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Unless I am losing my memory, I remember learning in geology class that seismic waves travel the least in clay and most in rock.

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Soil liquefaction is the real risk during earthquakes. Clay absorbs the energy and shakes a great deal more, and the soil can potentially destabilize further and allow structures to sink in.

For anyone not familiar, this is an effect where if you load wet soil just right it will break the structure of the soil grains and turn into a fluid. Demo here, you can skip the first minute if you're short on time: https://www.youtube.com/watch?v=-X9-4tWpMCo

Wikipedia has more information and some good photos: https://en.wikipedia.org/wiki/Soil_liquefaction

It's a really cool and equally terrifying phenomenon if you're used to thinking of the ground as a solid object. Plenty of videos on YouTube as well.

EDIT- Here's a car that sunk into the ground, which then resolidified around it: https://youtu.be/2WoKu5VxKgs?t=50

http://izismile.com/2012/08/31/christchurch_liquefaction_26_...

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