One more thing: when the M7.2 quake occured the other day, I noticed how far the top edge of the subduction zone (the trench) was from the coast. That quake only ruptured the upper part of the megathrust: none of the aftershocks to it was near the coast. The aftershocks to the M8.9 extend almost to the coast.

this is an issue for the Pacific Northwest: how much damage to places like Seattle and Portland depends in part on whether the rupture extends to the coast and inland at depth. At some depth the subduction zone is not locked, it creeps, so it will not accumulate stress and strain. I heard a talk a year or two where the speaker (I forget who it was) interpreted the locked part of Cascadia subduction zone to extend beneath the coast.

Chris