Five things folks on the Central Coast should know about tsunamis

Five things folks on the Central Coast should know about tsunamis : An interview with The National Tsunami Warning Center

KION viewers asked some great questions about tsunami watches and warnings. We called Dr. Kenneth MacPherson with the National Tsunami Warning Center following the cancellation of the tsunami watch that rattled some nerves on the Central Coast. He was up all night tracking the earthquake’s impact and took a break to talk to us:

What’s the difference between a watch and a warning?

MacPherson: The difference between a watch and a warning is that a watch just says that there’s been an earthquake large enough that historically it’s produced a tsunami in the past but the wave has not been observed yet. So if that’s the case we go ahead and put people in a watch just because there could be a wave out there and it puts people on notice and once we have more information, then we can upgrade that to a warning.

For California, at what point did it go from the watch to a cancelation?

MacPherson: California is a long, long way from this earthquake and so fortunately we had plenty of time to put California on a watch while we evaluated the hazard and then we could upgrade our forecast and decide whether to upgrade to a warning or to go ahead and cancel. And so in the hours following the earthquake, we knew a lot about the earthquake very quickly because seismic waves travel much more quickly than tsunami waves….but it took several hours to get a lot of data about the wave itself so once we had that we were able to determine that the wave was small enough that it would not affect California. We observed wave heights of about 15 centimeters in Kodak very near the source so based on those observations we felt comfortable downgrading the watch and canceling it for California.

If there were to be a warning in the future for coastal California, how would local people know there is a warning?

McPherson: That varies widely by region and how the local emergency managers choose to handle it so we send out those initial warnings to emergency managers and then depending on the procedures for the region they might ring a tsunami siren or however they choose to evacuate. That’s mostly up to emergency managers.

What we tell people in general is if you’re in a warning that means inundating waves are possible and so it’s best to a mile inland and up to a hundred feet upslope if possible. And so if you’re in a warning, it’s time to get away from the coastline as quickly as possible.

How fast can tsunami waves travel?

MacPherson: The tsunami speed is based on the depth of the water that it’s propagating through, so the deeper the water, the faster the tsunami wave moves. So in the open ocean, like the Gulf of Alaska, this morning a tsunami wave travels about the speed of a jet airliner…so on the order of 800 kilometers per hour and so it is a very fast moving wave.

Is there anything else you think is important to add?

MacPherson: If it is a warning and you evacuate, it’s important to stay in a safe location until local emergency managers tell you it’s safe to return because tsunami waves are very complex and usually the first tsunami wave arrival will not be the last and often it won’t be the largest. It’s more like a train of waves that can arrive for several hours following the initial arrival. So you don’t want to make the mistake of one wave arrives and then you go back to the coastline. Just stay put until told that it’s safe and the all clear sounds.