Two mentions of water storage caught my eye:

For the illustrative family of 4, they recommended keeping only 12 two liter bottles of the group, so only about 6 liters, or a gallon and a half per person.
FEMA recommends a minimum of 3 gallons per person for a possible need of 3 days; Seattle Emergency Management recommends 1 gallon per person per day for a possible need for 7-10 days.

The Tokyo book however recommends keeping the bathtub full at all times!

I'm wonder how much water would slosh out of the tub at what level of shaking on the Richter scale. . .