http://www.weather.gov/ptwc/
An 8.9 magnitude earthquake hit off the coast of Japan at around 9:45 p.m. PST, Thursday March 10, 2011 causing serious damage. It has generated a tsunami for the Pacific Ocean areas that is expected to hit the west coast of Vancouver Island and down the Washington-Oregon-California (& Alaskan) coasts between 6:30 & 8 a.m. PST.
A tsunami is not just *one* wave; it is a series of waves that can last several hours, with many minutes (up to 60 mins) between waves. Each individual wave crest can last 5 to 15 mins or more and extensively flood low-lying coastal areas. The first wave to hit may not be the largest. The height is measured from average ocean height, not crest-to-trough height. Also, due to local geological effects, tsunami wave heights can vary dramatically. I was reading on the bulletin for Hawaii that tsunami waves can 'effectively wrap around islands', so even if you're not on the 'direct' side of the incoming waves, there is still a risk.
When no major waves are observed for 2 hrs after the ETA or damaging waves haven't occurred for at least 2 hrs, then authorities can assume the threat has passed.
Even though our house is over 100 ft above sea level (we're about 500m to the beach), there are areas we drive through on our way into town that are directly at sea level. A few years ago, some official, in their infinite wisdom, thought to put up "Tsunami Hazard Zone" signs all along the road out here. You'd see an "Entering Tsunami Hazard Zone" sign, followed by a "Leaving THZ" less than half a km away as the coastal road around here has a lot of climbs and drops in elevation. I think the kids and I counted about 6 hazard zones on our way into town. People living in the designated THZs feared their housing prices would be affected and I'm not sure if that's why, but nearly all of the signs were gone within months of installation. If nothing else, the signs at least got people talking about emergency preparedness, something most of us put aside until it's too late to plan for.
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