Nor was the technology for surveying the aftermath of the tsunami very sophisticated. For a week, Fritz and his partners traveled along Sri Lanka's southern coast, measuring high-water marks on buildings by dangling yellow tape measures from the filigreed balconies of upper-class homes. They examined debris carried inland, trying to assess how far it had been transported by the water. And they interviewed survivors. In one village, a man demonstrated how he scrambled 16 feet up a slender palm tree to escape the rushing water. Photos show a tanned, khaki-clad Fritz balancing a tape measure against trees trimmed with flip-flops and ripped clothing. Using the data they gathered in this fashion, his group could roughly estimate the maximum height of the tsunami, how fast it moved, and where it struck.

These were the same methods that had been used in the past, including six years earlier, when a tsunami struck Papua New Guinea. But now there was a new technology: Video cameras had become ubiquitous among people of means.

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With macabre serendipity, the tsunami had hit during the peak of the holiday travel season. In Khao Lak, Thailand, dozens of foreign vacationers narrated their bewilderment, in German and English, as they walked toward the empty shoreline where the water was receding, pulled back by an unseen force. They zoomed in on ships ascending like balloons on the ocean's surface and fishing boats capsizing. Was ist das? they asked. As they realized what was happening, they began running inland. Then came the force of the wave and a terrifying chaos.

For days, footage from shaky camcorders looped on the TV news. As soon as Fritz saw the harrowing videos, he knew they would provide a scientific opportunity.

The back-of-the-envelope calculations made by the scientists relied on very rough information gathered by questioning those who had been on the scene—like the tree-climber in Sri Lanka. “How many waves, how quickly did the flood come in? What direction? Show us how deep it was,” Synolakis says. “I've spoken to hundreds of eyewitnesses around the world, and it's extremely difficult to get that information. People were running for their lives.” With the images, the scientists had a more reliable witness. “Videos are an incredible advancement.”

After Sri Lanka, the team was dispatched to the Maldives and then Banda Aceh, Indonesia, where the largest number of people had been killed. Though most of the tsunami footage came from tourists in Thailand, some survivors had filmed the water as it moved through the Indonesian city's streets, more than a mile and a half inland. “We don't have the scale of how big the wave was on the beach in Banda Aceh, because nobody survived it,” Fritz says.

In Thailand, vacationers narrated their bewilderment in German and English. They zoomed in on ships ascending like balloons on the ocean's surface and fishing boats capsizing. Was ist das? they asked.

Fritz bookmarked a video he found online of ocean water flowing through the streets of the city. It was filmed in a residential area by a local government worker who was trapped in his house when the tsunami arrived. In Banda Aceh, Fritz found the man and interviewed him. He also surveyed the locations in the video, calibrating the perspective and the motion of the camera. “It's incredibly detailed work,” says Synolakis. The result was a more precise measure of how fast the water moved through parts of the city: 10 to 15 meters per second. Many of the survivors Fritz spoke to had seriously underestimated the tsunami's speed as they watched the water flooding up streets from a distance, their eyes unable to gauge the wave's momentum. “It looks like it's moving slowly initially, similar to watching an airplane approach a runway,” Fritz explains.

By analyzing the videos, the survey team also made an important discovery about the waves. Before they saw the footage, Fritz and Synolakis couldn't understand why so many people had taken photos posing with the wave behind them, out on the open ocean. For some victims, those pictures would be the last of their lives. But the analysis showed that while normal waves slow when they hit dry land, tsunamis accelerate, catching people by surprise. “This is a huge observation that was made possible with the videos from 2004,” says Synolakis. Despite hundreds of interviews, “not a single eyewitness ever told us that the tsunami had accelerated once it hit the coastline.”