Great stuff so far, hopefully this can contribute as well.
Assuming your name SLO is a location reference, it might help to reassure your DS that earthquakes along the San Andreas (where a "big one" is most likely to occur) would happen below land anywhere south of San Francisco, so those won't trigger a tsunami. Furthermore, the San Andreas is a strike-slip fault, and this type of fault does not produce tsunamis.
There is a subduction zone off the coast roughly from British Columbia to Oregon which has the potential to produce much more powerful earthquakes and devastating tsunamis. It would be pretty bad news for Seattle, but if you guys are near SLO Bay, then geography means that would be coming from the landward side, and there's some natural protection there, so local effects would be significantly reduced.
It might even be worth working out the math with him for how long it would take a tsunami generated near Seattle to travel to you, so you could show him how much reaction time there is to get to high ground. Make sure you're subscribed to receive emergency alerts to your phone, and explain how that works.
