Say you're walking by the highway and you can feel the vibrations of cars moving along. How would you approximate the force that the ground is exerting on your feet due to these vibrations?
Just put some weight on a very sensitive scale before the car passes by. When the car passes by, note the maximum reading on the scale. If there is any upward force exerted, then the maximum scale change will give you the maximum upward force per (base area of the scale).
During the Christchurch earthquake in 2011, there was a peak ground acceleration of 2.2g - enough to throw people, vehicles and buildings into the air.