Chile-Earthquake Tsunamis Smaller Than Expected—But Why?

Richard A. Lovett, National Geographic News 28 Feb 10;

The giant earthquake in Chile that struck Friday—one of the most powerful ever recorded—killed more than 700 people and leveled cities. Yet the tsunamis spawned by the earthquake were smaller than expected, leaving experts speculating as to why. (See Chile earthquake pictures.)

Tsunamis reached only 4 feet (1.2 meters) in Japan and 6.5 feet (2 meters) in the South Pacific island of Tonga, according to scientists. Tsunamis can often become monster waves of more than 100 feet (30.5 meters).

Furthermore, despite a massive evacuation of Hawaii, tsunamis in Hawaii measured only about three feet (one meter), too small to do any damage.

But this doesn't mean the tsunamis in Hawaii fizzled, said Costas Synolakis, director of the Tsunami Research Center at the University of Southern California.

Rather, he said, the tsunamis were only slightly smaller than the 4-foot (1.25-meter) waves predicted by computer models.

"The main story here, I think, is that the full evacuation of Hawaii was unnecessary," Syolakis told National Geographic News by email.

Geophysicist Emile Okal agreed that the the tsunami spawned by the Chile earthquakei wasn't really all that small. "It is much larger than anything we've seen in the Pacific in 45 years," said Okal, of Northwestern University in Evanston, Illinois.

The tsunami only seems small, he added, because Hawaii and Japan were both spared its brunt—and because those places both remember much bigger tsunamis from 1960 and 1946. "[These] are what the public has the memory of, because [they] wrecked Hawaii," he said.

Although it will be a while before geophysicists understand for sure why the tsunami wasn't larger, Okal speculates that part of the reason may be that the tsunami was generated in a relatively shallow part of the Pacific off Chile.

"Locally this doesn’t change anything," he said. "But as the tsunami propagates into the really deep water of the Pacific Basin, [the shallower origin] does decrease its amplitude somewhat."

Predicting Tsunamis From Chile Earthquake Difficult

The decision to evacuate Hawaii was based purely on the tremendous power of the earthquake in Chile, which at magnitude 8.8, was the fifth largest in the past 110 years.

The 2010 earthquake also rocked the same region as a magnitude 9.5 earthquake in 1960, which sent waves racing across the Pacific. The 1960 tsunamis killed 61 people in Hawaii, 138 in Japan, and left 32 dead or missing in the Philippines.

Predicting a tsunami from seismic information alone is difficult, scientists say, because the earthquake might have vibrated the seabed up and down, or sideways.

"If all the movement is horizontal, you will have [no] tsunami," said Solomon Yim, interim director of the Hinsdale Wave Research Facility at Oregon State University.

To account for this, tsunami predictions combine seismic information with real-time measurements from seabed instruments so sensitive they can measure the pressure differences from passing waves thousands of feet above. (Take a tsunami quiz.)

"Measurements from these buoys, located in deep water, allow us to estimate the wave size and update the forecast models," the University of California's Synolakis said. "[They also] allow us to estimate the duration of the event and the numbers of waves that get triggered."

Accurate Tsunami Forecasts

And while these instruments are few and widely scattered, some of the predictions made about tsunamis from the earthquake in Chile were actually close to correct.

For example, he said, the model predicted that the earthquake would create small waves in Los Angeles harbor—exactly what happened.

"The forecast was quite accurate," Synolakis said.

Some scientists defend tsunami warnings
Gillian Flaccus Associated Press Google News 1 Mar 10;

HONOLULU — The warning was ominous, its predictions dire: Oceanographers issued a bulletin telling Hawaii and other Pacific islands that a killer wave was heading their way with terrifying force and that "urgent action should be taken to protect lives and property."

But the devastating tidal surge predicted after Chile's magnitude 8.8-earthquake for areas far from the epicenter never materialized. And by Sunday, authorities had lifted the warning after waves half the predicted size tickled the shores of Hawaii and tourists once again jammed beaches and restaurants.

Scientists acknowledged they overstated the threat but many defended their actions, saying they took the proper steps and learned the lessons of the 2004 Indonesian tsunami that killed thousands of people who didn't get enough warning.

"It's a key point to remember that we cannot under-warn. Failure to warn is not an option for us," said Dai Lin Wang, an oceanographer at the Pacific Tsunami Warning Center in Hawaii. "We cannot have a situation that we thought was no problem and then it's devastating. That just cannot happen."

Hundreds of thousands of people fled shorelines for higher ground Saturday in a panic that circled the Pacific Rim after scientists warned 53 nations and territories that a tsunami had been generated by the massive Chilean quake.

It was the largest-scale evacuation in Hawaii in years, if not decades. Emergency sirens blared throughout the day, the Navy moved ships out of Pearl Harbor, and residents hoarded gasoline, food and water in anticipation of a major disaster. Some supermarkets even placed limits on items like Spam because of the panic buying.

