What causes earthquakes?

You ask: What make ground shake?

Ground Caveman stand on not one piece. It many giant rock slabs, floating on hot soft rock below, all pushing slow — creep about as fast as Caveman fingernail grow, a couple finger-widths each year. Where two slab meet, edges catch and stick, like two heavy stones Caveman try to slide past each other. Slabs keep pushing, but edge stay stuck — strain build and build for many year. Then snap! Stuck edge break loose all at once, slabs jump. That jump send shake-waves rolling out through ground, fast, every direction. Waves reach Caveman cave, make floor tremble, rock fall. Cave wisdom: ground not shake from nowhere — it slabs that stuck too long, finally letting go.