Strong Earth Runes need a Earthstone along with a Blank Rune; a Silver Ingot and three Normal Earth Runes to make one Strong Earth Rune.
Strong Runes can be crafted at level twenty at the Rune Altar.
You need three of these to learn Earth Shatter and you will need extra to cast the spell.
The command to make a Strong Earth Rune is [nya!rpg rune altar strong earth]. For more info use [nya!info strong earth rune].