Several hours after the Pacific Tsunami Warning Center issued an all-clear for Hawaii following a 7.5-magnitude earthquake in the Aleutian Islands today, it sent out a tsunami advisory.
The initial all-clear was issued shortly after the 10:55 a.m. quake, and the advisory was effective 4:38 p.m. after above-average wave heights were measured in Hawaii.
Scientists watching the gauges in Hawaii determined the wave heights, though small, were of advisory level in some north-facing harbors, including Kahului (15 inches) and Hilo (12 inches) harbors, and issued a tsunami advisory after the fact.
Waves can continue for several hours. In Hanalei Bay, it continued after three hours. The final all-clear was issued shortly after 7 p.m.
“We wanted to make sure it didn’t pop up waves considerably larger,” said Pacific Tsunami Warning Center geophysicist Kanoa Koyanagi.
He said it was a nonevent in Hawaii, and has not heard of any injuries. A major issue would be if boats were in a harbor “bouncing around,” or possibly people getting overwhelmed by a wave.
The warning center uses models to predict the wave height, and based on the initial data, they were predicted to be less than 12 inches. But the scientists followed their procedures and issued an advisory after actual wave heights were at or above advisory level.
The minimum 12-inch wave height advisory level is fairly new, he said.
“We wanted a level the authorities could use to keep people off the beaches,” Koyanagi said. “It was an issue in the past.”
The only two options were “no warning at all and a full-on tsunami event,” Koyanagi said, and civil defense and emergency managers in Hawaii agreed to the change.
They now had authority to evacuate coastlines and certain beaches and parks that might be in a higher inundation area, he said.
While this event for Hawaii was by no means threatening, Koyanagi said, “we need to be conservative.”
He said the event was not large enough to trigger enough dart buoys to give scientists enough data to work with, triggering only one in Alaska, which was too close to the quake.
“It was so close to the earthquake, waves were still resonating on the buoy,” Koyanagi said.
One buoy in Hawaii was triggered seven minutes after the quake.
The buoys are tough to maintain and there aren’t enough of them, he said.
When an earthquake occurs, scientists can manually trigger the dart buoy to send data more frequently, Koyanagi explained. But he said there is always uncertainty even with good data.
Tsunami warning science is “still a fairly young science,” Koyanagi said.
The temblor warranted a tsunami warning for 950 miles from 40 miles southeast of Homer to Unimak Pass, 80 miles northeast of Unalaska. It was later downgraded to an advisory.
The small tsunami waves in Hawaii did not affect nuisance coastal flooding, which is expected to continue Tuesday.