When a magnitude 8.8 earthquake buckled the ocean floor off Chile on Saturday, there were concerns of a repeat of the 2004 disaster in which a giant earthquake off Indonesia generated a tsunami that killed thousands of people hours later in Sri Lanka and India.
In Hawaii, it was 8:34 p.m. Friday. The magnitude of an earthquake gives an expectation of the size of the resulting tsunami, and with an 8.8 magnitude, the scientists at the Pacific Tsunami Warning Center expected a dangerous tsunami.
Coastlines around the Pacific were put on alert, and the beaches in Hawaii were evacuated well in advance of the arrival of the tsunami. But the waves there turned out to be smaller than what was initially expected, causing little or no damage, pointing to the still incomplete knowledge in the art and science of tsunami forecasting.
Charles McCreery, the center’s director, said some early forecasts predicted that waves as high as eight feet could wash into parts of Hawaii. There were direct historical precedents. In 1960, a magnitude 9.5 earthquake off Chile, the largest earthquake ever recorded, generated a tsunami that killed 61 people in Hawaii and more than 100 in Japan. In 1837, a smaller quake, estimated at magnitude 8.5, also generated a deadly tsunami that hit Hawaii with waves as high as 20 feet.
But not all magnitude 8.8 earthquakes generate equally large tsunamis. If the earthquake occurs in shallower water, the uplift of the sea floor would displace less water, setting off a smaller tsunami. The seismic signals provide some clues, but not definitive information. Just five years ago, there would have been no midocean tsunami buoys between Chile and Hawaii, and forecasters would have been left guessing at the size of the waves until they hit.
This time, there was a buoy several hundred miles off Peru that recorded the tsunami as it sped by at more than 400 miles per hour, three hours after the earthquake.
“For this case, we had to pretty much base our forecast on one dot, because of the timing,” said Vasily V. Titov, a researcher at the Pacific Marine Environmental Laboratory in Seattle who developed one of the three models used by the warning center. In 2004, there were six tsunami buoys. Now there are 39.
The data from the one buoy was enough for the computer model to figure out that the tsunami was smaller and less destructive. At 6:24 a.m. Saturday in Hawaii, about five hours before the arrival of the tsunami there, the tsunami center put out a bulletin with predictions that the wave might reach four feet at Hilo, where the bay tends to amplify the waves, and much lower elsewhere.“In general, all of the numbers were bigger initially and went down,” Dr. McCreery said. The waves at Hilo were a bit less than three feet.
Dr. Titov said his model predicted the wave heights fairly accurately. This time, there were no deaths, and the tsunami pushed waters, at most, only a few feet above normal. “It looks like we nailed it, at least for U.S. coastlines,” Dr. Titov said.
But officials said the decision to order an evacuation in Hawaii, the first since 1994, was the right one given the uncertainties of the models.
“We’re still in the incipient stages of using these models to constrain our forecasts,” Dr. McCreery said. “There are still lots of improvements we need to make before we can rely on them totally for our decision making.” For one thing, the models do not provide estimates of how far off they might be.
“We had to do what we did, because there was too much uncertainty to say it was safe to not evacuate,” Dr. McCreery said. Dr. Titov agreed. He pointed out that the models indicated that the thrust of the tsunami’s energy passed south of Hawaii. When officials say that Hawaii dodged a bullet, “It’s almost literally true,” Dr. Titov said.