It is the slight elevation of the geoid over the ridge that would keep the ball from rolling toward the ridge. The deepest earthquakes occur in subduction zones where, for example, oceanic crust is subducted under continental crust. They can be as deep as 100 km. The deeper they are the more they tend to be found landwards of the ocean basin, under continents and island arcs. Justify your answer assuming that the principle of isostasy holds true.
Continental crust is less dense than oceanic crust.
We have evidence for this if we assume the crust is in isostatic balance above the plastic aestheonsphere because continental crust is thicker than oceanic crust and continents rise high above sea level. In other words, the mass per unit area above the level surface must be the same for both the thick and thin layers. The only way a thick high layer can be in isostatic balance with a thin layer is if the thin layer has higher density. An appropriate analogy here is the thick less dense ice cube rising above the surface of relatively dense water, both in isostatic balance. Earthquakes generate P waves and S waves which can travel throughout the earth.