This is the solid particles in the ground moving in place. As the wave passes through, any given volume of ground is displaced somewhat. In a balanced low-intensity wave, the soil or rock gets jostled around a bit. In a high-intensity balanced wave, the ground is yanked back and forth, potentially damaging foundations or buildings above the foundation. Particles will be displaced, but not permanently, with a net of 0.
In an unbalanced wave, the earth is permanently displaced in a particular direction. We can measure that net displacement in a particular direction using an anti-derivative if the total average velocity is nonzero (if we included negative velocities around a given axis). Earthquakes, of course, tend to have nonzero net displacement, and thus an extremely biased velocity waveform along a particular axis.
So in fact, the soil beneath you vibrating back and forth at 1 to 5 inches per second is not fun. At 118 inches per second? Catastrophe.
