Your son is right....but it would not be as deep as, say the M8+ in Bolivia at 600 or 700 km depth that did no damage but was felt in Canada. Someone check the following: Cascadia subduction zone is locked mainly west of, say, Seattle...so the great subduction earthquake would be mainly farther west....it creeps beneath Seattle...and for strong ground motion the problem might be faults like the Seattle fault??????
Chris