This term is used when referring to tremors. Full-blown earthquakes are called Exercises of Geb. Prayers are often made to Geb when these tremors become common in an area. It is said that he listens to some of these remonstrations, stopping what may become an Exercies of Geb.
The spread of this term across the realm started with the priests of Geb. They either truly believed that Geb was snoring and rocking the earth, or perhaps they just wanted to get his name out, and his influence on the realm.