What does strong motion mean?
The motion of a point on the ground during a small or distant earthquake can be so small that only specialized, precision instruments can record it. When the earthquake is larger (or closer), that motion will be larger. When the motion reaches the level where humans can feel it, typically a 1-2 %g, it is often called strong motion. This is an actually an arbitrary level, meant to communicate the level qualitatively, as when one says “heavy rain” vs. “light rain”.
