What term refers to more or less continuous motion in the Earth that is unrelated to earthquakes?

The term that describes more or less continuous motion in the Earth unrelated to earthquakes is "microseism." Microseisms are low-frequency vibrations produced by various natural phenomena, such as ocean waves interacting with the shoreline, or even human activities like construction. These motions are subtle and can be detected using sensitive seismometers, but they do not result from tectonic activity like earthquakes do.

In contrast, the other terms refer to different aspects of seismic activity. Tremors are generally small shaking events, often associated with earthquakes but can occur independently. Seismic activity is a broader term that encompasses all occurrences of seismic waves, including those from earthquakes. Ground motion is a general term that can refer to any movement of the Earth’s surface, whether from natural or human-induced sources, but it does not specifically denote the continuous nature implied by microseisms.