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Areas where the buildings are built on top of a deep layer of loose sediments are widely recognised as being more prone to earthquake damage (all else being equal) than areas where the bedrock is shallow and building foundations can easily be dug down to it, because the loose sediment cover amplifies the vibrations from earthquakes, increasing the amount of damage they cause. Why is this? Intuitively, one would expect that, since the vibrations from an earthquake originate in the bedrock or deeper, buildings bolted directly to bedrock would be the worst-affected, and having a thick sediment layer would dampen the vibrations considerably.

So why do soft sediments amplify earthquake vibrations, when one would expect the opposite effect?

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    $\begingroup$ Soft soils/basins do dampen, i.e., they effectively remove energy from the system. It's just that they do so do by shaking vigorously which is not good for buildings. Hard rocks OTOH allow energy to pass through efficiently. The interface between soft sediments and bedrock is where phase conversion happens (due to impedance contrast) and waves get trapped and cannot easily exit the system. $\endgroup$
    – stali
    Sep 25, 2018 at 18:54

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Areas where the buildings are built on top of a deep layer of loose sediments are widely recognised as being more prone to earthquake damage...

Yes. To be clear, this is not the same the same thing as saying that loose soils actually "amplify" seismic waves. If anything, both P waves and S waves are attenuated by unconsolidated materials.

However, like eSurfsnake mentions, buildings behave poorly when soils shift randomly beneath them: one corner of a tenement may drop several feet and shear off part of the structure injuring scores of people. On the other hand, the structure may be earthquake-proof, but may not have been built on an earthquake-proof foundation, in which case, shaking the loose soil effectively "liquifies" its load-bearing properties.

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You tend to find this situation in flat outwash plains like river valleys; places where land is easy to build on, and easy to populate, further "amplifying" an earthquake's effects:

enter image description here

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  • $\begingroup$ So loose sediments do actually dampen earthquake vibrations, but the dampening is outweighed by their tendency to, as you put it, "shift randomly" in an earthquake and literally pull the ground out from under buildings? $\endgroup$
    – Vikki
    Apr 1, 2018 at 18:20
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    $\begingroup$ Sure! That's a useful way to understand it: the safest place to be in an earthquake is also the one place you shouldn't be standing in! $\endgroup$
    – Knob Scratcher
    Apr 2, 2018 at 2:49
  • $\begingroup$ I'm confused by your last comment - could you please clarify it? $\endgroup$
    – Vikki
    Apr 2, 2018 at 17:39
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I think there are a few more options than bedrock/loose. But I owned a house 3 miles from the San Andreas fault, and a geophysical engineer explained it to me like this after telling me earthquake insurance would be a waste of money.

The worst situation is landfill, like the Marina district in San Francisco that took much of the brunt of the 89 quake. That is just loose sand and gravel, and when shaken it acts like a liquid - in fact, they call it liquefication. The foundation shifts and sinks in it, like if you had a jar of sand and gravel and put a heavy rock on top and shook it.

However, if you are on bedrock - or something tightly bound above it (like Franciscan shale), you get all the shaking transmitted, but the earth stays in place under and around the foundation.

Earthquake damage can happen a few ways: from shaking up and down as well as sideways. The Mendocino quake in the 90s (I think) had more side-to-side movement than ever seen before, and house held together but slid off their foundations. After that they upped the degree to which house are bolted to the foundation to avoid lateral shifts.

I am not sure soft soil 'amplifies' the shaking. There are a bunch of failure modes in buildings: they can come off the foundation; the foundation can shift and sink; the shaking can cause framing to collapse (which is why they put in shear walls); etc.

Incidentally, wood structures often do better because they can flex. You'll never see a real brick house being built in a seismic zone anymore.

In the end, it is about making the building live through the accelerations, and the hardest thing to deal with is loose soil that itself moves and flows under the structure.

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