Heart of valour - L.J. Smith

More than a year and a half has passed since Alys, Claudia, Jane and Charles helped to free the great sorceress, Morgana Shee, from imprisonment and close the passage between the Stillworld and the Wildworld. But when an earthquake hits California and continues to shake with lingering aftershocks, the Hodges-Bradley children begin to suspect this was no ordinary quake, and that the passage seal between the worlds may no longer be stable.

With their parents overseas and Morgana travelling north to find the epicentre of the quake the children are left on their own. But peril lurks around every corner, and strange happenings in their home town force them on journey that will test their skills to the very limit. It seems that Morgana's archrival, Thia Pendriel, has stolen the Forgotten Gem, the Heart of Valour, and is waiting to spring her trap…