The Indian Ocean tsunami in 2004, one of the deadliest events in recorded history, was an enigma to many survivors. Some experts were surprised to learn that a significant number of the people in the path of those lethal waves had never heard of such a destructive phenomenon until it came their way.

“Tsunami is a Japanese word,” said Syamsidik, an engineer who now directs the Tsunami and Disaster Mitigation Research Center at Syiah Kuala University in Banda Aceh, Indonesia, and, like many Indonesians, uses only one name. At the time, he assumed that meant that only Japan needed to worry about the natural disaster. “It misled a lot of people. Including me.”

That changed the day after Christmas in 2004, when a 9.1-magnitude earthquake west of the Indonesian island of Sumatra set off a mammoth wave that was recorded as high as 16 stories, and in some places as fast as 300 mph, as it raced toward the shorelines of South and Southeast Asia and East Africa.

Earthquake sensors hinted at the potential for destruction and death. But tsunami experts watching that data did not know whom to tell. Their system showed no threats to coastal communities on either side of the Pacific Ocean, which then was the only region monitored for tsunami threats. There were few, if any, monitors in the Indian Ocean, and no models of what could occur there.

“It was unsettling,” said Laura Kong, director of the International Tsunami Information Center, which is hosted by the National Weather Service in Hawaii. “We were blind.”

Two decades later, scientists have made great strides in tsunami monitoring, modeling and forecasting. And organizations have bolstered education and preparedness at both the local and global scales. Emergency preparedness planners are working to ensure that what happened in 2004 never happens again.

For all their work since the Indian Ocean event, other tsunamis have caused serious damage, including the disaster in 2011 in Japan, highlighting the difficulty of achieving a world with no tsunami deaths.

Most tsunamis are caused by earthquakes, but anything that displaces a large amount of ocean water, including landslides, volcanoes and meteor strikes, can trigger a big wave. According to the National Oceanic and Atmospheric Administration, 78% of tsunamis from 1900 to 2015 occurred in the Pacific Ocean. Only 5% originated in the Indian Ocean.

The tsunami on Dec. 26, 2004, quickly escalated into a global disaster. Across 17 countries, about 230,000 people died and 1.7 million were displaced, mostly from Aceh, an Indonesian province on the northern tip of Sumatra. Property damage, amounting to $13 billion, left some towns unrecognizable.

Vasily Titov, a scientist at NOAA’s Pacific Marine Environmental Laboratory who simulates tsunamis, was shaken. “Theoretical knowledge is one thing,” he said. “Seeing what it was in real life is completely different.”

The 2004 wave, Titov said, revealed that the world’s tsunami warning system, started in 1965 to monitor threats in the Pacific Ocean, needed a critical upgrade.

The first step was to get better data. In 2000, six sensors were deployed in the Pacific to experiment with detecting tsunamis in the open sea, rather than when they reached the shore. NOAA began to expand this effort in 2005, and 10 other nations have followed suit, leading to the worldwide Deep-ocean Assessment and Reporting of Tsunamis, or DART, network.

Today, more than 70 DART sensors measure water temperature and pressure in places that are considered tsunami zones. NOAA also operates coastal water stations and satellites to monitor ocean height, roughness and tides.

That information is transmitted to warning centers that, in addition to the Pacific, now cover the Indian Ocean, the Caribbean, the northeastern Atlantic Ocean and the Mediterranean Sea. The data gets input into forecasting models that have been updated to better predict tsunami activity from earthquakes and nonseismic sources.

In the past two decades, forecasters have gone from not being able to predict the height of a wave to giving a “pretty good” estimate up to half an hour before it hits, Kong said.

Experts have also worked to establish safety guidelines and emergency response systems for coastal communities facing risk.

One major effort is an expansion of NOAA’s TsunamiReady program. That has led to more sirens, signage and evacuation routes being set up along coastal communities in the United States, including parts of Northern California, which experienced a tsunami scare this month. A United Nations body established a global version of this initiative in 2015, which now includes communities in more than 30 countries.

Scientists are developing more targeted response plans by assessing how far away different communities are from potential tsunami sources, which changes the amount of time they need to evacuate. They are also studying the people living in tsunami zones to understand how they respond during natural disasters, and to better address different levels of vulnerability.

“The warning systems are the quarterback, and for years we’ve been figuring out how to help them chuck the ball farther down the field,” said Nathan Wood, a geographer at the U.S. Geological Survey. “It’s only been in the past couple of decades that we’ve realized maybe we should look at the wide receiver.”

But there is still work to do. Global early warning systems are either fast or accurate, but struggle to achieve both, said Ardito Kodijat, head of the Indian Ocean Tsunami Information Center at UNESCO.

Despite extensive efforts, fatal tsunamis have happened since 2004.

Two earthquakes off Samoa triggered a wave in 2009.

Two years later, in 2011, an earthquake and tsunami in Japan killed nearly 20,000 people and led to a nuclear meltdown, despite a robust warning system and preparedness measures.

In 2018, Sulawesi, an island in the center of the Indonesian archipelago, was struck by an earthquake and tsunami that killed thousands.

“The events keep happening and keep claiming people’s lives,” Titov said, which is why, he added, many tsunami experts are ambitiously aiming for a system that results in zero deaths.

That’s challenging, Titov conceded, but he said that it’s a good goal to set. “We are not done until that is the case,” he said.

This article originally appeared in The New York Times.

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