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TSUNAMI TRACKING


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Sparse data make it 'tricky'

Deep-water sensors did not help scientists make good forecasts

By Jim Borg

POSTED:
LAST UPDATED: 08:25 a.m. HST, Oct 29, 2012

<br />Craig T. Kojima / ckojima@staradvertiser.com<br />Officials are concocting ways to protect the large batteries that power emergency sirens at Haleiwa Beach Park after three thefts in the past three months.<br />

This was a horse of a different color.

The 7.7-magnitude earthquake that struck off British Columbia on Saturday — and which prompted a tsunami warning and statewide coastal evacuation — occurred in a spot where quakes like that are rare.

It was not, notably, a product of the Cascadia subduction zone, an active tectonic region that stretches from Vancouver Island to Northern California. Scientists have warned that Cascadia has the potential to generate a megaquake like last year's 9.0 off Japan, as it last did in 1700.

But few large quakes have been recorded just to the north, along the Queen Charlotte Fault, near the islands now known as Haida Gwaii. It was the largest quake in the area since an 8.1-magnitude quake in August 1949.

And none had triggered a tsunami warning — until Saturday.

"This was a tricky one," said Gerard Fryer, a geophysicist at the Pacific Tsunami Warning Center in Ewa Beach. "We knew this wasn't going to be a giant tsunami. The question was, Is it going to be big enough to cause flooding and require evacuation? It was right at the threshold."

The Queen Charlotte Fault is similar to the San Andreas Fault in California, where the plate boundaries slide horizontally. Earthquakes occur when pent-up pressure is released, but the mechanism usually doesn't produce tsunamis as do subduction zones, where part of the earth's crust creeps under another.

Initially the magnitude was estimated at 7.1. It also had an epicenter on land, which could mean that not a lot of water was displaced.

That's why there was no immediate alert regarding the quake, which hit at 5:04 p.m. Hawaii time.

After further analysis, the magnitude was raised to 7.7, and the estimated rupture zone was extended offshore.

Dennis Sinnott of the Canadian Institute of Ocean Science said a 27-inch wave was recorded off Langara Island on the northeastern tip of Haida Gwaii.

"It was not until we actually saw the tsunami on our deep-ocean gauges that we realized that it was bigger than we thought," said Fryer. "Unfortunately, the deep-ocean gauges gave us a bad angle on the tsunami."

Coastal tide gauges and deep-water sensors off Alaska and the West Coast didn't paint a good picture of the energy directed toward Hawaii.

When the scientists put what little data they had into computer models, some models predicted a wave run-up of 7 feet. After a discussion with state Civil Defense, a tsunami warning was issued at 7:09 p.m., more than two hours after the quake.

"Essentially there was no choice," said Fryer. "We had to go to a warning because we were uncertain. In retrospect the appropriate warning level for Hawaii would have been an advisory rather than a full-on warning."

An advisory means there is no threat inland, but people should stay off the beach and out of the water.

"We're trying to figure out if there is any way we could have known that with the information that we had," said Fryer. "We're still arguing about that."

If the quake had occurred in the Aleutian Islands — the source of the tsunami that hit Hawaii in 1946, or farther south near Washington or Oregon, there would have been "abundant measurements," said Fryer.

The lack of seafloor gauges meant the scientists had to take their best guess.

"We were prepared enough that we had models ready for the situation," said Fryer, "but in distributing the seafloor gauges, since there's only funding for a certain number of them, this was an area that was deemed of lower importance."

The deep-ocean gauges, called DART buoys, measure the tsunami as it passes by. There are eight off Alaska, four off the West Coast and one off Vancouver Island, but none between Alaska and Vancouver Island.

Charles "Chip" McCreery, director of the tsunami center, said Sunday, "We have to take another look at the orientation of our deep-ocean gauges to see if there are any changes we can make so we can cover an event like this. We've spent a lot of time figuring out how to deploy these around the Pacific so we get coverage. Unfortunately, for this event it went between the gauges."

The magnitude of the quake — not too big, not too small — also left room for doubt, Fryer said. "It was right there in the problem field."

Fryer said there are new analytical tools in the works that will help with earthquakes like Saturday's.

"What we don't know is the area that ruptured," said Fryer. "We don't exactly know how large an area of seafloor was deformed, and some of the new tools will help us with that."

When the waves arrived, beginning at about 10:30 p.m., they were about 12 to 15 minutes apart, an indication that much of the energy had bled off to the side.

Dangerous tsunamis typically have a period of 20 minutes, Fryer said.

"When it comes down to it in the end, I'm afraid there are going to be some unnecessary evacuations because we always want to err on the side of safety," he said. "But we are very aware that this was a major inconvenience to everybody and it involves significant costs to the state, so if we can avoid this in the future, we really want to."

He said the discussion will go on for some time.

"We'll let everyone sort of get some rest and probably (this) week talk about it," he said. "Typically we have a staff meeting on Wednesday, and we'll assemble our thoughts and compare notes with our partner warning center in Alaska."

Meanwhile Sunday, a 6.3-magnitude quake hit the Haida Gwaii region, following more than 40 aftershocks ranging up to 5.5.

———

Star-Advertiser reporter Sarah Zoellick contributed to this report.






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