At least five people were killed by the tsunami on Robinson Crusoe Island off Chile's coast and huge waves devastated the port city of Talcahuano, near hard-hit Concepcion on Chile's mainland.

But the threat of monster waves that left Hawaii's sun-drenched beaches empty for hours never appeared — a stark contrast to the tidal surge that killed 230,000 people around the Indian Ocean in 2004 and flattened entire communities.

This time, waves of more than 5 feet were reported in Kahului Bay in Maui and in Hilo, on the eastern coast of Hawaii's Big Island, but did little damage. Predictions of wave height in some areas were off by as much as 50 percent.

In Tonga, where up to 50,000 people fled inland hours ahead of the tsunami, the National Disaster Office had reports of a wave up to 6.5 feet hitting a small northern island, with no indications of damage.

And in Japan, where authorities ordered 400,000 people out of coastal communities, the biggest wave was a 4-foot surge that hit the northern island of Hokkaido, flooding some piers.

A Japanese official offered an apology to those affected after the government had warned that waves of up to 10 feet (three meters) could hit some northern regions.

"The tsunami estimates of the Meteorological Agency were too large, and so I'd like to apologize to individuals that were evacuated or inconvenienced," Sekita Yasuo, an official at the agency, told reporters Monday.

He said the agency compared its estimates to those from abroad and chose the larger of the two, leading to the overzealous forecasts, and that he wanted to improve accuracy in the future.

After the Pacific Tsunami Warning Center lifted its warning, some countries kept their own watches in place as a precaution. Early Monday, the Japan Meteorological Agency warned of a possible tsunami about a foot and a half in size along its entire Pacific coast and told people to stay away from the waterfront. That warning was cleared later Monday morning.

But scientists offered no apologies for the warnings and defended their work, all while worrying that the false alarm could lead to complacency among coastal residents — a disastrous possibility in the earthquake-prone Pacific Rim.

A similar quake in Chile in 1960 created a tsunami that killed about 140 people in Japan. The same surge hit Hawaii and devastated downtown Hilo, on the Big Island, killing 61 residents and wiping out more than 500 homes and businesses.

"If you give too many warnings and none of them materialize, then you lose your credibility," Wang said. "That's something that we have to deal with and we have to improve."

Despite some of the panic in Hawaii, public officials called the evacuation "perfect" and said it was a good test case that proved the system worked.

Chaos was at a minimum as people heeded evacuation orders and roads were free of the gridlock that can paralyze a region before a disaster. The smooth response occurred largely because the state had so long to prepare; Hawaii is nearly 7,000 miles from where the quake hit, and it took 15 hours for the tsunami to arrive.

"I hope everyone learned from this for next time, and there will be a next time," said Gerard Fryer, a geophysicist for the warning center.

The science of predicting tsunamis is difficult, given the vast size of the ocean and the volatile forces at work miles below the surface.

Scientists use an earthquake's magnitude and location as the basis for their predictions and then refine it constantly with data from more than 30 deep-water sensors stationed across the Pacific as the shock wave sweeps across the ocean floor.

The sensors, located at 15,000 to 20,000 feet beneath the surface, measure the weight of the water and beam it to buoys floating on the surface. Scientists then use the data to calculate the tsunami's wave height in the open ocean as it progresses toward shallower waters.

Coastal inundation models based on topographic mapping add another layer of analysis, helping scientists make assumptions about how the surge will behave in shallower waters and how it might affect shoreline communities.

"There are all sorts of assumptions that we make in trying to figure out how big the waves are going to be. If we can avoid some of those assumptions, maybe we can do a better job," said Fryer.

"If this event happened tomorrow, even with this knowledge, we would be forced to do the exact same thing."

Those models could be more accurate if scientists had more deep-water sensors and could build coastal inundation models for vast parts of the Pacific Rim where the topography hasn't yet been well-surveyed, Wang said.

Because complete data doesn't exist for every coastal area, scientists must play it safe in their wave predictions, he said.

"Even for Hawaii, we only have a forecast for less than 10 locations, we don't have inundation models for every coastal point in Hawaii and it's the same story for the U.S. mainland," Wang said. "We've got to be a little conservative. One point doesn't tell you that's going to be the maximum everywhere else."

In areas where inundation models exist, scientists' predictions were close to accurate, Wang said.

Residents and tourists alike in Hawaii said they weren't bothered by the evacuation and supported the scientists' actions — even though the waves never showed up.

Eugene Okamoto, 33, said he came to Honolulu from Hilo to visit some tourist attractions with his father and was disappointed the two had to cancel their plans because of the evacuation orders.

But Okamoto said his family understands the tsunami threat better than most because some of his relatives lived through the tidal surge in 1960. They remember how the water was sucked down the beach moments before the wave hit.

"My uncle was on the top floor when all the water washed away and all the kids ran out to grab the fish and before they could get back, the wave came. He was way up top, he saw all his friends get washed away and none of them were found, ever," Okamoto said, as he sat with his father in a hotel lobby. "They did the right thing."

