In our area we have basically two different soil conditions. The "flat" part of town is bay mud anywhere from a couple feet to a couple hundred feet deep. It's considered a zone of "moderate liquefaction" and will act like jello in an earthquake. Modern houses built on this typically have about 6 foot deep by 2-3 foot wide "slurry" poured prior to having a true foundation built with at least a 2 foot wide footing. The older houses that don't have an adequate footing are definitely sinking. Some a lot more than others, and some rather unevenly. I usually see about a dozen or so houses a year that have to have some kind of foundation work done. In the most extreme cases the house is lifted and a complete new foundation built under them. Others have had partial lifts, jacks, or helical piers added to stabilize/repair. The rest of town is built in/on/around the mountains that rise steeply out of the bay mud and the soil is a highly compacted mix of clay and sand/silt that is very stable and tends not to move at all. There are many 100+ year old houses on post and pier that haven't moved at all, even with the frequent, and sometimes large, earthquakes that we get here. We are a seismic zone D/E in northern CA.