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Frequently asked questions about tsunamis

Frequently asked questions about tsunamis

A lot of questions have been roaming around in the aftermath of the devastating earthquake and tsunami in Japan.

Here are some answers to some of the most common ones.

How can a tsunami strike the West Coast when the quake is thousands of miles away?

Just like when you throw a rock into a pond, or when you do the cannonball into the swimming pool, energy can travel great distances in a body of water. Take a look at this animation done by NOAA that is an excellent example of how the tsunami was expected to propagate across the Pacific Ocean:

And this graphic shows the amount of wave energy expected as the tsunami spreads:

Note how the model accurately predicted the stronger waves that headed toward the northern California and southern Oregon coast.

What's the difference between a tsunami advisory and warning?

Tsunami advisories work much the same way weather advisories are issued.

When a quake occurs, a tsunami watch might be issued, meaning a tsunami is possible but not certain.

If a tsunami is determined to be imminent and potentially damaging, it will be upgraded to a tsunami warning. If it seems conditions will not be as dire, a lesser tsunami advisory will be issued.

Here are more specific definitions from the National Weather Service:

Tsunami Warning:

Tsunami warnings mean that a tsunami with significant widespread inundation is imminent or expected. Warnings indicate that widespread dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after the initial wave arrival.

Tsunami Advisory

Tsunami advisories mean that a tsunami capable of producing strong currents or waves dangerous to persons in or very near the water is expected. Significant widespread inundation is not expected for areas under an advisory. Currents may be hazardous to swimmers... boats... and coastal structures and may continue for several hours after the initial wave arrival.

Why would some areas be under advisory and others a warning?

All of Oregon, and most of California from roughly Malibu north to the Oregon border were under a tsunami warning, as were parts of Alaska. Meanwhile, all of Washington state, and far southern California, and the other parts of coastal Alaska were under a lesser tsunami advisory.

Why the difference? It's likely related to the geography of the Pacific Coast and the angle at which the tsunami wave was generated. Models indicated the Oregon and North/Central California coastlines are better aligned to the main wave than the Washington coast, and thus the impact is greater there.

What can Washington expect?

A small tsunami wave hit Ventura, California harbor after an 8.8 quake in Chile last March. These are the similar effects we are likely to see around Washington under a tsunami advisory.

This video is from Officer John Higgins who works with the Harbor Patrol in the Ventura, California Port District. The harbor master told him the tide level went up 2 feet around 12:50 p.m. and then dropped 3.5 feet at 1 p.m. and that no one around there could remember any event like that happening before, going back to the 1960s.

This is a pretty good illustration of the expected effects when the quake is far enough away and we are not on a direct line to the quake's epicenter.

What should I do if a tsunami is heading toward my area?

Here is a good web site for some preparedness tips and what to do in case you're in a warning area.

How fast do tsunamis move?

Tsunamis move at about 500 mph, or roughly on par with a jet.