As the March 11 tsunami raced toward Hawaii, a NASA-French satellite called Jason-1 captured its progress from 830 miles up.
Jason’s sensitive radar altimeter, which measures small hills and valleys on the ocean’s surface, showed a phenomenon that startled scientists around the world: a long-hypothesized "merging tsunami" that doubled in intensity over ocean ridges.
The merging of separate arms of a tsunami was thought to be responsible for the deadly waves that hit Hilo in 1960 after a 9.5-magnitude quake in Chile. But for decades it was just a theory.
No more.
Jason-1 found not one wave, but at least two, merging into a single monster twice as high and capable of traveling long distances with undiminished power. Two other satellites, Jason-2 and EnviSAT, recorded the tsunami — moving at roughly 500 mph — before the merge.
"It was a 1-in-10-million chance that we were able to observe this double wave with satellites," said Yuhe T. "Tony" Song, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. "Researchers have suspected for decades that such ‘merging tsunamis’ might have been responsible for the 1960 Chilean tsunami that killed many in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now."
The study by Song and C.K. Shum, a professor of earth sciences at Ohio State University, first announced in December at a meeting of the American Geophysical Union in San Francisco, was published Thursday in the journal Geophysical Review Letters. Their collaborators include Ichiro Fukumori of JPL and Yuchan Yi at Ohio State.
The research sheds light on why tsunamis often travel long distances without losing power and devastate some coastal areas while leaving others relatively untouched. A process called "topographic refraction" occurs when tsunamis travel along ocean ridges and over seamount chains in the Pacific, causing the front to merge as it propagates, doubling the wave height and destructive potential in certain directions.
Jason-2 and EnviSAT measured wave heights of 8 inches and 12 inches, respectively. But as Jason-1 passed over a chain of seamounts southwest of Hawaii, it captured a wave front measuring 28 inches.
"We were very lucky not only in the timing of the satellite, but also to have access to such detailed GPS-observed ground motion data from Japan to initiate Tony’s tsunami model, and to validate the model results using the satellite data," Shum said. "Now we can use what we learned to make better forecasts of tsunami danger in specific coastal regions anywhere in the world, depending on the location and the mechanism of an undersea quake."
When the earth shook a year ago, the world wasn’t ready.
Japan, perched precariously on the Pacific’s "Ring of Fire," has more than 400 years of earthquake records, yet scientists never anticipated a "megaquake" of 9.0 magnitude in the Tohoku region off Honshu.
"In the past 400 years, there have been 13 magnitude-7s, five magnitude-8s — in the last 50 years also many large earthquakes," said Jim Mori of Kyoto University. "So people thought they understood quite well what was happening in this area."
As a result of the quake and tsunami a year ago today, scientists, engineers and civil defense experts have gone back to the drawing board as they assess future risks.
Hawaii Civil Defense, which has just finished new tsunami inundation maps based on a 9.0-magnitude quake in the Aleutians, is considering updating them yet again for a 9.2-magnitude quake. The Aleutians were the source of the most devastating tsunami to hit Hawaii, generated by a relatively mild 7.8-magnitude quake in April 1946.
"One major lesson that we learned from the tsunami was that the Japanese were not prepared for such a big event," said Kwok Fai Cheung, a Sea Grant researcher at the University of Hawaii at Manoa. "And I have been having discussions with state Civil Defense, trying to come up with a contingency plan for an event even more severe."
Vic Gustafson, interim vice director of state Civil Defense, said many aspects of the response to the 2011 tsunami went well.
Those include the "vertical evacuation" of Waikiki, with people leaving the lower floors of high-rises, a precaution also put into play during a tsunami alert on Feb. 27, 2010, when an 8.8-magnitude quake rocked Chile. The seismic waves that hit Hawaii that day were mild, but the response served as a useful dress rehearsal for last March 11, Gustafson said.
Another good thing: The city opened all 25 public evacuation centers, whereas the year before only five opened.
But Gustafson said the 2010 and 2011 quakes underscored a need for better directions for mariners.
"How far out do they need to go?" he said. "And when the all-clear is given, just because there is no threat of waves breaking and coming across the shore, that doesn’t mean the circulation of water and the eddies in the harbors has ended. You can say the land side is clear, but on the water you still have to be careful."
Boaters returned prematurely to Honokohau Harbor in North Kona, for instance.
"They hit some very, very bad surge," recalled Ed Teixeira, who left as vice director last fall. "We were lucky we didn’t lose anybody."
The Coast Guard is now conducting informational meetings on maps of proposed safety zones for commercial, recreational and military vessels.
And Civil Defense is working with the City and County of Honolulu on new ways to make sure warning sirens work. During the last verified siren test, on Feb. 1, 341 of 372 sirens sounded, or 92 percent, just above the typical rate of 91 percent. Of the 31 that didn’t work, 10 are permanently broken and due for replacement, Tom Simon of state Civil Defense said by email Thursday.
The agency has embarked on a pilot program to activate sirens via the Inmarsat satellite and Verizon cellular service, which also would give real-time information on which ones are not working, Simon said.