Associated Press Writer Jaymes Song contributed to this report.

Praise for nations' tsunami emergency response
Yahoo News 1 Mar 10;

WELLINGTON (AFP) – Officials in Australia, New Zealand and the Pacific Islands praised emergency authorities Monday for their handling of the tsunami warning following the massive earthquake in Chile.

But they slammed the many "stupid" members of the public who ignored the warnings for putting themselves and others at risk.

Alerts about a potentially destructive tsunami were issued throughout the region after the 8.8-magnitude quake struck just of the coast of Chile and in many areas people were evacuated from low-lying coastal areas. Related article: Search for Chile quake survivors

The worst fears were not realised with the resulting tsunami causing surges less than half a metre high in most places on Sunday New Zealand time, although there were reports in French Polynesia of damage from a series of two-metre (more than six feet) waves and one localised four-metre surge.

The warning was the third regionwide tsunami alert in five months and authorities said Monday lessons had been learned following criticism they had been slow to act in the past.

"It's a huge step up from where we've been," New Zealand's Civil Defence Minister John Carter said.

"Civil defence structures in the regions, along with the police, the fire service and the coastguard, and other authorities, all responded particularly well," he said.

In September last year a tsunami triggered by an 8.0-magnitude quake smashed into Samoa, American Samoa and northern Tonga, killing 186 people and sparking a regionwide tsunami alert.

Authorities in New Zealand and some Pacific Islands were criticised for their slowness in getting warnings out to the public, and there were problems again in October when a series of three quakes with magnitudes of more than 7.0 centred near Vanuatu triggered another regional alert.

But although happy with the response of emergency services after the Chile quake, New Zealand's Carter said some "stupid" people were lucky to been unhurt.

"Amid reports of people going to the beach or spectating, we also had reports of people getting caught in the powerful water surges," Carter said.

"There was definitely potential for loss of life in our waters and it is a credit to the team who managed this event that didn?t happen."

In Australia, many surfers and swimmers ignored warnings of surges and dangerous currents, despite the early warning system working well, experts said.

"A tsunami warning is a very important public announcement -- it is not made without a lot of careful consideration. Sadly, the public did not seem to agree," said James Goff, co-director of the Australian Tsunami Research Centre at the University of New South Wales.

Goff said more public education was needed to ensure people took tsunami warnings seriously.

In the Fijian capital Suva, many people gathered near the sea wall despite being warned away by emergency services.

But in Samoa and Tonga, with the memory of September's disastrous tsunami still fresh, residents were quick to flee for higher ground before dawn as sirens sounded.

"The public did not respond well in regards to that September 29 warning. This time around it was the complete opposite," Samoan Meteorological Office head Mulipola Ausetalia Titimaea told Radio New Zealand.

Japan apologises for major tsunami alert
Yahoo News 28 Feb 10;

TOKYO (AFP) – A Japanese official admitted Monday that the authorities may have been over-zealous in issuing their first major tsunami alert in more than 15 years for a wave that ended up causing almost no damage.

"The agency's tsunami forecasts turned out to be a bit too big. I'd like to apologise for the prolonged alerts," Yasuo Sekita, a Meteorological Agency official in charge of earthquakes and tsunamis, told a news conference.

Authorities on Sunday ordered more than half a million people to evacuate seaside areas and predicted that the tsunami sparked by Chile's massive earthquake might top three metres (10 feet) by the time it reached Japan.

When other Pacific-Rim nations had already sounded the all-clear, Japanese officials were still issuing warnings, as television stations provided non-stop live coverage with their cameras focused on the calm ocean.

When the tsunami arrived in the early afternoon, it was just 30 centimetres (one foot) high. Waves up to 1.2 metres high later inundated some port-side areas, but in caused no injuries or major property damage.

Cautious to the end, Japan's Meteorological Agency did not lift its last regional tsunami alerts until 10:15 am (0115 GMT) on Monday, after tens of thousands of people had spent the night in evacuation shelters.

But many Japanese would say authorities are right to err on the side of caution. Japan is a world leader in disaster preparedness, for good reason.

The island-nation of 128 million people, located on the intersection of several tectonic plates and dotted with a string of active volcanoes, is hit by about 20 percent of the world's most powerful earthquakes.

In the last major quake, in the Kobe area in 1995, about 6,400 people died.

Greater Tokyo, with 35 million people, is waiting for "The Big One", a monster quake of the scale of the 1923 Great Kanto Earthquake that claimed more than 140,000 lives, many of them burnt to death in wooden houses.

When news came Sunday of the tsunami from Chile, it brought back bad memories. In 1960, a 9.5-magnitude earthquake in Chile, the largest on record, sent a tsunami across the Pacific that killed more than 140 people in Japan.

Quake and tsunami experts stressed on Sunday that in the tsunami half a century ago, the initial waves had been deceptively small, but were later followed by monster waves up to four metres high.