Tsunami Science: Reducing the Risk
From Math to Maps
In communities such as Cannon Beach, Oregon, or Crescent City, California, or the Quinault Indian Nation in Washington, bright blue metal signs with a white, menacing ocean wave dot coastal streets. “Tsunami hazard zone,” they warn, or “evacuation shelter,” or “tsunami evacuation route.” A tsunami hasn’t affected the Pacific Northwest coast since 1964, when an earthquake and submarine landslides at Alaska’s Prince William Sound caused a significant one. Still, scientists are certain that these specific communities are at risk. Exhaustive computer modeling—mathematical simulations of where a likely wave will start, travel, and end up—tell NOAA, the U.S. government agency responsible for tsunami warning, where to stake the signs.
Communities that participate in NOAA's TsunamiReady program post warning and evacuation signs in low-elevation areas.
Lessons from Sumatra
Vasily Titov, a mathematician for NOAA’s Pacific Marine Environmental Lab in Seattle, is one of a handful of researchers worldwide who can craft a computerized tsunami. His expertise was put grimly to the test when NOAA learned that an earthquake had generated an enormous tsunami off the coast of Sumatra, Indonesia, on December 26, 2004. After eight hours of overnight work, Titov generated the first model to describe the wave’s travel speed, direction, and amplitude (height) in the open ocean.
But the tsunami raced faster than could Titov. It was slamming into Somalia and Kenya by the time the model was complete. “I had to start from scratch,” he explains. “If the event were in the Pacific Ocean, the model could have been done minutes after learning the magnitude and location of the quake.”
The delay was due to lack of data. The Indian Ocean has been studied far less than the Pacific, where the vast majority of tsunamis occur. The first piece of information Titov needed, he had—an initial seismic measurement of the earthquake. The quake’s magnitude roughly relates to how much seawater the shifting Earth’s crust could displace. (The first alert about the quake, after about fifteen minutes, described it as magnitude 8.0. The measure was refined four hours later to 8.9. New calculations published in Nature in March 2005, however, put it at 9.3—the second-largest earthquake ever recorded on a seismograph.)
Titov’s model applied equations incorporating Newton’s laws and wave physics to the data about this initial “bump” of water. The calculations described how the tsunami would likely propagate from its source. “But the nature of the tsunami wave is such that underwater topography, or bathymetry, defines the way it propagates,” explains Titov. The shapes of coastlines further transform a tsunami’s speed and size, as the sloping seafloor slows the wave and increases its amplitude.
