The Tremor spell is supposed to immobilize a unit for two seasons. While the spell itself lasts for that time, the effect itself only works for one season.

The reason for this is the order in which the player moves. As far as I can tell, Monster move first, then the AI player and lastly the human player. This means, that by the time the human player casts Tremor on a unit, it has already moved for that turn. The next turn works as expected. At the end of that turn Tremor stops to function and the unit is free to move again at the beginning of the next turn.

Essentially, casting Tremor prevents the target unit to move for only one turn.