of momentum (the product of mass and velocity of the object): $$F = \frac{dp}{dt}$$ where $$p = mv$$ is the momentum. An object with twice the mass has twice as much momentum compared to an object moving with its same velocity.

So a building of a million kilograms will have ten thousands times the momentum of a human weighing a hundred kilograms (assuming they are both planted to the ground so that the oscillating floor drives them both with the same velocity). As the floor oscillates, the velocity will be changing, but the mass doesn't.

So the change of momentum - the force - that goes to the building would also be ten thousand times the amount of force that goes to the human.

This is also why an ant can survive a fall from a 20 metres tree all safe, but a human will likely at least break some bones.

• Thank you, but humans have much stronger bones but expierence higher force so they should be able to survive the fall but ants have much weaker exoskeleton but expierence less force so both the human and the ant should survive the fall right? – Zheer Aug 25 at 9:50
• I have no idea how exoskeletons compare to human bones, but you hear about beetles' exoskeleton being as "strong as steel" - whatever quantity is used to quantify "strong" here. But in general, humans don't tend to survive 20 metres falls unharmed. 20 metres is ~ the height of an 8 story building. – Aiman Al-Eryani Aug 25 at 11:08

In addition to the other answers provided, resonance cannot be ignored (Resonance is where the object is vibrating at its natural frequency and as a result the vibrations are amplified). To put simply, people and buildings have different resonance frequencies and the lack of being anchored to the ground (for people) mitigates the damage done in the way.

The resonance frequencies of buildings play a large role in the destruction caused by earthquakes. Contrary to common misconceptions, the tallest buildings aren’t always the most damaged during an earthquake.

Check out this great demo for a visual representation of how resonance frequencies amplify building destruction. (Begin at the 1:25 mark for best experience).

BOSS Demo

• Thank you, very helpful, does resonance frequencies affect humans in the same way it affects buildings, for example it might affect tall humans more than short humans or the opposite based on resonance frequencies? – Zheer Aug 25 at 9:52
• @Zheer Yes it can. An example outside of earthquakes: the first Japanese cars didn't sell well in the West, because the natural frequency of the vehicles bouncing on the suspension didn't match any resonances of typical sized Japanese passengers, but larger Westerners got a much "bouncier" ride - specifically, parts of the female anatomy. (Work out which parts for yourself!) – alephzero Aug 25 at 9:55

As an addendum to other answers, pressure and volumetric energy density may be more useful in explaining the total effect on a small body vs large structure.

Since reasonably-sized buildings and people on the surface will be a small fraction of the overall earth mass being moved in a quake, the effective pressure $$P=F/L^2$$ in a localized region will be uniform. In other words, the effective force transmitted from the ground to an object will be proportional to the contact area of the object. A person's feet (or even a person lying prostrate on the ground) has a much smaller area than that of a building's rigid foundation.

Energy density has the same units as pressure, like $$[E/V] = [E/L^3] = [{FL}/{L^3}] = [P]$$ Similar calculations would show that the total energy transmitted to each object will be proportional to its mass, density and displacement. The conclusion is the same... that a small body (with less mass and volume) will receive less overall energy from the quaking ground than an entire rigid building of much greater mass. It does not matter that the human body is standing adjacent to or even leaning up against the larger building. The human body will still only receive its small portion of energy directly from the quaking ground.

• Thank you, so you dont receive a portion of energy from the building if you are leaning up against it, what about swaying buildings?Do You receive energy in this case? – Zheer Aug 26 at 11:16
• Of course it receives some energy, but a swaying motion itself will be slow and not likely to contribute to harming a person. Of course all of this is assuming that we're discussing only the quaking energy and motion. In other words, this neglects effects of the swaying motion causing the building to buckle, break, etc. which could harm someone. – C Perkins Aug 26 at 11:55