The Time Apprentice by Val Tyler
Old Father Tim and his grandson, Tid, have adopted young Sofi Mossel into the magical world of the Guardians. Born into the dirty and cruel underground world of the Wreccas, Sofi wants to forget her past and be a real Guardian. But then a massive quake strikes and Human and Guardian time are ripped apart. Before Sofi and Tid can help save the future of their world, Sofi must first face her past ...